Melting and Phase Relations of Carbonated Eclogite at 9-21 GPa and the Petrogenesis of Alkali-Rich Melts in the Deep Mantle

Melting and Phase Relations of CarbonatedEclogite at 9^21GPa and the Petrogenesis ofAlkali-Rich Melts in the Deep Mantle

EKATERINA S. KISEEVA1*, KONSTANTIN D. LITASOV2,3,GREGORY M. YAXLEY1, EIJI OHTANI4 ANDVADIM S. KAMENETSKY5

1RESEARCH SCHOOL OF EARTH SCIENCES, AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, ACT 0200, AUSTRALIA

2V. S. SOBOLEV INSTITUTE OF GEOLOGY AND MINERALOGY, SIBERIAN BRANCH, RUSSIAN ACADEMY OF SCIENCE,

NOVOSIBIRSK, 630090, RUSSIA

3NOVOSIBIRSK STATE UNIVERSITY, NOVOSIBIRSK, 630090, RUSSIA

4DEPARTMENT OF EARTH AND PLANETARY MATERIAL SCIENCE, FACULTY OF SCIENCE, TOHOKU UNIVERSITY,

SENDAI 980-8578, JAPAN

5ARC CENTRE OF EXCELLENCE IN ORE DEPOSITS AND SCHOOL OF EARTH SCIENCES, UNIVERSITY OF TASMANIA,

HOBART, TAS. 7001, AUSTRALIA

RECEIVED JUNE 15, 2012; ACCEPTED MARCH 15, 2013

The melting and phase relations of carbonated MORB eclogite have

been investigated using the multi-anvil technique at 9^21GPa and

1100^19008C.The starting compositions were two synthetic mixes,

GA1 and Volga, with the CO2 component added as CaCO3 (cc):

GA1þ10%cc (GA1cc) models altered oceanic crust recycled into

the convecting mantle via subduction, and Volgaþ10%cc (Volga-

cc) models subducted oceanic crust that has lost some of its siliceous

component in the sub-arc regime (GA1 minus 6·5 wt % SiO2).The

subsolidus mineral assemblage at 9 and 13 GPa includes garnet,

clinopyroxene, magnesite, aragonite, a high-pressure polymorph of

TiO2 (only at 9 GPa) and stishovite (only at 13 GPa). At 17^

21GPa clinopyroxene is no longer stable; the mineral assemblage con-

sists predominantly of garnet with subordinate magnesite (only at

17 GPa), Na-rich aragonite, stishovite, Ca-perovskite (mostly at

21GPa), and K-hollandite (mostly at 17 GPa). Na-carbonate with

an inferred composition (Na,K)2(Ca,Mg,Fe)(CO3)2 was present

in Volga-cc at 21GPa and 12008C. Diamond (or graphite) crystal-

lized in most runs in the GA1cc composition, but it was absent in ex-

periments with the Volga-cc composition. In Volga-cc, the solidus

temperatures are nearly constant between 1200 and 13008C over the

entire pressure range investigated. In GA1cc, the solidus is located at

similar temperatures at 9^13 GPa, but at higher temperatures of

1300^15008C at 17^21GPa. The difference in solidi between the

GA1cc and Volga-cc compositions can be explained by a change in

Na compatibility between 13 and 17 GPa as omphacitic clinopyroxene

disappears, resulting in the formation of Na-carbonate or Na-rich

melt in Volga-cc. The solidus temperature in GA1cc also increases

with increasing pressure as a consequence of carbonate reduction

and diamond precipitation, possibly brought on either via progressive

Fe2þ^Fe3þ transition in garnet at higher pressures or by a decrease

of the activity of the diopside component in clinopyroxene.The low-

degree melts are highly alkalic (K-rich at 9^13 GPa and Na-rich at

17^21GPa) carbonatites, changing towards SiO2-rich melts with

increasing temperature at constant pressure.The solidi of both com-

positions remain higher than typical subduction pressure^tempera-

ture (P^T) profiles at 5^10 GPa; however, at higher pressures the

flat solidus curve of carbonated eclogite may intersect the subduction

P^T profile in the Transition Zone, where carbonated eclogite can

produce alkali- and carbonate-rich melts. Such subduction-related

alkali-rich melts can be potential analogues of kimberlite and

*Corresponding author. Department of Earth Sciences, South Parks

Road, Oxford, OX13AN, UK; E-mail: [email protected]

� The Author 2013. Published by Oxford University Press. All

rights reserved. For Permissions, please e-mail: journals.permissions@

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JOURNALOFPETROLOGY VOLUME 0 NUMBER 0 PAGES1^29 2013 doi:10.1093/petrology/egt023

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carbonatite melt compositions and important agents of mantle meta-

somatism and diamond formation in theTransition Zone and in cra-

tonic roots. Melting of carbonated eclogite produces a garnet-bearing

refractory residue, which could be stored in the Transition Zone or

lower mantle.

KEY WORDS: high-pressure experiments; MORB eclogite; mantle;

Transition Zone; carbonate metasomatism; kimberlite formation;

diamonds

I NTRODUCTION

Subducted slabs of oceanic lithosphere, containing pelagic

sediments and hydrothermally altered basalts (MORB)

formed at a mid-ocean ridge by sea-floor spreading, are

one of the major sources for geochemical heterogeneity in

the mantle, transporting incompatible trace elements and

water. The amount of carbon in the primordial and

modern Earth, and the magnitude of carbon fluxes be-

tween the mantle, the crust, the hydrosphere and the at-

mosphere are highly uncertain (e.g. Zhang & Zindler,

1993; Sleep & Zahnle, 2001; Dasgupta & Hirschmann,

2010). Large amounts of carbon may be introduced into

the mantle by subduction of oceanic crust, which may con-

tain43wt % CO2 in its uppermost few hundred metres

(Alt & Teagle, 1999; Staudigel, 2003). Some of the sub-

ducted material may undergo partial melting in the sub-

arc regime, releasing the most incompatible and volatile

components back to the surface via arc magmatism, or

later by contributing to MORB, hotspot or continental

magmatism. However, both thermal modelling of sub-

ducting slabs and thermodynamic and experimental con-

straints on slab dehydration and decarbonation

indicate that decomposition of carbonate-bearing species

occurs at much higher depths than that of water-bearing

species, allowing preferential subduction of some slab car-

bonate relative to hydrous species (e.g. Yaxley & Green,

1994; Bebout, 1995; Poli & Schmidt, 1995; Kerrick &

Connolly, 2001). Consequently, slabs should transport

H2O-poor and carbonate-rich eclogite deep into the

Earth’s mantle.

Most previous experimental studies on carbonated

eclogite have been performed at pressures �10GPa

(Hammouda, 2003; Dasgupta et al., 2004, 2005; Yaxley &

Brey, 2004; Gerbode & Dasgupta, 2010; Kiseeva et al.,

2012). These studies reported a variety of solidus tempera-

tures and shapes, attributed to compositional differences

in the starting mixes, such as Na2O/CO2, Mg# [molar

Mg/(MgþFe)], Ca# [molar Ca/(CaþMgþFe)], the

abundances of alkali components, and minor but variable

amounts of water.

The solidi of carbonated eclogite at higher pressures

from 10 to 32GPa have been reported only for simplified

chemical systems, such as Na-CMASþ 5% CO2 by

Litasov & Ohtani (2010) and CMASþ 20% CO2 by

Keshav & Gudfinnsson (2010). A previous study of repre-

sentative carbonated eclogites at 3·5^5·5GPa (Kiseeva

et al., 2012) showed the great importance of minor compo-

nents, especially alkalis. Small amounts of K2O and P2O5

in subducted MORB can significantly decrease its solidus

temperature. The present study is the first to investigate at

P410GPa a complex natural composition, which includes

the additional and potentially highly influential compo-

nents FeO and K2O.

The focus of this study is to determine the phase rela-

tions (and particularly solidus temperatures) in the deep

upper mantle and Transition Zone (9^21GPa, correspond-

ing to a depth interval of 180^600 km) of carbonated

eclogite, modelling deeply subducted, altered MORB. The

effects of variable alkali and SiO2 contents on solidus tem-

peratures and phase compositions are examined. The re-

sults are applied to the stability of different carbon-

bearing phases in the deep mantle and their roles in

mantle melting and metasomatism and generation of kim-

berlitic and alkaline magmas.

EXPER IMENTAL AND

ANALYT ICAL PROCEDURES

Starting composition

Two eclogite compositions (GA1 and Volga) were used as

starting materials (Table 1). The GA1 composition repre-

sents altered oceanic basalt (MORB) and is somewhat en-

riched in alkalis compared with fresh MORB

compositions (Yaxley & Green, 1994). Volga is identical to

GA1, but with 6·5% less SiO2. To both compositions, 10 wt

% of pure CaCO3 (cc) was added, producing GA1þ10%

CaCO3 (GA1cc) and Volgaþ10% CaCO3 (Volga-cc).

The GA1cc composition models subducted, altered, mafic

oceanic crust. Volga-cc models subducted altered mafic

crust, which may have lost a siliceous component during

dehydration and/or silicate melting in the subduction

zone. Altered oceanic crust contains typically no more

than 3wt % CaCO3. The enhanced carbonate propor-

tions in the current experiments were designed to aid in

the detection of carbonate phases in experimental run

products. Details of the starting material preparation,

phase relations and mineral assemblage at 3·5^5·5GPa for

GA1cc have been given by Kiseeva et al. (2012).The prepar-

ation of the Volga-cc composition was identical to that for

GA1cc.

Experimental techniques

The experiments were conducted using a 3000 ton Kawai-

type multianvil apparatus at Tohoku University, Sendai,

Japan. For experiments at 9^13GPa, the truncated edge

length (TEL) of the tungsten-carbide anvils was 5·0mm,

and for experiments at 17^21GPa the TEL was 3·5mm.

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Each experimental charge contained two capsules with the

GA1cc and Volga-cc compositions (Fig. 1a). The assembly

design was similar to that of Litasov & Ohtani (2009a,

2009b) with two minor modifications: (1) an MgO insula-

tor (instead of BN) was used to separate the capsules from

the LaCrO3 heater; (2) ZrO2 (instead of MgO) was used

as a spacer on top of each capsule to separate it from the

Mo electrodes. It was also used as the pressure transmit-

ting material. The size of the sample chamber before com-

pression was 1·4 and 0·8 mm3 for TEL 5·0 and 3·5mm,

respectively.

Temperature was monitored with a W97Re3^W75Re25thermocouple located at the centre of the furnace, between

the two capsules. The temperature gradient in the runs

did not exceed 508C across the sample, according to two-

pyroxene thermometry determined for special temperature

gradient experiments at 4^6GPa (Litasov & Ohtani,

2009a). Pressure was calibrated based on in situ synchro-

tron X-ray diffraction experiments at the ‘SPring-8’ facility

using the gold pressure scale after Dorogokupets &

Dewaele (2007) and Sokolova et al. (2013). Both room-tem-

perature and high-temperature (1200^16008C) data with

durations of greater than 60min were used for this calibra-

tion. The pressure uncertainty was determined to be less

than 1GPa. This calibration was confirmed by laboratory

measurements using semiconductor to metal transitions at

room temperature and at high temperatures (�16008C)

using the forsterite ! wadsleyite and wadsleyite !

Fig. 1. Optical (a) andback-scattered electron (BSE) (b^d) images of the experimental runs. (a) Recovered and cut in half experimental chargesG1400-21andV1400-21 [read as starting composition (G indicates GA1cc,V indicatesVolga-cc), temperature,14008C, and pressure, 21GPa]". (b)RunG1300-21with largegarnet, diamondandcarbonate (inthe top side of the capsule) crystals surroundedbya fine-grainedmatrixof similar com-position material. Nomelt is observed. (c) RunV1300-21with Grt crystals surrounded by melt pools. (d) Run G1300-21. Run G1300-21. Magnifiedviewof part of the run shown in (b). Large garnet and stishovite crystals surroundedbya fine-grainedmatrixof similarmaterial.

Table 1: Compositions of experimental mixes from this and

other experimental studies

GA1cc Volga-cc Y W&T O&M L&O1 K&G L&O2

P (GPa): 9–21 9–21 3–20 2–27 10–19 18–28 12–25 10–32

SiO2 45·32 42·22 49·71 53·53 51·11 50·06 30·80 50·02

TiO2 1·34 1·43 1·71 1·44 1·76 1·47 — —

Al2O3 14·88 15·91 15·68 14·85 14·86 15·39 4·02 16·59

Cr2O3 — — 0·05 — — — —

FeOT 8·85 9·46 9·36* 7·92 10·30 9·61 — —

MnO 0·15 0·14 0·18 0·16 — — — —

MgO 7·15 7·64 8·43 7·64 7·68 7·59 22·49 14·82

CaO 14·24 14·85 11·73 9·12 11·23 11·28 21·23 11·49

Na2O 3·14 3·36 2·76 2·64 2·94 2·43 — 2·08

K2O 0·40 0·42 0·23 1·31 0·13 0·17 — —

P2O5 0·14 0·15 0·02 — — — — —

CO2 4·40 4·40 — — — — 21·46y 5·00

H2O — — — — 2·00 2·00 — —

Total 100·00 100·00 99·81 98·66 102·01 100·00 100·00 100·00

*Additional 0·95wt % Fe2O3 included in the value.yCO2 measured by difference.GA1cc andVolga-cc, this study; Y, Yasuda et al. (1994);W&T,Wang & Takahashi (1999); O&M, Okamoto & Maruyama(2004); L&O1, Litasov & Ohtani (2005); K&G, Keshav &Gudfinnsson (2010); L&O2, Litasov & Ohtani (2010).

KISEEVA et al. CARBONATED ECLOGITE PHASE RELATIONS

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ringwoodite transitions in Mg2SiO4 (Litasov & Ohtani,

2009a, 2009b).

Au75Pd25 capsules were used as sample containers,

which were found to be the best material to avoid a hydro-

gen flux into and out of the capsule during the experiments

(Nishihara et al., 2006). Sample parts were fired in the

oven at 8508C and pyrophyllite gaskets were heated at

2308C for several hours prior to the experiment.

Encapsulated starting mixtures were dried in the oven at

3008C for 1h before final sealing by arc welding. These

procedures minimized penetration of hydrogen into the

sample chamber during the experiments.

Experiments were conducted at 9, 13, 17 and 21GPa,

over a range of temperatures between 1100 and 18008C.

After recovery, the Au^Pd capsule was cut in two using

a 0·15 mm thick diamond saw and petroleum benzene cut-

ting fluid to preserve water-soluble phases. One half was

then mounted into epoxy resin and rough polished on

abrasive paper under petroleum benzene (Fig. 1a). The

samples were then reimpregnated with epoxy resin under

vacuum, followed by final polishing with oil-based dia-

mond paste.

Analytical techniques

Run products were analysed using both wavelength- and

energy-dispersive (WDS and EDS) spectroscopy. All

phases were analysed using aJEOL 6400 scanning electron

microscope fitted with an energy-dispersive detector at

the Centre for Advanced Microscopy, ANU. Spectra were

acquired using a 15 kVaccelerating voltage,1 nA beam cur-

rent, and an acquisition time of 120 s. Garnet and clinopyr-

oxene were also analysed on a Cameca SX100 at the

University of Tasmania, using a beam current of 30 nA

and accelerating voltage of 15 kV.

The compositions of garnets and pyroxenes measured

using both EDS andWDS differ by less than 5%, consist-

ent with the work of Spandler et al. (2010), who compared

multiple EDS and WDS analyses (using the electron

microprobe at James Cook University) obtained from the

same phases in experimental run products. The reported

values of garnet and clinipyroxene are averages of both

WDS and EDS analyses.

For crystalline phases, a 1 mm beam with an excitation

diameter of about 1·5 mm was used. To obtain the most

representative composition, at least 10 grains of each

phase were analyzed in each experiment, and only those

analyses close to the theoretical cation sum were accepted.

For the majority of melt analyses a larger area scan was

used. Most of the quenched melts present in the runs

were highly heterogeneous, so as many area scans as pos-

sible were performed on each melt-bearing run.

Detection limits were 0·1^0·2wt %. Analyses were ob-

tained for most melts and mineral phases; however, it

was not always possible to precisely analyse some very

fine-grained accessory phases and the extremely

heterogeneous melt patches present in some runs. Mass-

balance calculations were carried out for each of the

experiments.

Raman spectroscopy was used for identification of

carbon allotropes (graphite or diamond) and carbonates.

The Raman spectra were obtained using a Jasco NRS-

2000 microspectrometer at Tohoku University. A micro-

scope was used to focus the excitation laser beam (the

488 nm lines of a Princeton Instruments Arþ laser) on

the sample surface. Spectra were collected for 120^240 s,

using a laser operating at 12^20mW and a beam 1 mm in

diameter.

RESULTS

A summary of all run conditions and calculated phase pro-

portions for GA1cc and Volga-cc are given in Table 2.

Representative phase compositions are listed in

Tables 3^8. The observed phase assemblages were used to

construct an experimental P^T phase diagram (Fig. 2).

Most runs produced well-crystallized, chemically homoge-

neous mineral assemblages from the glass starting mater-

ial. The homogeneity of phase compositions in most runs

indicates a close approach to chemical equilibrium.

Evidence for disequilibrium was observed in some of the

lowest temperature runs, with incorrect garnet stoichiom-

etry, manifested by cation sums (12 oxygens per formula

unit) substantially less than eight.

Solidus position, phase assemblage andtypes of melt

The solidus temperature at a given pressure was bracketed

based on the presence of visible quenched melt products

and mass-balance calculations. In the case of the lowest de-

grees of melting an additional compositional criterion is

useful to distinguish stable mineral and metastable

quenched phases. Across the entire range of the experi-

ments, the quenched melt products are a mixture of car-

bonate and silicate components, whereas the solid

carbonates are homogeneous and free from silicate

components.

The estimated solidus is between 1250 and 13008C at

9GPa and between 1200 and 13008C at 13GPa for both

starting compositions (Fig. 2). Solidus temperatures are

estimated to lie between 1200 and 13008C at 17 and 21GPa

for the Volga-cc composition and between 1300 and

14008C at 17GPa for the GA1cc composition. Experiments

G1300-21 and G1400-21 [read as starting composition (G

indicates GA1cc), temperature, 14008C, and pressure, 21

GPa] exhibit unusual textures (Fig. 1b and d), distin-

guished by garnet, diamond and to a lesser extent large

crystals of stishovite and K-hollandite (high-pressure ana-

logue of KAlSi3O8) (10^20 mm) surrounded by a fine-

grained (1 mm) matrix of the same minerals. In this case,


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https://www.researchgate.net/publication/227116882_Experimental_phase_and_melting_relations_of_metapelite_in_the_upper_mantle_implications_for_the_petrogenesis_of_intraplate_magmas_Contrib_Mineral_Petrol?el=1_x_8&enrichId=rgreq-f2e27430-b2ba-45e9-b054-3ef554888b85&enrichSource=Y292ZXJQYWdlOzI2MDU2NTQyODtBUzoxMDEyNDg3OTk4NzA5NzlAMTQwMTE1MTAwMzE3OQ==

large grains of Na-rich aragonite are usually found segre-

gated to the edge of the capsule, although it is not possible

to rule out their presence within the matrix. These runs

are considered to be subsolidus and hence the GA1cc sol-

idus at 21GPa is located at a slightly higher temperature

than 14008C. However, there is a possibility that a small,

undetected melt fraction is present within the fine-grained

matrix.

The subsolidus phase assemblages for both starting ma-

terials at 9^13GPa consist of garnet, clinopyroxene, car-

bonates (aragonite and magnesite, or calcite^magnesite

solid solution), a high-pressure polymorph of TiO2, coesite

or stishovite, graphite or diamond (only in GA1cc runs)

and K-hollandite (only in the V1200-13 run). Na-rich ara-

gonite appeared in the V1300-13 run, whereas no alkali-

rich carbonates were observed in GA1cc runs at 9^13GPa.

At 17^21GPa, the phase assemblages consist of garnet,

stishovite, K-hollandite, magnesite (only at 17GPa), Na-

rich aragonite and Ca-perovskite. Carbonate with a high

Na content (around 20wt % Na2O) was detected in a sub-

solidusV1200-21 run.

Phase relations and compositionsMajor phases

Garnet is the major phase in all the experiments

(Fig. 3a^f). Its modal proportion increases from �40% at

9GPa (except V1050-9, which crystallized only 26%

garnet) to �70% at 13GPa and 80% at 17^21GPa. The

grain size differs significantly and varies from 55 mm in

low-temperature 9GPa runs to 40 mm in 17^21GPa runs.

In most experiments at 9 and 13GPa, garnet occurs as

well-shaped, equant grains, often containing inclusions of

clinopyroxene and coesite or stishovite (Fig. 3a^c). At 17

and 21GPa, large fractured crystals of garnet occupy most

of the experimental charge (Figs 1c and 3d^f), with acces-

sory phases (usually stishovite and K-hollandite) as inclu-

sions and intergranular carbonate or melt. At all

pressures, with increasing temperature the number of in-

clusions in garnet decreases and the grains become larger

and more compositionally homogeneous.

As in previous studies (Yasuda et al., 1994; Litasov &

Ohtani, 2010), high-pressure garnet is generally character-

ized by Si in excess of 3·00 cations per 12-oxygen formula

Table 2: Experimental results and run conditions

T (8C) dT P D (h) Exp. no. Phases present (GA1cc) Exp. no. Phases present (Volga-cc)

1050 50 9 72 G1050-9 Grt(42), Cpx(45), Arag(8), Mst(1·5), Co(3·5), TiO2 V1050-9 Grt(26), Cpx(62), Arag(6·5), Mst(3), Co(2), TiO2

1200 20 9 48 G1200-9 Grt(42), Cpx(45), Arag(5), Mst(4), Co(3·5), TiO2 V1200-9 Grt(49), Cpx(39), Arag(3), Mst(7), Co(1·5), TiO2

1250 60 9 48 G1250-9 Grt(48), Cpx(35), CMss(10), Co(6·5), TiO2 V1250-9 Grt(45), Cpx(44), CMss(10), Co, TiO2

1400 20 9 24 G1400-9 Grt(46), Cpx(40), Co(4), TiO2, LCarb(10) V1400-9 Grt(54), Cpx(35), Co, TiO2, LCarb(10·5)

1200 20 13 80 G1200-13 Grt(84), Cpx(4), Arag(5·5), Mst(4), St(2·5) V1200-13 Grt(83·5), Cpx(5), Arag(8), Mst(2), St(1), K-Holl

1300 20 13 48 G1300-13 Grt(70), Cpx(12), St(6), LSi-Carb(12) V1300-13a Grt(78), Cpx(8), St(2), LCarb(11·5)

1300 20 13 48 V1300-13b Grt(77), Cpx(11), St(2), LCarb(10·5)

1400 20 13 24 G1400-13 Grt(69), Cpx(13), St(6), LSi-Carb(12) V1400-13 Grt(73), Cpx(15), St(1), LCarb(11)

1550 70 13 12 G1550-13 Grt(59), Cpx(19), St(8), LSi-Carb(13) V1550-13 Grt(68), Cpx(16), St(4), LCarb(12)

1100 40 17 48 G1100-17 Grt(89), Arag(9), Mst(1), K-Holl(1), St, CPv V1100-17 Grt(88), Arag(9), Mst(1), St(1), K-Holl, CPv

1200 20 17 48 G1200-17 Grt(87·5), Arag(10), Mst(1), K-Holl(1), St, CPv V1200-17 Grt(85), Arag(10), St(3), K-Holl(2), Mst, CPv

1250 70 17 48 G1250-17 Grt(81), Arag(9·5), St(9), Mst, K-Holl V1250-17 Grt(83), Arag(3·5), St(5·5), K-Holl, Mst, LSi-Carb(7·5)

1400 20 17 24 G1400-17 Grt(74), St(10·5), LSi-Carb(15·5) V1400-17 Grt(81), St(3), LSi-Carb(16)

1500 70 17 12 G1500-17 Grt(73·5), St(8·5), LSi-Carb(18) V1500-17 Grt(81), St(2·5), LSi-Carb(17)

1200 20 21 48 G1200-21 Grt(89·5), Arag(9·5), St, K-Holl, CPv V1200-21 Grt(84·5), Arag(7·5), St(3), K-Holl(1), CPv(1·5),

Na-Carb(2·5)

1300 20 21 48 G1300-21 Grt(79), Arag(9), St(10), K-Holl(2), CPv V1300-21 Grt(80), St(8), CPv(1·5), LCarb(10·5)

1400 20 21 16 G1400-21 Grt(78), Arag(9·5), St(12·5), CPv V1400-21 Grt(78·5), St(10), CPv, LCarb(11)

1650 80 21 16 G1650-21 Grt(79·5), St(8), LSi-Carb(12·5) V1650-21 Grt(82), St(6), CPv, LCarb(12)

1900 100 21 12 G1900-21 Grt(80), St(7), LSi-Carb(14) V1900-21 Grt(83), St(4), LCarb(13)

dT, estimated temperature gradient. D (h), duration of the experiment in hours. We did not identify the structure of theTiO2 phase in the experiments and followed results by Withers et al. (2003) and Sato et al. (1991) for rutile or TiO2 II andfurther phase transitions. Numbers in parentheses are wt % of the phase, extracted from mass-balance calculations. Forphases with no wt % value included, it is considered to be51wt %. Grt, garnet; Cpx, clinopyroxene; Arag, aragonite;Mst, magnesite; Co, coesite; St, stishovite; CMss, calcite–magnesite solid solution; K-Holl, K-hollandite; CPv,Ca-perovskite; Na-Carb, Na-carbonate; LCarb, carbonate melt; LSi-Carb, silicate–carbonate melt.


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https://www.researchgate.net/publication/6066020_Baddeleyite-Type_High-Pressure_Phase_of_TiO2?el=1_x_8&enrichId=rgreq-f2e27430-b2ba-45e9-b054-3ef554888b85&enrichSource=Y292ZXJQYWdlOzI2MDU2NTQyODtBUzoxMDEyNDg3OTk4NzA5NzlAMTQwMTE1MTAwMzE3OQ==

Table 3: Compositions of experimental garnet

GA1cc

T (8C): 1200 1250 1400 1300 1400 1550

P (GPa): 9 9 9 13 13 13

n: 4 s 5 s 4 s 8 s 6 s 7 s

SiO2 38·66 0·86 42·67 1·02 41·27 0·42 43·00 0·76 43·27 0·37 42·31 0·40

TiO2 1·49 0·16 1·25 0·14 1·77 0·08 1·99 0·13 1·97 0·16 1·71 0·11

Al2O3 21·29 0·72 19·36 0·76 19·53 0·13 19·01 0·44 19·85 0·58 19·25 0·50

FeO 9·68 1·18 11·14 0·88 10·57 0·08 10·29 0·25 10·19 0·06 10·05 0·23

MnO 0·14 0·17 0·17 0·08 0·22 0·01 0·20 0·10 0·18 0·01 0·20 0·12

MgO 7·91 0·53 8·14 0·28 8·18 0·07 8·67 0·25 8·76 0·17 8·68 0·05

CaO 16·82 0·26 13·85 0·26 16·33 0·26 14·79 0·32 15·09 0·24 14·61 0·22

Na2O 0·71 0·26 1·69 0·59 0·80 0·13 1·76 0·09 1·64 0·11 1·63 0·14

K2O 0·13 0·01 0·20 0·09 0·14 0·04 b.d.l. — b.d.l. — b.d.l. —

P2O5 0·41 0·09 0·23 0·09 0·18 0·04 0·25 0·05 0·17 0·03 0·16 0·06

Total 97·23 1·70 98·70 1·01 99·00 0·34 99·95 1·36 101·12 0·71 98·60 0·64

Mg# 59·32 3·87 56·61 1·28 57·98 0·33 60·01 0·63 60·49 0·36 60·62 0·61

Atoms per 12-oxygen formula unit

Si 2·96 3·20 3·10 3·18 3·16 3·17

Ti 0·09 0·07 0·10 0·11 0·11 0·10

Al 1·92 1·71 1·73 1·66 1·71 1·70

Fe 0·62 0·70 0·66 0·64 0·62 0·63

Mn 0·01 0·01 0·01 0·01 0·01 0·01

Mg 0·90 0·91 0·92 0·95 0·95 0·97

Ca 1·38 1·11 1·32 1·17 1·18 1·17

Na 0·10 0·25 0·12 0·25 0·23 0·24

K 0·01 0·02 0·01 — — —

P 0·03 0·01 0·01 0·02 0·01 0·01

Total 8·02 7·99 7·98 7·99 7·98 7·99

GA1cc

T (8C): 1250 1400 1500 1300 1400 1650

P (GPa): 17 17 17 21 21 21

n: 6 s 5 s 6 s 5 s 7 s 5 s

SiO2 44·32 1·16 43·20 0·34 42·93 0·54 43·32 0·49 42·60 1·32 44·09 0·32

TiO2 1·60 0·04 1·73 0·06 1·69 0·14 1·54 0·07 1·46 0·23 1·60 0·03

Al2O3 18·43 0·58 20·26 0·22 19·45 0·29 19·28 0·24 19·70 1·28 20·16 0·08

FeO 9·61 0·32 9·46 0·06 9·01 0·17 8·92 0·23 9·69 0·52 9·18 0·09

MnO 0·15 0·12 0·16 0·01 0·18 0·05 0·20 0·09 0·16 0·07 0·16 0·01

MgO 8·29 0·31 8·88 0·06 8·48 0·14 8·55 0·10 8·42 0·37 8·84 0·09

CaO 13·45 0·25 14·82 0·18 14·45 0·19 14·36 0·19 14·73 1·08 15·00 0·06

Na2O 2·40 0·11 1·97 0·09 2·10 0·11 2·18 0·04 1·92 0·62 2·01 0·08

K2O 0·10 0·05 b.d.l. — b.d.l. — b.d.l. — b.d.l. — b.d.l. —

P2O5 0·23 0·05 0·22 0·03 0·28 0·11 0·23 0·07 0·31 0·06 0·19 0·01

Total 98·59 2·06 100·70 0·44 98·56 0·44 98·58 1·06 98·99 1·11 101·21 0·27

Mg# 60·59 0·80 62·57 0·29 62·65 0·75 63·07 0·45 60·78 1·15 63·17 0·31


Si 3·29 3·15 3·19 3·22 3·17 3·19

Ti 0·09 0·10 0·09 0·09 0·08 0·09

Al 1·62 1·74 1·71 1·69 1·73 1·72

Fe 0·60 0·58 0·56 0·55 0·60 0·56

Mn 0·01 0·01 0·01 0·01 0·01 0·01

Mg 0·92 0·97 0·94 0·95 0·93 0·95

Ca 1·07 1·16 1·15 1·14 1·17 1·16

Na 0·35 0·28 0·30 0·31 0·28 0·28

K 0·01 — — — — —

P 0·01 0·01 0·02 0·01 0·02 0·01

Total 7·97 8·00 7·99 7·99 8·00 7·98

(continued)


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Table 3: Continued

Volga-cc V1300-13a

T (8C): 1050 1200 1250 1400 1200 1300

P (GPa): 9 9 9 9 13 13

n: 3 s 6 s 4 s 5 s 2 s 7 s

SiO2 39·51 0·41 41·42 0·44 40·41 0·40 41·60 0·37 45·05 0·99 43·35 0·43

TiO2 1·07 0·32 1·70 0·09 1·27 0·06 1·48 0·17 1·71 0·10 2·00 0·13

Al2O3 20·29 0·71 20·02 0·21 21·12 0·25 19·96 0·29 17·59 0·30 18·89 0·60

FeO 12·90 0·58 12·58 0·30 14·60 0·30 12·51 0·25 11·11 0·08 10·50 0·29

MnO 0·38 0·06 0·33 0·02 0·38 0·01 0·32 0·01 0·23 0·04 0·20 0·04

MgO 6·75 0·67 6·85 0·23 8·62 0·26 8·65 0·25 7·73 0·74 8·64 0·31

CaO 16·60 0·48 16·99 0·34 13·72 0·21 13·57 0·15 13·05 1·46 14·63 0·61

Na2O 0·73 0·10 1·11 0·16 0·59 0·16 1·02 0·24 2·61 0·12 1·79 0·06

K2O 0·14 0·05 0·10 0·04 b.d.l. — b.d.l. — 0·22 0·03 b.d.l. —

P2O5 0·36 0·16 0·19 0·02 0·15 0·04 b.d.l. — 0·18 0·11 0·21 0·05

Total 98·73 0·82 101·29 0·31 100·86 0·09 99·10 0·66 99·47 0·04 100·21 1·22

Mg# 48·16 1·38 49·24 0·51 51·26 0·57 55·20 0·51 55·28 2·19 59·45 1·32


Si 3·02 3·08 3·01 3·12 3·34 3·20

Ti 0·06 0·10 0·07 0·08 0·10 0·11

Al 1·83 1·75 1·85 1·76 1·54 1·64

Fe 0·82 0·78 0·91 0·78 0·69 0·65

Mn 0·02 0·02 0·02 0·02 0·01 0·01

Mg 0·77 0·76 0·96 0·97 0·85 0·95

Ca 1·36 1·35 1·10 1·09 1·04 1·15

Na 0·11 0·16 0·09 0·15 0·37 0·26

K 0·01 0·01 — — 0·02 —

P 0·02 0·01 0·01 — 0·01 0·01

Total 8·03 8·02 8·02 7·99 7·98 7·98

V1300-13b

T (8C): 1300 1400 1550 1100 1200 1250

P (GPa): 13 13 13 17 17 17

n: 2 s 5 s 5 s 4 s 5 s 9 s

SiO2 42·81 0·64 43·87 0·56 42·46 0·99 45·05 0·80 44·32 0·93 43·09 0·98

TiO2 2·02 0·07 1·92 0·07 1·69 0·08 1·56 0·10 1·84 0·38 1·79 0·09

Al2O3 18·50 0·15 19·11 0·43 19·63 0·53 17·49 0·29 18·91 0·29 18·83 0·39

FeO 10·92 0·22 11·45 0·26 11·19 0·22 10·63 0·31 11·15 0·18 10·86 0·14

MnO 0·35 0·12 0·28 0·00 0·31 0·04 0·25 0·04 0·26 0·01 0·27 0·07

MgO 8·16 0·22 8·60 0·49 8·64 0·12 8·31 0·48 8·55 0·41 8·02 0·22

CaO 14·21 0·22 14·48 0·69 14·31 0·55 11·43 1·43 13·33 1·20 14·24 0·27

Na2O 1·70 0·02 1·70 0·18 1·56 0·17 3·08 0·51 2·27 0·32 1·96 0·08

K2O b.d.l. — b.d.l. — b.d.l. — 0·29 0·14 0·12 0·06 b.d.l. —

P2O5 0·11 0·10 0·13 0·03 0·11 0·01 0·19 0·02 0·22 0·03 0·21 0·04

Total 98·78 0·92 101·53 0·18 99·90 1·86 98·27 0·68 100·95 0·21 99·26 0·69

Mg# 57·09 1·17 57·21 0·87 57·89 0·32 58·19 1·38 57·71 0·80 56·81 0·59


Si 3·21 3·20 3·15 3·36 3·24 3·21

Ti 0·11 0·11 0·09 0·09 0·10 0·10

Al 1·63 1·64 1·72 1·54 1·63 1·65

Fe 0·68 0·70 0·69 0·66 0·68 0·68

Mn 0·02 0·02 0·02 0·02 0·02 0·02

Mg 0·91 0·94 0·95 0·92 0·93 0·89

Ca 1·14 1·13 1·14 0·91 1·04 1·14

Na 0·25 0·24 0·22 0·45 0·32 0·28

K — — — 0·03 0·01 —

P 0·01 0·01 0·01 0·01 0·01 0·01

Total 7·97 7·98 8·00 8·00 7·99 7·99

(continued)


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unit (p.f.u.), and significant amounts of Na (0·09^

0·45 p.f.u.) and low Al contents (1·54^1·92 p.f.u.). Within

the 9^13GPa pressure interval the amount of Si p.f.u. in

garnet sharply increases, from 3·00^3·05 in some of the

9GPa runs to 3·15^3·20 Si p.f.u. in most of the 13 GPa

runs (Fig. 4b), indicating significant increase in the major-

ite component in garnet. The strong positive correlation

between Si and Na is consistent with the Na-majorite

(Na2MgSi5O12) substitution (e.g. Presnall et al., 1978;

Dymshits et al., 2013). Garnet in subsolidus experiments

contains higher Si and Na contents than in partially

molten experiments. The Na content of garnet slightly de-

creases with increasing temperature and degree of melting.

The amount of Na in garnet generally increases with pres-

sure, but the rate of increase becomes lower as pressure in-

creases (Figs 4a, b and 5a).

Clinopyroxene is the dominant phase in the GA1cc

composition at 3·5^5·5GPa (see Kiseeva et al., 2012).

The amount of clinopyroxene decreases from �45^55 at

5GPa to �35^40% at 9GPa (exceptV1050-9 run with esti-

mated 62modal% of clinopyroxene). At13GPa the amount

of clinopyroxene drops steeply and reaches �4^19%

(Fig. 4a). The modal proportion of clinopyroxene increases

with increasing temperature.The clinopyroxene-outbound-

ary in both compositions lies just above 13GPa, because in

subsolidus runs at 13GPa only a few clinopyroxene grains

were observed. Further evidence for this is provided by the

garnet composition.The amount of the majorite component

ingarnet,manifestedbySicationsp.f.u.,doesnot increasesig-

nificantlyover thepressure interval between13and21GPa.

Clinopyroxene is Na-rich and with increasing pressure

the amount of Na increases from around 0·29^0·50Na

p.f.u. (4·3^8·0wt % Na2O) at 9GPa to 0·59^0·80Na p.f.u.

(8·7^12·2wt % Na2O) at 13GPa. It does not show any sig-

nificant correlation with temperature, and does not

change significantly across the solidus (Fig. 5b). However,

the Na content of clinopyroxene correlates with Na in

garnet (Fig. 4a). The amount of M2þ cations (Mg, Fe, Ca,

Mn) in clinopyroxene decreases with increasing

pressure (Fig. 6a), whereas the amount of Al does not

exceed �0·7^0·8 cations p.f.u. (Fig. 6b). Excess of Si (up

to 0·05 Si cations p.f.u. over 2·00) in clinopyroxene

Table 3: Continued

Volga-cc

T (8C): 1400 1500 1200 1300 1400 1650 1900

P (GPa): 17 17 21 21 21 21 21

n: 6 s 5 s 6 s 5 s 7 s 4 s 5 s

SiO2 42·78 0·64 42·89 0·54 44·88 0·74 42·76 0·80 42·24 0·41 43·56 0·18 44·34 0·41

TiO2 1·88 0·14 1·69 0·05 1·23 0·15 1·44 0·14 1·46 0·03 1·68 0·05 1·29 0·04

Al2O3 19·56 0·48 19·28 0·25 18·02 0·75 20·50 0·50 20·43 0·50 19·39 0·59 19·51 0·32

FeO 10·07 0·42 10·19 0·15 11·41 0·16 11·01 0·16 10·96 0·22 10·53 0·38 8·47 0·41

MnO 0·20 0·07 0·25 0·01 0·25 0·11 0·27 0·01 0·23 0·06 0·26 0·04 0·25 0·01

MgO 8·00 0·29 8·32 0·15 8·48 0·37 8·59 0·23 8·35 0·36 8·36 0·17 8·90 0·10

CaO 15·12 0·58 13·96 0·23 12·70 0·67 14·78 0·64 14·77 0·38 14·64 0·65 14·80 0·34

Na2O 1·87 0·16 2·02 0·09 2·29 0·28 1·55 0·15 1·52 0·09 2·03 0·19 2·25 0·08

K2O b.d.l. — 0·33 0·21 0·21 0·07 b.d.l. — b.d.l. — b.d.l. — b.d.l. —

P2O5 0·18 0·04 0·16 0·02 0·19 0·07 0·21 0·03 0·25 0·07 0·15 0·01 0·15 0·03

Total 99·66 0·95 99·07 0·55 99·65 0·97 101·10 0·16 100·20 1·20 100·60 1·36 99·96 0·23

Mg# 58·60 0·99 59·27 0·35 56·94 1·20 58·16 0·36 57·58 0·60 58·58 0·55 65·21 1·32


Si 3·17 3·20 3·32 3·13 3·12 3·20 3·24

Ti 0·10 0·09 0·07 0·08 0·08 0·09 0·07

Al 1·71 1·69 1·57 1·77 1·78 1·68 1·68

Fe 0·62 0·64 0·71 0·67 0·68 0·65 0·52

Mn 0·01 0·02 0·02 0·02 0·01 0·02 0·02

Mg 0·88 0·92 0·93 0·94 0·92 0·91 0·97

Ca 1·20 1·11 1·01 1·16 1·17 1·15 1·16

Na 0·27 0·29 0·33 0·22 0·22 0·29 0·32

K — 0·03 0·02 — — — —

P 0·01 0·01 0·01 0·01 0·02 0·01 0·01

Total 7·99 8·01 7·98 8·00 8·00 8·00 7·99

b.d.l., below the detection limit (taken as 0·1 for all EDS measured values).


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Table 4: Compositions of experimental clinopyroxene

GA1cc

T (8C): 1050 1200 1250 1400 1200 1300

P (GPa): 9 9 9 9 13 13

n: 11 s 3 s 10 s 8 s 2 s 7 s

SiO2 50·95 0·33 55·20 1·45 53·50 1·12 54·58 0·58 55·14 0·32 57·28 1·13

TiO2 1·43 0·12 0·78 0·16 0·92 0·10 0·80 0·08 0·81 0·00 0·36 0·01

Al2O3 17·23 0·21 16·95 0·48 16·15 0·38 14·16 0·74 17·36 0·06 17·55 0·48

FeO 8·03 0·56 3·65 0·90 4·67 0·50 3·71 0·31 3·64 0·06 2·34 0·19

MnO 0·13 0·01 b.d.l. — b.d.l. — b.d.l. — b.d.l. — b.d.l. —

MgO 7·68 0·31 6·20 0·11 6·90 0·17 7·42 0·25 4·78 0·17 4·67 0·14

CaO 10·23 0·54 9·55 0·62 10·98 0·44 11·65 0·29 7·67 0·32 6·88 0·44

Na2O 4·28 0·34 8·02 0·61 6·38 0·34 6·57 0·23 8·65 0·25 10·52 0·33

K2O 0·55 0·04 0·43 0·05 0·44 0·07 0·44 0·03 0·35 0·06 0·18 0·05

P2O5 0·22 0·02 b.d.l. — 0·14 0·04 b.d.l. — b.d.l. — b.d.l. —

Total 100·73 0·63 100·80 0·09 100·08 0·57 99·32 0·45 98·39 0·30 99·77 1·03

Mg# 63·05 1·32 75·35 4·17 72·54 1·76 78·13 1·02 70·05 1·07 78·06 1·01


Si 1·81 1·92 1·89 1·94 1·95 1·99

Ti 0·04 0·02 0·02 0·02 0·02 0·01

Al 0·72 0·70 0·67 0·59 0·73 0·72

Fe 0·24 0·11 0·14 0·11 0·11 0·07

Mn 0·00 — — — — —

Mg 0·41 0·32 0·36 0·39 0·25 0·24

Ca 0·39 0·36 0·42 0·44 0·29 0·26

Na 0·29 0·54 0·44 0·45 0·59 0·71

K 0·02 0·02 0·02 0·02 0·02 0·01

P 0·01 — 0·00 — — —

Total 3·94 3·99 3·97 3·98 3·96 4·00

GA1cc Volga-cc

T (8C): 1400 1550 1050 1200 1250 1400

P (GPa): 13 13 9 9 9 9

n: 6 s 6 s 8 s 8 s 8 s 9 s

SiO2 58·99 1·45 56·66 1·23 47·49 0·36 52·19 2·10 54·67 1·65 55·08 0·97

TiO2 0·40 0·02 0·50 0·11 1·36 0·16 0·75 0·11 0·73 0·32 0·59 0·07

Al2O3 17·17 0·83 16·46 0·46 17·62 0·39 16·08 0·44 14·77 1·04 14·77 0·33

FeO 2·36 0·11 2·49 0·10 9·40 0·36 6·04 1·02 4·67 0·46 4·48 0·32

MnO b.d.l. — b.d.l. — 0·22 0·01 0·11 0·04 b.d.l. — b.d.l. —

MgO 4·99 0·10 5·42 0·33 7·58 0·28 6·17 0·12 7·02 0·35 6·49 0·18

CaO 7·03 0·50 7·42 0·37 11·43 0·61 11·26 1·17 11·19 0·13 9·85 0·28

Na2O 10·21 0·31 9·74 0·29 4·31 0·23 6·94 0·79 6·90 0·36 7·21 0·30

K2O 0·13 0·06 0·11 0·04 0·58 0·03 0·55 0·09 0·31 0·15 0·27 0·03

P2O5 b.d.l. — b.d.l. — 0·18 0·03 0·13 0·02 b.d.l. — b.d.l. —

Total 101·27 1·24 98·80 1·21 100·16 0·52 100·21 0·61 100·26 0·64 98·74 0·57

Mg# 79·04 0·59 79·50 0·43 58·98 0·59 64·75 3·51 72·83 2·29 72·09 1·24


Si 2·01 1·99 1·73 1·87 1·93 1·96

Ti 0·01 0·01 0·04 0·02 0·02 0·02

Al 0·69 0·68 0·76 0·68 0·61 0·62

Fe 0·07 0·07 0·29 0·18 0·14 0·13

Mn — — 0·01 0·00 — —

Mg 0·25 0·28 0·41 0·33 0·37 0·34

Ca 0·26 0·28 0·45 0·43 0·42 0·38

Na 0·68 0·66 0·30 0·48 0·47 0·50

K 0·01 0·01 0·03 0·02 0·01 0·01

P — — 0·01 0·00 — —

Total 3·97 3·99 4·01 4·02 3·98 3·97

(continued)


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from this and other studies at 13^19GPa (Fig. 6b) can be

potentially explained by the additional clinopyroxene

component NaMg0·5Si0·5Si2O6, synthesized by Gasparik

(1988).

Minor and accessory phases

Crystalline carbonate (magnesite, aragonite, alkali-bear-

ing carbonates) is present at all pressures (Fig. 7a, c and

d). It usually occurs as relatively large (up to 25 mm) subhe-

dral crystals interstitial to garnet (or clinopyroxene at

9^13GPa).Where both phases are present, magnesite usu-

ally occurs along aragonite crystal boundaries (Fig. 7a).

Crystalline carbonates coexisting with low-degree sili-

cate^carbonate melts were observed in only one experi-

ment (V1250-17).

Thetypeandcompositionofcarbonatepresentvarieswith

runtemperatureandpressure (Fig.8).At 9GPaandtempera-

tures of �12008C, in both GA1cc andVolga-cc, nearly pure

aragonite (91·5^95·4mol%CaCO3) coexistswithmagnesite

(72·5^75·6mol % MgCO3) that has significant CaCO3

(10·9^12·1mol %) and FeCO3 (17·5^19·0mol %) compo-

nents. At higher temperatures, inG1250-9 andV1250-9, only

a single carbonate of siderite and magnesite-bearing calcite

composition (61·4^66·1mol % CaCO3, 21·3^27·7mol %

MgCO3, 8·1^9·7mol % FeCO3) is present. This is also the

case for theexperimentsat13GPa. In subsolidusexperiments

G1200-13 andV1200-13 two carbonates, aragonite andmag-

nesite, are observed. However, in theV1300-13 experiment

(in contrast to G1200-13), the carbonate is no longer pure

CaCO3butcontains significantNa-,K-,Mg-andFe-bearing

components. Although the structure of the crystallized car-

bonatehas notbeen determined, its composition is similar to

the aragonite crystallized at high pressures in alkali-car-

bonatite systems (Litasov et al.,2013).

In most experiments at 17GPa, pure aragonite was not

observed and Na-rich aragonite (4·60^8·42wt % Na2O;

Table 4: Continued

Volga-cc V1300-13a V1300-13b

T (8C): 1200 1300 1300 1400 1550

P (GPa): 13 13 13 13 13

n: 5 s 9 s 2 s 8 s 8 s

SiO2 58·27 0·73 56·69 1·77 56·90 0·75 59·45 0·79 58·52 0·74

TiO2 0·21 0·03 0·39 0·07 0·32 0·03 0·39 0·02 0·37 0·02

Al2O3 17·69 0·54 16·29 1·29 16·76 0·31 16·93 0·45 17·30 0·48

FeO 4·74 0·35 3·33 0·24 3·07 0·09 3·02 0·06 3·02 0·17

MnO b.d.l. — b.d.l. — b.d.l. — b.d.l. — b.d.l. —

MgO 2·64 0·23 4·43 0·28 3·98 0·07 4·91 0·17 5·02 0·36

CaO 3·98 0·33 6·73 1·08 6·13 0·54 6·78 0·25 6·80 0·51

Na2O 12·23 0·13 10·50 0·49 10·45 0·30 10·01 0·33 10·35 0·33

K2O b.d.l. — 0·25 0·24 0·28 0·11 b.d.l. — 0·11 0·01

P2O5 b.d.l. — b.d.l. — b.d.l. — b.d.l. — b.d.l. —

Total 99·76 0·69 98·61 2·45 97·87 0·82 101·48 0·36 101·49 0·31

Mg# 49·75 1·01 70·34 1·37 69·77 0·24 74·34 0·69 74·74 0·52


Si 2·03 2·00 2·02 2·02 2·00

Ti 0·01 0·01 0·01 0·01 0·01

Al 0·73 0·68 0·70 0·68 0·70

Fe 0·14 0·10 0·09 0·09 0·09

Mn — — — — —

Mg 0·14 0·23 0·21 0·25 0·26

Ca 0·15 0·26 0·23 0·25 0·25

Na 0·83 0·72 0·72 0·66 0·69

K 0·00 0·01 0·01 0·00 0·00

P — — — — —

Total 4·02 4·01 3·99 3·96 3·99


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Table 5: Compositions of experimental carbonate

GA1cc

Phase: Mst Arag CMss Mst Arag Arag

T (8C): 1200 1200 1250 1200 1200 1100

P (GPa): 9 9 9 13 13 17

n: 2 s 5 s 8 s 4 s 5 s 2 s

SiO2 0·16 0·06 b.d.l. — 0·95 0·53 3·15 0·96 b.d.l. — 0·25 0·21

TiO2 b.d.l. — b.d.l. — 0·23 0·13 b.d.l. — b.d.l. — b.d.l. —

Al2O3 0·18 0·04 b.d.l. — 0·65 0·27 1·17 0·60 b.d.l. — b.d.l. —

FeO 12·55 0·42 b.d.l. — 5·81 0·89 11·09 0·59 0·41 0·18 3·41 0·21

MnO b.d.l. — b.d.l. — b.d.l. — b.d.l. — b.d.l. — b.d.l. —

MgO 29·31 1·74 b.d.l. — 8·58 0·56 32·22 0·92 b.d.l. — 5·03 0·62

CaO 6·09 1·27 51·29 0·90 37·08 0·69 2·68 2·00 52·14 0·75 37·01 0·48

Na2O 0·22 0·07 b.d.l. — 0·20 0·11 0·48 0·12 b.d.l. — 5·37 1·65

K2O b.d.l. — b.d.l. — 1·15 0·26 b.d.l. — b.d.l. — 0·53 0·01

P2O5 b.d.l. — b.d.l. — 0·27 0·05 b.d.l. — b.d.l. — 0·12 0·01

CO2 51·49 0·79 48·71 1·02 45·08 1·31 49·21 1·29 47·46 0·80 48·29 1·92

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

GA1cc

Phase: Mst Arag Arag Arag Arag Arag

T (8C): 1200 1200 1250 1200 1300 1400

P (GPa): 17 17 17 21 21 21

n: 1 3 s 5 s 5 s 3 s 5 s

SiO2 0·63 b.d.l. — 0·44 0·28 0·41 0·24 b.d.l. — 0·18 0·10

TiO2 0·10 b.d.l. — b.d.l. — b.d.l. — b.d.l. — b.d.l. —

Al2O3 1·01 b.d.l. — b.d.l. — 0·14 0·06 b.d.l. — b.d.l. —

FeO 9·46 3·72 0·40 2·94 0·28 3·27 0·35 5·12 0·54 4·12 0·46

MnO b.d.l. b.d.l. — b.d.l. — b.d.l. — b.d.l. — b.d.l. —

MgO 33·21 5·90 0·23 5·51 0·26 5·36 0·72 4·80 0·49 5·93 0·32

CaO 1·84 37·89 0·76 36·04 0·35 36·92 1·63 33·67 0·97 36·23 1·27

Na2O 0·60 5·58 1·65 8·06 0·19 6·09 0·96 6·56 0·43 6·16 0·94

K2O b.d.l. 0·57 0·05 0·53 0·05 0·43 0·02 0·53 0·08 0·99 0·54

P2O5 0·09 0·15 0·07 b.d.l. — b.d.l. — b.d.l. — 0·17 0·08

CO2 53·06 46·19 0·83 46·48 0·56 47·39 1·83 49·32 1·03 46·21 1·03

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

Volga-cc

Phase: Mst Arag Mst Arag CMss Mst

T (8C): 1050 1050 1200 1200 1250 1200

P (GPa): 9 9 9 9 9 13

n: 1 2 s 1 2 s 3 s 3 s

SiO2 0·19 0·15 0·21 0·42 b.d.l. — b.d.l. — 1·03 0·15

TiO2 b.d.l. b.d.l. — 0·01 b.d.l. — 0·11 0·05 b.d.l. —

Al2O3 0·30 b.d.l. — 0·17 0·12 0·06 0·54 0·06 0·37 0·13

FeO 13·64 b.d.l. — 12·74 b.d.l. — 6·95 0·84 9·80 0·23

MnO 0·23 b.d.l. — 0·23 b.d.l. — 0·22 0·04 b.d.l. —

MgO 30·48 b.d.l. — 29·21 0·10 0·03 11·15 0·35 33·89 0·50

CaO 6·79 53·52 0·24 6·79 52·82 1·81 34·43 0·72 1·54 0·16

Na2O 0·30 b.d.l. — 0·30 b.d.l. — 0·27 0·08 0·31 0·05

K2O b.d.l. b.d.l. — 0·04 b.d.l. — 1·18 1·58 b.d.l. —

P2O5 b.d.l. b.d.l. — 0·02 b.d.l. — b.d.l. — b.d.l. —

CO2 48·07 46·33 0·03 50·07 46·97 0·69 45·16 1·49 53·06 0·94

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

(continued)


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0·38^0·53wt % K2O; 4·89^5·90wt % MgO; 2·94^3·89wt

% FeO) coexists with small amounts of magnesite

(�1 modal %) that usually has about 13·2mol % FeCO3

and 2·1^3·2mol % CaCO3. No magnesite was found in

experiments at 21GPa, leaving Na-rich aragonite as the

only solid carbonate, except in run V1200-21, where Na-

carbonate with the approximate composition

(Na,K)2(Ca,Mg,Fe)(CO3)2 (Fig. 7c) was detected. Owing

to the small grain size and the presence of multiple, tiny in-

clusions in garnet, we do not exclude the possibility that

unobserved magnesite is a subsolidus phase at 21GPa.

Stishovite or coesite (at 9GPa) is an accessory phase at

all pressures in both starting compositions, reaching 10

modal % at 21GPa in G1300-21 andV1400-21. It is usually

present as inclusions in garnet or clinopyroxene, and often

occurs as quench crystals in carbonate-rich melts. At

17^21GPa, the amount of stishovite is higher than at

9^13GPa. At subsolidus conditions it is anhedral to subhe-

dral (Fig. 1d), whereas with increasing temperature the

crystals become euhedral and slightly elongated. In some

of the experiments, stishovite consists of pure SiO2.

However, up to 5wt % Al2O3 was observed in stishovite

Table 5: Continued

Volga-cc

Phase: Arag Mst Arag Arag Mst Arag

T (8C): 1200 1100 1100 1200 1250 1250

P (GPa): 13 17 17 17 17 17

n: 4 s 1 4 s 3 s 2 s 7 s

SiO2 0·69 0·62 2·20 0·26 0·11 0·31 0·00 0·34 0·02 b.d.l. —

TiO2 b.d.l. — b.d.l. b.d.l. — 0·16 0·09 b.d.l. — b.d.l. —

Al2O3 b.d.l. — 0·94 b.d.l. — b.d.l. — b.d.l. — b.d.l. —

FeO 2·92 0·47 9·54 3·62 0·72 3·89 0·38 9·47 0·40 3·21 0·16

MnO b.d.l. — 0·19 b.d.l. — b.d.l. — b.d.l. — b.d.l. —

MgO 4·51 0·54 34·57 4·87 1·36 6·15 0·48 38·51 4·04 5·42 0·36

CaO 36·25 1·75 1·16 35·14 1·78 36·47 1·86 1·22 0·07 34·05 1·09

Na2O 6·41 1·82 0·49 4·60 0·59 6·20 1·26 0·70 0·27 8·42 0·81

K2O 1·50 0·04 b.d.l. 0·38 0·01 0·42 0·03 0·02 0·03 0·49 0·14

P2O5 b.d.l. — b.d.l. 0·13 0·04 0·17 0·08 b.d.l. — b.d.l. —

CO2 47·72 0·99 50·91 51·02 3·91 46·23 1·22 49·74 4·32 48·40 0·75

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

Volga-cc

Phase: Arag Na-Carb

T (8C): 1200 1200

P (GPa): 21 21

n: 4 s 2 s

SiO2 b.d.l. — 1·26 0·85

TiO2 b.d.l. — b.d.l. —

Al2O3 b.d.l. — b.d.l. —

FeO 3·90 0·76 2·13 0·30

MnO b.d.l. — b.d.l. —

MgO 5·65 0·93 3·48 0·03

CaO 38·08 1·70 13·57 1·03

Na2O 6·38 0·38 21·02 0·33

K2O 0·27 0·03 3·77 0·23

P2O5 b.d.l. — b.d.l. —

CO2 45·72 0·00 54·78 0·01

Total 100·00 100·00

CO2 content was calculated from the mass-balance calculations and EDS totals.


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from some experiments, although because of the small

grain size it is hard to measure the composition precisely.

Quenched melt pools exhibit a high proportion of crys-

talline SiO2. No other SiO2-bearing phase is present in

the quenching products. Usually, these crystals are small

and elongated.

K-hollandite is observed mainly in experiments at

17^21GPa, although small amounts were also observed at

subsolidus conditions at 13GPa (i.e.V1200-13). At pressures

of 17 and 21GPa (although more abundant at 17GPa),

K-hollandite is the most common accessory phase in the

subsolidus runs in both compositions. It persists to 21GPa

but in lesser amounts and as smaller crystals. Usually it

forms eudredral, elongated (up to 20 mm long) inclusions

in garnet (Fig. 3e).The amount of K2O in all the measured

K-hollandite crystals varies between 11·7 and 15·4wt %;

other components include CaO (0·61^1·90wt %),

TiO2 (0·27^1·82wt %), Na2O (0·42^1·09wt %) and FeO

(0^0·89wt %).

Ca-perovskite was observed at 21GPa. It is more

abundant in experiments with theVolga-cc starting mater-

ial. It usually forms tiny (�1^3 mm) well-shaped cubic

crystals. Its small grain size prevents precise analysis.

Nevertheless, all the values are consistent and range

within 25·2^37·7wt % TiO2, 28·8^40·4wt % CaO and

18·0^29·3wt % SiO2. The main impurities are Al2O3

Table 6: Compositions of experimental coesite, stishovite and K-hollandite

Composition: GA1cc GA1cc GA1cc GA1cc GA1cc GA1cc Volga-cc Volga-cc Volga-cc

Phase: Co St St St St St St St St

T (8C): 1250 1200 1400 1300 1400 1650 1550 1250 1200

P (GPa): 9 13 17 21 21 21 13 17 21

SiO2 97·20 92·92 94·31 95·62 92·14 96·57 96·39 97·03 89·56

TiO2 b.d.l. 0·83 b.d.l. 0·20 0·35 0·31 0·41 0·19 1·74

Al2O3 0·61 1·87 1·67 1·11 4·31 2·99 1·23 0·45 0·44

FeO 0·50 0·14 0·19 1·11 b.d.l. 0·32 0·66 0·54 0·78

MnO b.d.l. 0·13 b.d.l. 0·10 0·03 b.d.l. b.d.l. b.d.l. b.d.l.

MgO 0·26 0·24 0·19 0·30 0·77 0·59 0·70 0·13 0·54

CaO 0·48 1·00 0·56 0·34 1·30 1·49 1·22 0·57 2·27

Na2O b.d.l. 0·80 1·42 b.d.l. 0·23 0·21 0·27 0·41 1·11

K2O b.d.l. 0·32 0·65 b.d.l. 0·25 b.d.l. b.d.l. b.d.l. 0·50

P2O5 b.d.l. 0·21 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 0·11

Total 99·05 98·46 98·99 98·78 99·38 102·48 100·88 99·32 97·05

Composition: Volga-cc GA1cc GA1cc GA1cc Volga-cc Volga-cc Volga-cc Volga-cc

Phase: St K-Holl K-Holl K-Holl K-Holl K-Holl K-Holl K-Holl

T (8C): 1650 1200 1250 1300 1200 1100 1200 1250

P (GPa): 21 17 17 21 13 17 17 17

SiO2 96·43 63·79 67·51 65·13 66·01 62·45 63·24 63·81

TiO2 0·46 0·72 0·38 0·37 0·27 1·82 1·11 0·47

Al2O3 0·97 18·58 18·05 18·11 18·32 17·44 18·77 19·01

FeO 0·35 0·77 b.d.l. 0·54 0·62 0·88 0·62 0·32

MnO 0·12 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.

MgO 0·47 0·73 b.d.l. 0·20 0·27 0·61 0·18 b.d.l.

CaO 1·28 1·67 0·91 0·89 0·70 1·90 1·08 0·61

Na2O 0·92 0·96 0·98 1·09 0·42 0·76 0·86 0·90

K2O 0·48 13·16 13·87 13·67 15·39 11·71 13·72 14·87

P2O5 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 0·27 b.d.l. b.d.l.

Total 101·48 100·39 101·69 100·00 102·00 97·84 99·58 99·97


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(which may be related to overlapping crystals), Na2O and

FeO. The abundance of CaO and to a lesser extent TiO2

and Na2O in Ca-perovskite increases with increasing tem-

perature, whereas the amounts of SiO2, Al2O3 and MgO

decrease.

Most of the experiments with the GA1cc starting mater-

ial, at both subsolidus and above-solidus conditions, con-

tained accessory graphite or diamond crystals (Figs 1b, 7b

and 9a, b). Crystal size ranged from 10 to 40 mm. At

9^13GPa, grain shapes were anhedral or sometimes

rounded. Under subsolidus conditions the grains had a

clear basal cleavage indicating that they were graphite. At

17^21GPa, the shape of this phase appeared more crystal-

line, and the hardness of the grains while polishing indi-

cated the formation of diamond for both subsolidus and

supersolidus runs. The presence of diamond in some

runs (e.g. G1400-21) was verified by laser Raman

spectroscopy.

Experimental melts and their compositions

All the supersolidus experiments in this study contain 7·5^

18% melt. Melts did not quench to a glass but instead usu-

ally formed heterogeneous pools of quenched silicate and

carbonate phases, interstitial between coarser residual

crystals of garnet and other phases. The compositions of

these metastable quench crystals could not generally be

determined precisely because of their very fine grain size.

(Figs 1c and 3c, d, f). In some cases, melt partially segre-

gated to distinct zones in the capsules.

All of the melts produced are carbonate-rich (25^46%

CO2). The amount of CO2 in the melt has been estimated

from analytical totals that deviate from 100%, as well as

by mass-balance calculations. As previously reported

(Litasov & Ohtani, 2010), the melt composition evolves

from carbonatitic near the solidus to a more siliceous com-

position with increasing temperature and degree of melt-

ing. The heterogeneity of the produced melts is manifested

mainly by high variations in SiO2, Al2O3 and CaO con-

tents (Table 8). Low-degree melts in both compositions

at 9GPa are very similar to the solid carbonate

compositions (Fig. 8), but contain significant amounts of

TiO2 (2·67^3·18wt %) and SiO2 (1·82^4·64wt %). The

melts for GA1cc and Volga-cc differ slightly at 13GPa,

with different proportions of SiO2, Al2O3 and CaO. The

amount of alkali components, TiO2, FeO and MgO in

both melts is similar. All the melts at 17GPa for both com-

positions (except V1250-17) are silicate^carbonate at rela-

tively high (15·5^18%) degrees of melting. Similar to runs

at 13GPa, melt produced by the GA1cc composition at

21GPa is more SiO2-rich, whereas melts of the Volga-cc

starting material contain much higher concentrations of

CaO. The only low-degree melt (�7·5% melting) that

was analyzed (in experiment V1250-17) coexists with solid

magnesite and aragonite and is alkali-rich (�14·8wt %

Na2O and �3·6wt % K2O).

The Ca# of the melts decreases slightly and Mg# in-

creases slightly with increasing pressure and increasing

degree of melting (Fig. 10a and b). However, the Ca#

for melts at 17GPa is lower than for melts at 21GPa.

Table 7: Compositions of experimental Ca-perovskite

Composition: GA1cc Volga-cc

T (8C): 1300 1400 1200 1200 1300 1400 1400

P (GPa): 21 21 17 21 21 21 21

n: 3 s 4 s 2 s 1 5 s 2 s 2 s

SiO2 26·55 0·85 18·02 9·97 23·03 5·62 29·26 28·78 2·23 26·10 0·70 26·10 0·70

TiO2 26·51 1·67 34·70 11·43 37·73 6·51 25·17 26·82 1·85 26·82 1·70 26·82 1·70

Al2O3 4·98 1·23 1·69 5·91 3·13 0·65 5·56 3·19 1·14 3·55 0·63 3·55 0·63

FeO 1·47 0·46 1·48 1·28 2·63 0·69 2·28 1·03 0·25 1·26 0·33 1·26 0·33

MnO 0·12 0·07 0·11 0·08 b.d.l. — b.d.l. b.d.l. — b.d.l. — b.d.l. —

MgO 1·35 0·48 0·63 1·92 0·84 0·46 1·54 0·67 0·52 0·83 0·42 0·83 0·42

CaO 37·54 1·67 40·40 6·95 28·82 1·00 34·73 37·92 1·83 39·75 1·34 39·75 1·34

Na2O 1·16 0·29 2·44 0·87 2·49 0·26 1·08 1·31 0·63 1·32 0·84 1·32 0·84

K2O 0·33 0·07 0·52 0·25 1·33 0·17 0·38 0·27 0·19 0·38 0·29 0·38 0·29

P2O5 b.d.l. — b.d.l. — b.d.l. — b.d.l. b.d.l. — b.d.l. — b.d.l. —

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


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Table 8: Compositions of experimental melts

GA1cc

T (8C): 1400 1300 1400 1550 1400 1500

P (GPa): 9 13 13 13 17 17

Type of melt: Carb Si-Carb Si-Carb Si-Carb Si-Carb Si-Carb

n: 6 s 5 s 3 s 5 s 6 s 2 s

SiO2 4·64 1·96 17·28 16·29 18·67 2·94 20·35 7·44 25·90 2·76 25·97 2·24

TiO2 2·67 0·61 0·82 0·26 1·46 0·55 0·54 0·33 0·90 0·19 1·00 0·02

Al2O3 1·48 0·88 3·38 2·95 2·66 2·13 1·62 1·64 4·05 2·32 5·75 4·16

FeO 5·62 0·68 5·14 0·47 5·72 0·56 5·33 0·56 4·81 0·59 5·03 0·21

MnO b.d.l. — b.d.l. — 0·16 0·08 0·19 0·18 0·10 0·03 b.d.l. —

MgO 4·82 0·29 5·97 1·00 5·09 0·38 6·01 0·91 5·27 0·74 5·55 0·67

CaO 31·96 1·88 22·06 2·71 22·00 1·91 25·88 3·69 18·95 1·08 20·94 4·60

Na2O 0·78 0·29 5·12 0·54 5·41 1·03 2·56 1·55 9·06 1·00 8·57 2·14

K2O 2·89 1·77 2·12 0·14 1·43 0·36 1·20 0·84 1·92 0·36 1·88 0·47

P2O5 1·40 0·22 b.d.l. — 0·38 0·07 0·48 0·17 b.d.l. — 0·25 0·07

CO2 43·74 2·71 38·11 15·29 37·00 0·00 35·83 4·60 29·04 3·73 25·06 0·00

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

Element ratios

Mg# 60·46 67·45 61·31 66·79 66·14 66·28

CaO/SiO2 6·89 1·28 1·18 1·27 0·73 0·81

K2O/Na2O 3·69 0·41 0·26 0·47 0·21 0·22

GA1cc Volga-cc V1300-13a V1300-13b

T (8C): 1650 1400 1300 1300 1400 1550

P (GPa): 21 9 13 13 13 13

Type of melt: Si-Carb Carb Carb Carb Carb Carb

n: 4 s 5 s 5 s 1 s 7 s 8 s

SiO2 23·48 1·84 1·82 1·09 14·57 3·04 7·63 1·89 1·20 3·25 2·80

TiO2 1·17 0·17 3·18 1·34 0·65 0·21 0·21 0·73 0·24 0·93 0·48

Al2O3 0·91 0·50 0·63 0·42 2·24 1·35 0·20 0·73 0·54 1·04 1·18

FeO 3·17 0·24 7·00 0·21 4·72 0·27 5·25 7·13 1·20 7·96 1·36

MnO b.d.l. — 0·24 0·16 b.d.l. — 0·11 b.d.l. — b.d.l. —

MgO 3·88 0·26 5·18 0·83 4·72 0·47 5·39 5·90 1·46 6·74 2·04

CaO 18·84 0·64 32·66 1·05 23·53 1·50 28·71 30·64 1·42 33·00 3·49

Na2O 10·32 0·46 0·82 0·16 6·68 0·63 5·34 3·72 1·25 5·82 3·90

K2O 2·59 0·32 1·82 0·57 3·02 0·27 3·34 1·83 0·91 2·38 2·01

P2O5 0·36 0·03 1·22 0·38 0·37 0·05 0·32 0·63 0·20 0·76 0·34

CO2 35·30 1·34 45·42 1·54 39·49 2·95 43·50 46·79 1·39 38·12 0·00

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

Element ratios

Mg# 68·57 56·87 64·04 64·66 59·61 60·17

CaO/SiO2 0·80 17·90 1·61 3·76 16·18 10·15

K2O/Na2O 0·25 2·23 0·45 0·63 0·49 0·41

(continued)


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Table 8: Continued

Volga-cc

T (8C): 1250 1400 1500 1300 1400 1650 1900

P (GPa): 17 17 17 21 21 21 21

Type of melt: Si-Carb Si-Carb Si-Carb Carb Carb Carb Carb

n: 6 s 4 s 4 s 4 s 5 s 9 s 5 s

SiO2 15·20 8·35 28·08 4·60 28·25 14·08 1·98 0·49 4·50 3·08 5·02 3·61 9·58 3·52

TiO2 0·54 0·13 0·78 0·16 0·89 0·23 0·44 0·19 0·49 0·38 1·53 1·11 3·24 0·41

Al2O3 0·62 0·23 5·37 0·75 3·13 3·37 0·35 0·11 0·95 0·57 0·97 0·45 2·77 1·60

FeO 4·06 0·46 3·83 0·56 4·45 1·01 4·73 0·68 5·42 1·62 6·91 1·09 5·77 0·62

MnO b.d.l. — b.d.l. — b.d.l. — b.d.l. — 0·14 0·04 0·19 0·10 0·12 0·04

MgO 4·18 0·60 4·83 0·55 5·72 1·26 6·52 0·41 5·45 1·08 5·75 0·72 5·18 0·45

CaO 15·99 1·63 16·34 1·65 19·65 4·40 26·32 0·77 24·95 1·55 27·95 1·66 21·96 3·25

Na2O 14·79 1·16 8·56 0·99 7·91 1·72 10·85 0·50 12·42 1·52 9·67 2·82 11·38 3·37

K2O 3·62 0·34 1·97 0·33 3·10 1·35 3·23 0·11 3·49 0·46 3·41 0·69 3·60 0·62

P2O5 0·11 0·04 0·14 0·05 0·38 0·13 b.d.l. — 0·19 0·07 0·58 0·14 1·39 0·25

CO2 40·90 4·73 30·10 0·00 26·52 8·21 45·60 0·36 42·00 0·00 38·00 0·00 35·00 0·00

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

Element ratios

Mg# 64·74 69·19 69·61 71·08 64·20 59·72 61·53

CaO/SiO2 1·05 0·58 0·70 13·33 5·54 5·57 2·29

K2O/Na2O 0·24 0·23 0·39 0·30 0·28 0·35 0·32

CO2 content was calculated from the mass-balance calculations and EDS totals. The normalized values of melts totalshave 0·0 standard deviation for CO2, compared with those that have not been normalized.

Fig. 2. Experimental P^T phase diagram for GA1cc andVolga-cc. Abbreviations as inTable 2.Y, solidus of dry eclogite byYasuda et al. (1994);HF, solidus of dry eclogite by Hirose & Fei (2002); K, solidus for GA1cc in Au^Pd capsules at 5GPa (Kiseeva et al., 2012). Gr-D indicates graph-ite^diamond transition (Kennedy & Kennedy, 1976). Circles indicate experimental runs. Diamonds inside the circles indicate the presence ofdiamond or graphite in GA1cc runs.


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https://www.researchgate.net/publication/248794506_YasudaA_FujiiT_KuritaK_Melting_phase_relations_of_an_anhydrous_mid-ocean_ridge_basalt_from_3_to_20_GPa_Implications_for_the_behavior_of_subducted_oceanic_crust_in_the_mantle_J_Geophys_Res_99_9401-_941?el=1_x_8&enrichId=rgreq-f2e27430-b2ba-45e9-b054-3ef554888b85&enrichSource=Y292ZXJQYWdlOzI2MDU2NTQyODtBUzoxMDEyNDg3OTk4NzA5NzlAMTQwMTE1MTAwMzE3OQ==

Unlike Keshav & Gudfinnsson (2010), in this study any

increase in the Mg content of the melt with increasing

pressure is not observed. The amount of alkali compo-

nents in the melt increases dramatically with increasing

pressure, and the Na/K ratio increases up to the point of

K-hollandite saturation, and then subsequently decreases

(Fig. 11a and b).

DISCUSS ION

Solidus of carbonated eclogite andcomparison with previous studies

Experimental data on MORB-like compositions at pres-

sures above 8^10GPa are limited. Studies on volatile-free

MORB and K-rich MORB compositions (Yasuda et al.,

1994; Wang & Takahashi, 1999; Hirose & Fei, 2002)

Fig. 3. BSE images of the experimental runs. (a) RunV1400-9. Mineral assemblage at 9GPa above the solidus. (b) RunV1200-13 subsolidusmineral assemblage at 13GPa. (c) Run G1300-13 showing heterogeneously distributed melt and patches of carbonate melt separated fromareas of silicate^carbonate melt. (d) Run G1400-17. Heterogeneous melt pools. (e) Run G1100-17. K-hollandite crystal included in Grt. (f) RunG1650-21. Heterogeneous silicate^carbonate melt at 21GPa. Carbonate-rich matrix with quenched CAS and stishovite crystals.


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reported an increase in solidus temperatures from 16008C

at 8GPa to 2100^22008C at 20GPa. The melts in these

studies are silica-rich, with 50^60wt % SiO2 at pressures

below 20GPa (Yasuda et al., 1994; Wang & Takahashi,

1999). At higher pressures the amount of SiO2 in the

melts decreases to 44^48wt % (Wang & Takahashi,

1999; Hirose & Fei, 2002). The amount of alkali compo-

nents in the low-degree melts is strictly governed by the

Fig. 4. Na content of experimentally crystallized garnet and clinopyroxene. (a) Na distribution amongst the major phases within an eclogite as-semblage. Dashed line indicates 1^1 ratio; numbers indicate pressure in the experimental runs reported in literature. Arrows indicate pressureand the amount of clinopyroxene in the experiments with GA1cc andVolga-cc compositions. (b) Na vs Si in experimental garnet. Pt-Gr, experi-ments in Pt^graphite capsule; Au-Pd, experiments in Au^Pd capsule (see Kiseeva et al. 2012). O&M, hydrous MORB (Okamoto &Maruyama, 2004); L&O, carbonated MORB (Litasov & Ohtani, 2010); Y, dry MORB (Yasuda et al., 1994).


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stability of clinopyroxene and K-hollandite, as in the pre-

sent study.

The solidus of hydrous MORB has been estimated by

Okamoto & Maruyama (2004) to lie near 12008C at

19GPa, which is similar to the GA1cc and Volga-cc solidi

and about 10008C below the dry MORB solidus at the

same pressure. Litasov & Ohtani (2005) reported a much

higher solidus temperature for hydrous MORB at

18^28GPa, only �50^1008C lower than the dry MORB

solidus. Those researchers suggested that a small amount

of supercritical fluid was present in the runs even at the

lowest temperatures (10008C), and reported an ‘apparent

solidus’ based on extensive melting of the main silicate

phases, which occurred above 20008C at 20GPa. Similar

to the melts produced from dry MORB compositions, the

melts reported by Litasov & Ohtani (2005) are silica-rich.

There are only twopublishedexperimental studies oncar-

bonated eclogite compositions at pressures above 10GPa,

both in simplified systems: CMAS þ 20% CO2 (Keshav &

Gudfinnsson, 2010) and Na-CMASþ 5% CO2 (Litasov &

Ohtani, 2010). The solidus of carbonated eclogite at 10^

20GPa was reported to lie about 400^5008C below the dry

eclogite solidus (Litasov & Ohtani, 2010). At 10GPa, it was

about 50^1008C higher than the carbonated eclogite solidus

at 9GPaofDasgupta et al. (2004) (Fig.12).

Keshav & Gudfinnsson (2010) referred to their melts as

‘calcio-carbonatites’, with a substantial increase in the

MgCO3 component from low pressures (12^16GPa) to high

pressures (20^25GPa); the amount of SiO2 in themelts does

notexceed2·4wt%.Litasov&Ohtani (2010) facedproblems

with determination of partial melt compositions at 10·5 and

16·5GPa owing to possible coexistence of both carbonatitic

and carbonate-rich silicate melts, which indicate liquid im-

miscibility or heterogeneity across the sample. The Na2O

content in either of these melts does not exceed 3·1wt %

within the stability field of clinopyroxene, and is up to 7·2wt

% Na2O at higher pressures, where clinopyroxene is no

longer stable. This is similar to the melting style of GA1cc

andVolga-cc, which both exhibit a dramatic increase in the

alkali contents of themelts at pressures above clinopyroxene

stability. Both studies reportedmagnesite (ormagnesite and

aragonite together) coexistingwithcarbonatiticmelt.

Unlike at lower pressures (55^6GPa), all of the MORB

solidi except Volga-cc show a gradual increase in solidus

temperature with increasing pressure. However, the

slopes of the various solidi in P^Tspace are highly variable

(Fig. 12). Volatile-free eclogite solidi are very steep and

linear up to the point of clinopyroxene disappearance, but

significantly less steep at higher pressures (Yasuda et al.,

1994; Wang & Takahashi, 1999; Hirose & Fei, 2002). A

steep linear solidus was also reported for CMASþ 20%

CO2 (Keshav & Gudfinnsson, 2010) and MORBþ 2%

H2O (Litasov & Ohtani, 2005). On the other hand, most

alkali-bearing, carbonated eclogites and peridotites

(Ghosh et al., 2009; Litasov & Ohtani, 2009b, 2010) display

essentially flat solidi with increasing pressure from 10 to

20^30GPa, in good agreement with the solidi of GA1cc

andVolga-cc determined in this study.

The probable cause for the dramatic differences in sol-

idus temperatures between different carbonated eclogite

compositions lies in similar compositional parameters

identified in lower pressure studies (Dasgupta et al., 2004,

2005), which include Ca#, amount of CO2 and H2O, and

CaO/MgO and Na2O/CO2 ratios. Litasov et al. (2013) con-

sidered the true solidus of hydrogen-free carbonated eclog-

ite and peridotite to be strongly influenced by the amount

of alkalis and placed it at temperatures similar to this

study. Those researchers also reported a possible negative

slope from 15 to 21GPa for the Na-carbonatite solidus,

which is similar to the solidus determined forVolga-cc.

The main host for K in different starting compositions

differs significantly. In more K-rich compositions, such as

Fig. 5. Na2O content of the experimentally crystallized phases. (a)Garnet. (b) Clinopyroxene.


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https://www.researchgate.net/publication/222199969_Phase_relations_in_hydrous_MORB_at_18-28_GPa_Implications_for_heterogeneity_of_the_lower_mantle?el=1_x_8&enrichId=rgreq-f2e27430-b2ba-45e9-b054-3ef554888b85&enrichSource=Y292ZXJQYWdlOzI2MDU2NTQyODtBUzoxMDEyNDg3OTk4NzA5NzlAMTQwMTE1MTAwMzE3OQ==

https://www.researchgate.net/publication/257922634_Solidus_of_alkaline_carbonatite_in_the_deep_mantle?el=1_x_8&enrichId=rgreq-f2e27430-b2ba-45e9-b054-3ef554888b85&enrichSource=Y292ZXJQYWdlOzI2MDU2NTQyODtBUzoxMDEyNDg3OTk4NzA5NzlAMTQwMTE1MTAwMzE3OQ==

https://www.researchgate.net/publication/248216560_Solidus_and_phase_relations_of_carbonated_peridotite_in_the_system_CaO-Al_2O_3-MgO-SiO_2-Na_2O-CO_2_to_the_lower_mantle_depths?el=1_x_8&enrichId=rgreq-f2e27430-b2ba-45e9-b054-3ef554888b85&enrichSource=Y292ZXJQYWdlOzI2MDU2NTQyODtBUzoxMDEyNDg3OTk4NzA5NzlAMTQwMTE1MTAwMzE3OQ==

https://www.researchgate.net/publication/222886216_Dasgupta_R_Hirschmann_M_M_Withers_A_C_Deep_global_cycling_of_carbon_constrained_by_the_solidus_of_anhydrous_carbonated_eclogite_under_upper_mantle_conditions_Earth_Planet_Sci_Lett_227_73-85?el=1_x_8&enrichId=rgreq-f2e27430-b2ba-45e9-b054-3ef554888b85&enrichSource=Y292ZXJQYWdlOzI2MDU2NTQyODtBUzoxMDEyNDg3OTk4NzA5NzlAMTQwMTE1MTAwMzE3OQ==

dry CO2-bearing pelite with 2·21wt % K2O in the start-

ing mixture, a K-bearing phase that incorporates most of

the bulk K2O has been reported by Grassi & Schmidt

(2011a, 2011b) across the entire P^T range studied (e.g.

T¼ 900^15508C; P¼ 5·5^23·5GPa). Those researchers

observed K-feldspar, which crystallized at 59GPa and

was followed by K-hollandite at 49GPa, with other

phases such as clinopyroxene, carbonate or garnet contain-

ing very small amounts of K (usually �1wt %). K-feld-

spar was also reported at 900 and 10008C and 3GPa by

Tsuno & Dasgupta (2012) for carbonated pelite compos-

ition with 1·99wt % K2O in the starting mixture.

Fig. 6. Compositions of experimentally crystallized clinopyroxene. (a) Sum of divalent cations as a function of pressure. (b) Al content as afunction of Si content. Labels as in Fig. 4.


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For carbonated eclogite compositions, like those studied

by Litasov et al. (2013), the low solidus temperatures may

be attributed mainly to high alkali/CO2 ratios and to

the stability of alkali carbonates. Although at pressures

higher than around 6GPa in the simple KAlSi3O8 or

(K,Na)AlSi3O8 systems, K-feldspar was noted to trans-

form to wadeiteþkyaniteþ coesite (Urakawa et al., 1994;

Yagi et al., 1994), no potassium-bearing crystalline phase

was detected in most experimental runs on eclogite and

peridotite systems between the stability fields of sanidine

and K-hollandite (Wang & Takahashi, 1999; Ghosh et al.,

2009). Hence within the pressure range 5^13GPa potas-

sium is highly incompatible in silicate phases, and under

anhydrous conditions any potassium would partition into

the melt or a carbonate phase if not incorporated into

clinopyroxene. This was well documented by Wang &

Takahashi (1999), who reported 6·64wt % K2O at 5GPa

in the melt and up to 1·9wt % K2O in clinopyroxene at

about 7GPa. In CO2-bearing starting compositions, a K-

carbonate phase may be a more plausible host for K

than clinopyroxene; however, this needs more

clarification.

With increasing pressure up to the clinopyroxene-out

phase boundary at 14^16GPa, Na becomes more compat-

ible in clinopyroxene. The more Na is in the system, the

more jadeitic clinopyroxene is formed. The disappearance

of clinopyroxene roughly coincides with the appearance of

K-hollandite, which changes the compatibility of Na and

K in opposite senses. K-hollandite incorporates all the K,

whereas Na becomes highly incompatible until the stability

fields of NAL and CF phases (425GPa) are reached

(Hirose & Fei, 2002; Litasov & Ohtani, 2005). Although

some Na2O can be accommodated in majoritic garnet, in

the studied eclogitic systems the N2O concentration in

garnet does not exceed 2^3wt % (Yasuda et al., 1994;

Wang & Takahashi, 1999; Litasov & Ohtani, 2005) and

goes up to 3·1wt % Na2O for majoritic garnet in experi-

ment V1100-17. Thus, in the same manner as K, the ‘excess’

Na either fluxes the formation of low-degree melts, or, in

carbonated systems, it partitions into Na-rich crystalline

carbonates. This suggests that the Na- and K-bearing car-

bonated eclogite solidus will be largely controlled by the

melting of Na- and K-bearing carbonate phases, which

presumably are K-carbonates at 9^13GPa and Na-

Fig. 7. C-bearing phases at different P^T conditions. (a) Run G1200-9. Aragonite and magnesite. (b) Run G1400-13. Diamond or graphitewithin carbonate melt. (c) RunV1200-21. Na-carbonates coexisting with Na-rich aragonite. (d) Run G1400-21. Na-rich aragonite within fine-grained matrix.


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carbonates at 17^21GPa. This may be also one of the rea-

sons for the significant difference in GA1cc and Volga-cc

solidus temperatures at 17^21GPa. Whereas most of the

carbon in GA1cc partitions into Na-rich aragonite and dia-

mond, no diamond was observed inVolga-cc, and Na-car-

bonate (with, perhaps, a lower solidus temperature than

Na-rich aragonite) crystallized. This is consistent with the

more sodic character of the clinopyroxene and melts

formed in experiments on the Volga-cc composition.

Carbonate-rich melts in GA1cc and Volga-cc persist up to

the highest temperatures of the experiments, with rela-

tively minor participation of silicates in the melting pro-

cess. This explains the low melt productivity (518%) over

the large range of P^T conditions of the present

experiments.

The alkali/CO2 ratio of the bulk-rock and low solidus

temperatures can also affect the stability fields of the

main phases, including the transformation from eclogite

to garnetite. In carbonate-bearing systems, this transform-

ation should occur at lower pressures (�15GPa) relative

to carbonate-free systems, because of the increased parti-

tioning of Na into Na-aragonite with increasing pressure.

The absence of clinopyroxene in Na-CMASþ 5% CO2 ex-

periments at 16·5GPa (Litasov & Ohtani, 2010) may indir-

ectly indicate the presence of an additional Na-bearing

phase. In contrast, in dry and hydrous eclogite compos-

itions, clinopyroxene can be stable to higher pressures, in

the range of 16^19GPa (Yasuda et al., 1994; Okamoto &

Maruyama, 2004). This has important implications for

mantle melting and density profiles. Owing to the low

melting temperatures of Na-bearing carbonates (511508C

between 10 and 21GPa; Litasov et al., 2010, 2013; this

study), melting of carbonated eclogite may commence in

the deep upper mantle or at the very top of theTransition

Zone. This will effectively remove at least some of the car-

bonate from the system at depths within the upper part of

theTransition Zone.

Stability of carbon-bearing phases in thedeep mantle

Most experimental studies of carbonate stability in the

mantle show that at 5^9GPa dolomite breaks down to ara-

gonite plus magnesite (Martinez et al., 1996; Luth, 2001;

Sato & Katsura, 2001; Buob et al., 2006; Morlidge et al.,

2006). Although there is poor agreement regarding where

this reaction occurs in the CaO^MgO^CO2 system at

lower pressures, most experimental studies place it around

12008C at 9GPa. This study demonstrates that this reac-

tion for compositionally complex natural basaltic compos-

itions at 9GPa occurs at temperatures similar to those in

simplified compositions (i.e. between 1200 and 13008C).

At temperatures below 13008C, pure aragonite and magne-

site are present, whereas at higher temperatures calcite^

magnesite solid solution is observed. In more complicated

Fig. 8. Compositions of crystalline carbonate, melts, garnet and clinopyroxene.


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Na- and K-bearing systems at pressures 513GPa, alkali-

rich carbonates either may coexist with magnesite (as

inferred from this study) or may form a solid solution

between alkali-rich and calcite-rich carbonates (�13GPa;

this study) with an estimated composition of

(Na,K)2Ca(CO3)2^CaCO3 or (Na,K)2Ca2(CO3)3^

CaCO3. The amount of K- and Na- component in Na-rich

aragonite is buffered by coexisting K-hollandite and clino-

pyroxene (Grassi & Schmidt, 2011a, 2011b).

Despite some variation in the Na vs K content of Na-rich

aragonite, the proportions of alkali components relative

to (CaþMgþFe) remain constant from 13 to 21GPa

(Fig. 8). The fluctuations observed at 17GPa are within

analytical error, given that Na content may be underesti-

mated during SEM analysis. This may indicate (1) that

the capacity of aragonite to incorporate alkali components

or to create a solid solution with alkali-carbonates is lim-

ited and does not depend on pressure, and (2) the forma-

tion of an alkali-bearing carbonate with a different

structure.

The detailed study of the structure of this alkali-bearing

carbonate is beyond the scope of this study. In any case,

both hypotheses are commensurate with the observation

of Na-carbonate crystals Na2(Ca,Mg,Fe)(CO3)2 (Fig. 7c)

in run V1200-21 coexisting with Na-rich aragonite. The

excess of Na and K that could not be incorporated into

majoritic garnet or alkali-rich carbonate triggered crys-

tallization of additional Na-carbonate under subsolidus

conditions. Recently, Na^Ca carbonate containing

10·1^11·0wt % Na2O and 34·4^38·6wt % CaO was re-

ported by Grassi & Schmidt (2011b) in a carbonate-bear-

ing marine sediment bulk composition at 16^23·5GPa and

1200^14008C, and more Na-rich carbonate with c.

20·8wt % Na2O and c. 36 wt % CaO crystallized at

22^23·5GPa and 1350^14008C. (K,Na)2Ca4(CO3)5 and

(K,Na)2(Mg,Fe,Ca)(CO3)2 carbonates formed at 21GPa

in alkali carbonatite starting mixture have been reported

by Litasov et al. (2013). Similar K2Mg(CO3)2 carbonate

has also been synthesized at 8GPa and 12008C in the

study of Brey et al. (2011).

Another important observation is that alkali-rich carbon-

ates tend to form a solid solution with calcium carbonate

rather than magnesite. The apparent absence of magnesite

at 21GPa could be the result of incorporation of all the Mg

into majorite and Na-rich aragonite. Unlike the previous in-

terpretation that with increasing pressure to theTransition

Zone and lower mantle magnesite remains the only stable

carbonate (Takafuji et al., 2006; Isshiki et al., 2004; Litasov,

2011), there is apossibility that ifCa-bearing rocks of eclogitic

paragenesis are also present at that depth, aragonite (or

alkali-bearing aragonite in the case of high bulk alkali con-

tents)maybecome stable aswell.

The complex phase relations of carbonates at upper-

mantle^Transition Zone pressures are made more complex

by the presence of diamond or graphite. To our knowledge,

this is the first experimental demonstration of diamond

crystallization in a carbonated MORB composition. The

diamond aggregates coexist with either compositionally

variable crystalline carbonates (Fig. 9) or carbonate melts

(Fig. 7b). The fact that diamonds crystallized only in the

GA1cc bulk composition and not the Volga-cc bulk com-

position is of particular interest. A possible explanation

for diamond crystallization in GA1cc is the oxidation of

ferrous iron in silicate garnet as a consequence of the

increased stability of the andradite component (Simakov,

2006). However, the composition and modal proportions

of garnet in GA1cc and Volga-cc at 17^21GPa are almost

the same.

Unfortunately, it was not possible to directly measure

Fe3þ in the garnets from this study, although their cation

sums are consistent with most, if not all, iron being present

as Fe2þ. Another reason would be partial contamination

of the sample by hydrogen derived from cell assembly

parts during the experiment. This is not likely because

then diamond formation would have been expected in

both compositions, which is not the case.

Fig. 9. Carbonate inclusions in diamond or graphite. SEM images ofG1200-9 run.


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The following reaction governing diamond formation in

eclogite systems has been proposed by Luth (1993):

CaMg CO3ð Þ2þ2SiO2 ¼ CaMgSi2O6 þ 2CþO2

dolomiteþ coesite ¼ diopsideþ diamondþO2

Applying this reaction to the subsolidus experiments at

9GPa, in which magnesite and aragonite coexist, results in

CaCO3 þMgCO3 þ 2SiO2 ¼ CaMgSi2O6 þ 2CþO2:

The equilibrium constant for this reaction is

K ¼aCaMgSi2O6

� a2C � fO2

aCaCO3� aMgCO3

� a2SiO2

:

Given the presence of both coesite or stishovite and car-

bonates in all the experiments, it is possible to conclude

that the denominator can be assumed to be unity for

almost pure carbonate components. Because GA1cc and

Volga-cc compositions are run simultaneously (in the

same experiment), we assume that the oxygen fugacity in

the two charges is the same.Thus, diamond versus carbon-

ate stability will depend on the activity of the diopside

component. The fact that the diopside component in

GA1cc clinopyroxene from subsolidus experiments at

9GPa is �5^10mol % lower than theVolga-cc clinopyrox-

ene at the same P^Tconditions is consistent with this ana-

lysis. However, the influence of clinopyroxene composition

on diamond formation at constant oxygen fugacity needs

further experimental investigation.

Melting of carbonated eclogite in the deepmantle

There is general agreement, based on experimental studies

and on studies of natural rocks, that substantial amounts

of carbonate survive subduction beyond the sub-arc

regime and into the deeper upper mantle (Kerrick &

Connolly, 2001; Dasgupta & Hirschmann, 2010). Our re-

sults agree with previous studies and demonstrate that the

solidus of the carbonated mafic component of the subduct-

ing slab is at temperatures above most subduction

Fig. 10. Compositions of experimental melts as a function of pressure.(a) Ca#. (b) Mg#.

Fig. 11. Compositions of experimental melts as a function of pressure.(a) Molar Na/K ratio. (b) (NaþK) in mol %. (GrtþCpx) and Grtindicate stability field of the phases coexisting with melts.


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geotherms in the upper mantle, although it may intersect

subduction zone geotherms in the Transition Zone

(Fig. 12). Unlike all previous studies on MORB-like com-

positions byYasuda et al. (1994),Wang & Takahashi (1999),

Hirose & Fei (2002), Keshav & Gudfinnsson (2010) and

Litasov & Ohtani (2010), the solidi of GA1cc and Volga-cc

are significantly below the mantle adiabat, at least to

21GPa.

Low-velocity seismic anomalies on top of the 410 km dis-

continuity in the vicinity of subducted slabs have been re-

ported in some geophysical studies (Revenaugh & Sipkin,

1994; Song et al., 2004). Low P-wave velocity zones have

also been observed above the subducted Pacific slab along

almost its entire descending path into theTransition Zone,

in the depth range of 250^500 km (Zhao & Ohtani, 2009).

One possible interpretation of this may be partial melting

of the slab associated with dehydration, decarbonation or

their combined effects. Being located at the top and there-

fore hotter region of the subducted slab, carbonate-rich

(and perhaps some H2O-bearing) rocks are most likely to

be first in the melting sequence. On melting, they may

yield alkali-rich calcio-dolomitic melts across the whole

range of investigated pressures. The concentration of

alkali components in these melts will be dependent on

their content in the bulk-rock, on Na and K compatibil-

ities, and on the degree of partial melting.

Na and K compatibilities in MORB mineral phases

(garnet and clinopyroxene) at higher pressures have been

addressed by many experimental studies (Wang &

Takahashi, 1999; Spandler et al., 2008; Ghosh et al., 2009).

Sodium can be incompatible, partitioning into the melt

relative to clinopyroxene at fairly low pressures of

�53GPa (e.g. Blundy & Dalton, 2000; Dasgupta et al.,

2005; Yaxley & Brey, 2004). It may also be incompatible at

pressures beyond the clinopyroxene stability field (i.e.

P4�15GPa). However, between �4^5 and �15GPa,

sodium is compatible in clinopyroxene because of the high

stability of jadeite. Therefore low-degree, highly sodic

melts are unlikely to form in carbonate eclogite in the

depth range of 90^400 km. Given that the subducting

mafic oceanic crust is expected to have Na/K41, K-rich

and Na-poor low-degree melts may form in this

90^400 km depth range. Conversely, at Transition Zone

pressures, the melts are likely to have higher Na/K ratios

because of the disappearance of clinopyroxene from the

system.

Fig. 12. Comparison of GA1cc and Volga-cc solidi with other experimental studies and mantle and subduction geotherms (field marked withbold lines). OG, oceanic geotherm; CG, cratonic geotherm. Grey field indicates an approximate mantle adiabat with an average ofTp¼13158C (McKenzie & Bickle, 1988; McKenzie & O’Nions, 1991). Black and coloured lines mark the following solidi: WT, solidus of dryeclogite byWang & Takahashi (1999); KG, solidus of carbonated eclogite by Keshav & Gudfinnsson (2010); H, solidus of carbonated eclogiteby Hammouda (2003); D, solidus of carbonated eclogite by Dasgupta et al. (2004); YB, solidus of carbonated eclogite byYaxley & Brey (2004);LO, solidus of carbonated eclogite by Litasov & Ohtani (2010).With light grey lines the following solidi are marked: D, solidus of carbonatedperidotite by Dasgupta & Hirschmann (2006); L, solidus of carbonated peridotite by Litasov & Ohtani (2009b). Subduction geotherms are com-piled from van Keken et al. (2002) and Syracuse et al. (2010). Other labels as in Fig. 2.


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Thus, if partial melting of carbonate begins at

250^300 km depth (7·5^9GPa) the melts produced in the

top part of the subducted slab would have K-rich carbonate

compositions. These melts will segregate easily from the

eclogitic residue owing to their low density and low viscos-

ity, removing potassium and other incompatible elements

from the residual slab. The next pulse of partial melting

will most probably start at the eclogite^garnetite trans-

formation, which in carbonated eclogite systems will

begin near the top of the Transition Zone (about

400^450 km depth), corresponding to pressures of around

14^15GPa. Low-degree melts are expected to be extremely

Na-rich, consistent with the sodic nature (14·8wt %

Na2O, Na2O/K2O¼ 4·09) of the low-degree partial melt

(�7·5%) in experiment V1200-17. It is likely that most po-

tassium has been removed from the system before these

depths, during the lower pressure melting pulse described

above. Being of low viscosity, these Na-rich melts will rap-

idly segregate from the garnetite residue, metasomatizing

the surrounding peridotite, or even fluxing its partial melt-

ing, or forming diamond by ‘redox freezing’ (Rohrbach &

Schmidt, 2011). It should be noted that the eclogite^garne-

tite transformation occurs at different pressures depending

on bulk-rock composition.

After considerable partial melting and removal of alkalis

from the carbonated eclogite system, some carbon may

still be preserved in the rock as diamond. This argument

is supported by the formation of diamonds at 17GPa in

equilibrium with alkali-rich carbonate melt, and by the

presence of carbonate inclusions in diamonds from the

Transition Zone^lower mantle (Brenker et al., 2007). The

amount of diamond preserved in the rock will depend on

the bulk-rock composition and on oxygen fugacity.

Overall, we conclude that alkali-bearing carbonated

eclogite in the subducted slab will lose most of its volatile,

carbonate and alkali components during multiple partial

melting events upon descent, leaving only refractory eclog-

ite with small amounts of carbon (stored as diamond) and

water (in normally anyhdrous minerals) as it approaches

the Transition Zone and the top of the lower mantle. This

assumption is supported by low Na contents (�1·5wt %

Na2O) in majorite inclusions in diamonds (Collerson

et al., 2010) although at the pressures of majorite stability

this phase is capable of holding much higher amounts of

Na (Yasuda et al., 1994; Litasov & Ohtani, 2005).

Findings of various high-pressure minerals as inclusions

in natural diamonds (Harte, 2010) reinforce the results of

this study. Here we show that diamonds can crystallize

from Ca-rich carbonate melts (with various amount of

alkali components) that are produced by low-degree melt-

ing of carbonated MORB at all the pressures from 5 to

21GPa (Kiseeva et al., 2012; this study). These melts are

compositionally similar to those proposed as parental to

the Udachnaya-East kimberlites (Kamenetsky et al., 2004;

Litasov et al., 2010; Sharygin et al., 2013) and other kimber-

lites worldwide (Kamenetsky et al., 2009), and can also be

involved in the carbonatite magmatism.

CONCLUSIONS

We have investigated experimentally the melting and

phase relations of two MORB eclogite compositions with

4·4% CO2, at temperatures of 1100^19008C and pressures

of 9^21GPa. The solidus temperatures are above the sub-

duction geotherm but below the estimated mantle adiabat.

(1) The main subsolidus mineral assemblage consists of

garnet, coesite or stishovite, clinopyroxene (9, 13GPa)

and carbonate. Over the range of P^Tconditions stu-

died, carbon-bearing phases include the following:

magnesite and aragonite or calcite^magnesite solid

solution (similar to dolomite composition) at 9GPa;

magnesite and aragonite (GA1cc) or magnesite and

Na-rich aragonite (Volga-cc) at 13GPa; magnesite

and Na-rich aragonite at 17GPa; Na-rich aragonite

(GA1cc, Volga-cc) and Na-carbonate (Volga-cc) at

21GPa; diamond or graphite at 9^21GPa (GA1cc).

(2) Na-rich aragonite is an alternative to clinopyroxene as

a host for K and Na at pressures greater than 13GPa.

(3) In the Volga-cc bulk composition, the solidus curve is

almost flat and falls between 1200 and 13008C over

the entire investigated pressure range. In the GA1cc

bulk composition, the solidus is located at similar

temperatures at 9^13GPa, but lies at higher tempera-

tures (1300^15008C) at 17^21GPa.

(4) The difference in solidi between the GA1cc andVolga-

cc bulk compositions is related to a change in Na com-

patibility between 13 and 17GPa, owing to the dis-

appearance of omphacitic clinopyroxene, resulting in

the formation of Na-bearing carbonate in the Volga-

cc, to carbonate reduction and diamond precipitation,

induced either by progressive Fe2þ^Fe3þ transition in

garnet with pressure or by influence of the diopside

component in the clinopyroxene, in the GA1cc bulk

composition.

(5) Low-degree melts in both compositions are alkali-

rich. The amount of alkalis in the melts increases sig-

nificantly with pressure, and is buffered by the pres-

ence of clinopyroxene and K-hollandite in the system.

(6) Two melting pulses are proposed for subducted slabs

carrying carbonated eclogite in their upper sections.

The first melt pulse at �250^300 km depth, or

�8^9GPa, will produce K-rich carbonatite melts,

whereas the second melt pulse (near the top of the

Transition Zone, at �400^450 km depth, or

�14^15GPa) will produce very Na-rich carbonatite

melts. Some of the carbon will still survive in the

form of diamond and graphite.


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(7) Because of their low viscosity, the resulting carbona-

tite melts are assumed to segregate from the main

eclogite body at depths above the Transition Zone,

allowing refractory carbon-bearing eclogite to be

stored in the Transition Zone or lower mantle. These

melts can be involved in generation of such magmas

as kimberlitic or carbonatitic

ACKNOWLEDGEMENTSWe wish to thank Hugh O’Neill and Robert Rapp for valu-able suggestions on improving this paper. We also thankRaj Dasgupta and Oleg Safonov for constructive reviews.The experimental work was performed during Internshipto K.K. as a part of the 21st Century Center-of-Excellenceprogram ‘Advanced Science and Technology Center forthe Dynamic Earth’ at Tohoku University. The authorsgratefully acknowledge Hidenori Terasaki for his help insetting up experiments. Karsten Goemann (CentralScience Laboratory at University of Tasmania) is thankedfor assistance with electron microprobe analyses. FrankBrink and Hua Chen (Centre for Advanced Microscopyat ANU) assisted with the SEM analyses.

FUNDING

E.S.K. was funded by an ANU Postgraduate Scholarship.

The research was partly funded by an Australian

Research Council Discovery Grant to G.Y.

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Documents

Melting and Phase Relations of Carbonated Eclogite at 9-21 GPa and the Petrogenesis of Alkali-Rich Melts in the Deep Mantle