TY - JOUR

T1 - Carbonate melt interaction with natural eclogite at 6 GPa and 1100–1200 °C: Implications for metasomatic melt composition in subcontinental lithospheric mantle

AU - Shatskiy, Anton

AU - Bekhtenova, Altyna

AU - Podborodnikov, Ivan V.

AU - Arefiev, Anton V.

AU - Litasov, Konstantin D.

N1 - 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.

PY - 2020/12/30

Y1 - 2020/12/30

N2 - 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(Na0.97K0.03)2CO3∙82(Ca0.63Mg0.30Fe0.07)CO2–42(Na0.97K0.03)2CO3∙58(Ca0.46Mg0.45Fe0.09)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(Na0.96K0.04)2CO3∙75(Ca0.64Mg0.31Fe0.05)CO2, corresponding to 18–27 wt% Na2O in the melt on a volatile-free basis.

AB - 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(Na0.97K0.03)2CO3∙82(Ca0.63Mg0.30Fe0.07)CO2–42(Na0.97K0.03)2CO3∙58(Ca0.46Mg0.45Fe0.09)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(Na0.96K0.04)2CO3∙75(Ca0.64Mg0.31Fe0.05)CO2, corresponding to 18–27 wt% Na2O in the melt on a volatile-free basis.

KW - Carbonatite melt

KW - Continental lithosphere

KW - Eclogite

KW - High-pressure experiment

KW - Mantle metasomatism

KW - Mixed paragenesis

KW - BEARING ECLOGITE

KW - HIGH-PRESSURE

KW - EAST KIMBERLITE

KW - UDACHNAYA KIMBERLITE PIPE

KW - DIAMOND

KW - XENOLITHS

KW - FLUID INCLUSIONS

KW - MINERAL INCLUSIONS

KW - PHASE-RELATIONS

KW - POTASSIUM

UR - http://www.scopus.com/inward/record.url?scp=85092436389&partnerID=8YFLogxK

U2 - 10.1016/j.chemgeo.2020.119915

DO - 10.1016/j.chemgeo.2020.119915

M3 - Article

AN - SCOPUS:85092436389

VL - 558

JO - Chemical Geology

JF - Chemical Geology

SN - 0009-2541

M1 - 119915

ER -