sieve-textured plagioclase in volcanic rocks produced … · sieve-textured plagioclase in volcanic...

American Mineralogist, Volume 77, pages 1242-1249, 1992

Sieve-textured plagioclase in volcanic rocks produced by rapid decompression

SrnpHnN T. NBr,soN, Anr MoNrANrA.*Department of Earth and Space Sciences, UCLA, Los Angeles, California 90024-1567, U.S.A.

Ansrnlcr

Disequilibrium textures, particularly the coarse sieve texture ofplagioclase, are commonin orogenic volcanic rocks. The textures are usually interpreted as resulting from magmamixing, but they may occur by rapid decompression, where heat loss is minor relative tothe ascent rate. We conducted high-pressure piston-cylinder experiments on an andesiteto test this hypothesis. Experiments starting at 12 kbar, followed by isothermal pressurerelease in increments of 2, 4, and 6 kbar, produce sieve textures in plagioclase very muchlike those in many volcanic rocks. Therefore, the presence of sieve-textured plagioclaseshould not be taken as a priori evidence for magma mixing. Many volcanic systems prob-ably experience conditions of decompression similar to those simulated in this study, anddecompression is considered to be a simple mechanism to produce such textures, as itrequires no addition of heat or mass. Rapid decompression may also operate in conjunc-tion with magma mixing.

INrnonucrroN

Plagioclase commonly exhibits a variety of disequilib-rium textures in volcanic rocks, especially in orogenicandesites. These textures often include combinations ofcomplex zoning patterns and resorption features in pla-gioclase that record changing physical conditions in mag-matic systems. Thus, the potential exists to learn muchabout magmatic processes involved in their formation ifthe textures can be interpreted, with the caveat that er-roneous inferences may result if a particular texture canarise through more than one process. This study dealswith the origin ofcoarse sieve-textured volcanic plagro-clase (Fig. l) in the light of new experimental observa-tions and petrographic characteristics of resorbed naturalplagioclase.

We interpret the texture of the plagioclase in Figure Ias representing the remnants of an interconnecting net-work ofchannels, developed as the crystal underwent dis-solution in response to physical and chemical changes inthe magma reservoir or conduit. Initially, dissolution al-lowed diffusive and convective communication of the liq-uid in the channels with the bulk of the melt. However,in many volcanic rocks, sieve-textured plagioclase haseuhedral rims that may result from subsequent crystalgrowth in response to loss of volatiles or cooling prior toeruption. If the rims encapsulate the crystal, this mayleave the resorbed plagioclase with entrained liquid in-clusions that have no obvious means of communicationwith the bulk of the magma. Upon extrusion, quenchingof the magma leaves the network of channels filled withmicrocrystalline or glassy melt inclusions.

* Present address: P.O. Box I139, Pecos, New Mexico 87552,U.S.A.

One interpretation of coarse sieve textures similar tothose in Figure I is rapid skeletal growth resulting fromundercooling (Kuo and Kirkpatrick, 1982). However, meltinclusions are commonly elongate parallel to, or abuttedagainst, polysynthetic twin planes (Fig. l), or they cutboundaries between individual crystals in glomerocrysts.Given that glomerocrysts and polysynthetic twin planesform after crystallization (Dungan and Rhodes, 1978),the cross-cutting relationships indicate that these texturesmust form by resorption rather than rapid crystallization.

Two other mechanisms have been proposed to explainthe origin of sieve textures in plagioclase from volcanicrocks. They are magma mixing (e.g., Dungan and Rhodes,1978; Tsuchiyama, 1985) and magmatic decompression(e.g., Vance, 1965; Stormet 1972; Nelson, 1989). Thepossible effects of magma mixing have been experimen-tally investigated, although only under a limited set ofconditions (Tsuchiyama, 1985). To date, these experi-ments have not produced textures like those in Figure l,although future experiments may produce them if theyare carried out under an expanded range of pressures,temperatures, and compositions. Many studies haveshown the widespread occurrence of magma mixing as apetrogenetic process, and it is not our intent to disputeits importance. However, our results indicate that coarsesieve textures may result from the rapid ascent of mag-mas; therefore, invoking magma mixing solely on the ba-sis of coarse sieve-textured plagioclase may not be war-ranted. In the absence of other criteria indicative ofmagma mixing, such as trapped liquid enclaves, disequi-librium phenocryst assemblages (e.g., Mg-rich olivine +quartz), or other chemical and isotopic criteria, magmaticdecompression is a geologically simpler model, as it doesnot require an open system. However, the two processesneed not be mutually exclusive. Both mechanisms desta-

0003-004x/ 9 2 / | 1 | 2-r 242SO2.OO 1242

bilize relatively sodic plagioclase, and all magmas decom-press during ascent.

Magma mixing

Some of the relative merits of magma mixing and mag-matic decompression models can be evaluated by a re-view of pertinent phase equilibria and experimental stud-ies. Tsuchiyama's experiments model a mafic magmamixing with a more silicic, sodic plagioclase-bearing melt,which superheats and destabilizes the sodic plagioclase(Tsuchiyama, 1985). Any calcic plagioclase in the maficmagma would be supercooled, leaving a compositionallybimodal population of plagioclase crystals.

Mixing experiments produced dusty calcic mantles sur-rounding sodic cores (see Fig. 2 of Tsuchiyama, 1985),an effect produced by diffusional exchange between pla-gioclase and melt, assisted by the development of micro-channels in the partially resorbing mantle. However, Tsu-chiyama noted that his results did not produce the coarsesieve textures seen in Figure l, which are similar to tex-tures ascribed to magma mixing (see Fig. 5 of Dunganand Rhodes, 1978). Although it is apparent that somedisequilibrium textures in lavas may result from the mix-ing of magmas, it seems that coarse sieve textures mayalso form by decompression.

Magrnatic decompression

Plagioclase phase equilibria provide a framework forpredicting the possible effects of rapid magmatic decom-pression and for designing experiments to model it. InFigure 2a, decompression of the bulk composition, rep-resented by the filled squares, from A to B drives thesystem toward a more calcic plagioclase composition anda greater proportion of liquid, as represented by the rel-ative lengths of the tie lines connecting A and B to thesolidi and liquidi. Our experiments were designed tomimic this effect and to test whether rapid decompres-sion can produce resorption in volcanic plagioclase phe-nocrysts. The vigor with which a decompressing plagio-clase-bearing system begins to reequilibrate depends uponthe magnitude and rate of pressure release. Decompres-sion experiments modeled ascent rates >10 m/s (d.P/dt> 0.2 kbar/min) over absolute pressure intervals of 2,4,and 6 kbar.

The compositional changes produced by decompres-sion in the binary system (Fig. 2a) are not large. However,the effects of additional components in decompressingsystems can be evaluated in Figure 2b. The curves rep-resent the plagioclase loop with coprecipitating diopsidein the system Di-Ab-An. For decompression intervals ofequal magnitude, the simultaneous dissolution of diop-side produces a greater change in the equilibrium com-position of plagioclase and a greater proportion of liquidat the final pressure than in the system Ab-An (Fig. 2a).The increased sensitivity of the system Di-Ab-An tochanges in pressure and temperature results from the low-er slopes of the plagioclase liquidus and solidus in bothP-X and T-X projections caused by the coprecipitation

t243

Fig. 1. Photomicrograph of the starting material containinglarge resorbed plagioclase phenocrysts typical of many inter-mediate and mafic lavas. Arrow indicates a member of a secondpopulation of euhedral, often more calcic, plagioclase mrcro-phenocrysts.

of diopside (Wyllie, 1963). Thus, additional componentsand crystallizing phases in natural systems may enhanceresorption effects caused by decompression. Our experi-mental data and those of Meen (1987) confirm that, un-der nearly isothermal conditions, increasing pressure de-creases the An content of plagioclase in multicomponentnatural systems (Fig. 3). This agrees with the observationsof other workers (Green and Ringwood, 1967, 1968; Co-hen et al., 1967) and supports our model for decompres-sion-induced dissolution of plagioclase phenocrysts.

Although the curves in Figure 2 are for dry systems,no magma is strictly anhydrous, making it necessary toevaluate briefly the effects of volatiles. It is necessary tokeep in mind that the process of magmatic decompres-sion we describe is rapid, precluding a significant increasein volatile pressure due to crystallization of an anhydrousassemblage. Therefore, it is important to consider: (l) theeffects of decompression on vapor-undersaturated sys-tems, (2) the effects of decompression on vapor-saturatedsystems, and (3) volatile loss.

HrO lowers the temperature of the liquidi and especial-ly the solidi, but it does not significantly change the over-

NELSON AND MONTANA: SIEVE-TEXTURED PLAGIOCLASE

a, 'a i

f,na

Plagioclase+

Liquid

1244

1 atm

20

15

10

1 atm

o 'Lor" pil."''t ol'o.rnrr"

loo

Fig.2. (a) P-Xsection of rhe Ab-An system at 1325 .C (seeMorse, 1980, Fig. 18.8). (b) P-X section of the pseudobinaryplagioclase loop at about 1230 "C in the system Di-Ab-An con-structed by moving the l-atm loop up 100 "C to approximateits position at l0 kbar. This is a reasonable assumption, es-pecially for the An-rich side of the diagram (see Morse, 1980,Figs. 18.6, 18.9). A given isothermal pressure drop produces agreater change in the composition and proportion of liquid inthe system Di-Ab-An than in the system Ab-An.

all slope of the P-X and T-X loops when projected fromHrO. Thus, moderate volatile contents do not affect thegeneral phase relationships described in Figure 2, as longas the magma does not become vapor saturated and de-gas. The presence of HrO will assist decompression-in-duced resorption by reducing viscosity of the magma,which permits increased ascent and diffusion rates. De-compression of a vapor-saturated magma would hinderresorption ifit causes the system to approach a negativelysloping solidus in P-T space-a condition likely to pre-vail at crustal pressures. On the other hand, the rapid lossofvolatiles during eruption will arrest resorption by ele-vating the system's solidus temperature. Gradual volatileloss could induce a shift in plagioclase composition to-ward the Ab end-member and induce crystallization,thereby tending to counteract the effects of decompres-sion. However, it is difficult to envision that this is animportant process, except in very shallow magma reser-


I G L

l.i

J

qJ 10tiF<

U)a

l-i

(!

:thth

H

H

ooH

tr PL+CL

tr PL+CPX+GL O PL+I,IT+GL

1100 1200 1300

Temperature ('C)Fig. 3. P-Tprojection for andesite melts modified from Green

( I 972). Experimental phase assemblages from this study are alsoplotted for comparative purposes. Symbols the same as in Ta-ble 2.

voirs. Most mafic-intermediate orogenic magmas are notsufficiently HrO-rich (<3 wtolo) to exsolve a separate va-por phase until they ascend to less than 2-3 km from theEarth's surface (Gil1, l98l). Therefore, as intermediatemagmas rise through the crust, they are neither sumcient-ly HrO-rich to approach a negatively sloping solidus, norlikely to lose volatiles in transit until they reach veryshallow levels.

ExpnnrlrnxrAl pRocEDURE AND RESULTS

Methods

The starting material (Table l) was a high-K andesitefrom east-central Utah containing phenocrysts of plagio-clase, clinopyroxene, orthopyroxene, and olivine, butlacking primary hydrous phases (Nelson, 1989). Liquidusand solidus curves are well constrained because of a com-positional similarity to other experimentally investigatedandesites (Green and Ringwood, 1968; Green, 1972;Meen, 1987) (Table l). High-pressure experiments wereconducted in a piston-cylinder apparatus with a furnaceassembly 2.54 cm in diameter, similar to that describedby Boettcher et al. (1981). The starting material was sealedinside Pt capsules, which in turn were sealed within cap-sules containing hematite. The HM buffer was used tominimize Fe loss to Pt and to maintain anhydrous con-ditions. Nonetheless, moderate Fe loss did occur (4-4.5wtolo). Experiments at atmospheric pressure employed aCO/CO, gas-mixing furnace near the value of the HMbuffer. Temperatures were monitored with Pt-PteoRh,othermocouples.

We conducted two sets of experiments. The first setestablished the equilibrium phase assemblages and com-

PL+CPX+L

T = 1230 oC

Plagioclase + Diopside

Plagioclase + Diopside +Liquitl


TleLe 1. Comparison of the compositions of starting materials TABLE 2. Results and conditions of experiment set 1

56.080.94

16.989.140.132.855.993.363.300.38

99.1 6

P (kbar)

Nofe.' na: data not available. Numbers 1-4 designate the following:1 : starting material for this study (sample Gey-2, Nelson, 1989); 2 : Meen(1987) sample 2085; 3: Green and Ringwood (1968) basattic andesite;4: Green (1972) sample 68-66.

t245

Durationrfc) (h)

sio,Tio,Al,o3FeOMnOMgoCaONa.OKrOP,O.

Total

55 580.88

17.398.28na4.827 .133.812 . 1 1na

100.0

56.40 59.721.40 0.68

16.60 16.828.70 6.300.10 0 .134 . 3 0 3 1 08.50 7.163.00 3.991.00 1.28na 0.20

100.0 99.39

positions at atmospheric pressure and 6, 8, 10, and 12kbar (Table 2). All experiments were at 1250 "C for thefirst 24 h (above the liquidus at all pressures) to ensurecomplete melting of the crystalline starting material be-fore subsequent crystallization; they were then rapidlycooled to ll50 "C (between the liquidus and solidus atall pressures).

We designed the second set of experiments to modelmagmatic decompression (Table 3). The experiments wereheld at 12 kbar and 1250 "Cfor 24 h to completely meltthe starting material. The temperature was rapidlydropped isobarically to I 150'C for another 24 h to par-tially crystallize the liquid. For each experiment, the pres-sure was then dropped either 2, 4, or 6 kbar and held for3-12 h. This was accomplished for 2- and 4-kbar inter-vals by releasing piston pressure at a rate of approxi-mately 0.2 kbar/min. Because of thermocouple failure,experiments at 6-kbar intervals were reloaded into newfurnace assemblies and processed again at 6 kbar. De-compression rates were limited by the speed at whichpiston pressure could be released without thermocouplefailure.

Pressure dependence of plagioclase composition

The first set of experiments establishes the phase rela-tionships as a function of pressure. However, phase com-positions and the crystallization sequence also depend on

"f", (Hamilton et al., 1964). Because the experiments werecarried out under very oxidizing conditions, our resultsmay reflect a different crystallization sequence and rangeof mineral stabilities from those experienced in nature.Our experiments are generally consistent with the exper-imentally determined phase relationships of andesiticmelts (Fig. 3). However, the liquidus temperature at 12kbar of our starting material is somewhat higher than thatof Green (1972) and the stability of clinopyroxene in ourexperiments is also limited to somewhat higher pressures(Fig. 3).

Microprobe analyses show that plagioclase is more Abrich at increased pressure (Fig. a). Plagioclase composi-tion varies from about An' to An' between atmosphericpressure and l2 kbar. Ab and An contents vary at differ-

Notei All exoeriments were conducted with the HM buffer to minimizeFe loss to the capsule and keep the assemblage anhydrous. Temperaturewas monitored using a Pt-PLoRhlo thermocouple.

' GL: glass; PL: plagioclase; CPX: clinopyroxene; MT: magnetite.

ent rates because of the Or component, which increasesfrom 4 to 8 molo/o between I atm and 12 kbar. Despiteconsiderable scatter in the data, it appears that the com-position of plagioclase may change most rapidly as afunction of pressure when coprecipitating with diopside

Gig. a). This observation is consistent with the increasedpressure dependence of plagioclase and liquid composi-tions when coprecipitating with diopside in the systemDi-Ab-An (Fie. 2b) vs. the binary system (Fig. 2a). Thisprobably results from rapid chemical changes in the liq-uid brought on by the coprecipitation or resorption ofclinopyroxene with plagioclase. Although both clinopy-roxene and plagioclase are increasingly soluble under iso-thermal conditions as pressure decreases, clinopyroxenebecomes unstable at pressures less than about 8-10 kbar(Fig. 3). Therefore, at pressures approaching the stabilitylimit of clinopyroxene, the liquid becomes enriched in Caas the ratio of clinopyroxene to plagioclase rapidly de-creases. This allows the crystallization of more calcicplagioclases.

Development of resorption textures

The results of the second set of experiments confirmthat coarse sieve textures in volcanic rocks can be gen-

TABLE 3. Results and conditions of experiment set 2

Effect on plagioclase and other phases

24244848484896

1 21 21 21 0Io1 atm

125012251 1501 1501 1501 1501 150

GLGL, PL, CPXGL, PL, CPXGL, PL, CPXGL, PLGL, PLGL, PL, MT

Pinter- Dura-val. tion"

(kbar) (h)

2(12-101 60 (12)

4 (12-8) 60 (12)

4 (12-8) 52(4)

6 (12-6) 60 (12)

6 (12-6) 54 (6)

6 (12-6) s1 (3)

Approx. 10% resorption. A few resorptionchannels oer unit volumei in PL.

Approx.30o/" resorption. up to tens of resorFtion channels per unit volume.

Approx. 10% resorption. Up to tens of resorp-tion channels per unit volume.

Nearly complete resorption of PL. New euhed-ral calcic PL.

Approx. 30% resorption. Up to hundreds of re-sorption channels per unit volume. Pseudo-morphic MT after CPX.

Numerous resorption channels in PL. CPX be-comrng opaque.

* Numbers in oarentheses indicate the initial and final pressures.** Number in oarentheses indicates the duration of the exoeriment at

the final oressure.f Degree of resorption and number of channels estimated for a cubic

unit volume 100 pm in length per side.

1246

30 40 50 60 70

Mole Percent Anorthite

Fig. 4. Composition of plagioclase experiment products (opensquares) as a function of pressure in terms of mole percent An,as determined by electron microprobe. Data from Meen (1987)(filled circles) are shown for comparative purposes where tie linesjoin plagioclase compositions from four starting materials undernearly isothermal (- I 150 "C) conditions. Dashed line illustratesthat the change in plagioclase composition may vary morestrongly with pressure when clinopyroxene is stable.


I-r

Ti

aa;-.i

erated by rapid decompression (Fig. 5). In terms of min-imum time and pressure increments, sieve textures ap-pear after decompressions of 2 kbar and 12 h, and after4-kbar decompressions of just 4 h. The plagioclase crys-tals contain a mosaic of melt inclusions and dissolutionchannels strikingly similar to those in natural samples(compare Figs. I and 5).

The 6-kbar decompression experiments revealed a two-stage breakdown of clinopyroxene as the samples weredecompressed to well below its stability limit. The break-down of clinopyroxene, in a sequence of experimentswhere postdecompression conditions lasted 3, 6, and 12h (Table 3), began as metastable Fe-Ti oxides nucleate inthe clinopyroxene, rendering it opaque. After 6 h, pseu-domorphic aggregates of oxide after clinopyroxene weredispersed throughout the liquid. After 12 h, essentiallyall crystalline phases had resorbed into the melt, and afew new, relatively calcic plagioclase crystals were grow-ing in the liquid.

The nature of resorption in plagioclase depends on anumber of factors. The number of channels per unit vol-ume increases rapidly with the magnitude of the pressureinterval, and the percentage ofcrystal resorbed increaseswith time (Table 3). For decompression intervals of 6kbar, the density ofresorption channels exceeds that which

re$ion of clinopyroxenestability

Fig. 5. Backscattered electron images ofexperiment products produced by a 4-kbar pressure drop of 12 h. Note the resorptionin both plagi6sls5s and clinopyroxene. Symbols the same as in Table 2. l-arge scale bars are 100 pm.

is commonly found in orogenic lavas. The dissolution ofclinopyroxene may be important in the development ofresorption textures in plagioclase, as its increased solu-bility and disappearance at lower pressures (Fig. 3) causerapid changes in the composition of the melt that helpdrive resorption. We envision that a range of disequilib-rium textures could develop in plagioclase in response todecompression rates that vary from exceedingly rapid tothe infinitely slow rate required to produce equilibriummelting. These textures might include reverse zoning inaddition to resorption. However, slow ascent rates wouldfavor magmatic heat loss, which, in turn, would tend tocounteract the effects of decompression.

DrscussroN

Our experimental procedure involves three assump-tions in relation to the magmatic processes being mod-eled. First, decompression rates and intervals are reason-able analogues ofnatural conditions. Second, heat loss isinsignificant compared with the decompression rate, suchthat the process is nearly adiabatic. Third, over the pres-sure intervals considered, adiabatic decompression isnearly isothermal-otherwise significant cooling wil lcounteract the effects of decompression. On the otherhand, raising the temperature of the system by magmamixing will promote dissolution of plagioclase. However,we will consider the case of decompression without mag-ma mlxrng.

In regard to the third assumption, adiabatic coolingmust be less than the depression of the plagioclase liqui-dus caused by decompression. A liquid of compositionAb3jAnrjDi33 at 1425 "C has an adiabatic gradient of - 1.5'Clkbar (calculated from Rivers and Carmichael, 1987),cooling the liquid just 3-9 'C for the pressure intervals(2-6 kbar) in this study. In a crystal-bearing magma, thereis a second component to the total adiabatic gradient aslatent heat is extracted from the liquid as the crystalsresorb, resulting in further cooling ofthe system at equi-librium. Unlike the adiabatic gradient of a pure liquid,the extraction of latent heat is limited by the rate at whichthe crystals can resorb. Therefore, as long as resorptionis incomplete, a rapidly decompressed magma is apt tobe superheated with respect to its total adiabatic gradient.

Morse (1980) provided a convenient review of perti-nent experimental data regarding the relative effects ofisothermal decompression and adiabatic cooling. Albitehas a liquidus with a slope of > l0 "C/kbar, and the li-quidus of intermediate plagioclase (Anoo) is lowered byabout 4 "C/kbar, about three times that of the liquid adi-abat. However, as shown above, the effect ofpressure isenhanced in multicomponent systems, whereas adiabaticgradients are not nearly as compositionally dependent.Morse (1980) reviewed data showing that the Di-An eu-tectic is lowered on the order of l0 "C/kbar. Gill (198 l)noted that plagioclase is the liquidus phase in almost allandesites; thus, the whole rock liquidus represents a pla-gioclase liquidus, albeit independent of composition. At

1247

ffi nu.8" of .o.ditions where decomPression rate exceeds 10 m/s

Mean Flow Rate (m/s)

Fig. 6. Curves of mean flow rate or ascent rate of magmathrough a dikelike conduit as a function of conduit width andmagma viscosity for a density contrast of 0.2 g/cm3 between thewall rock and magma (Kushiro, 1980, p. ll7). Approximatevalues for dry andesite and basalt are shown for comparison.Note that ascent rates of >10 m/s (shaded region) are easilyachieved for a reasonable range of geologic conditions in thismodel.

pressures less than 15 kbar, where plagioclase is on theliquidus and the liquidus is least sensitive to pressure,dT/dP is still about 5 "C/kbar (Green and Ringwood,I 968). In addition, Figure 2 shows that changing pressurealso changes the equilibrium plagioclase composition,providing a further driving force for resorption. There-fore, we employed isothermal experiments as an approx-imation of natural processes, as the effect of pressure mustbe at least several times that of the adiabatic gradient.

We produced strong resorption textures in plagioclasefor decompression rates equivalent to rapid ascent rates(>10 m/s). Therefore, resorption must also occur for arange of decompression rates less than this. Previousstudies have determined magma ascent rates of up to 0.7m/s for Mount St. Helens (Scandone and Malone, 1985)and 1.7 m/s beneath Kilauea (Klein et al., 1987), whereasSpera (1980) calculated minimum ascent rates of 0.5 m/sbased upon the settling velocity of ultramafic nodules inbasaltic magma. These data indicate that ascent rates innature can be within at least an order of magnitude ofour experimental rates.

Because the upper range of magmatic ascent rates maynot be obvious to all, we constructed Figure 6 in orderto evaluate possible ascent rates in magmatic systems withestablished conduits. Figure 6 is intended to illustrate theapproximate scale (order of magnitude) of potential mag-ma ascent rates: It is not a rigorous model. We assumethe flow of a Newtonian fluid through a dikelike conduit,where the viscosities and density contrast between mag-ma and wall rock used to derive the curves were treatedas constants, although they vary somewhat with pressure(Kushiro, 1980). A model calculated for a tabular, ratherthan a cylindrical conduit, geometry provides a more


v

5 03 02 01 00

1248 NELSON AND MONTANA: SIEVE-TEXTURED PLAGIOCLASE

Or

range of feldspar compositionsAN

o

Or

t

oa resorbed phenocrysts

Q euhedralphenocrysts

Ab<-- \/ o"k,l/ -----+ An

30 40 50 60 70

Fig. 7. Compositions of coexisting euhedral and resorbedphenocryst interiors from the lava in Figure l.

conservative estimate ofascent rates because it takes intoaccount the larger component offrictional resistance be-tween the magma and wall rock.

The details ofthe calculation ofthe curves (Fig. 6) areonly modestly dependent on the model. Magma ascentrates in conduits of varying geometry (e.g., Spera, 1980,p. 280-281; Kushiro, 1980, p. l l7) are proportional tothe density contrast and inversely proportional to the vis-cosity. However, ascent rates are also proportional to thesquare of the conduit width or radius, depending uponthe geometry, causing this term to dominate the model.As a result, the calculations vary only by a factor of about2-3, given reasonable variations in the density contrast(0.2-0.3 g/cm') independent of conduit geometry. There-fore, it appears that magma ascent rates may be veryrapid, approaching our decompression rates over a rangeof geologically reasonable conditions. Magmas alwayscontain a volatile component, and modest concentrationsof HrO (0.25 wto/o) and CO, (0.5 wto/o) can result in thereduction of viscosity by an order of magnitude in silicateliquids (White and Montana, 1990), thereby enhancingascent rates.

Observations for natural systems

Our experimental results provide a basis for interpret-ing magmatic processes in volcanic rocks containing coarsesieve-textured plagioclase. The phenocrysts in Figure Iare, on average, l0-20 times larger in section than thosein our experiments, which develop similar textures after4-12h. If resorption scales linearly with crystal size, thesetextures may develop within a few days for large volcanicphenocrysts, following depressurization of at least 2-3kbar.

The lava in Figure I is strongly porphyritic, with pla-gioclase phenocrysts occurring in two distinct popula-tions: (l) resorbed crystals, and (2) euhedral phenocrysts.The phenocrysts show a fairly continuous size distribu-tion up to a few millimeters, although the resorbed phe-nocrysts are always the largest crystals in the rock. Com-

positional profiles in the resorbed crystals are quitecomplex, exhibiting normal, reverse, and oscillatory zon-ing. The euhedral phenocrysts never approach the size ofthe large resorbed crystals (<0.5 mm), and usually exhib-it normal zoning, although reverse and rare oscillatoryzoning are also present. We summarize the compositionsof crystal interiors of both populations in Figure 7 to helpinterpret this sample. Despite a compositional overlapbetween resorbed and unresorbed plagioclase, small eu-hedral plagioclase phenocrysts tend to range to signifi-cantly more calcic compositions. One analysis of re-sorbed plagioclase is distinctly An-rich and may representa Ca-rich sector in a zoned crystal.

Many of the compositional and textural aspects of thephenocrysts may be antecedent to those features imposedby decompression. Therefore, in light of our experiments,we outline a subset of the characteristics of the sample inFigure I that may result from rapid pressure release. Wesuggest that the large resorbed plagioclase samples rep-resent partial crystallization of a magma at depth. Theirsubsequent resorption was initiated by pressure releaseaccompanied by relatively little heat loss as the magmaascended to a shallow magma reservoir. As a result ofthis process, a coarse sieve texture was superimposed onthe complex zoning patterns that had already been ac-quired by these crystals. Unfortunately, it is nearly im-possible to quantify this process. The pressure at whichthe resorbed crystals formed is difficult to estimate with-out a suitable geobarometer. Even if a suitable geobarom-eter existed, the problem would not be solved becauseuncertainties in calculated pressures are often as large (-2-3 kbar) as the decompression interval required to pro-duce sieve textures. Likewise, the ascent rate is equallydifficult to estimate, as our experiments do not constrainthe relationship between resorption textures and the rateof pressure release. However, if the equilibrium compo-sition of plagioclase immediately prior to and after erup-tion could be determined, it might be possible to estimatethe minimum increment of pressure release from Fig-ure 4.

Although it is relatively easy to ascribe first-order fea-tures like coarse sieve textures to decompression, the ex-planation of other compositional and textural features ismore tenuous. Perhaps the nucleation ofeuhedral calcicmicrophenocryst cores was in response to new conditionsof equilibrium at a substantially reduced pressure, andnormal zoning of these crystals may have been in re-sponse to subsequent cooling and evolution ofthe liquid,as reflected in their compositional variation (Fig. 7). Onthe other hand, the reversely zoned euhedral crystals aremore problematic. They may reflect phenomena such ascrystal growth in a continually ascending magma in whichplagioclase compositions are increasingly calcic (Fig. 2),or crystallization under increasing HrO pressure (Jo-hannes, 1978) caused by the formation ofan anhydrouscrystal assemblage after decompression or by a numberof other phenomena, as reviewed by Gill (198 l). It isdifficult to interpret the presence of both normal and re-

verse zoning in the euhedral phenocrysts from the samesample. Zoning in plagioclase is a complex problem, un-derscoring the problematic nature of inferring magmaticprocesses from plagioclase phenocryst textures (Gill,l 98 l ) .

In general, the nucleation of new calcic crystals, ratherthan calcic overgrowths on the sodic plagioclase, is ne-cessitated by the concomitant dissolution of the sodiccrystals. Instances may occur in which resorption is ar-rested as dissolution kinetics become sluggish when thesystem approaches its new equilibrium. The resorbedcrystals could be subsequently rimmed by calcic plagio-clase. Alternatively, degassing or cooling of the magmamay also arrest resorption and allow the crystallizationof Ab-rich rims. A variety of processes may operate incombination at diferent times and places within a mag-ma body, giving rise to many of the complex texturalfeatures observed in the plagioclase of orogenic magmas.

AcxNowLnncMENTs

We thank the Geological Society of America for a grant to S.T.N. andthe Division of Physical Sciences at UCLA for financial support to A.M.We thank our colleague Jon Davidson for his careful reading of an earlyversion of the manuscript, and Robert Jones for assistance with the elec-tron microprobe. We also thank Kathy Cashman, Allen Glazner, RobertHildebrandt, James Myers, and Rosamond Kinzler for their constructivereviews. Institute ofGeophysics and Planetary Physics contribution num-ber 3438.

RornneNces crrEDBoettcher, A.L., Windom, K.8., Bohlen, S.R., and Luth, R.W. (1981) Low

friction, anhydrous, low- to high-temperature furnace sample assemblyfor piston-cylinder apparatus. Reviews of Scientific Instruments, 52,I 903-1904.

Cohen, L.H., Ito, K., and Kennedy, G.C. (1967) Melting and phase re-lations in an anhydrous basalt to 40 kilobars. American Journal ofScience. 265. 475-518.

Dungan, M.A., and Rhodes, M.J. (1978) Residual glasses and melt inclu-sions in basalts from DSDP legs 45 and 46:. Evidence for magma mix-ing. Contributions to Mineralogy and Petrology, 67,417-431.

Gill, J.B. (1981) Orogenic andesites and plate tectonics, 390 p. Springer-Verlag, New York.

Green, D.H., and Ringwood, A.E. (1967) The genesis of basaltic magmas.Contributions to Mineralogy and Petrology, I 5, 103- I 90.

Green, T.H. (1972) Crystallization of calc-alkaline andesite under con-trolled high-pressure hydrous conditions. Contributions to Mineralogyand Petrology, 34, 150-166.

Green, T.H., and Ringwood, A.E. (1968) Genesis of the calc-alkaline ig-

1249

neous rock suite. Contributions to Mineralogy and Petrology, I 8, 105-

162.Hamilton, D.L., Bumham, C.W., and Osborn, E.F. (1964) The solubility

of water and effects of oxygen fugacity and water content on crystalli-zation in mafic magmas. Journal of Petrology, 5, 2l-39.

Johannes, W. (1978) Melting of plagioclase in the system Ab-An-H'O and

Q-Ab-An-HrO at P',o : 5 kbar, an equilibrium problem. Contribu-

tions to Mineralogy and Petrology, 66, 295-303.Klein, F.W., Koynagr, R.Y., Nakata, J.S., and Tanigawa, W'R. (1987)

The seismicity of Kilauea's magna system. U.S. Geological Survey

Professional Paper, 1350 (2), l0l9-1185.Kushiro, I. (1980) Viscosity, density, and structure of silicate melts at

high pressures, and their petrological applications. In R.B. Hargraves,Ed., Physics of magmatic processes, p. 93-12O. Princeton UniversityPress, Princeton, New Jersey.

Kuo, L.C., and Kirkpatrick, R.J. (1982) Pre-eruptive history of phyric

basalts from DSDP Legs 45 and 46: Evidence from morphology andzoning patterns in plagioclase. Contributions to Mineralogy and Pe-

trology,79, 13-27 .Meen, J.K. (1987) Formation of shoshonites from calcalkaline basalt

magmas: Geochemical and experimental constraints from rhe type lo-

cality. Contributions to Mineralogy and Petrology, 97 , 333-351.Morse, S.A. (1980) Basalts and phase diagrams-An introduction to the

quantitative use ofphase diaS,rams in igneous petrology,356 p. Spring-er-Verlag, New York.

Nelson, S.T. (1989) Geologic map ofthe Geyser Peak quadrangle, Sevier

and Wayne Counties, Utah Geologic report included). Utah Geologicaland Mineral Survey Map Publication I14, scale l:24000.

Rivers, M.L., and Carmichael, I.S.E. (1987) Ultrasonic studies of silicatemelts. Journal of Geophysical Research, 92, 9247 -927 O.

Scandone, R., and Malone, S.D. (1985) Magma supply, magma discharge,and readjustment of the feeding system of Mount St. Helens during1980. Journal ofVolcanology and Geothermal Research, 23,239-262.

Spera, F.J. (1980) Aspects of magma transport. In R.B. Hargraves, Ed.,

Physics of magmatic processes, p. 261323. Princeton Univenity Press,

Princeton, New Jersey.Stormer, J.C. (1972) Mineralogy and petrology ofthe Raton-Clayton vol-

canic field, northeastern New Mexico. Geological Society of America

Bulletin. 83. 3299-3322.Tsuchiyama, A. (1985) Dissolution kinetics of plagioclase in the melt

system diopside-albite-anorthite, and origin of dusty plagioclase in an-desites. Contributions to Mineralogy and Petrology, 89, l-16.

Vance, J.A. (1965) Zoning in igneous plagioclase: Patchy zoning. Journalof Geology,'l 3, 637 -651.

White, B.S., and Montana, A. (1990) The effect of HrO and CO, on theviscosity of sanidine liquid at high pressures. Journal of GeophysicalResearch, 95, I 5683-15693.

Wyllie, P.J. (1963) Effecrs ofchanges in slope occurring on liquidus and

solidus paths in the system diopside-anorthite-albite. Mineralogical So-

ciety ofAmerica Special Paper, l,204-212.

MeNuscnrm RECETVED Dpcrrrrnen 30, l99lM,c.Nuscnrpr AccEprED Jutv 13, 1992


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