Carbonation of mantle peridotites at the Atlantis Massif: Insights from IODP Expedition 357

Abstract

Petrographic and major element investigations on carbonates from drill cores recovered during IODP Expedition 357 on the Atlantis Massif (AM) provide information on the genesis of carbonate minerals in the oceanic lithosphere. Textural sequences and mineralogical assemblages reveal three distinct types of carbonate occurrences in ultramafic rocks that are controlled by (i) fluid composition and flow, (ii) temperature of the system, and (iii) the presence of mafic intrusions. The first occurrence of carbonate consists of different generations of calcite that formed syn-to post-serpentinization. These calcites formed at temperatures between 30 and 185°C (based on clumped isotopes) and from a fluid influenced by interaction with mafic intrusions. The second occurrence consists of magnesite, dolomite, calcite and aragonite veins that also formed syn-to post serpentinization. These carbonates formed at temperatures between 4 and 188°C and from fluids with highly variable composition and Mg/Ca ratios, but overall high CO2 and moderate SiO2 concentrations. High FeO (3.3 wt%) and MnO (7.3 wt%) contents indicate high temperatures, high water/rock ratios, and low oxygen fugacity for both carbonate assemblages. The third occurrence consists solely of aragonite veins formed at low-temperatures (5°C) within the uplifted serpentinized peridotites. Chemical data suggest that aragonite precipitated from cold seawater, which underwent little exchange with the basement. Combining these observations, we propose a model that places different carbonate occurrences in a conceptual frame involving mafic intrusions in the peridotites and fluid heterogeneities during progressive exhumation and alteration of the AM.