Hydrous Carbonatitic Liquids Drive CO2 Recycling From Subducted Marls and Limestones

Abstract

Pelagic limestones are subducted in a variety of subduction zones worldwide. Despite the geochemical relevance of systems enriched in CaCO3, previous experimental investigations mostly focused on carbonated pelites, with low Ca/(Ca+Mg+Fe) ratio. We present the compositions and the formation conditions of liquids in the model system CaO‐Al2O3‐SiO2‐H2O‐CO2 (CASHC), building on phase relationships in the subsystems CHC and CSHC, where a second critical endpoint was suggested at temperatures as low as 515 °C, and 3.2 GPa. Multianvil experiments were performed at 4.2 and 6.0 GPa on five bulk compositions at variable Ca/Si/Al ratios. H2O contents vary from 5.6 to 21 wt%. Aragonite + kyanite + vapor and minor lawsonite form at 700 °C, replaced by zoisite/grossular at 800 °C. Between 850 °C and 950 °C, a complex sequence of textural features is observed upon quenching of a single volatile‐rich liquid phase formed at run conditions. Precipitates include dendritic CaCO3, silicate glass, and Al‐rich whiskers. The bulk composition of such hydrous carbonatitic liquids is retrieved by image analysis on X‐ray maps, showing Ca/Si ratio increasing with pressure and temperature. Hydrous Ca‐carbonatitic liquids are efficient media for scavenging volatiles from subducted crustal material and for metasomatizing the mantle wedge.