Carbonate melt interaction with natural eclogite at 6 GPa and 1100–1200 °C: Implications for metasomatic melt composition in subcontinental lithospheric mantle

doi:10.1016/j.chemgeo.2020.119915

Chemical Geology

Volume 558, 30 December 2020, 119915

https://doi.org/10.1016/j.chemgeo.2020.119915 Get rights and content

Abstract

Compositional ranges of carbonate melts stable under P-T conditions corresponding to the base of subcontinental lithospheric mantle (SCLM) are limited by alkali-rich near-eutectic compositions. In the present work, we investigated the interaction of such melts with the natural eclogite of Group A. It was found that the interaction is accompanied by decreasing Ca# in the melt (L) and increasing Ca# in garnet (Grt) according to the reaction: 3CaCO₃ (L) + Mg₃Al₂Si₃O₁₂ (Grt) = 3MgCO₃ (Mgs and/or L) + Ca₃Al₂Si₃O₁₂ (Grt), where Mgs is magnesite. The interaction with the Na-Ca-Mg-Fe carbonate melt increases amount of jadeite component in clinopyroxene (Cpx) consuming Al₂O₃ from garnet and Na₂O from the melt according to the reaction: Na₂CO₃ (L) + CaCO₃ (L) + 2Mg₃Al₂Si₃O₁₂ (Grt) + 2CaMgSi₂O₆ (Cpx) = 2NaAlSi₂O₆ (Cpx) + Ca₃Al₂Si₃O₁₂ (Grt) + 2MgCO₃ (Mgs, L) + 3Mg₂SiO₄ (Ol). As a result, garnet and omphacite compositions evolve from eclogite Group A to eclogite Group B. A byproduct of the reaction is olivine (Ol), which may explain the formation of inclusions of “mixed” eclogite (garnet + omphacite) and peridotite (olivine) paragenesis in lithospheric diamonds. The interaction with the K-Ca-Mg-Fe carbonate melt increases the K₂O content in clinopyroxene to 0.5–1.2 wt%, while the Na₂O content lowers to 0.3 wt%. The following range of carbonatite melt compositions can be in equilibrium with eclogite at the base of SCLM (1100–1200 °C and 6 GPa): 18(Na_0.97K_0.03)₂CO₃∙82(Ca_0.63Mg_0.30Fe_0.07)CO₂–42(Na_0.97K_0.03)₂CO₃∙58(Ca_0.46Mg_0.45Fe_0.09)CO₂. Our results also suggest that the partial melting of ‘dry’ carbonated eclogite, if any, at 1100 °C and 6 GPa yields the formation of a carbonate melt with the following composition (mol%) 25(Na_0.96K_0.04)₂CO₃∙75(Ca_0.64Mg_0.31Fe_0.05)CO₂, corresponding to 18–27 wt% Na₂O in the melt on a volatile-free basis.

Introduction

Many eclogite xenoliths carried by kimberlites from the base of the subcontinental lithospheric mantle (SCLM) exhibit a certain degree of enrichment with incompatible elements, attributed to the mantle metasomatism by ephemeral melts/fluids (Taylor and Neal, 1989) or in-situ partial melting (Kiseeva et al., 2017). X-ray 3D tomography of mantle xenoliths revealed that diamonds in eclogites grew in zones of later metasomatic alteration (Taylor et al., 2000; Taylor and Anand, 2004). The detailed study of these diamonds revealed the presence of numerous inclusions of CaCO₃, whereas dolomite was found among secondary minerals filling the intergranular space surrounding diamonds (Shatsky et al., 2008). This indicates the carbonatitic composition of metasomatic melt and implies that carbonate melts can coexist with eclogite under geothermal conditions typical for the base of SCLM, which are 1100–1200 °C at 6 GPa (Pollack and Chapman, 1977).

However, the experimental proofs for this possibility are controversial. Dasgupta et al. (2004) established a silica-poor carbonate melt on the eclogite-CO₂ solidus at 1080 °C and 6.1 GPa, while Yaxley and Brey (2004) observed no melting up to 1310 °C at 5.0 GPa. Earlier, Hammouda (2003) reported that the eclogite-CO₂ solidus temperature varying from 1125°С at 5 GPa to 1250 °C at 6 GPa, and to 980 °C at 6.5–7.0 GPa. Later, Kiseeva et al. (2012) studied the eclogite-CO₂ system with the higher K₂O content and found that the melting starts at 1050 °C at 5 GPa and incipient melt has potassic aluminosilicate composition, whereas the immiscible carbonate melt appears at 1100 °C. Thus, the minimum temperature of a carbonatite melt stability in eclogitic mantle domains, and hence the fundamental possibility of their stability at the base of SCLM, remain highly uncertain.

The most critical problem is that the compositions of carbonatite melt established at 5–7 GPa (Hammouda, 2003; Kiseeva et al., 2012) cannot be a liquid under the temperature conditions of SCLM, because they are below the solidus of the corresponding carbonate system (Fig. 1). Therefore, the compositional range of metasomatic carbonatite melts, which are true liquids under SCLM geothermal conditions and can coexist with eclogite, needs to be clarified.

The present experiments at 6 GPa and 1100–1200 °C address the above problems by studying the interaction of natural eclogite with the eutectic Na-Ca-Mg-Fe and K-Ca-Mg-Fe carbonate melts, defined in our previous studies (Arefiev et al., 2019b; Podborodnikov et al., 2019a).

Section snippets

Starting materials

Starting materials were prepared by blending synthetic carbonate mixtures and powder of natural eclogite. The powder of fresh eclogite xenolith (UD-45-02) from the Udachnaya kimberlite pipe (Yakutia, Russia) was provided by A.L. Ragozin and V.S. Shatsky (Ragozin et al., 2014). The clinopyroxene and garnet compositions given Table S1 in Supplementary data correspond to Group A eclogite after Taylor and Neal (1989). The bulk major-element composition was determined using X-ray fluorescence and

Phase relations

Backscattered electron (BSE) images of the sample cross-sections are shown in Fig. 2, Fig. 3, Fig. 4, Fig. 5. As can be seen, in samples with the eutectic carbonate mixtures (N2, K2, and K4), the carbonate melt segregates as a layer at the high-temperature (HT) side, while silicate residue recrystallizes to the low-temperature (LT) side independently on the direction of the gravity vector (Figs. 2h and 4). Although the amount of melt in the non-eutectic carbonate mixtures (N4 and K5) is

Eclogite interaction with eutectic carbonate melts

The carbonate melts derived by partial melting of oceanic slabs are alkali- and Ca-rich (Grassi and Schmidt, 2011a; Thomson et al., 2016) similar to the eutectic melts in the Na₂CO₃-CaCO₃-MgCO₃ and K₂CO₃-CaCO₃-MgCO₃ systems (Arefiev et al., 2019a; Podborodnikov et al., 2019b). The melting of hydrothermally altered basalts and recycled sedimentary rocks (carbonate-bearing metapelites) subducted to the mantle transition zone yields Na-rich carbonate melt (Grassi and Schmidt, 2011b; Thomson et

Conclusions

1. Interaction of eutectic Na-Ca-Mg-Fe or K-Ca-Mg-Fe carbonatite melt with natural eclogite at 1100–1200 °C and 6 GPa is accompanied by increasing Ca# in the garnet and decreasing Ca# in the melt according to the reaction: 3CaCO₃ (L) + Mg₃Al₂Si₃O₁₂ (Grt) = 3MgCO₃ (Mgs, L) + Ca₃Al₂Si₃O₁₂ (Grt).

2. The interaction with the Na melt increases jadeite in clinopyroxene consuming Al₂O₃ from garnet and Na₂O from the melt according to the reaction: Na₂CO₃ (L) + CaCO₃ (L) + 2Mg₃Al₂Si₃O₁₂ (Grt) + 2CaMgSi₂O₆

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We are grateful to O.G. Safonov and anonymous reviewer for constructive comments, D. Dingwell for editorial handling; V.S. Shatsky and A.L. Ragozin for providing natural eclogite powder; N.V. Sobolev for discussion; A.T. Titov, M.V. Khlestov, and N.S. Karmanov for assistance in SEM analyses. AS, AB, and IVP were supported by RFBR No. 20-05-00811. AVA was supported by RFBR No. 19-35-90082. KDL was supported by the state assignment of IHPP RAS.

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Change in carbonate budget and composition during subduction below metal saturation boundary
2023, Geoscience Frontiers
It is generally accepted that carbonates can be subducted to the mantle depths, where they are reduced with iron metal to produce a diamond. In this work, we found that this is not always the case. The mantle carbonates from inclusions in diamonds show a wide range of cation compositions (Mg, Fe, Ca, Na, and K). Here we studied the reaction kinetics of these carbonates with iron metal at 6–6.5 GPa and 1000–1500 °C. We found that the reduction of carbonate with Fe produces C-bearing species (Fe, Fe-C melt, Fe₃C, Fe₇C₃, C) and wüstite containing Na₂O, CaO, and MgO. The reaction rate constants (k = Δx²/2t) are log-linear relative to 1/T and their temperature dependences are determined to be
k_MgCO3 (m²/s) = 4.37 × 10⁻³ exp [−251 (kJ/mol)/RT]
k_CaMg(CO3)2 (m²/s) = 1.48 × 10⁻³ exp [−264 (kJ/mol)/RT]
k_CaCO3 (m²/s) = 3.06 × 10⁻⁵ exp [−245 (kJ/mol)/RT] and
k_Na2CO3 (m²/s) = 1.88 × 10⁻¹⁰ exp [−155 (kJ/mol)/RT].
According to obtained results at least, 45–70 vol% of carbonates preserve during subduction down to the 660-km discontinuity if no melting occurs. The slab stagnation and warming, subsequent carbonate melting, and infiltration into the mantle saturated with iron metal are accompanied by a reduction of carbonate melt with Fe. The established sequence of reactivity of carbonates: FeCO₃ ≥ MgCO₃ > CaMg(CO₃)₂ > CaCO₃ ≫ Na₂CO₃, where K₂CO₃ does not react at all with iron metal, implies that during reduction carbonate melt with Fe evolves toward alkali-rich. The above conclusions are consistent with the findings of carbonates in inclusions in diamonds from the lower mantle and high concentrations of alkalis, particularly K, in mantle carbonatite melts entrapped by diamonds from kimberlites and placers worldwide.
Towards composition of carbonatite melts in peridotitic mantle
2022, Earth and Planetary Science Letters
Citation Excerpt :
The K-rich carbonatite melts with Ca# < 30 indicate harzburgitic, 30-34 – lherzolitic, and > 34 wehrlitic wall-rock composition (Fig. 5). The melts obtained in equilibrium with wehrlitic assemblage here and in (Shatskiy et al., 2020c) resemble the melts established in the eclogite-carbonate system (Shatskiy et al., 2020b) and carbonated pelite system (DG2) in terms of Ca# (Grassi and Schmidt, 2011; Shatskiy et al., 2019) (Fig. S2f). It was previously shown that a K-rich carbonate melt is thermodynamically stable in the SCLM at a depth of 200 km (Arefiev et al., 2019) and can appear in coexistence with orthopyroxene, clinopyroxene, and olivine (Shatskiy et al., 2021).
It is generally accepted that carbonatite metasomatism in the subcontinental lithospheric mantle (SCLM) inevitably causes wehrlitization of the primary lherzolite substrate. However, the K-rich carbonatite inclusions in kimberlitic diamonds containing orthopyroxene indicate that this is not always the case. In the present study, we equilibrated natural garnet lherzolite with carbonate melts containing 33–38 wt% K₂O with various Ca# = 10, 20, 30, and 40 at 6 GPa and 1200–1500 °C, where Ca# = 100⋅Ca/(Ca+Mg+Fe). The original ratio of peridotite to carbonate was 58 to 42 by weight. In the studied temperature range, the melt retains essentially carbonate composition with silica content increasing from 1 to 11–12 wt%. The melt with Ca# 10 alters lherzolite to harzburgite, replacing clinopyroxene by orthopyroxene and decreasing CaO content in garnet below 4 wt%. The melts with Ca# 20-30 also consume clinopyroxene; although CaO content in garnet remains in the range of lherzolitic compositions. The melt with Ca# 40 yields wehrlitization, consuming orthopyroxene, increasing clinopyroxene fraction, and increasing CaO content in garnet above 6 wt%. After the interaction, the Ca# of the melt changes as follows 10 → 16–28, 20 → 20–33, 30 → 27–34, and 40 → 30–34. The olivine + orthopyroxene + clinopyroxene + garnet assemblage was found in equilibrium with carbonatite melt with Ca# 34 at 1200 °C and Ca# 30 at 1400 °C. Thus, K-rich (26–35 wt% K₂O) carbonatite melts with Ca# = 30-34 can appear in equilibrium with garnet lherzolite, while the melts with Ca# < 30 and > 34 can be in equilibrium with harzburgite and wehrlite, respectively, at 6 GPa and 1200–1400 °C. Considering that Ca-Mg-Fe carbonates do not melt at the geothermal conditions of the SCLM, while sodic, dolomitic melt causes wehrlitization, high-Mg (Ca# < 35) K-rich dolomitic melt is the only possible carbonatite fluids that are thermodynamically stable in equilibrium with garnet harzburgites and lherzolites in the SCLM at a depth of about 200 km. At higher temperatures corresponding to the underlying asthenosphere, the high alkalinity ceases to be a requirement for the stability of the carbonate melt. Nevertheless, the regularities established here for the K-rich melts remain valid for less alkaline (4–15 wt% Na₂O+K₂O) primary kimberlite (i.e., mantle carbonatite) melts in the sublithospheric mantle.
Solidus of carbonated phlogopite eclogite at 3–6 GPa: Implications for mantle metasomatism and ultra-high pressure metamorphism
2022, Gondwana Research
Citation Excerpt :
Representative pieces of xenoliths were ground with alcohol in a tungsten carbide mortar. The bulk major-element compositions were determined using X-ray fluorescence and verified using the energy-dispersive X-ray spectra (EDX) analysis of a polished tablet made of the eclogite powder (Table 1) (Shatskiy et al., 2020a). The ground xenolith was blended with synthetic mixtures modeling hydrous carbonatite melt.
The interaction of natural eclogite (Ecl) with synthetic hydrous carbonate melts with Na:K = 0:1 (KH2) and 1:1 (NKH2) was studied in multianvil experiments at 3–6 GPa and 850–1250 °C. The interaction with KH2 consumes garnet and clinopyroxene producing phlogopite and calcite-dolomite solid solution. Besides, the interaction yields a decrease in the jadeite component of clinopyroxene, evolving eclogite toward pyroxenite. This is consistent with a metasomatic alteration of eclogite xenoliths, manifested as Na-poor “spongy” clinopyroxene, replacing primary omphacite, and kelyphitic rims around garnet, containing phlogopite and carbonates. The interaction with NKH2 also produces phlogopite and carbonate, but the latter is more magnesian and represented by magnesite, above the solidus, and magnesite + dolomite below the solidus. The interaction with NKH2 increases the jadeite component in clinopyroxene and grossular component in garnet, evolving eclogite Group A to eclogite Group B.
The studied systems have H₂O/K₂O = 2, like that in phlogopite, and therefore correspond to carbonated phlogopite eclogite under fluid-absent conditions. Based on the obtained results its solidus is situated near 1050 °C at 3 GPa and decreases to 950 °C at 6 GPa. Thus, hydrous K- and Na-K-carbonatite melts can coexist with eclogite in SCLM at depths exceeding 120–170 km, and solidify as temperature decreases below 950–1050 °C according to the following solidus reactions: pyrope + diopside + melt → phlogopite + dolomite, below 6 GPa, and pyrope + diopside + melt → phlogopite + magnesite + grossular, at 6 GPa.
The melting reaction, involving phlogopite and dolomite, suggests the partial melting at the peak of ultrahigh-pressure metamorphism (UHPM) during continent–continent plate collision. The prograde P-T path of UHPM crosses the solidus of clinopyroxene + garnet + phlogopite + dolomite assemblage at 4.7–5.2 GPa and 970–990 °C and yields the formation of hydrous K-carbonatite melt-fluid in situ. This melt could be responsible for the formation of K-bearing clinopyroxenes and microdiamonds in the UHPM marbles in the Kokchetav massif, Kazakhstan. The retrograde P-T path intersects the solidus that has a negative Clapeyron slope in the diamond stability field. Thus, the hydrous K-carbonatite melt should disappear soon after the peak of metamorphism reacting with garnet to produce Ca-Mg carbonates and phlogopite.
Solidus and melting of carbonated phlogopite peridotite at 3–6.5 GPa: Implications for mantle metasomatism
2022, Gondwana Research
Citation Excerpt :
Representative pieces of xenolith were ground with alcohol in a tungsten carbide mortar. The bulk major-element composition was determined using X-ray fluorescence and verified using the energy-dispersive X-ray spectra (EDX) analysis of a polished tablet made of the lherzolite powder (Tables 1, S1) (Shatskiy et al., 2020c; Shatskiy et al., 2020b). Ground xenolith was blended with synthetic mixtures modeling mantle carbonatitic melt.
It is well known that water significantly lowers mantle solidi. But it turns out this paradigm is not always true. Here, we studied the interaction of K-rich carbonate melts with the natural garnet lherzolite from the Udachnaya kimberlite (Russia) in the presence of water at 3.0–6.5 GPa, corresponding to depths of 100–200 km. We found that at ≤ 1100 °C, the metasomatic interaction consumes garnet, orthopyroxene, and melt to produce phlogopite ± K-richterite + magnesite ± dolomite. Besides, primary clinopyroxene is replaced by one with a lower amount of jadeite component. Thus, the addition of water to the K-rich carbonate melt, infiltrating the subcontinental lithospheric mantle, should yield its partial or complete disappearance accompanied by phlogopitization and carbonation.
The studied systems have H₂O/K₂O = 2, like that in phlogopite, and therefore correspond to carbonated phlogopite peridotite under fluid-absent conditions. At 4.0–6.5 GPa, the solidus of carbonated phlogopite peridotite is controlled by the following reaction: phlogopite + clinopyroxene + magnesite = garnet + orthopyroxene + olivine + hydrous K-carbonatite melt, which is bracketed between 1100 and 1200 °C. At 3 GPa, the solidus temperature decreases to about 1050 °C presumably owing to the Ca-Mg exchange reaction, clinopyroxene + magnesite = orthopyroxene + dolomite, which stabilizes dolomite reacting with phlogopite at a lower temperature than magnesite.
Our results suggest that the phlogopite- and carbonate-rich metasomatic vein networks, weakening rigid lithosphere and promoting continental rifting, could be formed as a result of infiltration of hydrous K-carbonatite melt at the base of subcontinental lithospheric mantle. Stretching and thinning of the cratonic lithosphere make geotherms warmer and shifts their intersections with the solidus of carbonated phlogopite peridotite to shallower depths. Consequently, the successive erosion of the continental lithosphere and ascent of the lithosphere-asthenosphere boundary during continental rifting should be accompanied by remelting of phlogopite-carbonate metasomes, upward percolation of K-rich melt, and its solidification at the front of the magmatic-metasomatic mantle system.
Phlogopite-rich and phlogopite-poor kimberlite intrusions within the Du Toitspan kimberlite pipe, South Africa: Petrogenetic relationships and localised source heterogeneity
2021, Lithos
Citation Excerpt :
This conclusion is supported by the fact that a ubiquitous feature of all Group I kimberlites is a strong relative depletion in K (compared to other highly incompatible elements), (e.g. Becker and le Roex, 2006; Giuliani et al., 2020; le Roex et al., 2003) which likely requires involvement of a K-bearing phase during their petrogenesis. Since neither phlogopite nor K-richterite are known to be stable at P-T conditions of the asthenospheric mantle beneath continental keels, and relative K-depletion is not a feature of direct melting of the asthenospheric mantle, involvement of a K-bearing phase residing in the lithospheric mantle is a reasonable assumption and one that is experimentally supported (e.g. Brey et al., 2011; Shatskiy et al., 2020). Consequently, we investigate below whether the different geochemical characteristics of the D13 and D14 kimberlites could be accounted for through partial melting processes occurring within the underlying metasomatized lithospheric mantle.
Samples from three petrographically distinct, intrusive kimberlite bodies and associated kimberlite dykes from the eastern lobe of the Du Toitspan kimberlite pipe, Kimberley, South Africa, have been analysed for their bulk rock major and trace element compositions and their olivine and phlogopite compositions. The two dominant intrusive bodies (D13, D14) are distinguished by the one (D13) being phlogopite-rich and best classified as a macrocrystic hypabyssal phlogopite kimberlite, and the other (D14) being phlogopite-poor and best classified as a macrocrystic hypabyssal monticellite kimberlite. The minor D17 intrusive body is classified as a macrocrystic transitional hypabyssal serpentinized phlogopite kimberlite. The associated kimberlite dykes range texturally from aphanitic to macrocrystic and are classified as calcite kimberlites.
The major kimberlite intrusions and their associated dykes show no evidence of crustal contamination and are characterised by broadly overlapping geochemistry except for distinctly higher K₂O (> 2 wt%) and Al₂O₃ (>3 wt%) and flattening HREE patterns (Gd/Yb_N = 6.5–7.0) in the D13 – phlogopite kimberlite compared to the D14 – monticellite kimberlite and the calcite kimberlite dykes (Gd/Yb_N = 9.6–12.1). These distinguishing geochemical features of the D13 – phlogopite kimberlite are comparable to typical Group II kimberlites in southern Africa. However, their diagnostic incompatible trace element ratios (for example, Th/Nb, La/Nb, Ce/Pb, and Ba/Nb) are instead comparable to other kimberlite intrusions analysed in this study and to southern African Group I kimberlites in general.
Semi-quantitative modelling suggests that these kimberlite intrusions could have derived by low (<1%) degrees of partial melting of a source region that is enriched in LREE (La_n = ~6.1; Yb_n ~ 1.47) comparable to metasomatised peridotites from the underlying lithospheric mantle. The composition of the D13 phlogopite kimberlite is consistent with a partial melt of a modally metasomatised source containing a higher proportion of residual clinopyroxene relative to garnet (compared to that giving rise to the D14 monticellite kimberlite and calcite kimberlite dykes), as well as accessory amounts of phlogopite, i.e. a garnet phlogopite peridotite (GPP). The absence of K-anomalies on primitive mantle normalized diagrams for the D13 phlogopite kimberlite requires that phlogopite was not a residual phase during partial melting and was exhausted shortly before or at the moment of melt segregation. The higher Gd/Yb ratios and lower K₂O in the D14 monticellite kimberlite and calcite kimberlite dykes can be explained by partial melting of a cryptically metasomatized, phlogopite – free, garnet peridotite (GP) source, containing a higher proportion of garnet relative to clinopyroxene. The low absolute K and strong negative K-anomaly on primitive mantle normalized diagrams for the D14 monticellite kimberlite were inherited from a source region that previously experienced cryptic metasomatism by a differentiated fluid already carrying a negative K-anomaly.
Phase relations in carbonate component of carbonatized eclogite and peridotite along subduction and continental geotherms
2021, Gondwana Research
Citation Excerpt :
Besides, the authors also reported K2Mg and Mgs among near-solidus minerals. The results of this work are also consistent with our recent experiments on the equilibration of synthetic carbonate melts with natural eclogite and garnet lherzolite at 6 GPa and 1100 °C (Shatskiy et al., 2020a,b). The equilibration of K-Ca-Mg-Fe carbonate melt with garnet lherzolite yields melt with approximate composition [38(K0.97Na0.03)2CO3∙62(Ca0.54Mg0.37Fe0.09)CO3 + 1.1 wt% SiO2], which is similar to that established here in the “K‑carbonatite” system [35(K0.97Na0.03)2CO3∙65(Ca0.68Mg0.28Fe0.05)CO3 + 2.0 wt% SiO2].
Phase relations in the carbonate constituent of mafic and ultramafic suites are decisive in the deep carbon cycle and partial melting in the Earth's mantle. Here we present an experimental study of phase relations in the “Na‑carbonatite” and “K‑carbonatite” systems, which represent the carbonate constituent of mantle eclogites and peridotites, respectively.
The phase relations in the “Na‑carbonatite” system indicate that the carbonate constituent of the mafic suite consists of Na₂Ca₄(CO₃)₅ + dolomite + eitelite, Na₂Mg(CO₃)₂, at shallow mantle pressures and changes to Na₂Ca₄(CO₃)₅ + magnesite at depths exceeding 170 km. At the same time, the “K‑carbonatite” system indicates that the carbonate constituent of the ultramafic suite consists of K₂Mg(CO₃)₂ + dolomite + magnesite at shallow mantle pressures and changes to K₂Mg(CO₃)₂ + magnesite + aragonite at depths exceeding 170–180 km.
The “Na‑carbonatite” solidus is located near 850 °C / 3 GPa and 1100 °C / 6.5 GPa and has a Clapeyron slope of 14 MPa/°C. The “K‑carbonatite” solidus is situated near 825 °C / 3 GPa and 1000 °C / 6.5 GPa and has a Clapeyron slope of 20 MPa/°C. Both solidi are hotter than subduction geotherms. Consequently, under anhydrous conditions, the carbonate constituent of mafic and ultramafic suites can survive subduction into the deep mantle without melting. However, warming even to moderate temperatures corresponding to continental geotherms should be accompanied by partial melting yielding alkaline carbonatite melt.

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Carbonate melt interaction with natural eclogite at 6 GPa and 1100–1200 °C: Implications for metasomatic melt composition in subcontinental lithospheric mantle

Abstract

Introduction

Section snippets

Starting materials

Phase relations

Eclogite interaction with eutectic carbonate melts

Conclusions

Declaration of Competing Interest

Acknowledgments

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