Carbonate Mineralogy in Mantle Peridotites of the Atlantis Massif (IODP Expedition 357)

Abstract

Petrographic and major element investigations of carbonates from seven 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 (a) fluid composition and flow, (b) temperature, and (c) 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 184°C (based on clumped isotope thermometry) 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 ambient and 196°C, from fluids with highly variable FeO, MnO, and SrO 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 (<9°C) within the uplifted serpentinized peridotites. Major element analyses suggest that aragonite precipitated from seawater, which experienced little interaction with the basement. Combining these results, we propose a model that positions different carbonate occurrences in a conceptual framework considering mafic domains in the peridotites and fluid heterogeneities during progressive exhumation and alteration of the AM.