Journal of Volcanology and Geothermal Research 301 (2015) 116–127

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Journal of Volcanology and Geothermal Research

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Origin of the compositional diversity in the basalt-to-dacite serieserupted along the Heiðarsporður ridge, NE Iceland

Andrea Mancini ⁎, Hannes B. Mattsson, Olivier BachmannInstitute of Geochemistry and Petrology, Swiss Federal Institute of Technology (ETH Zürich), Clausiusstrasse 25, 8092 Zurich, Switzerland

⁎ Corresponding author. Tel.: +41 79 452 00 80; fax: +E-mail address: mancini.andrea@bluewin.ch (A. Manc

http://dx.doi.org/10.1016/j.jvolgeores.2015.05.0100377-0273/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 December 2014Accepted 19 May 2015Available online 24 May 2015

Keywords:Magmatic differentiationIcelandCumulate entrainmentSilicic magmas

TheHeiðarsporður ridge, located in the NorthernVolcanic Zone of Iceland,was formed approximately 9000 yearsago by a volcanic episode known as Lúdent Fires. The episode produced a broad spectrum of different magmatypes, forming approximately 50 small scoria cones and two larger craters (Lúdent and Hraunbunga). The bulkcompositions cluster in five distinct groups: (1) olivine basalts, (2) Fe-Ti basalts, (3) basaltic icelandites,(4) icelandites, and (5) dacites. Major and trace element trends, togetherwithmineral chemistry and isotopic ra-tios, suggest that the dominant process involved in generating the evolved magmaswas crystal fractionation oc-curring at variable depth. An origin by polybaric differentiation is confirmed byMELTSmodeling. Magmamixingplayed a dominant role in the formation of the basaltic icelandites. Additionally, the Fe-Ti basalts, which eruptedshortly after the dacites and used approximately the same vent area, display unusually high concentrations of Fe,Ti, P, and Sr. Their composition is best explained by some pyroxene-dominated fractionation (prior to Fe-Ti oxidestability), and by entrainment of some crystal cumulatematerial at shallowdepth,mostly left over from the silicicdifferentiation stage. Textural and chemical features of the minerals (e.g., presence of glomerocrysts, two popu-lations of plagioclase in these basalts) support this interpretation of evolved cumulate remobilization. Fe-Ti ba-salts with the same field, compositional and textural characteristics have also been erupted in the nearby butmagmatically independent Krafla Volcanic System, suggesting that a similar differentiation trend occurs also inthis larger central volcano.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

In Iceland, significant amounts of silicicmagmas have been producedin certain areas, typically fed by central volcanoes (Walker, 1964;Jónasson, 1994; Gudmundsson, 2000; Jónasson, 2005; Thordarson andLarsen, 2007). Understanding the processes that lead to the formationof these silicic rocks in an oceanic environment such as Iceland is essen-tial to improve our knowledge about the origin of continental crust(Jónasson, 1994; Gunnarsson et al., 1998; Jónasson, 2007; Jakobssonet al., 2008; Carley et al., 2014). As in most other magmatic provincesaround the world, two end-member models have been developed toexplain the formation of silicic rocks in Iceland: (1) fractional crystalli-zation of basaltic magma, with limited amounts of assimilation(Carmichael, 1964; Wood, 1978; Macdonald et al., 1990; Jónasson,2005); and (2) partial melting of hydrothermally altered crustal rocks(O'Nions and Grönvold, 1973; Sigvaldason, 1974; Sigmarsson et al.,1991; Gunnarsson et al., 1998; Jónasson, 2007; Bindeman et al., 2012).Both models often include magma mixing as an important process ingenerating intermediate magma compositions.

41 44 632 16 36.ini).

The question regarding the generation of silicic rocks is also inevita-bility linked to the controversial problem of basalt compositionalvariability, particularly in the case of differentiation dominated by frac-tional crystallization. If silicic magmas are genetically related to maficparents, then the variability of such parents fundamentally controlsthe geochemical composition of the more differentiated products (seerecent discussion in Rooney and Deering, 2014). Icelandic basaltsare clearly compositionally diverse, as described in multiple prior pub-lications (Holmes, 1918; Carmichael, 1964; Jakobsson, 1972; O'Nionsand Grönvold, 1973; O'Nions et al., 1976; Skovgaard et al., 2001;Sigmarsson and Steinthórsson, 2007). In general, two main end-member hypotheses have emerged to explain this diversity: (1) crustalcontamination of anunique parental basalt (Pálmason, 1973;Óskarssonet al., 1982, 1985; Hemond et al., 1988), and (2) different parental ba-salts related to mantle heterogeneities and/or the superposition of themid-Atlantic ridge and a mantle plume (Schilling, 1973; Jakobssonet al., 1978; Zindler et al., 1979). Although mantle heterogeneities canclearly affect basalt composition, entrainment of crustal material, in-cluding high crystallinity mush and/or cumulate remobilization is com-mon in Iceland (Hansen and Grönvold, 2000; Halldorsson et al., 2008;Passmore et al., 2012; Charreteur et al., 2013; Neave et al., 2013,2014) and elsewhere (Dungan and Davidson, 2004; Streck et al.,2007; Zellmer et al., 2014).

117A. Mancini et al. / Journal of Volcanology and Geothermal Research 301 (2015) 116–127

This paper focuses on the Heiðarsporður ridge, located in northernIceland. Heiðarsporður is a rare instance of Icelandic volcanismdevelop-ing silicic rocks despite not being directly associated with a central vol-cano. The ridge was formed ∼9000 years ago in a volcanic episodeknown as Lúdent Fires (Thorarinsson et al., 1960; Jónasson, 2005;Thordarson and Larsen, 2007). This episode produced two differenttypes of basalts as well as variable intermediate to silicic magmas(Walker, 1964; Nicholson and Latin, 1992; Jónasson, 2005; Thordarsonand Larsen, 2007). Following the pioneering work of Jónasson (2005),we here provide new geochemical andmineralogical data to better con-strain the processes that led to the geochemical diversity in both basaltsand differentiated products in this area.

2. Geologic Setting

The Heiðarsporður ridge is located in the Northern Volcanic Zone ofIceland (NVZ). The NVZ is the northernmost part of the Icelandic riftsystem and became active around 6–7 Ma (Björnsson et al., 1979;Brandsdóttir et al., 1997). The NVZ connects with the Eastern VolcanicZone to the south and in the north it joins the Kolbeinsey mid-oceanridge via the Húsavik-Flatey Fault (Sæmundsson, 1991; Bourgeoiset al., 1998; Gudmundsson, 2000; Thordarson and Larsen, 2007). TheNVZ is composed of five large volcanic systems with associated fissureswarms arranged en-échelon (Björnsson et al., 1977; Angelier et al.,1997). The fissure swarms trend NNE-SSW and are 60–100 km longand 5–20 km wide (Angelier et al., 1997).

TheHeiðarsporður ridge extends from theKrafla caldera in the northto the Fremri-Námur central volcano in south. The ridge is roughly15 km long and a 3 km wide, and covers an area of about 30 km2

(Jónasson, 2005). Sæmundsson (1991) estimated that ∼ 2 km3 ofmagma were erupted from Heiðarsporður. The volcanic episode inwhich the ridge was formed is referred to as Lúdent Fires, which werea large volume fissure eruption (Thordarson and Larsen, 2007) approx-imately 9000 BP (Thorarinsson et al., 1960; Sæmundsson, 1991).

All the five volcanic systems composing the NVZ (e.g. Kverkfjöll,Askja, Fremri- Námur, Krafla, Theistarekyr) have developed a centralvolcano. However, the Heiðarsporður has not despite the presence ofsilicic rocks. This is quite uncommon for Icelandic volcanism sinceevolved rocks are normally associatedwith central volcanoes. Scientistsargue still today whether the Heiðarsporður ridge forms a separate vol-canic system (Jónasson, 1994, 2005), or whether it shares a fissure anddraws magma from the Krafla volcanic system (Sæmundsson, 1991;Hjartardóttir et al., 2012). In a recent study of Hjartardóttir et al.(2012), the authors described the evolution and the formation of theKrafla fissure swarm. They noted that eruptive fissures are fartheraway from the Krafla caldera in the south than in the north. They sug-gested that this is due to both the presence of an additional magmasource coming from Heiðarsporður ridge and the contemporaneous ac-tion of the Húsavík-Flatey Fault (HFF; Fig. 1). The hypothesis ofHjartardóttir et al. (2012), based on structural and geophysical dataand on chemical analyses of Sæmundsson (1991) and Jónasson(2005), indicates that the Heiðarsporður ridge is likely to share fissureswith theKrafla volcanic systembut it is fed by a separatemagma source.

2.1. Field relations

Scoria cones composed of olivine basalt, Fe-Ti basalt, basalticicelandite, and icelandite delineate the Heiðarsporður ridge. Withinthe ridge, there are other volcanic forms such as lava shields and tuff-cones that seem to be formed by different small volcanic eruptions.Two prominent volcanic landforms (i.e. Lúdent and Hraunbunga,Fig. 1) outcrop in the middle of the ridge. Lúdent is an olivine basaltictuff-ring, which formed in a phreatomagmatic eruption at a timewhen proto-lake Mývatn had a larger extent than the present day lake(Lorenz, 1974; Sæmundsson, 1991; Jónasson, 2005). Hraunbunga, onthe other hand, is made up of a thick dacitic lava flow (with an

estimated volume of ∼0.025 km3) overlain by a Fe-Ti basalt scoriacone (Fig. 1). It is important to stress that the Fe-Ti scoria cone capsthe dacitic lava and therefore it has to be erupted through it. Due tothe high rate of erosion and the deposition of aeolian sands, is really dif-ficult to produce detailed stratigraphic and geological maps, and thecontact between the dacitic lava and Fe-Ti scoria is covered by sedimen-tary material.

3. Methodology

Forty-six samples from scoria cones and lavas from the Heiðarsporðurridge (and four from Krafla erupted between 1975–84) were sampledand analysed in the course of this study (see supplementary material).Each sample represents a single eruptive unit, except for dacite (forwhich several samples were collected). At least thirty grams of eachrock sample were crushed and pulverised using an agate mill. Aftersix hours of drying, 1.5 g of powder were fused into glass pills usingLi2B4O7 as a flux (1:5 flux to sample ratio) at 1000–1200 °C. Glass pillswere analysed for major elements by wave-length dispersive X-RayFluorescensce spectrometer (WD-XRF) using a PANalytical Axios oper-ating at 20–100 kV and 24–100 A. The standard deviations for XRF anal-yses are b0.5 wt% for all major elements. The glass pills were thenbroken and a piece of each sample was analyzed for trace element com-position using Laser-Ablation Inductively Coupled Plasma MassSpectrometry (LA-ICP-MS). A homogenized Argon-Fluorine Laser witha beam size between 30 and 60 μm was used for ablation coupled toquadropole ICP-MS (Elan 6100 DRC quadrupole ICP). A standardreference material (NIST610) was measured for calibration. Theprocedure described in Longerich et al. (1996)was followed for ablationand data reduction. For trace elements, the error (based on count rateand internal standards ratio) amounts to ca. 2% for concentrationsN100 ppm, ca. 5% for concentrations of 30–100 ppm, and N10% for con-centrations b20 ppm.

A JEOL JSM 6390LA Scanning Electron Microscope (SEM) equippedwith a BSE-detector and EDS-analyser was used to further investigatethe Fe-Ti basalts since they represent the crucial rock in this study.Quantitative analyses of mineral compositions were carried out usinganElectronMicroprobeMicroanalyser (JEOL, JXA-8200). Operating con-ditions included a voltage of 15 kV and beam current voltage of 20–28 nA. A defocused beamwith a diameter of 10 μmwas used for plagio-clase and a focused one with a diameter of 1 μm for pyroxene. Counttimes amount to 20 s for all elements, except V (40 s count time). Thetrace element composition of plagioclase in olivine basalts and Fe-Ti ba-salts was analysed in situ using a Resonetic Resolution 155 mounted ona LA-ICP-MS using an ablation time of 40 s and 43 μm beam diameter.NIST612 was measured as standard material for calibration.

4. Petrography

Plagioclase and clinopyroxene crystals occur in all samples both inthe groundmass and as phenocrysts. Their abundances increase and de-crease respectively with increasing degree of evolution of rocks. Olivinephenocrysts are a major constituent only in olivine basalt, whereasorthopyroxene phenocrysts appears as minor phases from basalticicelandites and more differentiated rocks. Fe-Ti oxides are always pre-sents in moderate amounts in the groundmass, while they form pheno-crysts in Fe-Ti basalt. Traces of apatite were detected in some dacitessamples (Table 1). The matrix consists of all the above phases, exceptfor olivine. Noteworthy is also the high amount of Fe-Ti oxides that con-stitutes the groundmass in the Fe-Ti basalts.

4.1. Olivine basalts

Olivine basalts are the most abundant rock type along theHeiðarsporður ridge. They have a vesicular, porphyritic, and microcrys-talline texture (Fig. 2a). Plagioclase and olivine are the most common

0 1 2 km

N

NVZ

EVZ

WVZ

RR

KR

100 km

500 m

Hraunbunga

Lúdent

Fe-Ti basalt

Dacite

Olivine basalt Fe-Ti basalt

Basaltic icelandite Dacite

Icelandite (Jónasson 2005)

Heiðarsporður lavaDacite lavaFe-Ti scoria cone

Samples:

HFF

Tr

KrFr

As

10 km

Krafla caldera 2km

A11

A54

A57

A15

A17A14A13

A51

A52

A53

A17

A47

A48

A49

A65

A4

A68

A42

A67

A59A3A1

A62

A63

A60

A20A21

A43A27

A40 A37A36A58

A38

A19A22

A44

A45A30

A26

Nám

afja

ll

A61

Fig. 1. Schematic map of the Heiðarsporður ridge (after Jónasson, 2005) with sample locations. On the right side, an aerial image of Lúdent and Hraunbunga craters (based on data fromNational Land Survey of Iceland), inwhich the existing spatial relationship betweendacite and Fe-Ti basalt is clear. NVZ=NorthernVolcanic Zone;WVZ=Western Volcanic Zone; EVZ=Eastern Volcanic Zone; KR = Kolbeinsey Ridge; HFF = Húsavík-Flatey Fault; RR = Reykjanes Ridge; Tr = Theistarekyr; As = Askja; Fr = Fremri- Námur, Kr = Krafla.

118 A. Mancini et al. / Journal of Volcanology and Geothermal Research 301 (2015) 116–127

phenocrysts (up to 1 mm), whereas clinopyroxene and opaque phasesare only present in the more chemically evolved samples, i.e. incoarser-grained samples (e.g., Fig. 2a).

4.2. Fe-Ti basalts

Fe-Ti basalts have only been sampled at the top of the Hraunbungacrater and in a few cones along the ridge. They are very similar tothe magmas sampled in the Krafla Central Volcano (see bulk rock

geochemistry in supplementary materials), which have been eruptedduring the Krafla Fires between 1975 and 1984 (Hollingsworth et al.,2012). They have a vesicular, porphyritic texture and are morecrystallized than olivine basalts. The equigranular groundmass is dom-inantly made up of microphenocrysts of titanomagnetite, plagioclase,clinopyroxene, and olivine (Fig. 2b). Titanomagnetite also appears asrelatively large phenocrysts (up to 100 μm). Characteristic features ofthe Fe-Ti rich basalts are large glomerocrysts (up to 3 mm, Fig. 2c) con-taining plagioclase, pyroxene and oxides (± olivine crystals).

Table 1Average phenocrysts proportion of the Heiðarsporður samples.

Phase Proportion ol cpx opx FeTi plag apt total

OB 0.4 0.4 0 0.05 0.15 0 1BI 0.1 0.4 0.1 0.05 0.35 0 1D 0 0.1 0.1 0.1 0.69 0.01 1FTB 0.01 0.45 0.1 0.03 0.4 0.01 1

Proportions determinedwith point-counting analyses (1000 point counted on each thin sec-tion). Note that the proportions are normalized to 1 excluding the amount in the ground-mass. OB olivine basalts; BI basaltic icelandites; FTB Fe-Ti rich basalts; and D dacites; ololivine; cpx clinopyroxene; opx orthopyroxene; FeTi Fe-Ti oxides; plagplagioclase; apt apatite

119A. Mancini et al. / Journal of Volcanology and Geothermal Research 301 (2015) 116–127

4.3. Basaltic icelandites

The basaltic icelandites from the Heiðarsporður ridge are rather het-erogeneous at all scales. Some samples are petrographically very similarto the olivine basalts, whereas other samples have distinct magmaticenclaves with a more mafic composition (Fig. 2d). These mafic inclu-sions vary from 1 mm up to 0.8 cm. Variations in vesicularity andcrystal-size define banding in someof the investigated samples. Crystalsof plagioclase, orthopyroxene, clinopyroxene, olivine, and oxides are setin a fine-grainedmatrix. All of themineral phases present in the basalticicelandites show pronounced textural disequilibrium features. Plagio-clase and pyroxene crystals are often resorbed, and some olivinemicrophenocrysts have developed orthopyroxene coronas.

4.4. Icelandites and dacites

Icelandites of the Heiðarsporður ridge are characterized by a trachytic,hypocrystalline texture composed of plagioclase, Fe-Ti oxides, pyroxenes

c

a

Fig. 2. Representative images showing the textures of the Heiðarsporður rocks. a) Photograph oclinopyroxene (plane polarized light). Given the presence of clinopyroxene, the samples are besTi basalt, showing the high abundance of Fe-Ti oxides. c) Characteristic glomerocryst in Fe-Ti bapolarized light) d) Typical fluidal texture in basaltic icelandites from the Heiðarsporður ridgmicrophenocrysts of plagioclase. Note a magmatic enclave of less evolved composition at top o

and glass (Jónasson, 2005). Dacites are homogeneous and crystal-poor(b5 vol% phenocrysts) with a fluidal, hypocrystalline texture defined byphenocrysts and microphenocrysts of plagioclase. The matrix is madeup mainly of plagioclase but also of oxides and pyroxenes set in a glassy(or finally crystalline) groundmass. Zircons were not observed indacite samples, but their presence is inferred by geochemical data (seegeochemical data in supplementary material).

5. Results

5.1. Whole-rock compositions

Whole-rock major and trace element compositions for theHeiðarsporður ridge are plotted in Figs. 3, 4, 5, and 6 (as well as supple-mentary material) and include data from Jónasson (2005). Also shownare data from the Krafla volcanic system (Nicholson et al., 1991;Nicholson and Latin, 1992; Jónasson, 1994). The Heiðarsporður rockshave been divided into five groups: olivine basalts, Fe-Ti rich basalts, ba-saltic icelandites, icelandites and dacites using the classification ofCarmichael (1964). Noticeable gaps in SiO2 occur between basalts,icelandites and dacites (Figs. 3 and 4). The Krafla rocks are composedof olivine-basalt, Fe-Ti basalt and rhyolites but lack intermediate rocks(Figs. 3 and 4).

5.1.1. Major and trace element variationsThe major element variation diagrams for the Heiðarsporður ridge

rock suites display the typical fractionation trends of increasing K2Oand decreasing MgO with increasing SiO2 (Fig. 3). Fe2O3

T first increaseand then decreases, as is expected in tholeiitic differentiation. Plots ofTiO2 and P2O5 against SiO2 also reveal inflected trends, initially increas-ing until icelandite stage and then decreasing towards the dacite field.

d

b

f a sample of vesicular olivine basalt with groundmass composed of olivine, plagioclase andt characterized as evolved olivine basalt. b) Equigranular texture (from BSE image) of a Fe-salt composed by plagioclase, olivine, clinopyroxene and oxides (optical microscope, planee. The texture is defined by the preferential orientation of lath-shaped phenocrysts andf the picture.

0.4

0.3

0.2

0.1

2.5

2.0

1.5

1.0

0.5

0

12

10

8

6

4

2

0

0.5

3.0

10

18

16

14

12

0

Olivine-basalt

Fe-Ti basalt

Basaltic icelandite

Icelandite ( Jónasson, 2005)Dacite

Krafla (published data) Heiðarsporður

Olivine-basaltFe-Ti basaltRhyolite

MgO (wt%)

TiO2 (wt%)

P2O5 (wt%)

8075706560555045SiO2 (wt%)

Al2O3 (wt%)

3.0

2.5

2.0

1.4

1.0

0.5

0

K2O (wt%)

8075706560555045SiO2 (wt%)

25

20

15

10

5

0

Fe2O3T (wt%)

Fig. 3.Variation diagrams for selectedmajor elements (wt%) versus SiO2 (wt%). Data for Heiðarsporður icelandites from Jónasson (2005) and for the Krafla Volcanic System from Jónasson(1994), Nicholson et al. (1991), and Nicholson and Latin (1992).

120 A. Mancini et al. / Journal of Volcanology and Geothermal Research 301 (2015) 116–127

Al2O3 is constant throughout the whole rocks series except for Fe-Ti ba-salts, where it is lower. Fe-Ti basalts are also characterized by highconcentrations of Fe2O3

T, TiO2, and P2O5, and low content ofMgOat com-parable SiO2 content (Fig. 3 and supplementary material).

Trace elements variation diagrams also clearly highlight the fraction-ation trend (Fig. 4). Compatible elements (Cr, V) rapidly decrease, whileincompatible elements (e.g., Rb) increasewith increasing SiO2. Sr showsthe same overall behavior as Al2O3. Again, the Fe-Ti basalts also plot intheir own field, being particularly enriched in V, Sr and Rb, and depletedin Cr (Fig. 4) and Ni. Variations of incompatible trace elements againstcompatible elements (e.g., Ba against Ni, Fig. 5a) show curvilineartrends, except for the basaltic icelandites, which always plots on lineararrays (mixing lines). All five rock groups show the typical greaterenrichment in LREE than HREE (Fig. 5b). The Fe-Ti basalts have some-what higher LREE/HREE in comparison to olivine basalts (La/Lu of 21and 11 respectively, see supplementary material). Finally, incompatibleelement ratios plotted against SiO2 showon average a constant behavior(flat trends) despite these elements have different fluid mobilities(Fig. 6; see supplementary material for errors due to analyticaluncertainties). Considering an average value for all samples, weobtained a constant ratio for every plot, at least until icelandite stages(dacites have slightly higher Rb/Zr and lower Zr/Ba, as Zr becomes less

incompatible with evolution, likely because of the crystallization ofsome zircon).

6. Mineral chemistry

6.1. Plagioclase

In olivine basalts, plagioclase phenocrysts display an inter-samplecomposition varying from An72 to An80. In the icelandites, it rangesfrom An55 to An70 and in the dacites from An35 to An55. Basalticicelandites have a wide range of An content (An55–An82), which spansthe fields from olivine basalts to icelandites, with crystals thatdisplay clear disequilibrium textures. Fe-Ti basalts show lowAn content(An52 to An70, similar to icelandite) despite having a basaltic bulk-rockcomposition, and contain two populations of plagioclase (Fig. 7).Population A comprises larger phenocrysts (up to 2 mm) with normalzonation pattern and typically higher An content (An65 to An70).Population B is characterized by smaller (up to 1 mm) and strongly re-sorbed phenocrysts, which show lower An content and reverse zoning(cores down to An52 and rims up to An66). All the presented data aresummarized in the supplementary material and in Fig. 8 a,b. Plagioclasetrace-element variation diagrams show that many plagioclase crystals

Cr (ppm)

Rb (ppm)Sr (ppm)

V (ppm)

SiO2 (wt%) SiO2 (wt%)

Sc/K2O

Olivine-basalt

Fe-Ti basalt

Basaltic icelandite

Icelandite ( Jónasson, 2005)Dacite

Krafla (published data) Heiðarsporður

Olivine-basaltFe-Ti basaltRhyolite

600

500

400

300

200

100

0

8075706560555045

60

40

20

0

300

200

100

0

400

500

80

500

08075706560555045

400

300

200

100

Zr (ppm)

400

200

0

600

80

200

160

120

Fig. 4. Variation diagrams for selected trace element (ppm) and Sc/K2O versus SiO2 (wt%). Data for Heiðarsporður icelandites from Jónasson (2005) and for the Krafla Volcanic Systemrocks Jónasson (1994), Nicholson et al. (1991), and Nicholson and Latin (1992). See Fig. 3 for legend.

121A. Mancini et al. / Journal of Volcanology and Geothermal Research 301 (2015) 116–127

in the Fe-Ti basalts have a higher concentrations of Sr, Pb and Ba thanthe olivine-basalts (e.g., Fig. 8 c,d).

6.2. Pyroxene

Clinopyroxene and orthopyroxene occur in various amounts in theHeiðarsporður rocks. They are largely unzoned (no significant intra-crystalline variation), as seen on the BSE images. Among grains, there isa range in Mg# (mole % Mg/(Mg+ Fe) from 39–54 (see supplementarymaterial).

7. Discussion

The following paragraphs discuss the results presented abovein light of hypotheses enunciated previously for the generation ofpetrological diversity in volcanic rocks of Iceland. This discussion in di-vided into 3main parts, focusing on (1)melting hydrothermally alteredcrust, (2) crystal fractionation and mixing, and (3) influence of magmacontamination by entrainment of cumulates.

7.1. Melting of hydrothermally altered crust?

The generation of dacites along the Heiðarsporður ridge has beensuggested to be the result of partial melting of mafic crust (Jónasson(2005). However, there are severe thermal constraints on the efficiencyof producing crustal melts in the relatively thin and refractory crust(i.e., of typically basaltic composition) of Iceland (Thompson et al.,2002; Dufek and Bergantz, 2005; Gelman et al., 2013), even takinginto account the enhanced thermal influence of the plume (Martinand Sigmarsson, 2007; Martin and Sigmarsson, 2010). The productionof dacites requires a relative high degree of melting (in comparison torhyolite; e.g. Sisson et al., 2005), which makes it thermally even morechallenging.

A possibility for making the crust more fusible is to hydrothermallyalter it, leaving a clear oxygen isotope signal (lowers the δ18O, seeTaylor, 1980). Partial melting of altered basaltic crust has been suggestedfor the generation of rhyolitic rocks in many places in Iceland, such asKrafla (Nicholson et al., 1991; Sigmarsson et al., 1991; Jónasson, 1994),Hekla (Sigmarsson et al., 1991; Sigmarsson et al., 1992), Törfajökull(O'Nions and Grönvold, 1973; Martin and Sigmarsson, 2007; Zellmeret al., 2008) and Askja (Sigmarsson et al., 1991). Similarly, holocene

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu

Sam

ple

/ C1-

cho

ndrit

e

10

100

1000Olivine basalt

Fe-Ti basalt

Basaltic icelandite

Dacite

Icelandite

a

b

Ni (ppm)

Ba (ppm)

300

250

200

150

100

50

0 0 20 40 60 80 100

350

Mixing line

Hypotetical fractionation curve

Fig. 5. a) Ba (ppm) versusNi (ppm) plot for theHeiðarsporður rocks (symbols as in Fig. 3).The curved solid line represents a fractional crystallization path for a primitive olivinebasalt (see text) whereas the dashed lines represent hypothetical magma mixing of aprimitive olivine basalt and an evolve icelandite. b) Average REE patterns of rocks fromthe Heiðarsporður ridge normalized to C1-chondrite (Sun and McDonough, 1989).

122 A. Mancini et al. / Journal of Volcanology and Geothermal Research 301 (2015) 116–127

dacites at Hekla and Askja are also thought to be formed by partial melt-ing of altered basaltic crust (Sigmarsson et al., 1991; Sigmarsson et al.,1992; Martin and Sigmarsson, 2007; Martin and Sigmarsson, 2010). The

Rb/Zr

Zr/Hf

SiO2 (wt%)

2

706560555045

0.06

0.04

0.02

0

0.08

0.10

0

1

1

2

20

30

40

50

60

zircon

Fig. 6. Incompatible trace element ratios plotted against SiO2 (wt%). Analytical uncertainities dumaterial for analytical errors). The effect of zircon crytallization is indicated by the arrow. See

Holocene dacites of Hekla andAskja exhibit chemical signs of the involve-ment of hydrothermal waters in their generation (low value of δ18O,higher abundance of Th; Sigmarsson et al., 1991, 1992; Martin andSigmarsson, 2007, 2010). However, a careful observation of textural andchemical parameters in the Heiðarsporður dacites suggest that they areunlikely to have been produced by melting hydrothermally alteredbasaltic crust. Holocene dacites potentially produced through partialmelting of hydrated basaltic crust show Th concentrations in the range6.5–9 ppm (Sigmarsson et al., 1991, 1992) whereas Th contents for theHeiðarsporður dacites varies between3.0 and 4.1 ppm (see supplementa-ry material). In addition, the relatively constant behavior of Th/U ratio(apart from some scattering in the low U magmas), Ba/Th and Rb/Zr asa function of magmatic evolution (Fig. 6 and supplementary material),are unlikely to be produced by melting hydrothermally altered material(all elements are strongly incompatible, but have variable mobilities inhot fluids). Moreover, δ18O of dacites from Heiðarsporður measured inprevious studies (Nicholson et al., 1991; Jónasson, 2005; Pope et al.,2013) are neither particularly low nor scattered for Icelandic magmas,hence not consistent with partial melting of hydrothermally altered ma-terial (Fig. 9). The only lithology at Heiðarsporður that exhibit low δ18Oare the Fe-Ti basalts.

7.2. Crystal fractionation and magma mixing

The alternative process to partial melting of hydrothermally-alteredcrust, crystal fractionation, is well supported by bulk-rock composition-al and δ18O data. Fractionation has been already suggested by Pope et al.(2013), and Jónasson (2005), although only for the rock suite from ba-salts to icelandites. Pope et al. (2013) base their interpretation mainlyon the homogeneity of the δ18O data (Fig. 9) and the major elementscomposition. However, variation diagrams formajor and trace elements(Fig. 3, 4, 5 and 6) also support this inference, particularly if a polybaricdifferentiation path is considered. The abrupt decrease in Sc/K2O withany differentiation indices (here with SiO2, Fig. 4) is an excellent indica-tor for clinopyroxene fractionation (Albarède et al., 1997) at least untilicelandite stage (from Icelandite to dacite, the Sc/K2O trend becomesflat, suggesting the dominant removal of plagioclase). These evidencestogether with the strongly nonlinear trends in Cr, Sr, TiO2, and P2O5

Th/U

Zr/Ba.5

0

9

6

3

0

12

.5

.0

.5

.0

SiO2 (wt%)706560555045

e to the low content of U and Th create the scattering in basaltic rocks (see supplementaryFig. 3 for legend.

An67

An66

a Plagioclase population A

An54

An63

c Plagioclase population B

An53

An62

d Plagioclase population B

An68

An65

An67

b Plagioclase population A

Fig. 7. Representative images of the two different populations of plagioclase observed in the Fe-Ti basalt. a) and b) Unzoned and euhedral plagioclase with normal zonation pattern (gen-eration A, see text for detail). c) and d) Reversely zoned and partially resorbed plagioclase crystals.

123A. Mancini et al. / Journal of Volcanology and Geothermal Research 301 (2015) 116–127

(Figs. 3 and 4) indicate chemical fractionation in magmatic liquids in-duced by the dominant removal of clinopyroxene and olivine for themafic stages, and, plagioclase, titanomagnetite, and apatite for themore silicic stages. In particular, the inflected trends displayed by P2O5

and TiO2 suggest the stabilization of apatite and titanomagnetite, re-spectively, in intermediate compositions, a key aspect in determiningthe importance of crystal fractionation in magmatic series (see recentcompilations of Geist et al., 1995; Wade et al., 2005; Deering andBachmann, 2010 and Lee and Bachmann, 2014). The influence of zirconis only indicated in dacites by the Rb/Zr plot (Fig. 6), which slightly in-creases due to the decrease of Zr in the liquid.

With the purpose of testing the fractionation pattern, we tried to re-produce the described polybaric behavior using the rhyolite-MELTS soft-ware (Gualda et al., 2012).We chose to use twodifferent steps at differentpressure and water content conditions in order to simulate the polybaricbehavior of an ascendingmagma. In thefirst step,we ran anolivine-basalt(our sample A59; supplementary material) at QFM conditions, 2.0 Kbarand 0.5 wt% water content. For the second one, we used an icelanditesample (KK47; from Jónasson, 2005) with slightly lower pressure (1.5Kbar) and higher water content (1.5 wt%). The resulting MELTS trendsat shallow depths reproduced quite well the major elements behaviour

observed along the Heiðarsporður ridge (Fig. 10; and supplementarymaterial for further results). However, some mismatches are noticeable(particularly for the Fe-Ti basalts). Those can be explained by small errorsinduced by the calculation of MELTS curves (e.g., Villiger et al., 2004;Rooney et al., 2012), some scatter in the compositions of the analyzedrocks samples, some variations in oxygen fugacity, and/or some entrain-ment of cumulate material and/or altered crustal material (magma con-tamination) during ascent to the surface (see discussion below).

As proposed since themid-1800's (Bunsen, 1851, but also see Larsenet al., 1938; Jónasson, 2005 and Charreteur and Tegner, 2013) , magmamixing must also play an important role in generating some of the pet-rologic diversity in Iceland and elsewhere. This also holds true for theHeiðarsporður ridge. In fact, basaltic icelandites show clear petrograph-ical, mineral chemical and geochemical indications of magma mixing:

(1) Largemafic enclaves and obvious textural disequilibria in crystals(2) Linear trends in the Ba vs. Ni (Fig. 5a) and in all the element-

element plots (Figs. 3 and 4; particularly significant in P2O5)(3) Large range of An content in the basaltic icelandite, which

covers the entire range from olivine basalt (An72-80) to icelandite(An52-70).

OB

BI

FTB

I

D

An %

B A

80

70

45 50 55 60 65SiO2 (%)

60

50

40

30

An

%

This studyJónasson (2005)

0.14

0.13

0.12

0.11

0.10

0.09

0.08

0.07

0.06

400

350

300

250

200

1508 10 12 14 16 18 20 22 24 8 10 12 14 16 18 20 22 24

Pb (ppm) Sr (ppm)

Ba (ppm) Ba (ppm)

Olivine-basalt

Fe-Ti basalt

a b

c d

30 40 60 70 80 90 10050

Fig. 8.Mineral chemistry of plagioclase fromHeiðarsporður. a) and b) Composition of plagioclase in olivine basalts (OB), basaltic icelandites (BI), icelandites (I), Fe-Ti rich basalts (FTB), anddacites (D). In Fe-Ti basalts: A = plagioclase of group A; B = plagioclase of group B (see text for details). Light grey field represents the data of Jónasson (2005) as reference. c) andd) Concentrations of trace elements (ppm) of plagioclase in olivine-basalts, and Fe-Ti rich basalts.

124 A. Mancini et al. / Journal of Volcanology and Geothermal Research 301 (2015) 116–127

7.3. Basalt diversity – influence of entrainment from crystal residues in thecrust

In bulk-rock chemical variation diagrams (Figs. 3 and 4), the Fe-Tibasalts clearly display unusual concentrations of both major and traceelements such Al, Mg, Fe, Ti, P, V and Sr. The simplest explanation forthose anomalous concentrations would be a mix between two differentmagmas, but the Fe-Ti basalts are always off any possiblemixing line be-tween different magmatic liquids in the area. Therefore, there must beanother process responsible for generating this basaltic variability atHeiðarsporður. These basalts could obviously come from differentmantle sources, but the mantle beneath Iceland is not particularly

18O (‰)

SiO2 (wt%)75706560555045

3.0

3.5

4.5

5.0

5.5

(Nicholson et al. 1991)

Fig. 9. δ18Ovalues (in‰) for theHeiðarsporður rocks suite. Fe-Ti basalts havedistinctly lowervalues than other magmas. Non-differentiated basalts means that Nicholson et al. (1991) donot make any distinction between Olivine-basalts and Fe-Ti basalts. Data from Nicholson etal. (1991)., Jónasson (2005), and Pope et al. (2013). Symbols and colors as in Fig. 3.

heterogeneous (Sigmarsson and Steinthórsson, 2007), and for magmaserupted so closely in space and time as the olivine and Fe-Ti basalts, itseems highly unlikely that their compositional variability is producedby mantle heterogeneities alone.

An alternative way to generate those basalts would be by crystalfractionation together with some entrainment of pre-existing crustand/or crystal cumulates produced during previous stages of differenti-ation in the upper crust. The hypothesis that Fe-Ti basalts havebeen partly generated by fractionation is supported by MELTS trends(Fig. 10; e.g., TiO2 and Fe2O3

TOT increase, while MgO decreases towardsthe Fe-Ti basalt field before Fe-Ti oxides become stable). Given that ap-atite is not well modeled by MELTS software (Rooney et al., 2012), wedid not attempt to model it here. However, the presence of apatiteleft-over in the crust is required by the strong drop in the P2O5 aticelandite stage (Lee and Bachmann, 2014). Al2O3 also tends to decreasetowards the Fe-Ti basalt field during plagioclase-dominated fraction-ation, but increases as Fe-Ti oxides join the crystallizing assemblage.Moreover, the Fe-Ti basalts present slightly lower δ18O values thanother rocks types in the area. Hence, on the basis of the textural andmineral chemistry features in the Fe-Ti basalts showing entrainmentsof glomerocrysts and evolved (low Al) but Sr-rich plagioclase, we sug-gest that some interaction occurred with previously formed cumulates(potentially partly altered) in the differentiation column (Fig. 11).Given that processes in the volcanic plumbing system are highly vari-able and controlled by local variations (Charreteur et al., 2013), cumu-lates are likely to be particularly heterogeneous, and therefore their“bulk” composition is impossible to estimate precisely. Such interac-tions between evolving mafic recharge and Fe-Ti oxide-rich (up to10 vol%) mafic to intermediate cumulates with interstitial apatiteshave already been described in Iceland by Thorarinsson and Tegner(2009) in the Austhurhorn intrusive complex (SE Iceland). Arrows inFig. 10 points towards fields, indicating somepossible cumulate compo-sition sampled by Thorarinsson and Tegner (2009). This process of

2.5

2.0

1.5

1.0

0.5

0

12

10

8

6

4

2

0

3.0

10

18

16

14

12

80757065605550458075706560555045

MgO (wt%)

TiO2 (wt%)

SiO2 (wt%)

Al2O3 (wt%)

SiO2 (wt%)

contamination

contamination

contamination

contamination

25

20

15

10

5

0

Fe2O3T (wt%)

Fig. 10. Results of petrogeneticmodellingwith rhyolite-MELTS software are shown for TiO2,MgO, Al2O3 and Fe2O3TOT for an olivine basalt (thick line: sample A59, QFM, 2 Kbar, 0.5wt%H2O)

and an icelandite sample (dashed line: sample KK47, QFM, 1.5 Kbar, 1.5 wt% H2O). Dashed arrows points to the possible field of cumulate material left-over in the crust. Cumulates whichpossiblyfit thismodel were sampled by Thorarinsson and Tegner (2009) inAusthurhorn intrusive complex (SE Iceland). Symbols and colors as in Fig. 3. See discussion and supplementarymaterial for further details.

125A. Mancini et al. / Journal of Volcanology and Geothermal Research 301 (2015) 116–127

magma contamination by (partly altered) cumulate left-overs wouldhelp explain the high concentrations of some elements (P, Sr, Fe, Ti)without requiring unrealistic fractionating assemblages.

Fe

a

Hydrothermal circulation

Evolved Plagioclase (low An)

Pyroxene Titanomagnetite

Apatite

Cumulate

Dacite

Fig. 11. Schematic cross section of the subvolcanic plumbing system beneath Hraunbunga cratereservoir, dacitic magmas are formed. The erupted dacite leaves some cumulate material (plagiorecharges themagma chamber, picking up some of the crystal residues. Following the recharge,assemblage characterized by olivine, clinopyroxene, titanomagnetite, two populations of plagio

According to Jónasson (2005), Fe-Ti rich basalts were injected intoHeiðarsporður from the Krafla central caldera as they share many simi-larities and are found only in the northern part of the ridge. However,

-Ti basalt

b

Titanomagnetite

Olivine

SmallerPlagioclase

Evolved Plagioclase core

Glomerocrysts with pyroxene and low An plagioclases

Cpx

Entrainment of crystal residues during ascent at shallow level

Deep differentation of mafic magma and recharge of magma chamber

Rim with higher An content

r (not to scale). a) Following some crystal fractionation and melt extraction from amushyclase, pyroxene, Fe-Ti oxide, and apatite) behind b) At a later time, a new batch of magmasome parts of themagma reservoir reaches a Fe-Ti basaltic composition,with a phenocrystclase (one with inverse An-content), and glomerocrysts of clinopyroxene and plagioclase.

126 A. Mancini et al. / Journal of Volcanology and Geothermal Research 301 (2015) 116–127

we provided evidence that these basalts have not only been erupted onthe northern part of the ridge, but alsomore to the south, as they cap thedacitic unit on the top of Hraunbunga (Fig. 1). We, nonetheless, concurthat Fe-Ti rich basalts erupted along Heiðarsporður are similar to thosefound near the Krafla central volcano (Fig. 3 and 4; Hollingsworth et al.,2012). Hence, we suggest that the Fe-Ti rich basalts in the Krafla volca-nic systemwere also produced bydifferentiation andpartial remobiliza-tion of cumulates following the production of large amounts of silicicmagmas in the area in Refilled-Tapped-Fractionated (RTF) magmareservoirs in the upper Icelandic crust (e.g., Thorarinsson and Tegner,2009).

8. Summary and conclusions

A semi-continuous series of magmas from olivine basalts to daciteshave been erupted along the Heiðarsporður ridge. Element variationdiagrams together with the mineral chemistry of the phenocrystsassemblages and MELTS modeling suggest a polybaric fractionalcrystallization trend. Icelandite and dacite compositions were predom-inantly formed by fractional crystallization of olivine basalts, in whichclinopyroxene played a dominant role at high temperature and pressureand was joined at more evolved compositions (and lower pressure) byplagioclase, titanomagnetite, apatite, and zircon. Basaltic icelandites, onthe other hand, appear to be hybrid rocks formed bymixing between ol-ivine basalt and icelandite magmas. Finally, there are several lines oftextural, isotopic and geochemical evidence suggesting that Fe-Ti ba-salts are formed by differentiation of recharge magmas, partly fraction-ating, and partly incorporating some hydrothermally altered crystalresidues left-over from the silicic differentiation stage. We believe thatsimilar processes also occur in the larger neighboring Krafla volcanicsystem.

Acknowledgements

Weare grateful to Natalia StammandMarcel Guillong for their assis-tanceduring laboratory analyses. In addition,wewould like to thank theNational Land of Survey of Iceland for providing us the aerial images oftheHeiðarsporður ridge. ChadDeering, CalvinMiller and an anonymousreviewer are thanked for their constructive comments on a previousversion of this manuscript. We would also like to thank editor MalcolmRutherford for his thoughtful comments, and the time he invested inthis manuscript.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.jvolgeores.2015.05.010.

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