tepley et al. (2013)

Magma Dynamics and Petrological EvolutionLeading to theVEI 5 2000 BP Eruption of ElMistiVolcano, Southern Peru

FRANKJ. TEPLEY, III1*, SHANAKA DE SILVA1 AND GUIDO SALAS2

1COLLEGE OF EARTH, OCEAN, AND ATMOSPHERIC SCIENCES, OREGON STATE UNIVERSITY, CORVALLIS, OR,

97331-5506, USA2DEPARTMENTO DE GEOLOGIA, UNIVERSIDAD NACIONAL DE SAN AGUSTIN, AREQUIPA, PERU

RECEIVED JULY 7, 2011; ACCEPTEDJUNE 17, 2013

Magma dynamics and time scales during the VEI 5, 2000 BP erup-

tion of El Misti volcano, southern Peru (EM2000BP) are investi-

gated to address cyclic explosive activity at this hazardous volcano.

The 1·4 km3 of pumice falls and flows have abundant mingled

pumice of high-K, calc-alkaline rhyolite and andesite composition.

Phenocryst zoning and compositions reveal mutual exchange of

plagioclase between the two magmas; amphibole in the rhyolite was

derived from the andesite. Amphiboles in the andesite are predomin-

antly unrimmed crystals whereas those in the rhyolite mostly exhibit

reaction rims. Phase equilibria indicate that the andesite formed at

�900^9508C and 2^3 kbar pressure and was water-saturated with

5·1^6·0 wt % H2O, broadly similar to El Misti magmas overall.

Amphibole, plagioclase, Ti-magnetite, and two pyroxenes were the

crystallizing phases. A separate rhyolite magma existed higher in

the crust at a temperature of 816� 308C and �5% H2O in which

only plagioclase and Fe^Ti oxides were stable.The lack of cognate

amphibole in the rhyolite despite H2O saturation requires that it

staged above the stability limit of amphibole (5100MPa).

Exchange reactions in amphibole (dominantly pargasitic) and

trace element partitioning in plagioclase indicate that both andesite

and rhyolite magmas were broadly constant in temperature and

H2O content. These constraints suggest that the initially separate

rhyolite and deeper andesite magmas interacted by an initial andesite

recharge event that resulted in mingling and crystal exchange. A

period of 50^60 days is required for amphibole introduced into the

rhyolite to develop reaction rims owing to decompression.These rims

are dominated by plagioclase, a consequence of the Al-rich nature of

the amphibole.The lack of reaction rims on amphibole in the andesite

implicates a second, more-forceful and voluminous eruption-trigger-

ing recharge event during which andesite rose rapidly from source to

surface in �5 days at ascent rates of at least 0·023 m s�1. Further

decompression-driven crystallization is recorded in plagioclase rims

and microlite growth that may have contributed to a rapid increase

in viscosity leading to explosive eruption.This VEI 5 plinian erup-

tion shares characteristics with other explosive events at El Misti

on a time scale of 2000^4000 years, suggesting periodic recharge-

driven explosive activity.

KEY WORDS: El Misti; explosive eruption; amphibole reaction rims;

trace element partitioning in plagioclase; magmatic time scales; recharge

I NTRODUCTIONMajor composite cones, among the most hazardous volca-noes on the planet, are the integrated product of a pro-longed history of effusive cone building activitypunctuated by explosive eruptions and edifice collapses(Davidson & de Silva, 2000). Although the eruptive styleand attendant hazard is dominated by effusion, the rareexplosive eruptions are often the most voluminous andhazardous. Understanding the controls on this transitionin activity is central to our efforts to fully address mag-matic and volcanic evolution and hazard mitigation. Twoimportant clues to this effort are that explosive activity iscyclic or quasi-cyclic (Matthews et al., 1997; Davidson &de Silva, 2000; Ruprecht & Wo« rner, 2007) and involvesrecharge, suggesting that the rhythm of open magmaticsystems is a dominant driver.

*Corresponding author. Telephone: 541 737 8199; Fax: 541 737 2064;E-mail: [email protected].

� The Author 2013. Published by Oxford University Press. Allrights reserved. For Permissions, please e-mail: [email protected]

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It has long been recognized that magmatic recharge cantrigger explosive eruptions of a perched magma throughthermal, mass, and volatile exchange that results in pres-surization of the system, and viscosity changes that resultin rheological and mechanical eruptive thresholds beingexceeded (e.g. Sparks et al., 1977; Blake, 1984). However, de-tailed studies of magmatic systems and single eruptionsreveal that these first-order results can be achieved in amyriad of ways: mafic^mafic, mafic^silicic, and silicic^sili-cic interactions (Sparks et al., 1977; Eichelberger, 1978;Feeley & Dungan, 1996; Eichelberger et al., 2000; de Silvaet al., 2008). Resident and recharge magma may or maynot achieve thermal and chemical equilibrium (e.g.Pichavant et al., 2007). Exchange of crystals and redistribu-tion is common (e.g. Davidson & Tepley, 1997; Ruprechtet al., 2008).The scale of mixing and its controls on thermalexchange and rates of equilibration, and changes in viscos-ity are particularly important (Huppert et al., 1982; Sparks& Marshall, 1986; Snyder & Tait, 1995; Ruprecht &Bachmann, 2010). All these processes are recorded in thejuvenile materials and revealed through detailed multi-scale petrological studies (e.g. Tepley et al., 1999). Whenbased on a strong stratigraphic and volcanological founda-tion, such petrological studies form a crucial part of theoverall hazard assessment.El Misti volcano (herein referred to as El Misti) in

southern Peru is one of the most hazardous volcanoes inSouth America (de Silva & Francis, 1991a, 1991b; Thouretet al., 2001; Harpel et al., 2011). Here, a population of4800 000 live in Peru’s second largest city, Arequipa,within 15 km of El Misti’s summit vents. During its �112kyr eruptive history, at least three major and several smal-ler explosive eruptions have punctuated the effusive back-ground activity. Reconnaissance of these eruptions hasrevealed macroscopic evidence for magma mingling(Legros, 1998; Thouret et al., 2001) and petrological studiesof plagioclase from various eruptions have revealed thatthese eruptions are preceded by multiple magma rechargeevents that eventually precipitated the respective eruptions(e.g. Ruprecht & Wo« rner, 2007). If these observations holdup to detailed scrutiny, the processes that drive explosivevolcanism at El Misti can be placed in the broader contextof the magmatic evolution of the system. To date no fullycontextual detailed study of an explosive eruption at ElMisti has been conducted.The most recent explosive eruption at El Misti is the

VEI 5, 2000 BP eruption (Thouret et al., 2001; Harpelet al., 2011), the products of which are exposed in multipledrainage canyons on the south and west flanks of the vol-cano. This was a plinian fall and flow mixed rhyolite^andesite tephra eruption, hypothesized to have involved arecharge event based on abundant macroscopic and micro-scopic evidence from mixed pumices. As such the eruptionserves as a potential model for the other explosive

eruptions at El Misti. Herein we report the results of a de-tailed petrological study of the 2000 BP eruption at ElMistiçthe first of its kind.We establish the magmatic con-ditions of the andesite and rhyolite reservoirs, and thephysical, chemical, and mineralogical signals of theirinteraction, and provide constraints on the timing of theevent that led to the eruption. This work provides valuablepetrological context to a case study of the stratigraphyand volcanology and hazard assessment of the eruption(Harpel et al., 2011).

GEOLOGICAL SETT INGEl Misti (16·2948S, 71·4098W; 5822m above sea level) is amajor volcanic edifice of the Central Volcanic Zone of theAndes (Bullard,1962; de Silva & Francis,1991a) in southernPeru lying less than 15 km from the city of Arequipa(Fig. 1). It is located within the Andean arc, and its historyis one of constructive dome growth, lava flows and explo-sive volcanism, endangering the growing populationcenter of Arequipa nearby (de Silva & Francis, 1991a;Thouret et al., 2001; Harpel et al., 2011).The geological history of El Misti is one typical of

Andean arc volcanoes. Based on extensive field mapping,40Ar/39Ar and 14C dating of rocks and organic material,Thouret et al. (2001, and references therein), PaquereauLebti et al. (2006) and Ruprecht & Wo« rner (2007) havepieced together a comprehensive volcanic history for ElMisti. The earliest remnant (c. 112 ka) of El Misti is aneroded stratovolcano (Misti 1) that unconformably overlieslavas and volcaniclastic deposits of Chachani Volcano(Paquereau Lebti et al., 2006; Ruprecht & Wo« rner, 2007).Upon this edifice lie successive edifices, termed Misti 2,Misti 3, and Misti 4, and lava flows and pyroclastic debriserupted since 112 ka. Historically, the volcano-buildingevents of El Misti are associated with alternating growthand destruction of andesitic and dacitic domes and lavaflows with dome collapses and associated pyroclasticflows, intermixed with explosive episodes, and avalanchedeposits (Thouret et al., 2001, and references therein;Ruprecht & Wo« rner, 2007). Thouret et al. (2001) suggestedthat, on average, ash falls occur every 500^1500 years,with pumice fallout-producing eruptions every 2000^4000years.The 2000 BP eruption is a plinian eruption producingpumice falls and flows with varying proportions ofbanded pumice of rhyolite and andesite compositionsamounting to �1·4 km3 of material (0·5 km3 dense rockequivalent; Thouret et al., 2001; Harpel et al., 2011). Exten-sive lahars were generated by interaction of pyroclasticflows with snow on the volcano, attesting to the potentialhazard of explosive eruptions at El Misti (Harpel et al.,2011).Over the course of its history, El Misti has produced

relatively homogeneous andesites and dacites with onlya few rhyolites. Thouret et al. (2001) noted that the

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heterogeneous textures of the banded andesites and rhyo-lites of the 2000 BP eruption are unique to El Misti com-pared with other volcanoes in the region, in both textureand the presence of a distinct mineral suite. For the pur-poses of our study, 50 samples were collected from through-out the eruption stratigraphy and studied to establish the

range of textures and mineralogy (compositions). Of thesefour were chosen to represent the end-member texturesand compositions: two were dominantly of the white rhyo-lite component, and two others were composed primarilyof the black to brown andesite component. Each represen-tative sample contains some mingled white and dark

(b)

(c)

10°

0°

10°

20°

30°

40°

50°

70°

70°

NVZ

CVZ

SVZNazca Plate

Antarctic Plate

Chile Rise

Nazca Ridge

Caribbean Plate

Peru-C

hile Tren

ch

El Misti

(a)

Fig. 1. (a) Map showing the location of El Misti in South America and its location in the CentralVolcanic Zone. (b) Image of El Misti and thesurrounding region (from Harpel et al., 2011). Irregular white regions in bajadas are pyroclastic-flow deposits from the EM2000BP eruption.Outlined by a white line is the city boundary of Arequipa. (c) Photograph of El Misti taken from downtown Arequipa, illustrating the proxim-ity of a large population center to a potentially explosive volcano.

TEPLEY et al. EL MISTI 2000 BP ERUPTION

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component.The results reported in this study are from thinsections of these four samples.

ANALYT ICAL METHODSA total of seven samples from the two lithological end-members were analyzed for their major- and trace-elementcompositions at the GeoAnalytical Lab at WashingtonState University, by X-ray fluorescence spectroscopy(XRF) and inductively coupled plasma mass spectrometry(ICP-MS) techniques. Details of the techniques and theirassociated analytical errors have been given by Johnsonet al. (1999) and Knaack et al. (1994), respectively.Petrographic descriptions of the four representative sam-

ples provide records of the constituent phases, their abun-dance, and their textural relationship to the other phases.Detailed analyses of minerals and glasses were performedat Oregon State University using a CAMECA SX-100electron microprobe (EMP) equipped with five wave-length-dispersive spectrometers (WDS) and high-intensitydispersive crystals for high-sensitivity trace element ana-lysis. Minerals and groundmass glasses were analyzedusing 15 keV accelerating voltage, 30 nA sample current,and 1 mm beam diameter for mineral phases and 5 mm forgroundmass glasses. Counting times ranged from 10 to60 s depending on the element and desired detection limit.In all cases, zero-time intercept functions were applied toreduce the effects of alkali migration. Data reduction wasperformed online using a stoichiometric PAP correctionmodel (Pouchou & Pichoir, 1984). Back-scattered electron(BSE) images were obtained using the same instrumentusing the CAMECA Peak Site software. Precision meas-urements for the most significant elements in the glass,feldspar, amphibole, pyroxene, and Fe^Ti oxides routinesare listed inTables 5, 8, 6, 3 and 4, respectively.Because some amphiboles in the selected samples exhibit

reaction rims whereas others do not, several amphibolesfrom each lithology were selected for in situ trace elementanalysis to determine population identity. Following EMPanalysis, analysis spots were chosen where EMP dataexisted and in selected cores, mid-sections and rims of theamphiboles. The analyses were carried out by laser abla-tion (LA)-ICP-MS in the Keck Collaboratory for PlasmaSpectrometry, Oregon State University, using a NewWaveDUV 193 nm ArF Excimer laser at 5 hz frequency, 15 nspulse duration and 50 mm beam size attached to aVG PQExCell Quadrupole ICP-MS system and following thetechniques outlined by Kent et al. (2004). Concentrationsof single trace elements were calculated employing 43Caas an internal standard relative to the USGS glass stand-ard BCR-2G. External errors are dependent on elementalconcentrations in the samples; however, calculated errorsare typically �5% for Sc, Cr, Rb, Y, Zr, Nb, La, Ce, Pr,Nd, Sm, Eu, Gd, Dy, Er,Yb, and Pb, and �10% forV, Sr,and Ba (1s).

RESULTSLithology and whole-rock texturesBoth pyroclastic flow and fall deposits contain juvenileclasts that display abundant evidence for magma mingling.Two end-member lithologies, a plagioclase^amphibolerhyolite and a plagioclase^amphibole andesite, are foundintimately mingled at different scales. Both are moderatelyporphyritic. No pure end-member clasts were found, andall the clasts show some mingling.The rhyolite forms a dis-tinct pervasively micro-vesicular pumiceous lithology,whereas the andesite occurs as a more obviously vesicularscoriaceous lithology. A wide range of mingling relation-ships can be seen, from rhyolite-dominated to andesite-dominated (Fig. 2). Evidence of mingling is abundant inhand specimen as millimeter-scale wisps and selvages.Andesite within dominant rhyolite tends to be in linearwisps, selvages, and bands, but more complex relationshipsare displayed as the lithologies become more andesite-dominated. Complex sheath folding relationships can beseen and thicker (centimeter-scale) bands of rhyolite showclear evidence of ductile deformation with recumbentfolds (Fig. 2). Complex crenulation develops on rhyoliteselvages included in andesite. In several instances wefound that some of the rhyolitic wisps were rooted indense rhyolitic clasts (centimeter scale), which were beingdisaggregated at their margins and being incorporatedinto the andesite. Some grey selvages may represent ahybrid lithology. We did not find any systematic strati-graphic variations in the distribution of the lithologies ineither the fall or flow deposits.Diverse textural features characterize both the rhyolitic

pumice and andesitic scoria in thin section. Some clastsshow uniform distribution of a range of vesicle sizesthroughout the slide, but more commonly, particularly inthe rhyolite, heterogeneous clasts show distinct regionswhere small bubbles (diameters �5^25 mm) predominateand are surrounded by a matrix with intermediate-size tocoarse vesicles (75^100 mm and 175 mm diameters, respect-ively). Independent of the degree of heterogeneity, amarked predominance of intermediate-size to coarse ves-icles is conspicuous within some slides. The andesiticscoria is characterized by largely equant to sub-sphericalvesicles with limited evidence for bubble deformation.However, bubble deformation is ubiquitous in the rhyolitepumice, which typically exhibits bands of elongated ves-icles crossing larger regions with more equant bubbles.The bands tend to range in width from �50 to 500 mm,suggesting the presence of localized shear zones on arange of scales.

Whole-rock geochemistryOf the 50 collected samples of the eruption, seven sampleswere chosen for major- and trace-element analysis. Thesewere chosen to check and supplement existing data fromthis


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eruption.All samples are typicalmedium- tohigh-K calc-al-kaline CentralVolcanic Zone (CVZ) andesites and rhyolites(Fig. 3; Table 1). Our samples from the EM2000BP eruptionfall ina similar range inaplotofK2Ov.SiO2asother samplesfrom the�112 kyr history of the volcano and from the CVZin general (Legros, 1998; Legrende, 1999; Ruprecht &Wo« rner, 2007;Mamani et al., 2010; Fig.3). Similarly, for othermajor or trace elements, our samples fall within the data en-velopeofotherCVZvolcanoes.Theyarecharacterizedbyse-lective enrichment in large ion lithophile and alkaline earthelements, attesting to the probable involvement of subduc-tion-zone fluids, and lowerabundancesof rareearthelements(REE) andhigh field strengthelements, comparedwithtyp-icalmid-ocean ridgebasalt, confirming their arc affinity.

Summary of rhyolite and andesitepetrologyThe rhyolite-dominant samples are �50% vesicles, 40%groundmass, including glass and microlites, and 10%phenocrysts (4500 mm) and microphenocrysts (100^500 mm) (Fig. 4). The dominant phenocryst and microphe-nocryst phases include sub-equal amounts of plagioclase(6%) and amphibole (2%) with lesser amounts of pyrox-ene (1%), Fe^Ti oxides (1%), and high-SiO2 rhyoliticglass (72^78wt % SiO2). Plagioclase phenocryst andmicrophenocryst compositions range from An30 to An85and display simple to complex normal and oscillatoryzoning. Plagioclase microlite (�100 mm) compositionsrange from An28 to An63 encompassing two populationsof normally zoned microlites: one in the range An43 toAn63 and another in the range An28 to An44.The andesite-dominant samples contain 40% vesicles,

50% groundmass of equal proportions of glass and micro-lites, and 10% phenocrysts and microphenocrysts (Fig. 4).The groundmass comprises plagioclase, pyroxene,Ti-mag-netite, and andesitic to rhyolitic glass (60^72wt % SiO2).Plagioclase phenocryst and microphenocryst compositionsrange from An30 to An88, showing a similar but slightlygreater compositional range than the rhyolite samples;they exhibit similar complex textural features. The plagio-clase microlites range from An63 to An43 (Table 2).Amphiboles occur in both lithologies as strongly pleo-

chroic crystals 51·4mm in length and are pargasiticin composition (Mg# �0·75). They commonly containFe^Ti oxide inclusions. Amphiboles in the andesite are eu-hedral and slightly zoned, whereas those in the rhyoliteare rimmed by plagioclase, pyroxene and Fe^Ti oxide re-action products of variable thickness (50^600 mm). The re-action rim occurs where the amphibole is in contact withmelt rather than with other crystalline phases.Pyroxene phenocrysts and microphenocrysts are present

in both lithologies, accounting for51% of the phenocrystsin the rocks. Orthopyroxenes in the rhyolite are51mm insize, are euhedral to subhedral, and sparsely distributed.They range in composition from En78 to En82 with an

(a)

(b)

(c)

Fig. 2. Hand samples illustrating the various textures of the tephra:(a) sample containing thick globs of rhyolite in andesite matrix; (b)thin wisps of rhyolite in a gradational rhyolite^andesite matrix; (c)sample containing examples of both.


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average composition of En80 (Table 3). In the pumiceousandesite rocks, microphenocrysts of both clinopyroxeneand orthopyroxene are present as51% of the crystals inthe rocks. Orthopyroxene occurs as euhedral or subhedralmicrophenocrysts, and ranges in composition from En79to En82, with an average composition of En80. The clino-pyroxene occurs as microphenocrysts, and like the ortho-pyroxene, is euhedral to subhedral and51mm in length.Its compositions range between Wo42 and Wo47, with anaverage composition of Wo45 (Table 3). There are noFe^Ti oxide pairs in the andesite, therefore we used coex-isting pyroxene pairs to determine magma temperatures.These coexisting pyroxenes yield temperatures of940� 408C for the andesite based on the thermometer ofPutirka (2008).In the rhyolite samples, Fe^Ti oxides (both ilmenite and

magnetite) occur as discrete microlites, as inclusions inamphibole and pyroxene, and as symplectites in the reac-tion rims of amphibole. Crystals are typically small (�2^20 mm), accounting for �1% of the mode. For temperaturecalculations in the rhyolite, groundmass ilmenite and mag-netite were used, and yielded temperatures of 816�308Cin the rhyolite based on the oxide thermometer of Ghiorso& Evans (2008) (Table 4). The Fe^Ti oxides are in equilib-rium based on the method of Bacon & Hirschmann (1988).We determined glass compositions in the four targeted

thin sections by EMP analysis. The rhyolite glass compos-itions have a compositional range varying between72·5 and 76·4wt % SiO2 whereas the glass in the ‘andes-ite’ ranges from �62wt % SiO2 to �72wt % SiO2

(Table 5). The full dataset of whole-rock XRF analysesand EMP phase chemistries is provided as SupplementaryData.

PHENOCRYST TEXTURES ANDCOMPOSIT IONAL PATTERNSAmphiboleAmphibole occurs as ubiquitous crystals throughout bothrhyolite and andesite lithologies with a modal abundanceof about 2% in each lithology.The most obvious differencebetween the two is that most (490%) of the amphibolesthat reside in the rhyolite have reaction rims of plagioclase,pyroxene, and Fe^Ti oxides, or occur as ragged clusters,whereas most (490%) of those in the andesite do notdisplay a reaction rim. Different rim widths may reflectdifferential sectioning of crystals rather than any process-related phenomenon. Most amphiboles in both lithologiesshow some evidence of minor compositional zoning basedon EMP analyses and BSE images.Amphiboles in both lithologies of the EM2000BP erup-

tion show a relatively small range of major element vari-ation, forming relatively tight trends over �3wt %absolute spread in SiO2 (Fig. 5; Table 6). Compositions ofthe amphiboles from both lithologies are similar, althoughamphibole from the andesite defines the full range foralmost all major oxides.Cation abundances, used to constrain the amphibole

classification and decipher petrogenetic processes, werecalculated assuming a formula cation sum of 15 excluding

0

1

2

3

4

5

6

50 55 60 65 70 75 80

K2O

wt%

SiO2 wt%

CVZ WR rho WR and EMP rhyo glass EMP and glass

MEDIUM-K

LOW-K

HIGH-K

Fig. 3. K2O vs SiO2 diagram for whole-rocks (filled circles and squares distinguished as rhyolite and andesite) and glasses (determined byEMPA; open circles and squares) from EM2000BP, and their positions relative to a complete sampling of the Central Volcanic Zone (Mamaniet al., 2010). Most rocks are High-K calc-alkaline.


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Na and K (15eNK). Amphiboles in both the rhyolites andandesites are pargasitic [nomenclature of Leake et al.(1997)]. They show a moderate but significant range inAlIV from �1·65 to 2 atoms per formula unit (p.f.u.), al-though, as in the major oxides case, the amphiboles fromthe andesites tend to anchor the high and low end of the

trends. Variations of AlVI and (NaþK)A with AlIV aresmall and form nearly horizontal trends (Fig. 6). Mg#[Mg/(MgþFe2þ)] varies between �0·7 and �0·8 with nodistinction between amphiboles in rhyolite or andesite,and decreases with increasing AlIV, as seen in other studies(e.g. Rutherford & Devine, 2003). Core and rim datafrom both lithologies show no preference for higher orlower AlIV in variation with AlVI, (NaþK)A and Mg#.Compositional zoning within any single amphibole

phenocryst represented by either core-to-rim transects orsingle EMP spots is illustrated in Fig. 7. The chemical vari-ations within single crystals are shown in relation to BSEimages, and representative amphibole samples in bothrhyolite and andesite are illustrated. Representative com-positions are given inTable 6.Selected amphibole crystals from both rhyolite and an-

desite were chosen for detailed in situ LA-ICP-MS traceelement analysis primarily to determine whether a correl-ation exists between amphiboles in the different lithologies.Further, analysis locations were chosen coincident withelectron microprobe locations to utilize the major-elementmicroprobe data for trace element calibration (see Kentet al., 2004) and to evaluate the chemical differences be-tween different zones in the phenocrysts. Chondrite-nor-malized REE patterns of in situ LA-ICP-MS data foramphiboles from both rhyolites and andesites show a clas-sic convex form; the patterns and normalized concentra-tions are nearly identical for 495% of the samples,regardless of the host-rock lithology (Fig. 8; Table 7). LightREE (LREE) and middle REE (MREE) abundances(La/SmN vs LaN; not shown) and LREE and heavy REE(HREE) abundances (La/YbN vs LaN; not shown) alsodemonstrate that although variations in normalized con-centrations are present, they are minor. Lastly, there areminor variations between compatibleYand slightly incom-patible Sr when the grouped data are considered. In allcases, there is no distinction between the chemical signa-tures of amphiboles hosted in the rhyolite or andesite.

PlagioclaseBased on EMP analyses and backscattered electron images,plagioclase phenocrysts, microphenocrysts and microlitesin both rock types define a large compositional range.Phenocryst sizes range from �0·1 to �1·5mm; we definethe boundary between phenocryst and microphenocrystat 0·5mm and microlites as �0·1mm. We group thecrystals based on composition into two broad groups: aLow-An group, which ranges from An60 to An30, and aHigh-An group, which ranges from An88 to An65. Thisclassification is based on An content frequency analyses ofthe total plagioclase dataset (Fig. 9), supported by a MgOwt % frequency histogram. Mirroring the compositionalvariations are two broad classes of textural varieties basedon crystal morphology and texture: clear crystals, andcrystals with alternating sieved or dusty and clear portions.

Table 1: Representative whole-rock major and trace element

compositions from the 2000 BP eruption of El Misti Volcano

EM

007

EM

008

EM

009

EM

085

EM

094

EM

098

EM

099

EM

0401

SiO2 59·97 60·76 60·56 61·21 60·64 60·93 69·63 59·99

TiO2 0·890 0·799 0·822 0·774 0·804 0·807 0·359 0·776

Al2O3 17·71 17·75 17·70 17·67 17·86 17·79 15·64 17·57

FeO* 5·74 5·41 5·52 5·29 5·40 5·31 2·64 5·59

MnO 0·09 0·09 0·09 0·09 0·09 0·09 0·07 0·10

MgO 2·95 2·59 2·67 2·46 2·56 2·52 1·05 3·31

CaO 5·89 5·63 5·71 5·53 5·70 5·63 2·92 5·98

Na2O 4·31 4·43 4·40 4·38 4·41 4·34 3·86 4·25

K2O 2·15 2·25 2·22 2·31 2·23 2·28 3·70 2·18

P2O5 0·30 0·30 0·30 0·29 0·30 0·30 0·14 0·25

Total 100·00 100·00 100·00 100·00 100·00 100·00 100·00 100·00

La 24·75 25·46 25·20 25·68 25·27 25·51 31·72 25·99

Ce 50·01 51·03 50·72 51·04 50·90 51·21 57·38 51·28

Pr 6·14 6·18 6·13 6·17 6·18 6·19 6·13 6·11

Nd 24·17 24·22 23·85 23·82 24·11 24·13 21·18 23·44

Sm 4·54 4·45 4·45 4·39 4·48 4·43 3·53 4·27

Eu 1·26 1·23 1·26 1·25 1·26 1·25 0·90 1·22

Gd 3·57 3·39 3·40 3·30 3·38 3·39 2·64 3·30

Tb 0·48 0·45 0·46 0·45 0·46 0·45 0·37 0·46

Dy 2·42 2·35 2·41 2·36 2·36 2·35 2·17 2·46

Ho 0·44 0·43 0·44 0·42 0·44 0·42 0·42 0·46

Er 1·08 1·07 1·09 1·07 1·04 1·08 1·16 1·18

Tm 0·15 0·15 0·15 0·15 0·15 0·15 0·18 0·16

Yb 0·91 0·90 0·91 0·86 0·90 0·90 1·22 1·02

Lu 0·14 0·14 0·14 0·14 0·14 0·14 0·20 0·16

Ba 907 928 917 925 921 927 1092 941

Th 2·32 2·55 2·48 2·67 2·39 2·57 7·63 3·30

Nb 5·85 5·99 6·06 6·11 6·06 6·13 7·08 5·22

Y 11·28 11·00 11·08 10·91 10·98 11·07 11·53 11·93

Hf 4·01 4·06 4·09 4·01 4·03 4·03 4·04 3·91

Ta 0·36 0·37 0·36 0·37 0·37 0·37 0·61 0·30

U 0·41 0·43 0·42 0·46 0·41 0·43 1·22 0·44

Pb 13·14 13·70 13·50 13·98 13·41 13·81 23·95 12·99

Rb 37·5 40·3 39·6 42·4 39·4 41·1 94·8 44·2

Cs 0·86 0·91 0·90 0·97 0·86 0·92 2·48 0·66

Sr 836 834 835 829 850 840 513 840

Sc 9·3 8·1 8·6 7·7 8·1 7·8 5·1 12·5

Zr 151 153 153 154 154 155 145 150

*Total Fe given as FeO.


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Low-An plagioclase crystals are morphologically clear andsimple, and tend to represent the rhyolite, whereas High-An plagioclase crystals are complexly zoned and textured,and tend to reside in the andesite. Occasionally these gen-eral host^plagioclase relationships are reversed, attestingto crystal exchange between the two hosts. Figure 10

illustrates some of the various plagioclase types and sizes,andTable 8 gives the representative compositions.

Low-An plagioclase group (An60^30)

The Low-An plagioclase phenocryst group have maximumcore An contents of �An60, and minimum rim An contents

(b)

(d)

(c)

(e)

(f)

(a)

rhyolite

andesite

rhyolite

andesite andesite

andesite

rhyolite

Fig. 4. Photomicrographs of amphibole with and without reaction rims and Low-An and High-An Group plagioclase in both rhyolite and an-desite. Field of view in all images is 2mm, and all images are in crossed polars. (a) Boundary (dashed line) between rhyolite and andesitewith reacted and unreacted amphibole, respectively. (b) A rhyolite-hosted amphibole with reaction rim. (c) A clear elongate amphibole residingin andesitic melt. (d) A complexly zoned plagioclase crystal in rhyolite host from the Low-An group. (e) A plagioclase crystal with complexdusty core and clear outer rim from the High-An group, in andesitic host. (f) A predominantly clear plagioclase crystal with minor zoning, amember of the Low-An group, in andesitic host.


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of �An30. This population of phenocrysts is generally nor-mally zoned, although most of those either imaged (BSE)or analyzed show one zone of increased An content out-board of the core before decreasing to values �An40^30near the rims (Fig. 10a). Backscattered electron imagesshow that the low-An cores have rounded interior bordersthat changed immediately to higher An values, althoughthese higher values are a few mol % An higher.Texturally, these phenocrysts are generally simple, euhe-dral, clear crystals, although there are sparse crystals withmottled cores.The trace elements Mg,Ti and Fe were measured simul-

taneously with major elements during analysis transects.Generally, their concentrations are low, given the incom-patibility of these elements in plagioclase. Transects ofFeO show two patterns: one pattern shows little variationregardless of changing An content (2B and 11E), whereasthe other pattern is antithetic to An content (e.g. samples5I and 10G). MgO concentrations are more variable thanFeO, but they display the same patterns relative to Ancontent.

High-An plagioclase group (An88^65)

The second broad group of plagioclase phenocrysts in-cludes those with compositions between An88 and An65,with average core An contents of An80. Rim compositionsdepend on whether the crystals are hosted in rhyolite orandesite. This crystal population is also normally zoned,but most crystals have complex zoning patterns extendingto the rim. This complex compositional zoning is reflected

in their complex textural features, characterized by obvi-ous dusty or sieved portions alternating with clear por-tions. In most examples, the cores of these crystals aresieved or dusty and alternate with clear portions outwardstowards the rim; in a few cases, the cores of these crystalsare clear.We see no compositional differences between thecrystals with clear cores versus dusty cores.Trace element concentrations are generally higher and

their distribution patterns are different from those of theLow-An group. In contrast to the Low-An group, FeOvariations in all cases are regular and unchanging regard-less of the variations in An content (Fig. 10b). However,MgO appears to be sensitive to changes in An content,producing an anti-correlation in MgO.

Microlites

Microlites in the rhyolite and andesite also show somecompositional heterogeneity. Frequency histograms showthat there are two populations of microlite compositions,one in the rhyolite, and one in the andesite. Both popula-tions are normally zoned. The microlite population in theandesite has cores of An54^63 and rims of An43^63, whereasthe microlite population in the rhyolite has cores ofAn41^44 and rims of An28^38 (Fig. 9). These populations ofmicrolites are distinct in composition from the phenocrystsin their respective host lavas: they are slightly moreevolved. We attribute these compositional characteristicsto lower pressure final equilibration and lower magmapH2O values.

Table 2: Summary of rhyolite and andesite petrography and compositions

Rock type, proportions Phases Characteristics

Rhyolite

50% vesicles small and intermediate to coarse �5–25 mm, 75–100 mm

40% groundmass glass 72–78wt % SiO2

microlites An28–44; An43–63 bimodal

10% phenocrysts, plagioclase (6%) An30–85 complexly zoned

microphenocrysts amphibole (2%) pargasite with reaction rims

Fe–Ti oxides (1%) 8168C� 308C

pyroxene (1%) En78–82, av. En80

Andesite

40% vesicles equant to sub-spherical

50% groundmass glass 62–72wt % SiO2

microlites An43–63

10% phenocrysts, plagioclase (8%) An30–88 complexly zoned

microphenocrysts amphibole (2%) pargasite euhedral

pyroxene (51%) En79–82, av. En80 Wo42–47, av. Wo45

2-pyx temperature 9408C� 408C


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DISCUSS IONThe details of magma minglingThe macroscopic and microscopic lithological, petro-graphic, and petrological observations presented above areall consistent with extensive mingling of a relatively hot(940� 408C) andesite and a cooler (816�308C) rhyolitemagma prior to the 2000 BP eruption of El Misti. Thesemagmas are typical of the calc-alkaline high-K suite ofmagmas that have erupted in the Central Volcanic Zone

during the Pleistocene. The pumice of the 2000 BP eruptionof El Misti is extensively ‘banded’ and heterogeneous atmacroscopic and microscopic scales, and vesicle textures inthe respective lithologies record differences in rheology andthe results of the interaction (shearing, vesicle trains, etc.).Petrographic evidence for crystal exchange is supportedby phenocryst compositions that indicate two populationsof plagioclase phenocrysts and microlites based on compos-ition and texture. Low-An plagioclase, morphologicallyclear and simple crystals formed in the rhyolite, and

Table 4: Representative compositions of magnetite and ilmenite phenocrysts

magnetite ilmenite

Sample: EM 2 O EM 2 O EM 2 O EM 2 O EM 2 N EM 2 N EM 2 O EM 2 O EM 2 O EM 2 N EM 2 N EM 2 N

SiO2 0·05 0·05 0·07 0·05 0·04 0·04 0·00 0·01 0·01 0·03 0·03 0·04

TiO2 6·53 6·28 6·20 6·25 6·69 6·22 38·43 37·95 37·08 37·21 36·91 37·25

Al2O3 1·92 1·51 1·53 1·50 1·50 1·47 0·14 0·15 0·16 0·20 0·17 0·25

V2O3 0·44 0·48 0·45 0·50 0·44 0·48 0·27 0·32 0·31 0·34 0·30 0·35

Cr2O3 0·10 0·10 0·11 0·07 0·08 0·05 0·00 0·01 0·02 0·00 0·00 0·00

FeO* 81·82 82·47 83·64 83·45 82·94 82·23 54·79 54·56 55·64 54·29 52·95 53·57

MnO 0·39 0·51 0·52 0·53 0·49 0·58 0·57 0·60 0·52 0·57 0·51 0·56

MgO 1·84 1·73 1·76 1·67 1·67 1·58 2·37 2·38 2·41 2·62 2·53 2·76

CaO 0·03 0·01 0·02 0·01 0·04 0·01 0·02 0·01 0·01 0·01 0·03 0·03

ZnO 0·18 0·07 0·11 0·06 0·09 0·17 0·10 0·03 0·02 0·04 0·07 0·00

Total 93·30 93·21 94·42 94·11 93·99 92·84 96·70 96·04 96·18 95·32 93·95 94·83

*Total Fe given as FeO.Typical 1SD: (magnetite) SiO2� 0·01; TiO2� 0·01; Al2O3� 0·005; V2O3� 0·02; Cr2O3� 0·025; FeO*� 0·3; MnO� 0·02;MgO� 0·02; CaO� 0·004; ZnO� 0·01; (ilmenite) SiO2� 0·01; TiO2� 0·4; Al2O3� 0·02; V2O3� 0·03; Cr2O3� 0·01;FeO*� 0·65; MnO� 0·1; MgO� 0·01; CaO� 0·01; ZnO� 0·01.

Table 3: Representative compositions of clinopyroxene and orthopyroxene phenocrysts

clinopyroxene orthopyroxene

Sample: EM 10 L EM 10 L EM 10 N EM 10 N EM 11 M EM 11 M EM 2 J EM 2 J EM 10 M EM 10 M EM 11 L EM 11 L

SiO2 50·32 52·46 49·61 51·73 52·09 52·34 53·68 52·94 52·89 53·01 53·61 53·22

Al2O3 3·20 1·15 3·71 1·55 1·21 1·14 0·98 1·12 1·14 1·00 0·85 0·98

FeO 8·77 8·12 10·04 9·80 9·29 9·12 19·45 19·82 19·98 20·04 18·31 18·46

MgO 14·67 15·69 13·69 15·84 14·45 14·79 24·81 24·29 24·45 24·40 25·24 24·94

CaO 20·86 21·04 20·50 18·96 21·10 21·13 1·00 1·00 1·09 1·09 1·04 1·04

Na2O 0·45 0·43 0·50 0·36 0·46 0·39 0·03 0·04 0·02 0·01 0·03 0·03

TiO2 0·88 0·43 1·05 0·59 0·19 0·14 0·12 0·12 0·30 0·26 0·19 0·21

MnO 0·38 0·37 0·29 0·32 0·44 0·35 0·67 0·70 0·66 0·60 0·58 0·61

Total 99·53 99·68 99·40 99·14 99·24 99·41 100·75 100·03 100·52 100·41 99·86 99·50

Typical 1SD: SiO2� 0·12; Al2O3� 0·04; FeO*� 0·01; MgO� 0·04; CaO� 0·05; Na2O� 0·04; TiO2� 0·01; MnO� 0·03.


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high-An plagioclase, complexly zoned and textured crystalsformed in the andesite, now occur in both andesite andrhyolite, attesting to crystal exchange. Based on the mineralcomposition data, we infer that amphibole grew in the an-desite magma at depth. However, amphibole can now befound in both the rhyolite and andesite, with the amphibolein the rhyolite exhibiting reaction rims.Having established these baseline characteristics, below

we explore the deeper issues of the complex trace elementsystematics of the plagioclase and reaction rim develop-ment on amphibole in the rhyolite, and how these relateto the timing and development of the system as a whole.

Plagioclase trace element systematics

Plagioclase compositions in the 2000 BP eruption display awide range of variability; we have distinguished twogroups, the High-An Group and the Low-An Group,based on their predominant compositions and textures.However, overlap in An content and, in some cases, tex-tural features limits our ability to definitively discriminatebetween the two populations of plagioclase phenocrysts.In this case, we have turned to trace element concentra-tions in the plagioclase as an efficient discriminator.Plagioclase compositions are controlled by the melt

composition and its H2O content, and the intensive par-ameters, temperature and pressure, of the crystallizingsystem (Bowen, 1928; Tsuchiyama, 1985; Housh & Luhr,1991). Changes in these variables can lead to variations inplagioclase composition (crystal zoning), and possibly inthe rate of crystal growth (i.e. crystal growth kinetics).Changes in the temperature of the system will change theequilibrium composition of the plagioclase in that system,

as will increases or decreases in the pH2O of the system(e.g. Housh & Luhr, 1991; Lange et al., 2009). Closed-system processes that effect compositional changes, such ascrystal entrainment in convective currents within amagma chamber (Singer et al., 1995) or density currentsinduced from overburdened sidewall or roof crystallization(Marsh, 1989), may occur without the interaction of differ-ent magmas. Open-system processes, such as magma re-charge, change not only the composition of the systemand its temperature but also the equilibrium plagioclasecomposition.Discriminating between competing intensive and exten-

sive variables requires evaluation of the minor and traceelement compositions of the plagioclase, which are less sus-ceptible to changing intensive parameters. In closed sys-tems, equilibrium crystallization of plagioclase and theassociated partitioning of trace elements into plagioclasewill be governed by crystal chemical controls on elementalpartitioning (e.g. Blundy & Wood,1994) and melt compos-itional controls (Nielsen & Drake, 1979; Nielsen &Dungan, 1983). An exception to these rules is the non-equi-librium effects of variable diffusion of trace elements toand from the crystal^melt interface during rapid crystalgrowth that may lead to significant departures from ‘equi-librium’ element partitioning (Albare' de & Bottinga, 1972;Shimizu, 1983; Singer et al., 1995). Recharge events bringabout changes in temperature, pressure, pH2O and meltcomposition, which may change plagioclase compositionsand the trace element composition of the melt, and there-fore the equilibrium partitioning of that trace elementinto plagioclase. Recharge may also mix two populationsof plagioclase crystals with different compositions.

Table 5: Representative glass compositions

Sample: 2 2 5 5 10 10 11 clear 11 dark 11 dark

SiO2 74·66 75·69 74·32 74·67 66·81 68·5 73·58 66·07 69·46

TiO2 0·33 0·33 0·34 0·33 0·79 0·77 0·56 0·75 0·76

Al2O3 13·34 13·47 14·02 14·29 15·21 15·95 14·42 12·68 14·11

FeO* 1·49 1·47 1·48 1·41 4·91 4·18 2·26 5·88 3·76

MnO 0·05 0·07 0·00 0·08 0·08 0·09 0·03 0·13 0·10

MgO 0·30 0·29 0·18 0·18 1·96 0·96 0·41 4·20 2·17

CaO 1·15 1·03 0·73 0·82 2·88 3·04 1·55 2·84 2·55

Na2O 2·00 2·36 3·07 2·93 2·93 3·51 2·84 2·72 2·81

K2O 4·64 4·66 4·81 4·71 3·38 3·36 4·26 3·56 3·83

P2O5 0·04 0·04 0·03 0·02 0·37 0·33 0·26 0·32 0·33

Cl 0·15 0·13 0·19 0·18 0·11 0·14 0·14 0·13 0·13

Total 98·18 99·56 99·17 99·63 99·46 100·84 100·35 99·29 100·02

*Total Fe given as FeO.Typical 1SD: SiO2� 0·07; TiO2� 0·01; Al2O3� 0·03; FeO*� 0·12; MnO� 0·02; MgO� 0·01;CaO� 0·03; Na2O� 0·04; K2O� 0·03; P2O5� 0·01.


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Plotted in Fig. 11 are equilibrium partitioning concentra-tion curves of MgO, TiO2 and FeO based on the plagio-clase^melt trace element partitioning experiments ofBindeman et al. (1998) and Tepley et al. (2010). In modeling

the plagioclase concentrations, we use starting melt compos-itions obtained from the rhyolite and andesite whole-rockcompositions (see Table 1), and temperatures calculatedfrom oxide pairs in the rhyolite and pyroxene pairs in the

0.0

0.2

0.4

0.6

0.8

1.0

4.5 5.0 5.5 6.0 6.5 7.0 7.5

Mg/

(Mg+

Fe2+

)

Si pfu

Amphiboles in rhyolite melt Amphiboles in andesite melt

magnesiosadanagaite pargasite edenite

ferropargasite ferro-edenite

11.0

11.5

12.0

12.5

13.0

13.5

14.0

41.0 42.0 43.0 44.0

Al 2O

3

SiO2

Amphiboles in rhyolite melt

Amphiboles in andesite melt

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

41.0 42.0 43.0 44.0

FeO

*

SiO2

10.0

10.5

11.0

11.5

12.0

12.5

13.0

41.0 42.0 43.0 44.0

CaO

SiO2

12.0

13.0

14.0

15.0

16.0

17.0

18.0

41.0 42.0 43.0 44.0

MgO

SiO2

Fig. 5. Upper diagram shows EM2000BP amphibole phenocryst compositions from both rhyolite- and andesite-dominated samples plotted inthe classification scheme of Leake et al. (1997) using the Mg# [Mg/(MgþFe2þ)] vs Si p.f.u. (per formula unit) diagram. All are pargasiticamphibole. Lower diagrams show amphibole variations in Al2O3, FeO* (total iron), CaO and MgO vs SiO2.


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Table 6: Representative compositions and structural formulae of rhyolite- and andesite-hosted hornblende types

rhyolite-hosted amphiboles

Sample: EM 2 EM 2 EM 2 EM 2 EM 2 EM 2 EM 2 EM 5 EM 5 EM 5 EM 5 A EM 5 EM 5 EM 5 EM 5

I I I(2) I(2) I(2) I(3) I(3) cl1 cl1 cl2A cl2A cl2A cl2B cl2B cl2B

SiO2 42·28 42·30 42·47 42·43 42·38 42·50 42·75 42·84 42·97 43·03 42·32 42·72 42·50 42·53 42·36

TiO2 2·48 2·53 2·44 2·46 2·54 2·43 2·37 2·43 2·40 2·30 2·41 2·40 2·35 2·55 2·50

Al2O3 12·92 12·98 12·60 12·56 12·54 12·36 12·19 12·59 12·36 12·22 12·52 12·28 12·26 12·34 12·51

Cr2O3 0·02 0·02 0·02 0·00 0·01 0·00 0·00 0·00 0·00 0·17 0·02 0·00 0·07 0·02 0·06

FeO* 12·03 12·22 11·39 12·00 11·91 12·07 11·75 11·81 11·99 10·64 11·71 11·67 11·33 11·92 11·93

MnO 0·10 0·13 0·10 0·15 0·07 0·08 0·11 0·11 0·15 0·11 0·10 0·11 0·09 0·15 0·15

MgO 14·58 14·24 14·80 14·58 14·48 14·58 14·69 14·68 14·88 15·48 14·76 14·88 14·97 14·63 14·50

CaO 11·81 11·73 11·58 11·56 11·56 11·62 11·55 11·71 11·73 11·41 11·57 11·45 11·37 11·47 11·37

Na2O 2·28 2·30 2·24 2·32 2·31 2·23 2·25 2·31 2·29 2·25 2·26 2·28 2·30 2·27 2·27

K2O 0·53 0·63 0·60 0·60 0·56 0·52 0·51 0·55 0·55 0·59 0·54 0·53 0·58 0·53 0·58

Cl 0·02 0·02 0·02 0·02 0·03 0·02 0·02 0·03 0·03 0·02 0·02 0·02 0·01 0·02 0·02

Total 99·09 99·18 98·34 98·74 98·43 98·46 98·25 99·11 99·40 98·26 98·25 98·36 97·89 98·49 98·31

Si 6·103 6·122 6·170 6·157 6·166 6·175 6·221 6·184 6·184 6·230 6·153 6·203 6·199 6·181 6·172

AlIV 1·897 1·878 1·830 1·843 1·834 1·825 1·779 1·816 1·816 1·770 1·847 1·797 1·801 1·819 1·828

SUM T 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000

AlVI 0·301 0·336 0·328 0·307 0·318 0·293 0·312 0·328 0·281 0·316 0·299 0·305 0·307 0·295 0·321

Ti 0·265 0·272 0·263 0·265 0·274 0·262 0·255 0·260 0·256 0·247 0·260 0·258 0·254 0·275 0·270

Fe3þ 0·320 0·228 0·224 0·236 0·203 0·279 0·218 0·213 0·281 0·192 0·282 0·227 0·212 0·225 0·202

Cr 0·002 0·002 0·002 0·000 0·002 0·000 0·000 0·000 0·000 0·019 0·002 0·000 0·008 0·002 0·007

Mg 3·137 3·072 3·204 3·153 3·142 3·157 3·186 3·158 3·192 3·341 3·197 3·221 3·256 3·170 3·150

Fe2þ 0·974 1·090 0·979 1·039 1·062 1·011 1·029 1·042 0·994 0·885 0·960 0·989 0·964 1·033 1·050

Mn 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000

SUM C 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000

Fe2þ 0·158 0·161 0·181 0·180 0·185 0·177 0·183 0·172 0·169 0·212 0·182 0·201 0·207 0·191 0·201

Mn 0·012 0·016 0·013 0·018 0·009 0·010 0·013 0·013 0·018 0·014 0·012 0·013 0·012 0·018 0·019

Ca 1·827 1·819 1·803 1·797 1·802 1·809 1·800 1·811 1·809 1·770 1·802 1·781 1·777 1·786 1·775

Na 0·004 0·004 0·004 0·004 0·004 0·004 0·004 0·004 0·004 0·004 0·004 0·004 0·004 0·005 0·005

SUM B 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000

Na 0·635 0·641 0·628 0·648 0·647 0·624 0·632 0·642 0·634 0·627 0·634 0·638 0·645 0·635 0·636

K 0·097 0·117 0·112 0·110 0·104 0·096 0·096 0·102 0·100 0·109 0·099 0·097 0·108 0·099 0·108

SUM A 0·733 0·757 0·739 0·758 0·750 0·720 0·727 0·744 0·735 0·736 0·733 0·736 0·753 0·734 0·745

Mg/(Mgþ Fe2þ) 0·735 0·711 0·734 0·721 0·716 0·727 0·724 0·722 0·733 0·753 0·737 0·730 0·736 0·721 0·716

andesite-hosted amphiboles

Sample: EM 10 EM 10 EM 10 EM 10 EM 10 EM 10 EM 10 EM 10 EM 10 EM 10 EM 10 EM 11 EM 11 EM 11 EM 11

H H H I I I K K H H H I I K(2) K(2)

SiO2 41·99 41·95 41·62 43·88 42·74 43·80 43·03 42·27 42·72 42·58 42·45 43·21 42·69 42·77 42·63

TiO2 2·58 2·53 2·64 2·30 2·43 2·29 2·39 2·47 2·51 2·54 2·54 2·22 2·35 2·44 2·33

Al2O3 12·86 12·84 13·13 11·87 12·59 11·98 12·62 12·54 12·37 12·55 12·74 12·67 11·95 12·53 12·64

Cr2O3 0·03 0·01 0·00 0·12 0·11 0·50 0·07 0·04 0·05 0·02 0·02 0·14 0·00 0·17 0·02

FeO* 11·41 11·76 11·92 10·11 11·62 10·18 11·47 11·89 11·20 11·26 12·04 10·51 11·65 11·52 12·35

(continued)


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andesite, and then calculate the trace element equilibriumconcentration in the plagioclase. On each diagram, twoequilibrium-partitioning curves are plotted representingthe equilibrium conditions of plagioclase crystals growingin the rhyolite (Low-An type) and those growing in the an-desite (High-An type), labeled as lowTand highT, respect-ively. Plotted with these equilibrium-partitioning curves areMgO, TiO2 and FeO compositions measured simultan-eously with An content via EMP analysis. In the MgO andTiO2 diagrams, two swaths of data are prominent, whichplot on or near the equilibrium concentration lines. Thefirst observation is that trace element concentrations inplagioclase allow us to discriminate between the two

populations of crystals. The second observation is that, forthe most part, equilibrium crystallization of plagioclaseoccurred, and the large variations in An content in bothclusters are consistent with closed-system evolution asso-ciated with small variations in H2O and/or temperature ofthe host magma. In contrast, FeO shows large variations inAn content with small or no changes in FeO, which suggestthat other factors, such as fO2, contributed to the partition-ing of Fe in plagioclase phenocrysts that did not affect MgorTi partitioning. Based on these observations, we concludethat plagioclase phenocrysts in both the rhyolite and andes-ite grew independently of each other in relatively consistentenvironments before being mingled together and erupted.

Table 6: Continued

andesite-hosted amphiboles

Sample: EM 10 EM 10 EM 10 EM 10 EM 10 EM 10 EM 10 EM 10 EM 10 EM 10 EM 10 EM 11 EM 11 EM 11 EM 11

H H H I I I K K H H H I I K(2) K(2)

MnO 0·12 0·14 0·08 0·07 0·16 0·07 0·09 0·11 0·09 0·11 0·11 0·10 0·11 0·10 0·12

MgO 14·86 14·67 14·41 16·15 14·87 16·03 15·03 14·42 15·32 14·87 14·42 15·86 14·68 15·02 14·51

CaO 11·21 11·53 11·34 11·41 11·59 11·61 11·75 11·75 11·60 11·66 11·75 11·23 11·55 11·45 11·39

Na2O 2·32 2·39 2·37 2·28 2·31 2·29 2·35 2·21 2·36 2·28 2·28 2·39 2·20 2·30 2·23

K2O 0·54 0·57 0·58 0·52 0·60 0·61 0·53 0·54 0·60 0·56 0·58 0·59 0·59 0·59 0·54

Cl 0·02 0·01 0·02 0·02 0·02 0·03 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·01 0·02

Total 97·99 98·46 98·16 98·75 99·06 99·42 99·42 98·37 98·89 98·47 99·01 98·96 97·83 98·97 98·87

Si 6·119 6·098 6·077 6·298 6·168 6·260 6·181 6·152 6·165 6·173 6·144 6·204 6·241 6·175 6·174

AlIV 1·881 1·902 1·923 1·702 1·832 1·740 1·819 1·848 1·835 1·827 1·856 1·796 1·759 1·825 1·826

SUM T 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000

AlVI 0·328 0·299 0·337 0·307 0·310 0·278 0·318 0·303 0·269 0·318 0·318 0·348 0·300 0·308 0·332

Ti 0·278 0·273 0·285 0·244 0·260 0·242 0·255 0·267 0·268 0·273 0·272 0·234 0·254 0·260 0·250

Fe3þ 0·227 0·270 0·229 0·156 0·223 0·167 0·225 0·275 0·245 0·208 0·236 0·178 0·214 0·215 0·258

Cr 0·004 0·002 0·000 0·013 0·013 0·056 0·008 0·005 0·005 0·002 0·002 0·016 0·000 0·019 0·003

Mg 3·228 3·180 3·136 3·456 3·199 3·416 3·219 3·128 3·297 3·214 3·112 3·394 3·198 3·233 3·133

Fe2þ 0·935 0·977 1·014 0·824 0·995 0·841 0·976 1·023 0·916 0·985 1·060 0·829 1·037 0·964 1·024

Mn 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000 0·000

SUM C 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000

Fe2þ 0·228 0·182 0·212 0·233 0·184 0·209 0·178 0·150 0·191 0·172 0·160 0·255 0·174 0·211 0·214

Mn 0·015 0·018 0·010 0·008 0·019 0·008 0·011 0·014 0·011 0·013 0·013 0·012 0·013 0·012 0·014

Ca 1·751 1·796 1·773 1·754 1·793 1·778 1·808 1·833 1·794 1·811 1·822 1·728 1·809 1·772 1·767

Na 0·005 0·004 0·005 0·005 0·004 0·004 0·004 0·003 0·004 0·004 0·004 0·005 0·004 0·005 0·004

SUM B 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000

Na 0·650 0·668 0·666 0·629 0·643 0·631 0·651 0·622 0·656 0·638 0·636 0·662 0·619 0·640 0·622

K 0·100 0·105 0·107 0·096 0·111 0·112 0·097 0·101 0·110 0·104 0·108 0·109 0·109 0·109 0·099

SUM A 0·750 0·773 0·773 0·724 0·754 0·743 0·748 0·722 0·766 0·742 0·744 0·770 0·728 0·748 0·721

Mg/(Mgþ Fe2þ) 0·735 0·733 0·719 0·766 0·731 0·765 0·736 0·727 0·749 0·735 0·718 0·758 0·725 0·733 0·717

*Total Fe given as FeO.Typical 1SD: SiO2� 0·11; TiO2� 0·04; Al2O3� 0·05; Cr2O3� 0·02; FeO*� 0·17; MnO� 0·02; MgO� 0·07; CaO� 0·06;Na2O� 0·07; K2O� 0·04; Cl� 0·005.


14




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The advantage of these geochemical discriminators isthat regardless of whether a plagioclase crystal is found inthe rhyolite-dominated end-member or the andesite-domi-nated end-member, the trace element characteristicscoupled with the An content can reveal the original pre-mixed environment of crystallization: the andesite reser-voir or the rhyolite reservoir.With a system dominated bymixed tephra containing mixed crystal populations, thisgives us the ability to elucidate the mixing process.

Amphibole textures: the significance of reaction rims

The reaction rims on amphibole in the rhyolite lavas arecomposed of intergrowths of plagioclase, orthopyroxene,clinopyroxene and Fe^Ti oxides; the rims occur onlywhere amphibole edges are in contact with melt, not othercrystals. The rims are of relatively uniform thicknessaround selected amphiboles and generally retain the pre-cursor euhedral shape of the amphibole. These observa-tions suggest that the reaction rims grew inward from theamphibole edge such that the host melt plays an integralrole in the development of the rim (e.g. Rutherford &Hill, 1993; Browne & Gardner, 2006; Buckley et al., 2006).These gabbro-type reaction rims on amphibole are often

interpreted as resulting from volatile exsolution as a conse-quence of H2O loss during magma decompression duringmovement to or storage at shallow depth (e.g. Garcia &Jacobson, 1979; Rutherford & Hill, 1993; Rutherford &Devine, 2003). The major percentage of amphiboles in theandesitic host magma have no reaction rims, whereas themajority of amphiboles in the rhyolite have reaction rims.We evaluate the re-equilibration process through a detailedmass-balance analysis of the amphiboles, their reactionrims, and the surrounding melt, because this informationhas bearing on the mechanism, timing and evolution ofthe reaction rims.We determined the distribution and proportion of phases

in the reaction rims using high-resolution X-ray elementmapping on the OSU electron microprobe. X-ray intensitymaps of Al, Fe, Ca and Mg were produced for selectedreacted amphiboles to determine the spatial distributionand proportion of phases, in which Al is diagnostic forplagioclase, Fe for Fe^Ti oxides, and Mg and Ca for clino-pyroxene and orthopyroxene. The X-ray images were im-ported and modified in Adobe PhotoshopTM. The layersfor each element were stacked, the reaction rim on theinside and outside was outlined, and the interior and exter-ior pixels were cut away leaving only reaction rim pixels.Within each element layer, pixels of a limiting thresholdwere highlighted and counted, and the total pixels fromeach layer were summed. To obtain the areal proportionof a phase in the reaction rim, each layer’s pixels wereratioed to the summed pixels of the reaction rim. Table 9lists the relative proportion of plagioclase, orthopyroxene,clinopyroxene and Fe^Ti oxides in five amphibole rims.

0.0

0.1

0.2

0.3

0.4

0.5

0.6 A

lVI (

pfu)

Amphibole in rhyolite melt

Amphibole in andesite melt

(a)

10K10H

0.5

0.6

0.7

0.8

0.9

1.0

(Na+

K)A

(pfu

)

(b)

0.1

0.2

0.3

0.4

Ti (p

fu)

(c)

0.5

0.6

0.7

0.8

0.9

1.0

1.6 1.7 1.8 1.9 2.0 2.1

Mg/

(Mg+

Fe2+

AlIV (pfu)

(d)

Fig. 6. Amphibole atomic (p.f.u., per formula unit) compositions andevaluation of substitution mechanisms. (a) AlVI shows no changewith increasing AlIV in the pressure-sensitive Al-Tschermak substitu-tion, reflecting no change in pressure at time of crystallization andgrowth. Included in the diagram are lines labeled 10K and 10H indi-cating the range of AlIV for two amphibole crystals (10K and 10H)from the andesite.Temperature-dependent exchanges, such as the ede-nite exchange (b) and the Ti-Tschermak exchange (c), indicate slighttemperature fluctuations. (d) A slight decrease in Mg# [Mg/(MgþFe2þ)] with increasing AlIV is indicative of growth in a fractio-nating liquid.


15




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1000 um 500 um

200 um1000 um

500 um 200 um

Laser spot Microprobe spotMicroprobe transect

0.65

0.70

0.75

0.80

1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2

Mg#

AlIV pfu

EM10 andI

0.65

0.70

0.75

0.80

1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2

Mg#

AlIV pfu

EM11 rhyI

0.65

0.70

0.75

0.80

1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2

Mg#

AlIV pfu

EM11 rhyK2

0.65

0.70

0.75

0.80

0 200 400 600 800

Mg#

Distance (μm)

EM10 andK

0.65

0.70

0.75

0.80

0 200 400 600 800

Mg#

Distance (μm)

EM10 andH

0.65

0.70

0.75

0.80

0 200 400 600 800

Mg#

Distance (μm)

EM11 rhyH

(a)

Fig. 7. BSE and compositional diagrams for representative amphibole phenocrysts from rhyolite and andesite. Illustrated are BSE imagesplotted with EMP traverse or spot point locations and LA-ICP-MS spot locations. In traverses, the Mg# data are plotted versus distancefrom rim, and in spot analyses, the Mg# data are plotted versus AlIV p.f.u.


16

(continued)




ownloaded from


200 um

500 um200 um

500 um

200 um

200 um

Laser spot Microprobe spotMicroprobe transect

0.65

0.70

0.75

0.80

0 200 400 600 800

Mg#

Distance (μm)

EM2 rhyI2

0.65

0.70

0.75

0.80

1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2

Mg#

AlIV pfu

EM2 rhyI

0.65

0.70

0.75

0.80

1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2

Mg#

AlIV pfu

EM2 rhyI3

0.65

0.70

0.75

0.80

0 200 400 600 800

Mg#

Distance (μm)

EM5 rhy2A

0.65

0.70

0.75

0.80

1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2

Mg#

AlIV pfu

EM5 rhy cl1

0.65

0.70

0.75

0.80

0 200 400 600 800

Mg#

Distance (μm)

EM5 rhy2B

(b)

Fig. 7. (Continued)


17




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Mass-balance calculations were performed using a mul-tiple linear regression least-squares mixing algorithmcoded in MATLAB (Dymond et al., 1973). The code usesthe chemical compositions of the reaction rim phases(plagioclase, pyroxene, and Fe^Ti oxides) in oxide weightper cent to calculate a modal best-fit solution to a targetcomposition (amphibole composition) with the lowest re-siduals. The algorithm in this code is similar to Petmix(Wright & Doherty, 1970) used by both Rutherford & Hill(1993) and Buckley et al. (2006), and reproduces solutionsto mineral proportions in amphibole reaction rims inthese studies accurately.Rutherford & Hill (1993) noted that there was no com-

bination of reaction rim phases in their calculation equiva-lent to the amphibole. To balance their equation and

reduce their residuals, Rutherford & Hill (1993) needed toinclude melt compositions in the equation that took theform

hblþmelt! cpxþ opxþ plagþ ilm: ð1Þ

However, Buckley et al. (2006) re-evaluated the Mount St.Helens data and determined that using amphibole com-positions close to the rim instead of an averaged amphibolecomposition, the reaction equation can be written

hbl! cpxþ opxþ plagþmagþ ilm: ð2Þ

They applied this equation to amphiboles from Soufrie' reHills Volcano and found that the mass balance similarlyfollowed equation (2), but required an open system inwhich some components in the amphibole are exchangedwith the melt, and vice versa.The main difference betweenequation (1) and equation (2), therefore, is that wholesalemelt interaction is required in the former, whereas selectivecomponent interaction occurs in the latter.Applying this method to the El Misti amphiboles, it is

noted that the mineral phase mode in the reaction rim onEl Misti amphiboles is dominated by plagioclase followedby orthopyroxene, clinopyroxene and Fe^Ti oxides(Table 9). This is in contrast to amphibole reaction rimsfrom Mount St. Helens (Rutherford & Hill, 1993) andSoufrie' re Hills (Buckley et al., 2006) in which the dominantphase is clinopyroxene, followed by orthopyroxene, plagio-clase and some oxides. Furthermore, using reaction rimphase compositions, we were not able to reproduce thetarget amphibole composition with similar observed min-eral modes in the reaction rim or with low residuals. Inthe El Misti case, host melt is required to balance the equa-tion, acting as both an element supplier and element reser-voir as observed in the case of the Soufrie' re Hillsamphiboles (Buckley et al., 2006). The plagioclase-domi-nated mode in the reaction rims of the EM2000BP rhyoliteamphiboles is probably controlled by the breakdown ofthe Al-rich amphibole pargasite; excess Al and Cafrom the decomposing amphibole may contribute to thepreferential growth of plagioclase and then pyroxenerespectively.

Magma evolution and dynamicsThe detailed petrological evidence, in particular the dis-parate plagioclase populations and the complex amphiboleprovenance involving transfer from andesite to rhyoliteand reaction rim growth, provides a framework withinwhich we now attempt to piece together the magma dy-namics that led to the 2000 BP eruption of El MistiVolcano.

The andesitic magma: phase equilibria constraints

The main crystallizing phases in the EM2000BP andesiteare amphibole and plagioclase with lesser amounts ofFe^Ti oxides and pyroxenes. Notably, amphibole and, insome cases, plagioclase contain small inclusions of Fe^Ti

1

10

100

La Ce Pr Nd Sm Eu Gd Dy Er Yb

Con

cent

ratio

n/C

hond

rite

EM2 Rhyolite

1

10

100


Con

cent

ratio

n/C

hond

rite

EM10 Andesite

1

10

100


Con

cent

ratio

n/C

hond

rite

EM11 Rhyolite and Andesite

Fig. 8. Chondrite-normalized REE element patterns illustrating thesimilarity in trace element abundances between reacted (mostly rhyo-lite) and unreacted (mostly andesite) phenocrysts. Each frame repre-sents an single hand sample with several LA-ICP-MS analysis pointswithin one or two amphibole phenocrysts from the listed sample,regardless of the composition of the glass.


18




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Table7:

REELA-ICP-M

Scompositionsofam

phibole

Sam

ple:

EM

21S

EEM

21S

EEM

21S

EEM

21S

EEM

21S

EEM

21S

EEM

21S

EEM

21S

EEM

21S

EEM

101S

EEM

101S

EEM

101S

E

I3-1

I3-2

I3-3

I2-1

I2-2

I2-3

I-1

I-2

I-3

H-1

H-2

H-3

La

4·29

0·20

9·00

1·04

11·65

2·94

3·56

0·10

4·28

0·10

5·06

0·17

4·56

0·16

4·21

0·17

6·63

0·17

2·98

0·07

3·52

0·22

3·39

0·22

Ce

19·55

0·63

28·22

2·41

36·75

6·20

17·76

0·63

20·01

0·63

21·36

0·81

20·60

0·70

19·23

0·69

21·45

0·69

14·53

0·37

16·98

0·28

15·49

0·28

Pr

3 ·84

0·11

4·02

0·25

5·35

0·78

3·24

0·12

3·50

0·12

3·52

0·17

3·74

0·14

3·62

0·19

3·41

0·19

2·63

0·06

2·93

0·05

2·79

0·05

Nd

20·24

0·36

21·73

1·43

27·82

2·81

18·26

0·34

20·92

0·34

19·39

0·43

23·24

0·35

20·16

0·56

18·21

0·56

16·02

0·38

18·45

0·44

16·60

0·44

Sm

6·32

0·61

5·13

0·26

7·70

0·87

5·38

0·27

6·21

0·27

5·03

0·47

5·89

0·34

6·20

0·29

5·25

0·29

4·35

0·39

4·71

0·07

5·08

0·07

Eu

1·98

0·05

1·82

0·16

2·22

0·17

1·83

0·05

1·76

0·05

1·58

0·14

1·81

0·02

1·78

0·07

1·72

0·07

1·60

0·06

1·73

0·07

1·54

0·07

Gd

5·40

0·26

4·12

0 ·68

5·88

0·69

4·47

0·42

5·04

0·42

4·63

0·26

5·64

0·19

5·39

0·33

4·20

0·33

3·41

0·22

4·66

0·46

3·93

0·46

Dy

3·81

0·24

3·58

0·16

4·21

0·12

3·17

0·20

3·72

0·20

2·79

0·21

3·41

0·16

3·81

0·23

3·43

0·23

2·70

0·16

3·15

0·17

2·62

0·17

Er

1·47

0·10

1·69

0·11

1·59

0·07

1·14

0·09

1·21

0·09

1·34

0·08

1·33

0·08

1·43

0·07

1·48

0·07

1·05

0·08

1·19

0·07

1·12

0·07

Yb

1·08

0·06

1·11

0·15

1·22

0·13

0·98

0·15

1·10

0·15

0·92

0·07

1·38

0·14

0·72

0·13

1·02

0·13

0·65

0·11

0·94

0·10

0·81

0·10

Sam

ple:

EM

101SE

EM

101SE

EM

101SE

EM

101SE

EM

101SE

EM

101SE

EM

101SE

EM

101SE

EM

101SE

EM

101SE

EM

101SE

EM

111SE

H-4

H-5

I-1

I-2

I-3

J(2)-1

J(2)-2

K-1

K-2

K-3

K-4

H-1

La

3·33

0·06

4·95

0·84

3·65

0 ·09

3·46

0·14

4·60

0·14

4·70

0·15

4·80

0·55

2·87

0·07

4·09

0·21

6·76

0·21

8·28

0·47

3·48

0·13

Ce

15·72

0·34

20·14

1·72

18·62

0·56

17·32

0·34

22·04

0·34

20·98

0·41

19·77

1·30

14·59

0·44

19·50

0·33

32·87

0·33

24·87

1·15

15·34

0·47

Pr

2·85

0·08

3·52

0·24

3·53

0·12

3·14

0·11

3·88

0·11

3·75

0·15

3·54

0·18

2·48

0·05

3·53

0·07

5·51

0·07

3·59

0·23

2·97

0·10

Nd

16·32

0·77

19·40

0·93

18·25

0·98

17·71

0·47

20·95

0·47

21·78

1·51

19·50

0·57

14·79

0·57

20·29

0·55

29·12

0·55

17·24

1·45

17·15

0·30

Sm

4·42

0·33

5·39

0·48

5·40

0·29

4·32

0·15

6 ·47

0·15

5·67

0·51

5·47

0·24

4·00

0·44

5·28

0·25

8·22

0·25

4·08

0·37

5·55

0·20

Eu

1·70

0·09

1·73

0·06

1·94

0·09

1·83

0·05

1·92

0·05

1·92

0·06

1·83

0·11

1·37

0·07

1·69

0·09

2·30

0·09

1·36

0·15

1·53

0·08

Gd

4·55

0·34

4·97

0·30

4·32

0·23

3·74

0·12

4·67

0·12

6·03

0·18

4·60

0·09

3·51

0·29

4·73

0·16

6·64

0·16

4·34

0·32

5·27

0·18

Dy

2·86

0·10

3·24

0·18

2·76

0·09

2·69

0·33

3·54

0·33

3·66

0·32

2·87

0·20

2·27

0·20

3·49

0·16

4·17

0·16

2·29

0·16

2·94

0·22

Er

1·19

0·07

1·23

0·12

1·30

0·23

1·36

0·09

1·40

0·09

1·44

0·09

1·28

0·09

0·79

0·09

1·17

0·09

1·68

0·09

0·98

0·07

1·61

0·10

Yb

0·64

0·16

0·73

0·08

0·75

0·08

0·75

0·08

1·07

0·08

0·99

0·14

0·95

0·07

0·75

0·07

0·83

0·09

1·15

0·09

0·79

0·18

0·84

0·05

Sam

ple:

EM

111SE

EM

111SE

EM

111SE

EM

111SE

EM

111SE

EM

111SE

H-2

H-3

I-1

I-2

I-3

K(2)-1

La

3·80

0·13

3·96

0·08

2·58

0·11

2·88

0·14

4·59

0·07

3·87

0·09

Ce

16·73

0·47

16·97

0·35

12·82

0·28

14·07

0·37

20·56

0·38

17·33

0·22

Pr

2·96

0·10

3·26

0·14

2·29

0·16

2·70

0·05

3·82

0·17

3·31

0·10

Nd

18·12

0·30

19·11

0·27

13·91

0·35

15·12

0·71

20·55

0·36

18·53

0·47

Sm

5·02

0·20

5·29

0·43

3·97

0·30

4·48

0·21

5·91

0·37

5·01

0·11

Eu

1·78

0·08

1·77

0·04

1·41

0·07

1·47

0·09

1·81

0·07

1·53

0·07

Gd

5·12

0·18

4·93

0·24

3·77

0·09

3·70

0·31

5·36

0·42

5·43

0·43

Dy

3·81

0·22

3·51

0·22

2·26

0·20

2·91

0·25

3·62

0·31

3·36

0·14

Er

1·42

0·10

1·40

0·05

0·87

0·09

1·11

0·05

1·31

0·10

1·45

0·18

Yb

0 ·84

0·05

1·00

0·05

0·65

0·04

0·55

0·11

0·77

0·14

0·95

0·15

Sam

ple

values

andstan

darderrors

arein

mgg�1.


19




ownloaded from


oxides revealing a paragenetic sequence.There are no rela-tive crystallization clues between plagioclase and amphi-bole (i.e. no inclusions of plagioclase in amphibole or viceversa) to indicate saturation order.Experimental data for andesite and dacite phase equili-

bria reveal the effect of pressure and melt H2O content(Eggler, 1972; Eggler & Burnham, 1973; Moore &Carmichael, 1998; Martel et al., 1999), and fO2

(Rutherford & Devine, 1988; Martel et al., 1999) on amphi-bole stability. These studies show that crystallization ofplagioclase as the main liquidus phase at low pressure issuppressed by increasing pH2O, and that the amphibolestability field increases at high pH2O and temperatureand fO2�Ni^NiOþ1 at the expense of clinopyroxene,orthopyroxene and plagioclase (Moore & Carmichael,1998; Martel et al., 1999). Water-saturated conditions alsochange the plagioclase phase equilibria such that plagio-clase compositions increase in An content with increasingpH2O (Housh & Luhr,1991; Lange et al., 2009). Phase equi-libria for a starting material similar to the El Misti andes-ite indicate that hornblende is the first crystallizing phase,followed closely by plagioclase and Fe^Ti oxides at pres-sures between 2 and 2·5 kbar at H2O-saturated conditions(�5^6wt % H2O) and temperatures between 950 and9758C (Moore & Carmichael, 1998). This temperaturerange is similar to our two-pyroxene thermometer calcula-tion for the EM2000BP andesite. A similar relationship isfound in the experiments of Martel et al. (1999) using silicicandesites from Mount Pele¤ e, although at somewhat higher

pressures of 3·5 kbar. These experimental studies suggestthat the EM2000BP andesite magma reservoir was locatedat �2^3·5 kbar pressure (�7^12 km depth), �9408C(calculated in this study) and was H2O saturated with�5^6wt % H2O.We confirm these data using the plagio-clase^liquid hygrometer of Lange et al. (2009), whichyields 5^6wt % (H2O), and the amphibole thermobarom-eter of Ridolfi et al. (2010), which yields similar temperatureand water saturation values (Fig. 12). Our results for theEM2000BP andesite are concordant with conditions ofcrystallization of �900^9508C and 2^3 kbar pressure(Legrende, 1999) under conditions of maximum watersolubility for andesite to dacite magmas of 5·1^6·0wt %(Ruprecht & Wo« rner, 2007) for El Misti overall.

The andesite: constraints from amphibole compositions

Variations in formula cation abundances in the amphibolesallow us to describe the pre-mixing thermal and pressurehistory of the crystals and the andesite in which theygrew. Experimental studies by Spear (1981) and Blundy &Holland (1990) found that variations in AlIV in amphiboleare strongly temperature dependent, expressed in the ede-nite exchange [SiIVþœA

¼AlIVþ (NaþK)A)], and theTi-Tschermak exchange (2SiIVþMnVI

¼ 2AlIVþTiVI)(Fig. 6c), which is applicable as long as a Ti-rich phasesuch as magnetite or ilmenite is present in the mineral as-semblage (Spear,1981). In EM2000BP amphiboles, the ede-nite exchange accounts for most of the total observable Alvariation (Fig. 6a), whereas Ti (p.f.u.) vs AlIV (p.f.u.)

Phe

nocr

ysts

Tota

l # a

naly

ses

= 18

50To

tal #

cry

stal

s =

47

Mic

rolit

esTo

tal #

ana

lyse

s =

63To

tal #

cry

stal

s =

35

0 20 40 60 80

100 120 140 160

20 40 60 80 100

Relative probability

Num

ber

An Content


0 2 4 6 8

10 12 14 16

20 30 40 50 60 70 80

Num

ber

An Content

0 20 40 60 80

100 120 140 160

0.00 0.02 0.04 0.06 0.08 0.10 0.12


Num

ber

MgO wt%

0 1 2 3 4 5 6 7 8 9

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14


Num

ber

MgO wt%

±2σ

±2σ

Fig. 9. Cumulative abundance plots of plagioclase phenocryst and microlite An content and corresponding MgO contents. The relativefrequency of occurrence of compositions and 2s errors for MgO are plotted.


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shows a slightly positive correlation, indicative of the Ti-Tschermak exchange. Further support for temperaturecontrol on the EM2000BP amphibole compositions is seenin the trend of decreasing Mg# (�0·5Mg# values) andincreasing AlIV (�0·4 AlIV p.f.u.) (Fig. 6a and d), consist-ent with the work of Rutherford & Devine (2003). Finally,our data show no increase in AlVI with any other geochem-ical indicator obviating a role for the Al-Tschermakexchange (2SiIVþMgVI

¼ 2AlIVþAlVI) favored byincreasing pressure (Johnson & Rutherford, 1989; Thomas& Ernst, 1990; Schmidt, 1992).Thus, on the basis of previously published experimental

results and our observed mineral chemistry variations, wesuggest that the EM2000BP amphiboles crystallized in anear isobaric environment with modest temperature

fluctuations during crystal growth, possibly as a result ofconvective rotation in a small magma body or as a resultof repeated small recharge events of similar compositioninto a small magma body.This model is consistent with the zoning characteristics

of two amphibole crystals (Fig. 6) that show large variationin AlIV, Mg# and other geochemical parameters.Amphibole 10H and 10K, a non-rimmed and a rimmedamphibole both in mixed andesite 10, respectively, togetheraccount for the same range in (NaþK)A, AlVI, andMg# vs AlIV as the complete amphibole dataset. Theirvariation in AlIV also covers the full range exhibited bythe complete dataset. This type of zoning, increasing AlIV

and decreasing Mg# followed by decreasing AlIV andincreasing Mg#, has been shown in experimental studies

Low An Content Plagioclase

200 um

500 um

An

Con

tent

Distance (μm)

±1σ ±1σ

An

Con

tent

Distance (μm)

±1σ ±1σ

(a)

Fig. 10. BSE images, An transects, and corresponding MgO (open circles) and FeO (filled squares) concentration profiles for representativeLow-An (a) and High-An group (b) plagioclase phenocrysts from EM2000BP tephra.White lines on plagioclase (BSE images) represent tran-sect locations. Dashed black line represents average limit of detection for MgO. Also plotted are 1s errors for MgO (open circle) and FeO(filled square).


21

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to represent crystallization in a hotter, more Al-richmagma followed by crystallization in cooler, moreAl-poor magma (Scaillet & Evans, 1999; Rutherford &Devine, 2003).Similar zoning has been interpreted by Humphreys et al.

(2006) to be the result of changes in pH2O and its associatedeffect on plagioclase composition. In effect, increasing thepH2O of the melt at constant temperature promotes crystal-lization of higher An plagioclase and Al-poor amphibole.In the study by Humphreys et al. (2006), this mechanism ofamphibole^plagioclase compositional exchange is mirroredin the edenite exchange in (NaþK)A vs AlIV (their fig. 12cand d). In the EM2000BP case, (NaþK)A shows onlysmall variation with AlIV (Fig. 6). Additionally, the plagio-clase exchange (SiIVþNaA¼AlIVþCaA) (not illustrated)also shows no variation over the full range of AlIV vari-ations. Both of these observations suggest that plagioclasecrystallization did not affect the amphibole compositions

and that zoning and changes in amphibole compositionwere the result of crystal growth in a melt of fluctuatingtemperature. The amphibole adjusted its composition asthe temperature fluctuated, utilizing whatever exchangemechanism necessary to maintain chemical and thermalequilibrium with the surrounding melt. Plagioclase datafrom the andesite (and rhyolite) support these conclusionsas trace element partitioning between plagioclase and meltgenerally falls along equilibrium partitioning curves(Fig. 11). Although changing pH2O may play a role in thechanging An contents, it did not play a major role in amphi-bole crystallization and zoning.

The rhyolite: bulk geochemistry and phase equilibriaconstraints

Above we showed that whole-rock major element vari-ations follow similar evolutionary trends to previouslypublished data from various eruptions in El Misti’s past

High An Content Plagioclase

500 um

200 umDistance (μm)

±1σ ±1σ

Distance (μm)

±1σ ±1σ

(b)

Fig. 10. (Continued)


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Table 8: Representative compositions of Low- and High-An plagioclase phenocrysts and microlites

Low-An plagioclase phenocrysts

Sample: EM 2 B EM 2 B EM 2 B EM 5 I EM 5 I EM 5 I EM 10 G EM 10 G EM 10 G EM 11 E EM 11 E EM 11 E

Core Inter Rim Core Inter Rim Core Inter Rim Core Inter Rim

SiO2 57·47 55·62 57·00 55·11 53·76 58·91 56·55 54·98 59·69 54·28 52·90 56·20

Al2O3 26·61 27·92 27·20 28·48 28·87 25·70 27·49 28·27 25·82 28·59 29·60 27·53

FeO* 0·36 0·33 0·40 0·28 0·29 0·56 0·29 0·31 0·35 0·40 0·44 0·43

MgO 0·02 0·02 0·01 0·01 0·01 0·05 0·01 0·01 0·01 0·03 0·02 0·02

CaO 8·71 10·11 9·07 10·44 11·31 7·95 9·36 10·41 7·12 10·85 12·11 9·70

Na2O 5·82 5·03 5·26 4·84 4·58 5·21 5·38 4·91 6·21 4·69 3·96 4·67

K2O 0·58 0·42 0·51 0·37 0·32 0·57 0·50 0·37 0·77 0·35 0·27 0·43

TiO2 0·03 0·04 0·02 0·02 0·01 0·05 0·02 0·01 0·02 0·02 0·02 0·02

Total 99·59 99·50 99·48 99·54 99·14 99·01 99·60 99·26 99·98 99·21 99·32 99·02

An content 44 51 47 53 57 44 48 53 37 55 62 52

High-An plagioclase phenocrysts

Sample: EM 2 D EM 2 D EM 2 D EM 5 H EM 5 H EM 5 H EM 10 E EM 10 E EM 10 E EM 11 C EM 11 C EM 11 C

Core Inter Rim Core Inter Rim Core Inter Rim Core Inter Rim

SiO2 47·64 49·04 54·71 46·65 50·77 53·64 45·99 48·16 50·02 47·86 48·29 50·82

Al2O3 33·14 32·35 28·16 34·03 30·94 29·72 33·94 32·25 31·28 32·76 32·78 31·39

FeO 0·55 0·62 0·53 0·54 0·64 0·58 0·59 0·57 0·58 0·57 0·57 0·58

MgO 0·04 0·06 0·06 0·04 0·09 0·07 0·03 0·05 0·06 0·05 0·05 0·07

CaO 16·55 15·34 10·71 17·07 13·84 11·86 17·34 15·47 14·12 15·85 15·69 14·01

Na2O 1·98 2·51 4·83 1·60 3·25 4·33 1·48 2·41 3·06 2·20 2·40 3·14

K2O 0·06 0·10 0·31 0·05 0·15 0·20 0·04 0·08 0·11 0·07 0·08 0·11

TiO2 0·03 0·03 0·03 0·01 0·03 0·03 0·02 0·02 0·03 0·02 0·02 0·03

Total 99·99 100·05 99·33 99·99 99·71 100·43 99·42 99·02 99·27 99·37 99·87 100·15

An content 82 77 54 85 70 59 86 78 71 80 78 71

Low- and High-An plagioclase microlites

Sample: EM 2 EM 2 EM 2 EM 5 EM 5 EM 5 EM 10 EM 10 EM 10 EM 11 EM 11 EM 11

SiO2 56·86 59·68 61·00 54·64 54·85 54·27 54·11 54·50 58·75 52·86 53·01 53·45

Al2O3 26·36 24·56 24·00 28·69 27·67 28·10 28·21 27·79 24·92 29·32 29·44 29·09

FeO 0·40 0·25 0·31 0·66 0·75 0·69 0·78 0·79 1·36 0·63 0·65 0·60

MgO 0·01 0·01 0·00 0·07 0·10 0·08 0·07 0·10 0·14 0·08 0·06 0·08

CaO 8·42 6·22 5·34 10·78 10·33 10·70 10·97 10·50 8·00 12·09 11·98 11·75

Na2O 5·81 6·62 6·94 4·96 5·07 4·81 4·71 4·93 4·93 4·30 4·28 4·47

K2O 0·63 0·94 1·32 0·28 0·32 0·30 0·30 0·36 1·01 0·19 0·20 0·26

TiO2 0·00 0·02 0·03 0·04 0·05 0·04 0·05 0·06 0·18 0·04 0·03 0·04

Total 98·59 98·32 98·96 100·14 99·15 98·99 99·23 99·07 99·35 99·52 99·67 99·74

An content 43 32 27 54 52 54 55 53 44 60 60 58

*Total Fe given as FeO.Typical 1SD: SiO2� 0·14; Al2O3� 0·15; FeO*� 0·03; MgO� 0·01; CaO� 0·1; Na2O� 0·05; K2O� 0·015; TiO2� 0·01.


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and within the Central Volcanic Zone as a whole (Fig. 3).Although the rhyolite is related to the system as a whole,its petrogenesis is not wholly understood. For El Misti asa whole, all compositions fall on a liquid line of descent

and thus a case can be made that the rhyolite end-member is related to the andesite magma by crystalfractionation of plagioclase, amphibole, pyroxene andmagnetite. However, we have no definitive evidence that

20

30

40

50

60

70

80

90

100

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

An

Con

tent

MgO wt%

Phenocrysts Microlites Hi T eq Lo T eq

(a)

±2σdete

ctio

n lim

it

20

30

40

50

60

70

80

90

100

0.00 0.02 0.04 0.06 0.08 0.10

An

Con

tent

TiO2 wt%


(b)

±2σ

dete

ctio

n lim

it

Fig. 11. Trace-element [MgO (a), TiO2 (b), FeO*(c)] variations vs An content in plagioclase from EM200BP tephra. Plotted are phenocryst(open symbols) and microlite (shaded symbols) values. Also plotted are equilibrium partitioning curves for andesite (Hi Teq) and rhyolite(Lo Teq) and partitioning curve uncertainties based on the partitioning behavior of these elements into plagioclase as reported by Bindemanet al. (1998) and Tepley et al. (2010).Values for each element used to determine the equilibrium partitioning curves are whole-rock values of theandesite (EM0401) and the rhyolite (EM099), as an estimate of the melt composition, and T is determined through Fe^Ti oxide (rhyolite;8168C) and two-pyroxene (andesite; 9408C) geothermometry. Limit of detection for each trace element is depicted as a gray dashed line. Alsoplotted are 2s errors.


24

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the EM2000BP rhyolite was derived from the EM2000BPandesite.Whatever its exact relation to the EM2000BP an-desite, the rhyolite magma must have separated from anEl Misti andesite magma sometime in the past and stagedat shallower levels in the crust as required by its separateplagioclase phenocryst population, the lack of cognateamphibole, its lower temperatures of equilibrium, and itsevolved residual liquid composition. This magma sat inthe upper crust fractionating plagioclase and Fe^Tioxides, stagnated and partially solidified. It seems that aninjection of andesitic magma ‘reactivated’ the rhyolite on alocal level, mingled with it intimately, and a later rechargeinduced it to erupt explosively.The main crystallizing phases in the rhyolite are plagio-

clase, Fe^Ti oxides and pyroxene, which are similar tothose crystallizing in the rhyolitic component of a zonederuption deposit from the 1912 eruption at Novarupta,Alaska (Hildreth, 1983). Phase equilibria experimental re-sults from the Novarupta rhyolite (Coombs & Gardner,2001) demonstrate that at temperatures and pressures simi-lar to the El Misti system (T¼ 8168C, P5100MPa), asimilar mineral assemblage was produced. The experi-ments also show that amphibole is not on the liquidus5100MPa in pressure despite being water saturated.

Plagioclase^melt equilibria (plagioclase rims^adjacentglass compositions) indicate that the rhyolite was watersaturated with �5wt % H2O (Lange et al., 2009). Thisprobably represents the conditions of the rhyolite when itfirst formed and not just before eruption, as otherwise itwould have been water-saturated with the potential toerupt through crystallization-driven overpressure. The

EM2000BP rhyolite must then represent a degassed rem-nant of some prior episode in El Misti’s past, or it had pas-sively degassed. In either case, based on phase equilibriaexperiments and plagioclase^melt equilibria, the lack ofcognate amphibole requires that the magma was storedabove the stability limit of amphibole (�100MPa or53 km at c. 816�308C). This is consistent with the corres-pondence of the EM2000BP rhyolitic glass to the 0·5^1kbar granite ternary eutectic in Petrogeny’s ResiduaSystem (Tuttle & Bowen, 1958).

Time scales of magma dynamics during EM2000BP:chemical equilibration and amphibole rim development

Three observations can be used to provide time scales forthe magmatic interactions preceding and during theEM2000BP eruption of El Misti volcano: (1) the lack ofpervasive chemical equilibration despite minimal diffusivelength scales; (2) the absence of reaction rims on amphi-bole in the andesite; (3) the presence of reaction rims onthe andesite-originated amphibole mixed into the rhyolite.The lack of chemical equilibration of andesite and rhyo-

lite melt despite intimate mixing of the two magmas sup-ports very short time scales of interaction prior toeruption. Macroscopic and microscopic textural evidenceshows that mingling on the crystal scale and significantfolding and stretching of the magmas occurred.The equili-bration process is dependent on the diffusivities of majorelements, which are of the order 10^12m2 s�1 (Liang et al.,1996). Using t� x2/D, for length scales of 1mm to 1cm,which would be necessary for pervasive equilibration,chemical equilibration would be reached in the order of

20

30

40

50

60

70

80

90

100

0.0 0.2 0.4 0.6 0.8 1.0

An

Con

tent

FeO wt%


(c)

dete

ctio

n lim

it

±2σ

Fig. 11. (Continued)


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10 days to 1^2 years over those length scales. Meanwhile,thermal equilibration (10^6m2 s�1) and cooling wouldhave been achieved orders of magnitude fasterçin amatter of seconds to minutes. Thus, because very limitedhybridization seems to have taken place, the process musthave been arrested rather rapidly, in the order of days.Although not meant to be definitive, the point here is thateven though diffusive length scales were minimal, chem-ical equilibration is too slow to compete with freezing ofthe system. The interaction between the rhyolite andandesite could not have been prolonged and had to havehappened just prior to eruption.The lack of reaction rims on amphibole in the

EM2000BP andesite allows us to assign an upper timelimit on the mixing process during this eruption. Multi-step decompression experiments for starting materialsfrom the 1989 eruption of Redoubt volcano, Alaska, USA,by Browne & Gardner (2006) suggest that as few as 4days and as many as 7 days elapsed before reaction rimsdeveloped on amphiboles that were moved outside theirstability range, depending on the rate of decompression. Asimilar time frame was demonstrated for the 1980 eruptionof Mount St. Helens volcano, Washington, USA, in thedecompression experiments of Rutherford & Hill (1993).Therefore, for the EM2000BP andesite, we suggest a

conservative time frame of �5 days for magma migrationfrom the storage reservoir to the surface.Reaction rim development on andesite-originated

amphiboles in the EM2000BP rhyolite is controlled by dif-fusion of constituent material from the melt to the crystalsurface and diffusion of unused elements away from thecrystal surface into the melt (Liang, 2000; Coombs &Gardner, 2004). Whereas the rate of the reaction willdepend on the temperature and pressure of the systemand the amount of dissolved water in the melt, the rates ofcrystal decomposition and crystal rim growth should begoverned by the diffusive exchange of amphibole rim ma-terial and the surrounding melt. In a study by Browne &Gardner (2006), amphibole decompression rim growthwas determined for various systems at a range of tempera-tures. Those researchers found that reaction rim growthrate is probably related to melt viscosity and associatedtemperature, and that it would take �50^60 days as a min-imum for the reaction rims on amphiboles to develop.Given that the EM2000BP rhyolite has a similar tempera-ture to the 1989 Mount Redoubt dacite (�8168Cand �8408C, respectively), we infer a similar time scale

550

650

750

850

950

1050

1150

2 4 6 8 10 T

(°C

)

H2Omelt (wt.%)

EM rhyolite EM andesite

maximum thermal stability limit

0

200

400

600

800

1000

700 800 900 1000 1100

P (M

Pa)

T (°C)

EM rhyolite EM andesite

maximum thermal stability limit

Fig. 12. P (MPa) vsT (8C), andT (8C) vs H2Omelt (wt %) based onthe amphibole reduction of Ridolfi et al. (2010), showing the coherenceof data from amphiboles residing in both the rhyolite and andesite.This figure also illustrates T and P of formation for the amphibole,and the water content in the andesitic melt.

Table 9: Amphibole reaction rim phase proportions, and

comparison with other studies

Sample plagioclase pyroxene* oxidesy

EM 2 I 68 25 7

EM 2 I3 64 26 9

EM 2 I2 64 30 6

EM 5 K 61 36 3

EM 10 K 55 40 5

Soufriere Hills (B) 18·2 77·4 4·5

Soufriere Hills (B) 18·1 78·3 3·6

MSH (B)z 23·7 67·5 8·7

MSH (R&H)§ 43 53 3

*Clinopyroxene and orthopyroxene are combined as‘pyroxene’.yIlmenite and magnetite are combined as ‘oxides’.zRecalculation of Rutherford & Hill (1993) amphibole reac-tion rim mineral modes of Buckley et al. (2006) using themethod described by those researchers.§Original calculation of reaction rim mineral modes ofRutherford & Hill (1993).B, Buckley et al. (2006); R&H, Rutherford & Hill (1993);MSH, Mount St. Helens amphibole.


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of 50^60 days for the development of rims on the amphi-boles in the EM2000BP rhyolite.

Microlite crystallization

Plagioclase microlites dominate the groundmass mineralassemblage and constitute a significant proportion of thetotal crystal fraction in both lithologies. Microlite compos-ition histograms show two populations of microlites(Fig. 9), the compositions of which correlate with the hostmagma composition and are lower in An than the pheno-cryst compositions in the respective host magmas. Thissuggests that the microlites grew at lower pH2O duringmagma ascent (e.g. Geschwind & Rutherford, 1995).Moreover, the microlites are normally zoned and textur-ally display elongate, tabular and swallowtail morpholo-gies that indicate rapid crystallization. This suggests thatthe plagioclase microlites crystallized during ascent intheir respective host magmas rather than in a shallowholding chamber such as in the 1980^1986 eruptions ofMount St. Helens volcano (e.g. Geschwind & Rutherford,1995).

Magma dynamics and physical model of the EM2000BPeruption

Based on the above, a petrological model for the El Misti2000 BP eruption can now be proposed (Fig. 13). Two separ-ate, compositionally distinct magma reservoirs existed be-neath El Misti, of andesite and rhyolite composition, eachwith their own sets of phenocryst and microphenocrystpopulations. The andesite reservoir was located at�7^12 km depth in the crust (�200^350MPa) and themagma within it was at �9408C and was water saturated(5^6wt % H2O). Although sparse, the crystallizingphases were amphibole, plagioclase, Fe^Ti oxides and pyr-oxenes. Conditions of crystallization in the reservoir wererelatively constant with minor perturbations in tempera-ture perhaps reflecting small-scale convection currents orsmall-volume recharge of similar composition magma.The rhyolite reservoir is less well constrained but it ap-pears that the only crystallizing phases were plagioclaseand Fe^Ti oxides. The rhyolite reservoir was located at�3 km depth in the crust (5100MPa) and the magmawithin was at 816�308C and either degassed or partiallydegassed. The lack of stable hydrous phases and the tem-perature of the system also suggest low-pressure(5100MPa), shallow crustal residence (3 km depth).We envision the development of the eruption as a two-

stage process (Fig. 13a). The first stage initiated as a dikeof andesitic magma intruding into the rhyolite magma.The andesite was water-saturated and would have beenvesiculating as it rose and, on encountering the rhyolite, itmay have vigorously mixed with the latter. Exchange ofminerals occurred between the two magmas during thisintrusion event, but was limited by the large temperatureand viscosity contrasts. Given that the rhyolite resides in

the upper crust above the stability limit of amphibole, it isat this stage that amphiboles and plagioclase from the an-desite are mixed into the rhyolite along a thin interactivezone, and the amphibole, now out of its stability field,develops reaction rims over a period of 50^60 days.Eruption was precluded by the relatively small volume ofthe recharging magma with respect to the host rhyolitemagma.Around 50^60 days after the initial recharge event,

spurred on either via continued recharge or by simplebuoyant rise, another larger pulse of vesiculating andesitemagma forced its way through the earlier stalledrecharged zone in the perched rhyolite and initiated theeruption. In this second stage, further limited crystal ex-change may have occurred at the margins of the andesiticdike or eruption conduit, with amphiboles grown in theandesite being added to the rhyolite and plagioclase fromeach lithology being mutually exchanged.The time for the EM2000BP second pulse of andesite to

reach the rhyolite at �3 km depth (above the amphibolestability limit) is indeterminate, but the absence of reactionrims on the amphiboles in the andesite recharge magmarequires that it travelled from its storage reservoir to thesurface within �5 days. This yields an average ascent rateof at least 0·023m s�1.The presence of abundant microlites in the EM2000BP

rhyolitic and andesitic groundmasses is consistent withsuch ascent rates from low pressures (m s�1 and5100MPa; e.g. Klug & Cashman, 1994; Metrich &Rutherford, 1998; Cashman & Blundy, 2000; Martel &Schmidt, 2003). The formation of plagioclase microliteswas most probably driven by magma undercooling owingto exsolution of volatiles associated with decompression(e.g. Muncill & Lasaga, 1988; Hammer, 2008). The loss ofdissolved volatiles has the effect of increasing the relativeliquidus temperature of the magma, thereby decreasingthe An content of any crystallizing plagioclase. The lowerAn contents of the rims on both High-An and Low-Anplagioclase populations, when compared with their coreAn contents, attest to this process. This process similarlyaffects andesite and rhyolite microlite compositions, whichin El Misti’s case, show an overall reduction in An contentin comparison with their respective plagioclase phenocrystpopulations (Figs 9 and 10).Given the relatively small volume of the EM2000BP

tephra deposits, we prefer a model in which the magmaticinteractions take place along and within a dike of water-saturated andesite. Mixing between the hot (9408C) andes-itic magma and a cooler rhyolitic (8168C) magma willinitially be limited, given the temperature and viscositycontrasts between them (Huppert et al., 1982; Campbell &Turner, 1985; Sparks & Marshall, 1986; Turner & Camp-bell, 1986; Snyder & Tait, 1995); however, shearing alongthe edges and progressive physical mixing will allow


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Stage 1 Stage 2

Limited magma mixingand exchange of crystals

lnitial dike emplacementand stagnation

Duration of stagnation:50-60 days Duration of event:

<5 days

Second more forceful dike emplacementof recharge magma

More in-conduitmagma mixingand exchange

of crystals

Eruption

(a)

andesite reservoir

rhyolite crystal mush reservoir

hbl stability limit (in and)

hbl stability limit (in rhy)

Tmax

minmax

minmax

min

Vm/Vf

μ

Tmax

minmax

minmax

min

Vm/Vf

μ

(b)

conduitcross section

conduitcross section

amphibole stability(in andesite)

Pre

ssur

e

Time

eruption

amphibole stability(in rhyolite)

100 MPa

>200 MPa

50-60 days

~5 d

ays

Stage 1

Sta

ge 2

Stag

e 1

dike

em

plac

emen

t

Sta

ge 2

dik

e em

plac

emen

t

Fig. 13. Petrogenetic model illustrating andesite reservoir location and perched rhyolite magma lens. (a) Schematic model and simplified devel-opment of the 2000 BP eruption of El Misti. Diagram illustrates the initial conditions of each reservoir in relationship to the amphibole stabilitylimit. Stage 1 is initiated with dike emplacement into and stagnation in the existing rhyolitic mush. Limited magma mixing occurs during thisstage, resulting in mixed crystal populations and development of reaction rims on amphibole. Stage 2 occurs when a stronger recharge pulse re-activates the emplaced dike, causing more magma mixing, mixed crystal populations and eruption. (b) Right panel illustrates the detail asso-ciated with mixed magma and crystal exchange. Included are box models schematically illustrating the interaction between andesite andrhyolite at the initial contact deep in the system (left), and interactions in the conduit during eruption (right). The box models offer across-dike and conduit qualitative assessments associated with variations in temperature (T), volume of mafic to felsic magma (Vm/Vf) and viscosity(m). These gradients are steep at the initial contact deep in the system and become more gentle higher in the system with continued shearingand diffusion of material and heat. The left panel illustrates the pressure^time relationship showing the time scales of eruption based on amphi-bole stability. Elongate dike represents the initial intrusion of andesite into the rhyolite magma reservoir (Stage 1). This magma residesabove the amphibole stability limit for �50^60 days, before being recharged by another pulse of andesite magma, which initiates evacuationand eruption in the time frame of �5 days (Stage 2). Diagram is not to scale.


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thermal equilibration to commence rapidly (of the order ofseconds to minutes for length scales of millimeters to centi-meters respectively), reducing the viscosity contrastbetween the two magmas (e.g. Ruprecht et al., 2012;Fig. 13b). With time (and decreasing depth of the risingmagma in the dike), this will promote magma mingling.Shear along the interface between the magmas might aidmixing of liquid and crystals. As seen in the ejecta, inter-action between the two magma compositions takes manyforms, from thick toothpaste-like globs of rhyolite in amatrix of andesite, to thin wisps of alternating andesiteand rhyolite (e.g. Fig. 2), to intermixing melt fractions andcrystal population transfers. We see this as reflecting anevolving gradient in mingling in space and time from theedges to the center of the dike, with more intimate min-gling the further upwards it travels (Fig. 13b).Texturally, vesicle shapes and glass distortion provide

evidence for the viscosity differences between the rhyoliteand the andesite. Bubble coalescence, merging of smallerbubbles into larger bubbles, can be seen in most of therhyolite thin sections [‘donut-like’ features of Klug et al.(2002)]. As in Mazama pumices (Klug et al., 2002), inter-action between equal-sized bubbles often results in verythin, planar melt films (�1 mm), inferred to be caused byapproximately equal pressures acting on the film frominside each bubble. These textures suggest significantshear within the rhyolitic melt that may have been bufferedin the andesite by its lower viscosity. The combined effectsof groundmass crystallization and loss of volatiles fromthe melt lead to increased magma and melt viscosity, butcombine with vesiculation to increase the potential for ex-plosive eruption, overcoming the ‘viscous death’ describedbyAnnen et al. (2006).Magma recharge and associated mixing with a pre-

existing magma is often cited as a triggering mechanismfor volcanic eruptions (Sparks et al., 1977; Eichelberger,1978; Huppert et al., 1982; Pallister et al., 1992; Suzuki &Nakada, 2007; de Silva et al., 2008; Kent et al., 2010). Thismay be due to a simple hydraulic pressure increase inducedby addition of mass to a magma reservoir (e.g. Blake, 1981,1984), by exsolution of volatiles from a resident felsicmagma induced through superheating owing to rechargeby hot mafic magma (Sparks et al., 1977), or by cooling ofthe more mafic recharge magma forcing saturation andvapor phase exsolution (e.g. Huppert et al., 1982; Tait et al.,1989; Pallister et al., 1992; Folch & Marti, 1998). In allthese cases, over-pressurization of small magma chambersbeyond the tensile strength of the wall-rocks is thought tobe the trigger for explosive eruptions (see Gregg et al.,2012). Alternatively, volatile exsolution and syn-eruptivecrystallization driven by depressurization during adiabaticrise of magma to the surface can drive explosive eruptions(Geschwind & Rutherford, 1995; Hammer et al., 1999;Nakada & Motomura, 1999; Cashman & Blundy, 2000;

Blundy & Cashman, 2001). In the case of the 2000 BP erup-tion of El Misti, all these processes probably conspired tocause the explosive eruption, but the fundamental triggerfor the eruption was andesite recharge.

The magmatic architecture at El Misti

Lastly, we consider the size and nature of the El Mistimagmatic system over the lifetime of the volcano.Ruprecht & Wo« rner (2007) concluded that a single, large,often-recharged magma reservoir existed below El Mistirather than a plexus of smaller, interconnected magma res-ervoirs and dikes. Given that the eruption history of ElMisti is one dominated by effusive edifice-building andesiteand dacite domes and flows, this interpretation is plausible.However, the 2000 BP eruption of El Misti was an explosiveeruption precipitated by a recharge event(s) of andesiteinto rhyolite. Rhyolite at El Misti is rare and is found onlyin the explosive eruptions that punctuate its effusive-domi-nated eruption history on a time scale of 2000^4000 years(e.g. Thouret et al., 2001). The similarity of the EM2000BPjuvenile material in composition and magmatic conditionsand in physical appearance to those from the other (lessstudied) explosive events during the history of El Mistiallows a model for the explosive events to be presented.We modify the model of Ruprecht & Wo« rner (2007) to in-clude periods when a small rhyolitic reservoir develops atshallower levels in the crust. Recharge by andesite resultsin an explosive eruptionça fast and transient event in thehistory of El Misti.We suggest that these events do not tapa large single reservoir but perhaps represent the inter-action between a deeper long-lived andesitic reservoir anda small transient shallow rhyolitic magma that may formcyclically on a 2000^4000 years time scale. This timeframe may represent the period required to produce therhyolite from the andesite and segregate it to a high levelin the plumbing system shortly before eruption.Interaction does not have to be chamber wide, but moreprobably occurs along a dike that penetrates, interacts lo-cally and erupts at the surface. If, as in the case of the2000 BP eruption, the explosive eruption coincides withperiods when El Misti has significant snow cover, the inev-itable ash-fall hazard would be magnified by the triggeringof extensive lahars by small pyroclastic flows (Harpelet al., 2011).

CONCLUSIONSThe architecture, dynamics, and time scales of andesite^rhyolite interaction during the 2000 BP, VEI 5 eruption ofEl Misti in southern Peru have been revealed throughdetailed petrological study. Bulk-rock chemistry, mineraltextures and compositions reveal macroscopic and micro-scopic evidence for magma mingling and crystal exchangethat record how an initial dike tapping a deep (7^12 km),hot, water-saturated andesite magma reservoir intruded


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into a cooler, dryer, shallow (�3 km) rhyolitic magmahigher in the crust and stalled. During the initial intrusion,limited exchange of crystals from the two magmasoccurred. Amphibole crystals grown in the andesitemagma were transported into a cooler, shallower, andchemically different environment where over a period ofat least 50^60 days they decompressed in both the rhyoliticand andesitic melt to form plagioclase-dominated reactionrims. A subsequent recharge via an andesitic dike remobi-lized the small magma storage system and resulted in ex-tensive magma mingling and crystal exchange at a varietyof scales with mingling diminishing away from the andes-ite dike^rhyolite magma interface. Explosive eruption ofpervasively to minimally banded pumice reveals that al-though decompression crystallization of plagioclase micro-lites occurred, there was no wholesale equilibration ofmelt and no reaction rims developed on amphiboles in theandesite from the second recharge event. These observa-tions require that during this latter stage, transport of an-desite magma above the amphibole stability zone,interaction with the rhyolite, and eruption all happenedwithin a period of �5 days at an average ascent rate of�0·02m s�1.The 2000 BP VEI 5 plinian eruption shares characteris-

tics with other explosive events that punctuate the back-ground effusive activity at El Misti with a period of2000^4000 years. It may therefore serve as a model for ex-plosive events at this hazardous volcano. Our model forthe El Misti system includes the interaction of a deeperand larger body of andesitic magma with a small rhyoliticreservoir resulting in cyclic explosive eruptions. The peri-odicity may represent the time scales required for rhyolitedevelopment, rapid andesite recharge and eruption.

ACKNOWLEDGEMENTSJ. Permenter, C. Harpel, W. Scott, B. Anders, Y. Lavalleeand J. Burns, as well as Ms. C. Harpel-Avendano andother students from UNSA, were helpful during variousfieldwork sessions at El Misti when these samples were col-lected. We thank S. Marcott for help with MATLABcode, H. Diettrich for help with EMPA work, and C.Bouvet de la Maisonneuve for an internal review of thepaper. A. Allan, P. Ruprecht, M. Rutherford, and M.Streck provided very thorough and constructive reviewsthat are appreciated. These, and G. Wo« rner’s editorialhandling of the paper, have helped clarify and strengthenour ideas.

FUNDINGThis work has been variously supported by the NationalScience Foundation (EAR 0087181 to S.d.S.) and theVolcano Disaster Assistance Program (VDAP) of the USGeological Survey. This work was initiated when G.S. was

a visiting scientist supported by Oregon State University,Department of Geosciences.

SUPPLEMENTARY DATASupplementary data for this paper are available at Journalof Petrology online.

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Documents

magmas amphibole

deeper andesite magmas

andesiteand rhyolite

explosive eruptions

explosive events

whichonly plagioclase

misti magmas

andesite composition