TY - JOUR

T1 - The system KCl−CaCO3−MgCO3 at 6 GPa: A link between saline and carbonatitic diamond-forming fluids

AU - Shatskiy, Anton

AU - Podborodnikov, Ivan V.

AU - Arefiev, Anton V.

AU - Bekhtenova, Altyna

AU - Litasov, Konstantin D.

N1 - Funding Information: The authors are grateful to two anonymous referees for constructive reviews. This work is financially supported by Russian Science Foundation (project No 21-17-00024 ). Publisher Copyright: © 2022 Elsevier B.V.

PY - 2022/8/20

Y1 - 2022/8/20

N2 - Alkaline chlorides and carbonates are abundant in melt inclusions in diamonds, mantle xenoliths, and igneous minerals from kimberlites. Despite this, the phase relationships in chloride‑carbonate systems remain poorly constrained. Here we studied phase equilibria in the system KCl–CaCO3–MgCO3 in multianvil experiments at 6 GPa and 1000–1600 °C. It was found that at 1000 °C, subsolidus assemblage consists of KCl, magnesite, and aragonite. At higher temperatures, the stabilization of dolomite splits the system into two partial ternaries: KCl + dolomite + aragonite and KCl + magnesite + dolomite. Both ternaries start to melt near 1200 °C. The melting of the first ternary is controlled by the KCl-dolomite-aragonite eutectic situated at 30 mol% (24 wt%) KCl and Ca# 92, where Ca# = 100∙Ca/(Ca + Mg). The melting of the second ternary is controlled by the peritectic, KCl + dolomite = magnesite + liquid, producing chloride‑carbonate melt with 33 mol% (27 wt%) KCl and Ca# 69. The established melting temperature of the KCl + Ca-Mg carbonate assemblages is 200–500 °C higher than the subduction geotherms. This indicates that chlorides and carbonates can survive subduction to a depth of 200 km. However, the stagnation and warming of slabs to the ambient mantle temperature must result in partial melting of KCl + Ca-Mg carbonate assemblages producing a KCl-rich low-Mg carbonate melt. Percolation of this melt into the subcontinental lithospheric mantle should yield crystallization of KCl and Ca[sbnd]Mg carbonates. In the presence of water, KCl will form hydrous saline melt or fluid, while Ca and Mg carbonates, which are poorly soluble in water, will remain solid. The obtained results imply that the hydrous saline fluids (brines) found as inclusions in diamonds are a lower temperature derivative of mantle carbonatite melts and do not support the hypothesis of chloride melt generation owing to the chloride‑carbonate liquid immiscibility since no such immiscibility was established.

AB - Alkaline chlorides and carbonates are abundant in melt inclusions in diamonds, mantle xenoliths, and igneous minerals from kimberlites. Despite this, the phase relationships in chloride‑carbonate systems remain poorly constrained. Here we studied phase equilibria in the system KCl–CaCO3–MgCO3 in multianvil experiments at 6 GPa and 1000–1600 °C. It was found that at 1000 °C, subsolidus assemblage consists of KCl, magnesite, and aragonite. At higher temperatures, the stabilization of dolomite splits the system into two partial ternaries: KCl + dolomite + aragonite and KCl + magnesite + dolomite. Both ternaries start to melt near 1200 °C. The melting of the first ternary is controlled by the KCl-dolomite-aragonite eutectic situated at 30 mol% (24 wt%) KCl and Ca# 92, where Ca# = 100∙Ca/(Ca + Mg). The melting of the second ternary is controlled by the peritectic, KCl + dolomite = magnesite + liquid, producing chloride‑carbonate melt with 33 mol% (27 wt%) KCl and Ca# 69. The established melting temperature of the KCl + Ca-Mg carbonate assemblages is 200–500 °C higher than the subduction geotherms. This indicates that chlorides and carbonates can survive subduction to a depth of 200 km. However, the stagnation and warming of slabs to the ambient mantle temperature must result in partial melting of KCl + Ca-Mg carbonate assemblages producing a KCl-rich low-Mg carbonate melt. Percolation of this melt into the subcontinental lithospheric mantle should yield crystallization of KCl and Ca[sbnd]Mg carbonates. In the presence of water, KCl will form hydrous saline melt or fluid, while Ca and Mg carbonates, which are poorly soluble in water, will remain solid. The obtained results imply that the hydrous saline fluids (brines) found as inclusions in diamonds are a lower temperature derivative of mantle carbonatite melts and do not support the hypothesis of chloride melt generation owing to the chloride‑carbonate liquid immiscibility since no such immiscibility was established.

KW - Carbonate

KW - Chloride

KW - Earth's mantle

KW - High-density fluid

KW - High-pressure experiment

UR - http://www.scopus.com/inward/record.url?scp=85131461399&partnerID=8YFLogxK

U2 - 10.1016/j.chemgeo.2022.120931

DO - 10.1016/j.chemgeo.2022.120931

M3 - Article

AN - SCOPUS:85131461399

VL - 604

JO - Chemical Geology

JF - Chemical Geology

SN - 0009-2541

M1 - 120931

ER -