ABSTRACT

The Andaman ophiolite preserves widespread carbonate mineralization as massive ophicarbonates and carbonate veins hosted in ultramafic and mafic rocks, providing a natural laboratory for investigating carbon transfer and sequestration in supra-subduction-zone oceanic lithosphere. We combine petrography, mineralogy, trace-element geochemistry, radiocarbon dating, and C-O-Sr isotopic analyses to constrain the origin, fluid sources, and conditions of carbonate formation. Ophicarbonates are dominated by calcite with minor quartz and ankerite and represent carbonation of previously serpentinized mantle peridotites, whereas carbonate veins record prolonged carbonate precipitation across the ophiolite sequence. Both carbonate types exhibit negative Ce anomalies, positive Eu and Y anomalies, and high Y/Ho ratios, indicating precipitation from seawater-derived hydrothermal fluids. Their isotopic compositions (δ¹³C_PDB = -14.6 to +2.7‰, δ¹⁸O_SMOW = +16.3 to +27.1‰, and ⁸⁷Sr/⁸⁶Sr = 0.704468-0.708684) are consistent with submarine hydrothermal carbonation involving seawater that interacted with mantle-derived CO₂ and Sr. Rayleigh fractionation modeling of calcite precipitation suggests formation temperatures in the range of 70-200^oC for both types of carbonates. The carbonating fluids contained a significant mantle CO₂ component, whereas contributions from slab-derived fluids, methane oxidation, or meteoric waters were negligible. Radiocarbon activities below detection limits indicate carbonate formation predates ~50 ka, supporting carbonation prior to recent obduction of the ophiolite. Our results demonstrate that extensive calcite-dominated carbonation of oceanic mantle and crust can occur in low-temperature submarine hydrothermal systems, highlighting the role of hydrothermal seawater circulation in carbon sequestration within oceanic lithosphere and providing an important natural analog for mineral carbonation processes relevant to long-term carbon storage.