Storage and Evolution of Mafic and Intermediate Alkaline Magmas beneath Ross Island, Antarctica

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Storage and Evolution of Mafic and Intermediate Alkaline Magmas beneath Ross Island, Antarctica

Kayla Iacovino, Clive Oppenheimer, Bruno Scaillet, Philip Kyle

To cite this version:

Kayla Iacovino, Clive Oppenheimer, Bruno Scaillet, Philip Kyle. Storage and Evolution of Mafic

and Intermediate Alkaline Magmas beneath Ross Island, Antarctica. Journal of Petrology, Oxford

University Press (OUP), 2016, 57 (1), pp.93-118. �10.1093/petrology/egv083�. �insu-01294605�

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Storage and Evolution of Mafic and

Intermediate Alkaline Magmas beneath Ross Island, Antarctica

Kayla Iacovino

¹

*, Clive Oppenheimer

¹

, Bruno Scaillet

²

and Philip Kyle

³

1

Department of Geography, University of Cambridge, Cambridge CB2 3EN, UK;

²

Institut des Sciences de la Terre d’Orleáns, Universite´ d’Orleáns–CNRS–BRGM, 45071 Orleáns Cedex 2, France and

³

Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA

*Corresponding author. Present address: US Geological Survey, Mail Stop 910, 345 Middlefield Road, Menlo Park, CA 94025, USA. Telephone:þ1 (650) 329-4986. E-mail: kiacovino@usgs.gov

Received June 9, 2014; Accepted December 24, 2015

ABSTRACT

We present the results of phase equilibrium experiments carried out on basanite and phonoteph- rite lavas from Ross Island, Antarctica. Experiments were designed to reproduce the P–T–X–fO

₂

conditions of deep and intermediate magma storage and to place constraints on the differentiation of each of the two predominant lava suites on the island, which are thought to be derived from a common parent melt. The Erebus Lineage (EL) consists of lava erupted from the Erebus summit and the Dry Valley Drilling Project (DVDP) lineage is represented by lavas sampled by drill core on Hut Point Peninsula. Experiments were performed in internally heated pressure vessels over a range of temperatures (1000–1150

C) and pressures (200–400 MPa), under oxidized conditions (NNO to NNO þ 3, where NNO is the nickel–nickel oxide buffer), with X

H2O

of the H

2

O–CO

2

mixture added to the experimental capsule varying between zero and unity. The overall mineralogy and mineral compositions of the natural lavas were reproduced, suggesting oxidizing conditions for the deep magma plumbing system, in marked contrast to the reducing conditions (QFM to QFM – 1, where QFM is the quartz–fayalite–magnetite buffer) in the Erebus lava lake. In basanite, crystal- lization of spinel is followed by olivine and clinopyroxene; olivine is replaced by kaersutitic amphibole below 1050

C at intermediate water contents. In phonotephrite, the liquidus phase is kaersutite except in runs with low water content (XH

2

O

^fluid

< 02) where it is replaced by clinopyrox- ene. Experimental kaersutite compositions suggest that the amphibole-bearing DVDP lavas differentiated below 1050

C at 200–400 MPa and NNO þ 15 to NNO þ 2. Olivine- and clinopyrox- ene-bearing EL lavas are consistent with experiments performed above 1050

C and pressures around 200 MPa. The plagioclase liquidus at <1–2 wt % H

₂

O suggests extremely dry conditions for both lineages (XH

2

O

^fluid

approaching zero for EL, 025 for DVDP), probably facilitated by dehydra- tion induced by a CO

2

-rich fluid phase. Our results agree with previous studies that suggest a single plumbing system beneath Ross Island in which DVDP lavas (and probably other peripheral volcanic products) were erupted through radial fractures associated with the ascent of parental magma into the lower crust. The longer travel time of the DVDP lavas through the crust owing to lateral move- ment along fractures and the lack of a direct, sustained connection to the continuous CO

₂

-rich gas flux that characterizes the main central Erebus conduit is probably responsible for the lower tem- peratures and slightly wetter conditions and hence the change in mineralogy observed.

Key words: alkaline volcanism; basanite; CO

2

; phase equilibria; phonotephrite; Ross Island

V^CThe Author 2016. Published by Oxford University Press. 93

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

J O U R N A L O F

P E T R O L O G Y

doi: 10.1093/petrology/egv083 Original Article

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INTRODUCTION

Ross Island, Antarctica (77

32’S, 167

10’E) is part of the Erebus Volcanic Province (EVP) and comprises five pre- dominant volcanic centers. The currently active and volumetrically dominant Erebus volcano is surrounded by the now extinct subsidiary volcanic centers of Mts Bird, Terror, Terra Nova, and Hut Point Peninsula (Fig. 1). Erebus is known for its persistently active pho- nolitic lava lake, which has been at the focus of recent active monitoring and petrological studies as it repre- sents a window into the shallow regions of the alkaline intraplate volcanic plumbing system. Recent investiga- tions employing experimental petrological work (Moussallam et al., 2013) and seismic tomography (Zandomeneghi et al., 2013) have placed tight con- straints on the physical and petrological properties of the shallow plumbing system and phonolitic lava lake including its geometry, temperature, pressure, and oxy- gen fugacity. These studies have the potential to refine the results of numerical models developed for the shal- low Erebus system, such as those of Burgisser et al.

(2012) and Molina et al. (2012). The intermediate and deep plumbing system beneath Erebus (>8 km depth) has been investigated via study of a suite of melt inclu- sions (Oppenheimer et al., 2011) and mixed volatile solubility experiments (Iacovino et al., 2013), which have placed constraints on the depths of degassing and the equilibrium fluid contents of primitive and inter- mediate composition magmas. Little is known, how- ever, about the specific P–T–X–fO

₂

conditions of

magma storage and differentiation processes at these depths. Understanding the deep system at Erebus is crucial because the constant supply of deeply derived CO

₂

-rich gases combined with a sustained energy and mass input into the lava lake suggests a direct link be- tween the phonolitic lava lake and the mantle source of the parental magmas via a deep mafic plumbing system (Kyle et al., 1992; Oppenheimer & Kyle, 2008;

Oppenheimer et al., 2011).

Gas emissions from the Erebus lava lake are rich in CO

₂

(30 mol %) and comparatively low in H

2

O (65 mol

%; Oppenheimer et al., 2011; Burgisser et al., 2012), con- sistent with the volatile compositions deduced from melt inclusions for the deeper system. The longevity of the lava lake itself is hypothesized to reflect, in part, the con- stant fluxing of heat and CO

2

-rich fluid (Oppenheimer et al., 2011) that originates from depths greater than those recorded by the most primitive melt inclusions (Iacovino et al., 2013a). Indeed, many separate studies have concluded that various aspects of the Erebus sys- tem are controlled by the influx of heat and CO

2

, includ- ing polybaric fractional crystallization throughout the magmatic system (Oppenheimer et al., 2011), the bi- modal eruptive behavior in the lava lake (Oppenheimer

& Kyle, 2008), shallow magma chamber processes (Moussallam et al., 2013), and fumarolic degassing and the formation of ice caves and ice towers (Curtis & Kyle, 2011). CO

2

flushing has been invoked to explain the char- acteristics of degassing and melt inclusions at other vol- canoes including Stromboli (Allard, 2010), Vesuvius

Fig. 1.Map showing the location of Ross Island within the McMurdo Volcanic Province, Antarctica. Erebus volcano is surrounded by the peripheral volcanic cones Bird, Terra Nova, Terror, and Hut Point Peninsula. The locations of the DVDP and EL samples collected for this experimental study are indicated.

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(Marianelli et al., 2005), Etna (Spilliaert et al., 2006;

Shinohara et al., 2008; Tagliani et al., 2012) and volca- noes in southeastern Guatemala (Walker et al., 2003) that sit atop large sedimentary carbonate sequences, the devolatilization of which could provide the necessary ex- cess carbon (Iacono-Marziano et al., 2009). As Erebus has no such shallow carbon reservoir and probably de- rives all of its volatiles from the same mantle source re- sponsible for magma production, we propose Erebus as the archetype for mantle-sourced carbon-controlled vol- canism on Earth.

Subsidiary extinct volcanic centers and cones on Ross Island are distributed radially around Erebus at c.

120 from each other. This symmetrical distribution has been hypothesized to be the result of radial fractures created by the pressure of updoming mantle material on the crust directly beneath Mount Erebus (Kyle &

Cole, 1974; Kyle et al., 1992). The rare earth element pat- terns in primary Ross Island basanitoids indicate the generation of a basanitic parent magma through low- degree partial melting of the mantle source (Sun &

Hanson, 1975, 1976). Major and trace element modeling suggests that lavas from Erebus [the Erebus Lineage (EL)] and the peripheral Hut Point Peninsula [the Dry Valley Drilling Project (DVDP) lineage] both evolved from a common basanite parent melt, which underwent two distinct fractional crystallization trends (Kyle, 1981;

Kyle et al., 1992). So far experimental work has focused on the evolved magmas (Moussallam et al., 2013) and no experimental work is available to constrain storage and differentiation conditions of mafic lavas at Erebus (or other subsidiary volcanoes).

In this experimental study, one lava from each of the two Ross Island lava lineages (EL phonotephrite AW- 82038 and DVDP basanite KI-04) were chosen as start- ing materials for equilibrium crystallization experiments to establish the P–T–X–fO

₂

conditions necessary to pro- duce the observed natural rock series. Phase equilib- rium experiments are commonly used to constrain the pre-eruptive storage conditions of magmas, as well as their liquid line of descent (Sack et al., 1987; Moore &

Carmichael, 1998; Freise et al., 2003; Martı´ et al., 2008;

Andu´jar et al., 2010; Di Carlo et al., 2010). Although non- equilibrium processes such as magma mixing and re- peated replenishment of the system are important to consider, high pressure–temperature experimentation nevertheless illuminates the fundamental processes that control magma evolution from mantle to surface, allowing us to evaluate to what extent crystal–liquid equilibrium has been attained or maintained at depth.

Experiments were performed over a range of tem- peratures (1000–1150

C) and at pressures between 200 and 400 MPa to reproduce the intermediate region of the magma plumbing system where much early crystal- lization (differentiation of a basanitic parent to form intermediate lavas) is thought to occur. A temperature of 1000

C was chosen as a thermal lower bound, as the temperature of the surficial lava lake, which should be the coolest region of the plumbing system, has been

constrained to 1000

C by several studies (Kyle, 1977;

Caldwell & Kyle, 1994; Dunbar et al., 1994). The result- ing experimental charges thus span the entire range of compositions seen in the natural lavas, allowing eluci- dation of the evolutionary processes occurring through- out the Ross Island magmatic plumbing system. With this approach, our aim has been two-fold: (1) to experi- mentally establish the conditions necessary to derive the distinct Erebus and DVDP lineages from a hypothe- sized single basanite parent; (2) to place constraints on the intermediate Erebus and DVDP plumbing systems with respect to pressure, temperature, oxygen fugacity, and dominant volatiles (H

2

O and CO

2

).

Experiments with H

₂

O–CO

₂

fluid compositions in the range of XH

2

O ¼ 0–1 not only help elucidate the role of H

₂

O in controlling phase equilibria, but also constrain the effect of CO

2

on crystallization processes via melt dehydration. Given that carbon dioxide is known to play an important role in the genesis of alkaline magmas, in both the mantle (Spera, 1984) and crust (Kaszuba &

Wendlandt, 2000), it is important to understand its role at Erebus throughout the magmatic system (from man- tle to surface).

Mineralogy and petrology of Ross Island lavas The majority of lavas on Ross Island are represented by two strongly undersaturated sodic differentiation lin- eages: the Erebus Lineage (EL), made up of lavas erupted from Erebus volcano, and the Dry Valley Drilling Project (DVDP) lineage, made up of lavas on the Hut Point Peninsula and sampled via drill coring. The rarer Enriched-Iron Series (EFS) lavas are less silica- undersaturated and constitute only a small volume of Ross Island rocks. The EFS will not be discussed in this study. The DVDP and Erebus lineages are similar in terms of whole-rock chemistry as demonstrated by the similar variation in major element evolution of the DVDP and EL lavas with the exception of P

2

O

5

(Fig. 2), the variation of which is probably being controlled by apatite crystallization. Major element mass balance and trace element modeling of DVDP and EL rocks (Kyle, 1981; Kyle et al., 1992) suggests fractional crystallization from a common basanitic parent melt, with slight differ- ences in differentiation conditions resulting in the two similar yet distinct lineages.

The mineralogy of the EL and DVDP lineage rocks from Ross Island has been extensively documented by Kyle et al. (1992) and Kyle (1981), respectively, whose observations are summarized below. These lineages share the same assemblage with two notable excep- tions. First, kaersutitic amphibole is present in almost all DVDP lavas (except in basanites where it is rare) but is absent in EL lavas. Second, olivine is present in all EL lavas and in DVDP basanites, but occurs only as xeno- crysts in more evolved DVDP lavas. In the EL rocks, oliv- ine ranges in composition from Fo

88

in basanites to Fo

₅₁

in phonolites. Olivine xenocrysts are rare and occur only in the most primitive lavas. In the DVDP

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lavas, olivine occurs only in basanites as euhedral to subhedral phenocrysts and in the groundmass.

Clinopyroxene is ubiquitous in all EL and DVDP lavas and occurs as euhedral to subhedral phenocrysts.

Pyroxenes with green cores, which show irregular rims, possibly owing to resorption, are occasionally found in DVDP lavas. In the EL lavas, clinopyroxenes show oscil- latory zoning in basanites and intermediate lavas, but no zoning in phonolites.

Kaersutite is absent in EL lavas but is an important phase in all DVDP lavas except the basanites where it is rare. In some phonotephrites, kaersutite ranges from euhedral grains to those showing strong resorption and oxidation. It was suggested by Kyle (1981), based on previous experimental work, that temperature and PH

₂

O probably exert a strong control on the appearance or absence of kaersutite in DVDP and Erebus lineages, respectively.

Fig. 2.Variation diagrams illustrating the evolution of the DVDP lineage (red diamonds;Kyle, 1981) and Erebus Lineage (EL; blue tri- angles;Kyleet al., 1992). For whole-rock compositions the major element totals are normalized to 100%. One lava sample from each of these two lineages (red star: DVDP KI-04; blue star: EL AW-82038) was used as the starting material for the experimental work.

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Feldspar is the modally dominant phenocryst phase in EL lavas (up to 40% of the mode in anorthoclase- bearing phonolites) with compositions ranging from An

72

to An

59

in basanites, becoming increasingly albitic in the more evolved lavas. The feldspar in the EL phonolite is primarily anorthoclase. In the DVDP lava, feldspar occurs mostly as microphenocrysts or in the groundmass and is mainly labradorite (An

_50–60

) in bas- anites, and andesine to oligoclase in all other lava types.

Feldspathoids occur in the groundmass of the EL lavas, as nepheline, which contains up to 24% norma- tive Ne. Microphenocrysts of apatite are common in most rocks of both lineages and are often found as in- clusions in kaersutite and magnetite in the DVDP rocks.

Pyrrhotite is present in all rocks from both lineages and occurs as small round blebs, suggesting the presence of an immiscible sulfide liquid in the Ross Island mag- mas. Opaque Fe–Ti oxides are common in both lava suites and are typically titanomagnetite as phenocrysts and in the groundmass, plus rare ilmenite phenocrysts.

EXPERIMENTAL METHODS Starting materials

To investigate the evolution of the Ross Island magmas and the two dominant lava lineages, two samples were chosen, one from each lineage (Table 1): AW-82038 is an Erebus Lineage phonotephrite lava (SiO

2

¼ 4801 wt %;

MgO ¼ 318 wt %) from Turk’s Head (Kyle et al., 1992) representative of intermediate EL lavas; KI-04 is a basan- ite (SiO

2

¼ 4306 wt %; MgO ¼ 892 wt %) collected from Hut Point Peninsula for this study and represents the less evolved magmas of the DVDP lineage. Figure 3 illus- trates our starting material compositions relative to those of a suite of melt inclusions in Erebus Lineage rocks, which provides a representative cross-section through the evolutionary chemical trend of lavas on Ross Island (Oppenheimer et al., 2011).

The AW-82038 phonotephrite [also referred to as Ne hawaiite by Kyle et al. (1992)] is weakly porphyritic (<8% phenocrysts) with a phenocryst assemblage of plagioclase, olivine, clinopyroxene, Fe–Ti oxides, apa- tite and rare accessory nepheline. AW-82038 is consist- ent with the fractionation of 12% olivine, 35%

clinopyroxene, 7% Fe–Ti oxides, and 1% apatite from a basanitic parent, yielding a 43% residual melt (basanite DVDP 2-105.53; Kyle et al., 1992).

In the KI-04 basanite, phenocrysts constitute around 12 vol. % and include olivine (6 vol. %), plagioclase feld- spar (2 vol. %), augite (2 vol. %), apatite (1 vol. %), and titanomagnetite (<1 vol. %). Although kaersutitic amphi- bole is common in almost all DVDP lavas, it is rare in DVDP basanites and is absent in KI-04. Crystal distribu- tion is relatively homogeneous throughout the sample.

This sample is consistent with 17% fractionation from basanitic parent DVDP 2-105.53 yielding an 83% re- sidual melt.

The lavas were crushed to a powder and melted in a Pt crucible in a 1 atm furnace at 1300

C. Sample AW- 82038 was held in the furnace for 30 min under a stream of nearly pure CO

2

(corresponding to an oxida- tion state near the Ni–NiO buffer, NNO) and then drop- quenched into cold water [sample prepared by Iacovino et al. (2013a)]. Sample KI-04 was held in furnace for 1 h in air (i.e. extremely oxidizing conditions) and then removed from the furnace and immediately placed into cold water to quench. The resulting glassy samples were extracted from their crucibles, re-crushed, and re- melted under identical conditions to ensure a homoge- neous and crystal-free glass. A portion of each starting material was polished and analyzed by electron micro- probe to verify that no compositional changes had occurred during melting (see Table 1).

Experimental techniques

Between 30 and 60 mg of glassy starting material pow- der was added to Au

₈₀

Pd

₂₀

capsules with lengths of 20 mm, an outer diameter of 29 mm, and an inner diameter of 25 mm. Liquid H

2

O was added via a micro- syringe (0–6 mg), and CO

2

was added as silver oxalate powder (Ag

₂

C

₂

O

₄

, 0–18 mg). The X

_H2O

in the H

₂

O–CO

₂

mixture of pre-run charges was varied at steps between zero and unity (i.e. X

_H2O

¼ 0, 025, 05, 075, and 1), and the fluid proportion in the charge was always 10% to ensure fluid saturation (see Supplementary Data Appendix 1; supplementary data are available for down- loading at http://www.petrology.oxfordjournals.org).

Once filled, capsules were weighed, crimped at their open end, arc welded shut with a graphite electrode, and then subsequently re-weighed to check for any mass loss or gain during welding. After welding, capsules were placed in an oven at 110

C for several minutes and then re-weighed to check for any leaks in the capsule. As a third method to check for leaks, some capsules were held in a beaker of oil at 300

C for a few seconds. If bub- bles were observed escaping from the capsule, it was considered to have leaks and was thus discarded. After completion of the experiment, capsules were weighed to check that no fluid loss occurred during the run. Any cap- sules that showed a change in mass >02 mg either after welding or after heating were considered to have leaked at least part of their fluid phase and were discarded.

Experiments were run in vertically working internally heated pressure vessels (IHPVs; see Holloway, 1971) at the Institut des Sciences de la Terre Orle´ans (ISTO) at isothermal and isobaric conditions between 1000 and 1150

C and between 200 and 400 MPa with the oxygen fugacity of the vessel at about one log unit above the Ni–NiO buffer (NNO þ 1; Table 2). Each run con- tained 5–10 sample capsules loaded into a vertically working IHPV equipped with a double-wound Kanthal furnace. An Ar–H

₂

gas mixture loaded sequentially at room temperature was used as the pressure medium with the Ar/H

₂

ratio fixed so as to achieve an experimen- tal fO

2

in the vessel of NNO þ 1 (Scaillet et al., 1992),

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although the fO

₂

within each experimental charge var- ied slightly depending on its specific H

₂

O/CO

₂

fluid mix- ture (see below). It is noteworthy that, because the experimental fO

2

is lower than that imposed during melting of the sample KI-04 starting material, some additional H

₂

O was created in the capsule during KI runs via the reduction of Fe

^3þ

. Thus, even nominally

‘dry’ runs will contain some H

2

O (typically < 1–2 wt %), and this has been taken into account for the calculation of experimental oxygen fugacity. Pressure was meas- ured with a transducer calibrated against a Heise–

Bourdon tube gauge with an accuracy of620 bars.

Temperature was measured with two chromel–alumel (Type K) thermocouples, which allow for precise control over the length of the hotspot (4 cm) to within 65

C. Experiments were run for 17–24 h and then quenched by dropping the sample into the lower, cold part of the pressure vessel (<40

C), which gives a quench rate of

>100

C s

^–1

(Roux & Lefevre, 1992). The very low

viscosities of our mafic alkalic glasses facilitated the at- tainment of equilibrium even given these relatively short run times. In similar studies on mafic melts, the at- tainment of equilibrium was observed in runs of similar duration (Di Carlo et al., 2006; Feig et al., 2010;

Pichavant et al., 2014).

ANALYTICAL TECHNIQUES Fluid phase composition

For capsules that did not leak, the conventional weight- loss method was employed as a first-order measure of the composition of the equilibrium H

₂

O–CO

₂

fluid phase (see Behrens et al., 2009). After the first post-run weigh- ing, the capsule was submerged in a liquid nitrogen bath, thus freezing any liquid water or water vapor but allowing the CO

2

to remain as a gas. The capsule was then punctured to allow for the escape of the CO

2

gas, and a hissing noise and bubbles at the puncture site

Table 1: Composition of AW-82038 and KI-04 starting glass, whole-rock and representative analyses of phenocrysts and microphenocrysts in the natural rock

AW- Whole- St. Gls² Ol-6 Ol-10 Plag-5 Plag-12 Ap-1 Sp-1 gnd gnd

82038 rock¹ Ol-5 Plag-5

SiO2 4801 4803 3872 3847 5410 5369 026 3627 5546

TiO2 277 284 014 015 2618 016

Al2O3 1856 1812 2854 2924 434 2856

FeO* 949 959 2047 2254 035 031 033 6315 2908 039

MnO 022 023 028 045 000 004 010 080 064 003

MgO 318 308 4179 3984 003 003 026 529 3509 005

CaO 765 757 030 030 1111 1153 5511 004 044 1062

Na2O 599 604 490 466 007 004 520

K2O 288 307 062 050 070

P2O5 124 141 4202

Total 9829 9954 10172 10180 9982 10021 9816 10004 10174 10119

Mg# 37 36 78 76 68

Or 23 18 26

An 698 719 675

Ab 279 263 299

KI-04 Whole- St. Gls² Ol-6 Ol-9 Aug-5 Aug-24 Plag-1 Plag-4

rock¹

SiO2 4306 4315 3804 3762 4709 4235 4991 5010

TiO2 426 443 257 499 023 018

Al2O3 1380 1394 492 916 2995 3091

FeO 973 1235* 1761 1983 798* 699* 066 053

Fe2O3 287

MnO 019 017 022 031 025 010 000 000

MgO 892 846 4356 4066 1334 1210 010 009

CaO 1107 1116 029 039 2249 2270 1411 1462

Na2O 364 381 046 045 341 331

K2O 163 168 035 029

P2O5 083 085

Total 9861 9768 10000 9899 9910 9906 9872 10004

Mg# 55 55 82 79 75 76

Or 12 10

An 811 822

Ab 177 168

Glass analyses are given in wt % and normalized to 100% with original analysis totals given. Mineral analyses are not normalized;

original analysis totals are given. St. Gls, starting glass after vitrification; Ol, olivine; Plag, plagioclase; Ap, apatite; Sp, spinel; gnd Ol, groundmass olivine; gnd Plag, groundmass plagioclase.

*Total iron given as FeO; Mg#¼100[Mg/(MgþFe*)] in mol %.

1Whole-rock composition. AW-82038 analyzed byKyleet al.(1992); KI-04 analyzed in this study, using X-ray fluorescence (XRF).

2Electron microprobe (EMP) analyses of glass composition after vitrification in 1 atm furnace, but before run in pressure vessel.

AW-82038 vitrified at an oxygen fugacity of NNOþ1 [analysis fromIacovinoet al.(2013)]; KI-04 vitrified in air (analysis from this study).

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were typically observed. After warming at room tem- perature for up to 1 min, the capsule was weighed to de- termine the mass of CO

2

lost after puncture. This was then recorded as the mass of CO

₂

in the fluid phase.

The capsule was then placed in a drying oven at 110

C for several minutes to evaporate the remaining H

₂

O and then weighed to determine the mass of the H

2

O fluid.

During this step, capsule weight was checked periodic- ally until its mass remained constant. It should be noted that this method is an indirect measure of the compos- ition of the fluid phase in our runs and can be prone to significant error owing to uncertainties in the weighing of the capsules before and after opening. Behrens et al.

(2009) estimated an error in XH

₂

O determination of around 6003 using this method. The composition of the fluid phase was also calculated by mass balance, where the mass of dissolved H

₂

O or CO

₂

in the glass [as measured by Fourier transform IR (FTIR) spectroscopy]

is subtracted from the amount of fluid (liquid H

₂

O or CO

2

as silver oxalate) added to the capsule (see Supplementary Data Appendix 1). A comparison of fluid compositions calculated both by weight loss and by mass balance shows reasonable agreement, with a cor- relation coefficient between the two methods of R ¼ 079 and an average error in XH

2

O determination between the two methods of 6011. Here, calculation of the fluid phase composition by weight loss is preferred, with the mass-balance method used for sample KI-07, for which weight-loss data were not obtained. After fluid mass measurement, the AuPd capsule was peeled away, and the sample was mounted in epoxy and pol- ished for analysis. Selected samples were doubly pol- ished for analysis by FTIR spectroscopy.

SEM and electron microprobe

Charges were initially studied using a JEOL WINSET JSM 6400 scanning electron microscope (SEM) at ISTO for preliminary phase identification, modal abundance determination and textural analysis. Most phases could be identified by visual inspection, and several crystals from each phase were analysed using energy-disper- sive spectrometery (EDS). The EDS analyses are not re- ported here as they were used only for mineral identification.

Representative SEM images from each sample were digitally processed to measure total crystallinity and the modal abundance of all phases. Images were converted to grayscale and a threshold was applied and adjusted to isolate each phase. The total area of each phase was then calculated as a percentage of the total image. Any areas of the image that contained no sample (holes, cracks, etc.) were not included in the total sample area.

Electron microprobe (EMP) analyses of the major element compositions of the glass and crystal phases in the natural samples, starting glasses and experimental run products were made primarily using a Cameca SX- 100 electron microprobe at the University of Cambridge. Some samples were analyzed on a Cameca SX-100 at the University of Arizona or on a Cameca SX- 50 at the Universita` degli Studi di Padova. Glass was measured using an accelerating voltage of 15 keV, a beam current of 3 nA, a defocused beam size of 10 lm, and a peak counting time of 10 s, analyzing Na first to minimize any measurement error owing to Na migra- tion. Crystal phases were analyzed with an accelerating voltage of 15 keV, a beam current of 10 nA, a focused beam with a diameter of 1 lm, and a peak counting time of 20 s. In each experimental charge, typically 20–30 analyses were performed on glass and 8–12 analyses were performed on each crystal phase.

FTIR spectroscopy

Whenever possible, the H

₂

O and CO

₂

concentrations in the glass phase of each experimental charge were measured using a Thermo Scientific Nicolet iN10 MX mapping FTIR system at the US Geological Survey in Menlo Park, California. Transmission IR spectra of dou- bly polished glass chips ranging in thickness from 20 to 150 lm were obtained in the 6000–1000 cm

^–1

wave- number range using a KBr beamsplitter with a variable aperture size between 25 and 500 lm

²

. The sample stage was continuously purged with dry air, and a back- ground measurement was made before the collection of each spectrum to minimize and correct for the contri- bution of atmospheric H

₂

O and CO

₂

. Moderately to highly crystal-rich charges were characterized with FTIR mapping to identify crystal-free regions for the analysis of dissolved volatiles.

The concentrations of OH, H

₂

O, CO

₂

, and CO

₃

species were calculated with the Beer–Lambert Law (Ihinger et al., 1994), using near-IR (4500 and 5200 cm

^–1

), total water (3500 cm

^–1

), molecular H

2

O (1640 cm

^–1

), CO

2

Fig. 3.Total alkalis vs silica (TAS) diagram showing the compositions of the starting materials (stars) relative to the fractional crystallization sequence seen in Erebus Lineage rocks represented by a suite of melt inclusions [data from Oppenheimeret al.(2011)].

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Table 2: Experimental conditions and run products Run Experiment P

(MPa) T (C)

XH2O^{in 1} XH2O^{fluid 2} H2O glass (wt %)³

CO2glass (ppm)³

fO2

(DNNO)⁴

Phases (vol. %)⁵

AW 82038 phonotephrite experiments

PV-4B AW-19B 200 1000 018 014 049 — 09 Gl 5, Cpx 10, Plag 81, Tit 3, Ap 1

PV-4B AW-20B 200 1000 049 035 179 1077 17 Gl 81, Kaers 16, Tit 3

PV-4B AW-21 200 1000 072 056 322 (23) 931 21 Gl 87, Kaers 9, Tit 4

PV-4B AW-22 200 1000 088 074 404 (37) 817 24 Gl 87, Kaers 8, Tit 5

PV-4B AW-23 200 1000 100 100 555 (10) 0 26 Gl 96, Tit 4

PV-3 AW-06 200 1050 — — 142 (7) 1298 (137) — Gl 92, Plag 7, Tit<1

PV-3 AW-17 200 1050 075 043 314 (11) 443 (17) 12 Gl 99, Tit 1

PV-3 AW-18 200 1050 087 071 400⁶ — 16 Gl 99, Tit 1

PV-3 AW-10 200 1050 100 100 557 (11) 0 19 Gl 99, Tit 1, Ap<1

PV-2 AW-14 200 1100 078 020 186 831 12 Gl 99, Tit 1

PV-2 AW-13 200 1100 091 068 427 (20) 937 23 Gl 99, Tit 1

PV-8 AW-24B 250 1100 021 013 114 (1) 1700 (74) 10 Gl 100

PV-11 AW-42 300 1000 021 022 073 (3) 1145 (107) 16 Gl 78, Kaers 13, Plag 2, Tit 7

PV-11 AW-41 300 1000 049 049 178 1180 23 Gl 80, Kaers 15, Tit 5

PV-11 AW-40 300 1000 072 064 275 (34) 1975 (329) 26 Gl 94, Kaers 2, Tit 4

PV-11 AW-39 300 1000 086 086 430 (16) 1227 (366) 28 Gl 91, Tit 9, Ap<1

PV-11 AW-38 300 1000 100 100 726 (22) 0 30 Gl 95, Tit 5

PV-6 AW-29 300 1050 018 — 101 (3) 1653 (150) — Gl 93, Plag 7

PV-6 AW-33 300 1050 073 071 294 (6) 1702 (358) 19 Gl 99, Tit 1

PV-6 AW-34 300 1050 100 100 590 (78) 0 22 Gl 97, Tit 3

PV-10 AW-37 300 1100 021 014 017⁶ — 12 Gl 99, Tit 1

PV-5 AW-25 300 1100 053 025 225 (9) 1186 (97) 17 Gl 99, Tit<1

PV-5 AW-27 300 1100 088 067 303 (113) 1622 (311) 26 Gl 99, Tit<1

PV-5 AW-26 300 1100 075 043 359 (3) 982 (59) 22 Gl 99, Tit<1

PV-5 AW-28 300 1100 100 100 665 (85) 0 30 Gl 99, Tit<1

PV-15 AW-43 400 1000 021 018 002⁶ — 17 Gl 36, Cpx 22, Plag 35, Tit 7

PV-15 AW-44 400 1000 049 051 221 — 26 Gl 75, Kaers 19, Tit 6

PV-15 AW-45 400 1000 070 071 404 (13) 3375 (824) 29 Gl 80, Kaers 13, Tit 7

PV-15 AW-46 400 1000 086 074 476 (76) 3439 (134) 29 Gl 87, Kaers 9, Tit 4

PV-15 AW-47 400 1000 100 100 684 (49) 0 32 Gl 74, Tit 26

KI-04 basanite experiments

PV-7 KI-09 200 1050 019 017 073 (17) 1154 (271) 04 Gl 70, Cpx 23, Tit 7

PV-7 KI-10 200 1050 075 055 152 (11) 1233 (137) 14 Gl 53, Cpx 44, Tit 3þrare Ol

PV-7 KI-11 200 1050 070 060 218⁶ — 14 Gl 60, Kaers 37, Tit 4

PV-7 KI-12 200 1050 086 078 355⁶ — 17 Gl 65, Ol 6, Cpx 25, Tit 5

PV-2 KI-05 200 1100 100 — — — — All quench

PV-2 KI-06 200 1100 089 — — — — Quenchþrare Ol

PV-2 KI-08 200 1100 058 — — — — Gl 86, Ol 11, Tit 2

PV-2 KI-07 200 1100 076 058 290 (8) 1131 (100) 22 Gl 93, Ol 4, Tit 3

PV-8 KI-14 250 1100 017 020 045⁶ — 14 Gl 52, Ol 26, Cpx 23

PV-8 KI-15 250 1100 049 043 191 (2) 3074 (627) 21 Gl 91, Ol 9, Tit<1

PV-8 KI-16 250 1100 069 054 333 4338 23 Gl 72, Ol 27, Tit 1

PV-8 KI-17 250 1100 087 077 384⁶ — 26 Gl 58, Ol 21

PV-8 KI-18 250 1100 100 100 — — 28 All quench

PV-10 KI-23 300 1100 021 014 111 (4) 2628 (257) 12 Gl 80, Ol 15, Q 5

PV-10 KI-22 300 1100 049 026 213⁶ — 18 Gl 92, Ol 8, Tit<1

PV-10 KI-21 300 1100 070 048 279 (58) 2798 (573) 23 Gl 91, Ol 9, Tit<1

PV-10 KI-20 300 1100 087 061 221 (31) 1256 (364) 25 Gl, Qþrare Ol

PV-10 KI-19 300 1100 100 100 — — 30 All quench

PV-12 KI-28 300 1150 021 021 059⁶ — 09 Gl 98, Ol 2, Tit<1

PV-12 KI-27 300 1150 049 032 180⁶ — 13 Gl 99, Ol 1, Tit<1

PV-12 KI-26 300 1150 070 043 — — 16 All quench

PV-12 KI-25 300 1150 086 083 — — 22 All quench

PV-12 KI-24 300 1150 100 100 — — 23 All quench

PV-13 KI-29 400 1000 021 013 — — 14 Subsolidus

PV-13 KI-30 400 1000 048 031 — — 21 Subsolidus

PV-13 KI-31 400 1000 071 059 272⁶ — 27 Gl 38, Kaers 60, Tit 2

PV-13 KI-32 400 1000 085 080 305 (5) 1601 (139) 30 Gl 57, Kaers 41, Tit 2

(continued)

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(2345 cm

^–1

), and carbonate (1430 and 1525 cm

^–1

) bands.

The total water 3500 cm

^–1

band was always preferred for retrieving H

₂

O concentrations; in five samples the peak at this band was unusable and so near-IR bands were used. The H

2

O concentrations in samples meas- ured with both the total water and near-IR bands agree well, with an average standard deviation of 057 wt % and an R

²

value of 083.

Sample thicknesses were measured to an accuracy of 62 lm using a digital tabletop micrometer equipped with a fine tip that allowed for measurement at multiple spots on the sample surface. Additional sample thick- nesses for some samples were also measured using an ADE Phaseshift Microxam surface-mapping microscope (with white light interferometer) to an accuracy of 605–

2 lm. In very thin samples, the presence of interference

fringes in reflectance IR spectra allowed for the meas- urement of the sample thickness in the precise spot that was analyzed with transmitted light, following the method of Nichols & Wysoczanski (2007, and references therein; see Supplementary Data Appendix 2 for details).

Glass densities were calculated for each sample using the model of Lange & Carmichael (1987), assum- ing an H

2

O molar volume of 229 cm

³

mol

^–1

(Ochs &

Lange, 1999). Molar absorption coefficients strongly depend on glass composition (Pandya et al., 1992).

Because of variation in the crystallinity of our samples, and therefore significant variation in glass chemistry, an attempt was made to use absorption coefficients appropriate for each sample. All absorption coeffi- cients used in this study are listed in Table 3. Where possible, the model of Dixon & Pan (1995) was used to calculate e

1525

and e

1430

(carbonate absorption) for sin- gle samples. This calculation can only be applied to glasses with an Na/(Na þ Ca) ratio between 025 and 05 (in our case, most glasses classified as phonoteph- rite and tephriphonolite fall in this range). For glasses with Na/(Na þ Ca) < 025 or > 05, e

1525

and e

1430

values were selected from the literature (see Table 3). (See Supplementary Data Appendix 2 for details of FTIR spectral background fitting, absorbance measure- ments, sample thicknesses, and glass density calculations.)

Dissolved volatile concentrations could not be meas- ured in nine crystalline samples (29–75 vol. % glass) where glass pools were too small to analyze by FTIR spectroscopy. A further six glassy samples (95–99 vol. % glass) could not be successfully prepared for FTIR analysis. For these 15 samples, H

2

O and CO

2

contents were calculated using the mass-balance method (see Supplementary Data Appendices 3 and 4 for details), and the 35 samples for which FTIR measure- ments were obtained were used as internal standards.

The standard error for these calculations is large at 614 wt % and 61000 ppm for calculated H

₂

O and CO

₂

values, respectively.

Table 3: Absorption coefficients used to determine concentrations of dissolved volatiles in experimental charges via FTIR spectroscopy

Composition and Species Coefficient Reference wavenumber (cm^–1) [l (mol cm)^–1]

Phonolite

1640 H2Om 2563 Dixonet al., 1994

2345 CO2 890 Morizetet al., 2002

3500 H2O^tot 6468 Cioni, 2000

Tephriphonolite

1430 CO3 Calculated Dixon & Pan, 1995 1525 CO3 Calculated Dixon & Pan, 1995

1640 H2Om 2563 Dixonet al., 1994

3500 H2O^tot 6468 Cioni, 2000

4500 OH 0626002 Behrenset al., 2009

5200 H2Om 1026006 Behrenset al., 2009

Phonotephrite

1430 CO3 Calculated Dixon & Pan, 1995 1525 CO3 Calculated Dixon & Pan, 1995

1640 H2Om 2563 Dixonet al., 1994

3500 H2O^tot 639654 Behrenset al., 2009

4500 OH 0626002 Behrenset al., 2009

5200 H2Om 1026006 Behrenset al., 2009

Basanite

1430 CO3 375620 Fine & Stolper, 1986

1525 CO3 375620 Fine & Stolper, 1986

1640 H2Om 2563 Dixonet al., 1994

3500 H2O^tot 639654 Behrenset al., 2009

4500 OH 0586002 Cocheo, 1993

5200 H2Om 0566004 Cocheo, 1993

Table 2: Continued Run Experiment P

(MPa) T (C)

XH2O^{in 1} XH2O^{fluid 2} H2O glass (wt %)³

CO2glass (ppm)³

fO2

(DNNO)⁴

Phases (vol. %)⁵

PV-13 K1-33 400 1000 100 100 430⁶ 0 32 Gl 38, Kaers 59, Tit 3

PV-14 KI-36 400 1100 072 053 338⁶ — 26 Gl 95, Ol 5, Tit<1

PV-14 KI-37 400 1100 085 080 356⁶ — 30 Gl 29, Q 51, Ol 20

PV-14 KI-38 400 1100 100 100 — — 32 All quench

1Mole fraction of H2O in the H2O–CO2fluid added to the experimental capsule prior to running atPandT.

2Mole fraction of H2O in the equilibrium H2O–CO2fluid phase during the run, measured by weight-loss method. Sample KI-07 calculated by mass balance.

3Weight per cent H2O or ppm CO2 dissolved in the glass, measured via FTIR spectroscopy. Standard deviation shown in parentheses.

4Calculated oxygen fugacity of the experiment in terms ofDNNO (log units from the Ni–NiO buffer). (See text for details.)

5Phase proportions calculated as volume per cent; Gl, glass; Ol, olivine; Cpx, clinopyroxene; Kaer, kaersutite; Fsp, feldspar; Ap, apatite; Tit, titanomagnetite; Q, quench crystals.

6Volatile concentration calculated by mass balance using values in Table 3. Corrected values using the calibration lines in Supplementary Data Fig. A2.2are reported here.

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Oxygen fugacity

The fO

2

of all IHPV runs was controlled by loading the pressure vessel with sufficient H

₂

to achieve an appar- atus fO

₂

of NNO þ 1. The fO

₂

inside each experimental capsule will vary depending on the H

2

O/CO

2

ratio of the fluid phase contained therein. Assuming that the fH

2

of the pressure vessel will be the same as the fH

2

inside an experimental capsule owing to the very fast diffusion rates of hydrogen, we can calculate the fO

₂

inside each capsule following the method of Iacovino et al. (2013a).

First, because the partial pressure of H

2

is fixed for each experiment, we can calculate the fH

2

of the pressure vessel for each run using the equation

f H

₂

¼ /P

_H2

(1)

where / is the fugacity coefficient of hydrogen for the P and T of a given experiment (Shaw & Wones, 1964), and P

H2

is the partial pressure of hydrogen gas added to the IHPV at the start of the run. If the XH

2

O

^fluid

is known, we can also then calculate the fH

₂

O inside the capsule as

f H

₂

O

^inside

¼ f H

₂

O

^pure

X H

₂

O

^fluid

(2) where fH

2

O

^pure

is the fugacity of pure H

2

O vapor at the experimental P and T conditions, assuming ideal mix- ing between H

2

O and CO

2

in the fluid [calculated with the modified Redlich–Kwong equation of state of Holloway (1977) as modified by Flowers (1979)]. Now, it is possible to calculate the oxygen fugacity for each ex- periment as follows:

f O

^inside₂

¼ f H

₂

O

^inside

f H

₂

K

_F

!

2

(3) where K

F

is the equilibrium constant of formation for H

2

O (Robie et al., 1979). Oxygen fugacities in our experi- ments are reported in Table 2 and range from a DNNO of þ03 to þ32.

RESULTS

Textural and mineralogical observations

Optical and electron microscope images revealed homogeneously distributed euhedral crystals in most experimental charges, with crystals ranging in size

from <5 mm to 80 mm in length. Phases identified in-

clude olivine, clinopyroxene, plagioclase, amphibole, Fe–Ti oxides, apatite, glass and fluid inclusions. In the most highly crystalline charges, glass tends to collect in pools and small subhedral crystals (‘quench’ tex- ture) are ubiquitous. Quench texture is especially com- mon in KI samples with high H

2

O contents, probably owing to the extremely low-viscosity melt allowing for the rapid crystallization upon quench. In samples where crystals were too small to analyze by electron microprobe (<25 mm

²

), phases were analyzed quantita- tively for identification purposes only. Those analyses are not reported here.

Phase relationships

The equilibrium phase relations for the AW and KI ex- periments are shown in Figs 4 and 5. In both compos- itions, titanomagnetite is ubiquitous in nearly all experimental charges independent of P/T/H

₂

O and is probably stabilized owing to the relatively oxidizing conditions of our experiments. Experiments are plotted in terms of the mole fraction of H

2

O in the H

2

O–CO

2

fluid added to the capsule, XH

2

O

ⁱⁿ

.

In AW, kaersutite is the liquidus phase at median to high water contents, crystallizing below about 1025

C. Amphibole is common in the AW experiments and is stable to melt H

₂

O contents as low as 1 wt %. A reac- tion relationship between kaersutite and clinopyroxene is inferred to exist around XH

2

O

^fluid

¼ 01–02, below which clinopyroxene is the dominant calcic ferromag- nesian phase (experiments AW-19B and AW-43). As with kaersutite, clinopyroxene is restricted to below about 1025

C. Plagioclase feldspar is the dry liquidus phase and is restricted to low water contents, with its stability field increasing with decreasing temperature.

Phase equilibria at 300 MPa are similar to those at 200 MPa [the stability field for clinopyroxene at 300 MPa (Fig. 4b) is inferred based on that at 200 MPa (Fig. 4a)].

Although apatite was not always detected, it is inferred to crystallize below 1100

C at both pressures. Its stabil- ity is mostly constrained by water-saturated experi- ments, but apatite is probably stable but undetectable in drier runs (see Harrison & Watson, 1984). This hy- pothesis is supported by the steady decrease in the P

2

O

5

content of residual liquids in phonotephrite experi- ments. The polybaric section at 1000

C (Fig. 4c) shows that in the pressure range 200–400 MPa amphibole is al- ways the liquidus phase, followed by Pl and Cpx, in that order.

In KI experiments at 200 MPa (Fig. 5a), olivine is the liquidus phase followed by clinopyroxene and kaersu- tite. Although loosely constrained, the liquidus is located above 1100

C except at very high H

2

O contents (XH

2

O

ⁱⁿ

approaching unity), where it drops to 1075

C. A peritectic-like relationship exists at intermediate melt water contents and 1050

C at 200 MPa where olivine is replaced by kaersutite at low temperatures. At these P–

T conditions, clinopyroxene replaces kaersutite as the dominant ferromagnesian phase at low and high melt water contents where it coexists with olivine, except at very low melt water contents (XH

2

O

ⁱⁿ

approaching zero) where clinopyroxene is the only stable silicate phase.

The phase relationships observed at 400 MPa (Fig. 5b) are broadly similar to those at 200 MPa except that the kaersutite stability field is enlarged towards H

2

O-rich conditions, this mineral being in reaction relationships with both olivine and clinopyroxene as temperature de- creases. The polybaric section at 1100

C (Fig. 5c) illus- trates that olivine is the liquidus mineral in the pressure range 200–400 MPa, for a large range of fluid compos- itions (melt water content), followed by clinopyroxene at XH

2

O

in

< 025.

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Fig. 5.Isothermal (a, b) and isobaric (c) phase diagrams illustrating the stability fields of different phases in KI experiments.

Numbers next to points indicate wt % H2O dissolved in the glass of the experimental charge as measured by FTIR spectroscopy. Although not shown, titanomagnetite is ubiquitous in all charges. Apatite was not always detected but is inferred to be stable in runs below1100C. A dashed line indicates an inferred phase boundary.

Fig. 4.Isothermal (a, b) and isobaric (c) phase diagrams illustrating the stability fields of different phases in AW experiments. Numbers next to points indicate wt % H2O dissolved in the glass of the experimental charge as measured by FTIR spectroscopy. Although not shown, titanomagnetite is ubiquitous in all charges. Apatite was not always detected but is inferred to be stable in runs below1100C. A dashed line indicates an inferred phase boundary.

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Experimental phase compositions

Compositions of the experimental mineral phases and residual glasses are given in Table 4. Chemical ana- lyses for all natural and experimental phases are given in Supplementary Data Appendix 5. Titanomagnetite, apatite, and some clinopyroxene crystals were too small to be analyzed by electron microprobe (<25 mm

²

). In six charges, clinopyroxenes were large enough to be analyzed, and their compositions are re- ported in Table 4. In most samples, the K

_D^Fe–Mg

values of experimental clinopyroxenes, which range from 031 to 046 and average 036 (calculated using Fe

^2þ

as XFeO), are similar to the K

DFe–Mg

values observed in natural KI-04 basanite clinopyroxenes, which average 040, suggesting equilibrium crystal growth. Overall, clinopyroxene is characterized by elevated contents of Al

₂

O

₃

and TiO

₂

, which do not show any obvious correl- ation with experimental parameters in the explored range.

Experimental olivine compositions in KI range from Fo

77

to Fo

90

(Table 4). No olivine crystals were observed in AW experimental samples. Experimental olivine Fo contents become more forsteritic with increasing H

2

O

^melt

(Fig. 6), reflecting the increase in the melt frac- tion of the charge and a change in oxygen fugacity caused by an increase in the water activity in the melt (e.g. Feig et al., 2010). Crystal–melt Fe–Mg partition co- efficients (K

_D

) for most experimental olivines range from 033 to 039, with an overall average value (includ- ing outliers) of 033. Our olivines are expected to have a higher K

D

value than the canonical 03 owing to the al- kalinity and redox state of the charges. Using the model of Carmichael & Ghiorso (1990), which accounts for the effect of melt composition on K

_D

, we calculate that our experimental K

_D

values should range between 034 and 036, suggesting that the majority of the olivine crystals in our experiments are approaching equilibrium with their host melts. Four outliers, which do not appear to have attained equilibrium, are samples KI-06 (K

D

¼ 022), KI-08 (022), KI-10 (052), and KI-36 (043). In samples KI-06 and KI-10, olivines were rare and small, suggesting that they grew late during the experiment, possibly during quench. Some olivines may have been affected by Fe loss to the experimental capsule, which is difficult to avoid at high T even given the relatively short run times of 24 h.

Amphibole is classified as sodic-kaersutite on the basis of 23 oxygens and crystallized in both phonoteph- rite and basanite experiments (compositions reported in Table 4). Al contents in kaersutites show a positive correlation with pressure, as documented for other sys- tems (e.g. Prouteau & Scaillet, 2003) but in contrast to the results of experiments with Erebus phonolite (Moussallam et al., 2013). Kaersutites synthesized at the lowest fO

₂

conditions have markedly lower Mg# and higher Ti contents. At all pressures, the modal propor- tion of amphibole in AW runs increases as the water content of the melt decreases.

Plagioclase feldspar was observed only in the driest AW experiments and ranges from An

57

to An

65

. It was not found in any of the KI experiments.

Glass compositions

The compositions of residual experimental glasses highlight the evolution of the melts from the starting composition to intermediate and evolved compositions.

FeO, MgO, CaO, and TiO

2

all show continuously decreasing trends with melt evolution reflecting the crystallization of pyroxene or kaersutite, olivine, plagio- clase, and titanomagnetite. In contrast, Al

₂

O

₃

, and K

₂

O show continuously increasing trends. In detail, Harker- type plots clearly show the effect of the fractionating as- semblage on the liquid lines of descent of each starting material (Figs 7 and 8).

KI-related melts in charges held above 1050

C show limited compositional evolution relative to the starting composition, except for FeO and MgO contents, which are affected by olivine and titanomagnetite crystalliza- tion to some extent. In contrast, charges held at or below 1050

C show the prominent roles of either kaer- sutite or Cpx–Ol-dominated assemblages. In both cases, derivative liquids produced by crystallization at these temperatures reach SiO

2

contents of 50–52 wt % (i.e. are more evolved than the AW starting compos- ition), trending toward the phonotephritic to phonolitic fields. Charges in which kaersutite is stable are charac- terized by higher TiO

2

, FeO, and CaO melt contents rela- tive to those in which Cpx and Ol are stable (at the same SiO

2

content). In contrast, the crystallization of a Cpx 6 Ol assemblage produces liquids with higher Al

2

O

3

, Na

2

O, and K

2

O contents relative to those in which kaersutite predominates. It is worth noting that the role of kaersutite increases at high pressure;

charges crystallized at 400 MPa have up to 60 wt % amphibole, whereas at 200 MPa, amphibole does not exceed 37 wt % of the crystal cumulate (Table 2).

The relationship between the evolution of CaO and MgO in KI liquids is illustrated in Fig. 9. Such a plot is useful for illuminating the evolution of basaltic compos- itions sensu lato in which olivine and Ca-pyroxene (or amphibole) play a dominant role (e.g. Pichavant et al., 2009, 2014). The KI trend can be divided into two seg- ments: the first runs broadly parallel to the MgO axis, primarily reflecting depletion in MgO of the liquid owing to olivine crystallization. The second segment corresponds to liquids in which either kaersutite or Cpx 6 Ol crystallize, defining either a kaersutite control line or the Cpx–Ol cotectic (at 1050

C), along which both CaO and MgO contents decrease. It is apparent that li- quids lying on the Cpx–Ol cotectic are slightly less CaO- rich than those driven by kaersutite crystallization. Also shown in Fig. 9 are the Cpx–Ol cotectics experimentally determined for Stromboli and Vesuvius primitive bas- alts (Di Carlo et al., 2006; Pichavant et al., 2009, 2014), which, like the magma compositions investigated here, are mafic and alkali-rich. It is clear that basaltic liquids

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Table4:Compositionsofexperimentalphases PhaseSiO2TiO2Al2O3FeOTMnOMgOCaONa2OK2OP2O5TotalMol% AW82038experiments AW-6PT(9)4803(074)284(024)1812(062)960(089)023(005)308(028)757(038)604(012)308(011)141(022)9920(059)Mgno.364 plag(4)5038(09)175(030)2274(126)529(085)013(004)162(034)883(031)500(020)189(028)n.d.9764(054)An61Al31Or8 AW-10PT(7)4945(03)264(007)1874(026)826(036)025(004)273(007)762(007)598(009)308(011)126(003)9500(069)Mgno.371 AW-13PT(9)4884(044)274(007)1864(024)889(027)023(004)284(010)764(011)589(011)300(006)130(005)9608(080)Mgno.363 AW-14PT(7)4875(049)272(008)1859(015)894(034)018(004)289(008)763(013)599(012)299(007)131(006)9708(072)Mgno.366 AW-17PT(7)4929(026)263(008)1875(017)843(036)021(002)285(007)759(019)594(017)303(007)129(006)9752(043)Mgno.377 AW-18PT(7)4960(041)256(014)1876(028)821(016)021(004)289(011)760(008)596(015)297(009)124(005)9641(051)Mgno.385 AW-19BP(4)5374(087)153(004)1946(010)405(029)015(007)103(011)421(023)829(023)587(009)168(021)9663(070)Mgno.311 cpx(4)4509(13)345(051)775(045)801(047)034(002)1119(030)2117(071)116(013)026(017)n.d.9843(065)En36Wo49Fe15 plag(1)551610622613801164972507126n.d.10043An65Al30Or5 AW-20BT(10)5530(064)172(004)1986(049)425(025)020(007)130(009)415(033)805(013)474(023)044(019)9476(130)Mgno.353 kaers(7)3738(091)633(035)1268(041)1032(055)023(004)1212(036)1133(064)276(005)118(008)012(010)9448(232)Mgno.677 AW-21T(10)5393(044)201(007)2007(036)479(028)020(008)156(009)580(019)736(015)380(008)048(019)9424(084)Mgno.367 kaers(6)3675(056)596(028)1358(044)1092(033)023(006)1197(023)1192(009)264(008)109(008)025(033)9535(077)Mgno.661 AW-22T(12)5251(058)237(007)1969(017)523(021)022(005)224(010)712(014)657(013)347(011)059(017)9211(071)Mgno.433 kaers(10)3756(069)527(023)1300(037)1015(017)016(005)1322(022)1216(030)257(008)109(006)022(030)9542(071)Mgno.699 AW-23T(11)5191(053)257(004)1962(025)541(029)020(007)252(016)746(023)637(015)332(011)062(010)9083(060)Mgno.454 AW-24BPT(11)4977(063)285(005)1897(023)736(025)019(006)300(015)747(014)625(012)295(007)118(010)9335(089)Mgno.421 AW-25PT(5)4880(038)285(004)1880(044)923(006)023(004)295(003)759(008)587(017)280(012)088(004)9545(045)Mgno.363 AW-26PT(6)4854(038)280(006)1849(017)934(021)023(006)281(007)758(010)587(009)303(005)131(003)9649(052)Mgno349 AW-27PT(27)4854(036)287(003)1895(044)952(024)025(003)291(004)762(005)572(035)283(012)079(004)9353(066)Mgno.352 AW-29PT(12)4844(049)310(010)1803(056)826(053)025(009)310(017)772(031)649(014)335(018)125(012)9484(076)Mgno.401 plag(3)5184(14)137(076)2480(260)414(230)010(008)107(067)972(101)483(021)140(058)n.d.9927(061)An65Al29Or6 AW-33PT(6)4907(04)261(005)1841(018)863(033)024(008)274(003)770(029)618(011)319(011)120(027)9466(085)Mgno.362 AW-34PT(6)4878(079)285(005)1878(015)837(029)023(005)256(008)788(009)607(012)306(005)142(008)9041(105)Mgno.353 AW-37PT(10)4839(077)302(007)1864(031)876(026)025(006)295(008)769(013)594(011)305(005)130(006)9686(147)Mgno.375 AW-38PT(6)4997(051)268(007)1964(021)792(013)019(007)282(003)758(012)511(026)296(006)114(006)9242(098)Mgno.388 AW-39PT(11)5130(055)240(007)1995(030)649(020)019(006)270(016)747(020)565(019)312(011)071(004)9425(092)Mgno.426 AW-40PT(11)5051(062)246(007)1955(028)720(011)023(005)276(010)742(017)589(011)315(006)083(006)9574(123)Mgno.406 AW-41T(12)5373(082)172(005)2018(038)597(014)022(006)157(007)525(014)686(014)403(008)046(003)9579(140)Mgno.320 kaers(5)3819(03)608(021)1361(017)1144(031)024(002)1145(026)1179(033)273(005)114(004)n.d.(000)9666(025)Mgno.641 AW-42T(9)5277(046)163(005)2030(030)616(024)023(007)194(003)425(011)737(021)468(007)067(006)9788(095)Mgno.359 kaers(2)3878(08)703(036)1315(012)1138(035)028(005)1127(009)1186(040)257(008)132(004)n.d.9763(045)Mgno.638 plag(4)5557(2)069(034)2575(128)142(061)005(003)028(013)915(142)596(025)114(027)n.d.10003(158)An60Al36Or4 AW-43PT(1)5200221125960221214957624050869855Mgno.270 cpx(4)4537(066)246(050)668(109)872(040)038(002)1144(046)2163(071)101(012)008(001)n.d.9777(068)En36Wo49Fe15 plag(3)5635(1)091(058)2412(113)258(143)008(005)084(068)797(050)536(032)187(013)n.d.10009(065)An57Al35Or8 AW-44T(11)5457(08)163(005)2059(032)529(013)017(006)128(006)471(012)707(016)430(008)038(004)9674(146)Mgno.302 kaers(6)3820(041)592(019)1358(016)1234(019)028(003)1192(016)1144(020)275(007)127(004)021(019)9793(065)Mgno.633 AW-45T(4)5303(089)191(005)2069(026)555(023)021(001)168(002)634(006)695(015)337(005)026(004)9294(132)Mgno.351 kaers(5)3703(083)506(026)1444(020)1211(071)024(001)1142(043)1191(055)273(015)121(011)n.d.9618(067)Mgno.627 AW-46B-TA(9)5298(087)218(009)2049(067)554(039)020(008)200(021)710(040)568(029)316(019)066(021)9374(186)Mgno.392 kaers(4)3854(058)458(010)1407(036)1064(027)021(005)1328(016)1223(018)266(003)115(006)n.d.(000)9741(038)Mgno.690 AW-47B-TA(10)5180(02)253(004)2016(026)656(012)022(007)279(007)731(007)466(023)317(007)079(005)9226(043)Mgno.432 (continued)

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Table4:Continued PhaseSiO2TiO2Al2O3FeOT MnOMgOCaONa2OK2OP2O5TotalMol% KI-04basaniteexperiments KI-06B(3)4592(021)437(002)1442(011)1022(005)025(004)781(018)1303(031)335(032)064(001)n.d.9087(003)Mgno.577 ol(5)4067(008)009(001)003(001)941(025)025(002)4909(023)035(001)001(001)001(000)n.d.9993(013)Fo90 KI-07B(15)4461(038)437(009)1441(020)1061(040)017(004)769(019)1155(025)393(011)174(013)092(004)9688(063)Mgno.564 ol(3)3833(135)065(045)050(038)1411(245)022(001)4511(217)049(010)010(002)003(001)n.d.9975(109)Fo85 KI-08B(16)4404(049)454(013)1446(044)1151(061)016(005)711(131)1159(046)390(014)179(008)091(005)9824(079)Mgno.524 ol(5)3956(052)021(013)012(011)1293(055)027(002)4609(059)040(003)002(001)001(000)n.d.9963(022)Fo86 KI-09PT(9)4966(037)287(006)1878(037)701(038)014(005)313(009)642(014)701(014)376(008)122(011)9467(072)Mgno.443 cpx(4)4434(082)344(037)829(103)782(057)012(004)1235(076)2082(073)065(008)018(012)n.d.9806(018)En39Wo47Fe14 KI-10B(8)4727(079)365(026)1632(120)796(038)020(008)589(161)928(134)544(057)260(032)139(023)9205(104)Mgno.569 ol(3)3882(018)019(007)017(014)1798(040)033(001)4125(049)071(028)002(000)002(000)n.d.9951(022)Fo80 cpx(5)4308(046)438(022)998(031)846(043)012(003)1114(021)2096(033)082(009)018(005)n.d.9923(045)En36Wo49Fe15 KI-11B(5)4787(026)378(006)1671(024)825(022)020(007)486(020)918(009)535(022)234(013)147(009)9299(025)Mgno.512 kaers(5)3768(147)575(038)1208(015)1179(245)011(003)1525(031)1166(070)251(007)107(006)n.d.9797(122)Mgno.700 KI-12B(11)4559(048)454(008)1601(015)854(030)016(011)589(034)1132(038)488(013)190(020)117(011)9156(098)Mgno.552 ol(7)3865(032)014(004)009(006)1164(016)025(003)4716(050)043(023)005(002)001(001)n.d.9848(036)Fo88 cpx(4)4423(098)305(028)592(047)695(049)012(001)1445(048)2275(014)065(012)007(001)n.d.9826(052)En42Wo47Fe11 KI-14B(7)4462(039)511(006)1575(028)1025(044)021(013)558(017)1056(014)468(013)220(012)103(009)9657(100)Mgno.493 ol(4)3805(016)016(002)009(006)2019(031)032(005)4021(047)053(013)005(004)003(001)n.d.9971(026)Fo78 KI-15B(15)4355(037)494(008)1437(024)1128(032)019(006)673(022)1211(028)406(012)181(014)097(009)9370(069)Mgno.515 ol(8)3872(013)011(002)004(002)1669(014)026(006)4284(027)041(002)001(001)001(001)n.d.9917(029)Fo82 KI-16B(15)4381(030)483(009)1435(023)1152(023)019(012)673(030)1220(048)403(012)140(020)095(008)9420(059)Mgno.510 ol(8)3925(027)013(003)007(003)1527(008)026(003)4390(023)040(003)007(002)002(001)n.d.9942(021)Fo84 KI-17B(1)4480443148811170266811663841730579516Mgno.516 ol(4)3942(052)011(002)005(002)1351(029)024(003)4576(027)033(001)002(002)000(001)n.d.9958(092)Fo86 KI-20B(5)4505(041)444(010)1440(011)1123(017)021(002)774(060)1231(088)311(034)091(027)060(004)9255(061)Mgno.551 KI-21B(8)4360(036)469(007)1417(009)1177(023)015(004)803(009)1138(019)362(005)166(011)093(007)9528(069)Mgno.549 ol(4)3900(044)012(003)005(003)1438(033)023(002)4675(097)034(003)002(002)000(001)n.d.10101(068)Fo85 KI-22B(10)4342(043)474(010)1446(021)1171(034)020(005)732(025)1160(013)381(009)182(006)092(006)9602(119)Mgno.527 ol(8)3907(045)012(009)013(023)1590(083)026(004)4524(097)052(046)003(003)001(001)n.d.10136(072)Fo83 KI-23B(10)4363(061)495(008)1504(030)1126(031)019(005)649(012)1149(012)411(007)188(004)096(003)9721(130)Mgno.507 ol(5)3871(037)013(001)005(001)1874(019)026(004)4325(023)051(001)001(001)001(001)n.d.10174(069)Fo80 cpx(6)4357(160)444(054)1133(199)829(121)017(001)982(189)1767(274)160(078)079(048)n.d.9796(125)En36Wo47Fe17 KI-26B(6)4376(058)448(011)1375(019)1145(049)016(007)933(138)1104(159)349(017)164(068)090(004)9665(035)Mgno.592 KI-27B(6)4348(063)448(004)1463(021)1079(017)015(005)906(015)1174(011)409(009)157(015)n.d.9224(082)Mgno.599 KI-28B(6)4461(041)455(005)1425(016)989(011)021(004)898(007)1140(008)376(012)169(011)066(002)9599(033)Mgno.618 KI-31PT(3)5187(066)268(016)1859(033)822(031)023(005)298(008)770(005)519(013)255(001)n.d.9206(066)Mgno.392 kaers(8)3982(073)488(017)1194(049)1105(045)015(005)1404(040)1158(014)264(006)121(005)009(010)9749(044)Mgno.694 KI-32PT(7)4992(075)322(030)1706(057)894(053)023(001)443(130)914(041)419(036)218(020)069(009)9146(171)Mgno.469 kaers(6)3967(078)404(019)1191(042)1045(133)018(002)1494(124)1184(012)258(007)124(013)015(017)9709(050)Mgno.717 KI-33TB(7)4739(070)369(006)1609(020)981(032)018(005)557(027)999(015)383(014)218(009)126(006)8981(141)Mgno.503 kaers(4)3982(082)438(007)1180(056)1059(035)014(004)1479(031)1177(027)260(005)125(005)012(019)9737(020)Mgno.713 KI-36B(10)4375(061)470(008)1424(028)1133(027)018(008)821(031)1146(035)356(010)166(011)091(007)9404(140)Mgno.564 ol(13)3922(021)009(002)004(001)1530(035)024(003)4580(028)033(002)002(001)001(001)n.d.10117(057)Fo84 KI-37ol(10)3972(035)009(002)004(001)1417(043)022(004)4699(041)028(002)001(001)001(001)n.d.10170(066)Fo86 Analysesgiveninwt%andnormalizedto100%withoriginalanalysistotalgiven.Numbersinparenthesesnexttoanalysesarestandarddeviations;thosenexttophasenamesare numberofanalyses.n.d.,notdetermined.Reportedvaluesforglassesareaveragecompositionsforeachcharge.FeOT,totalirongivenasFeO.Glassnameclassifications:B,basanite; PT,phonotephrite;T,tephriphonolite;P,phonolite;B-TA,basaltictrachyandesite;TB,trachybasalt.Mgno.calculatedasmolarMgO/(FeOTþMgO).