Moho carbonation at an ocean-continent transition

Abstract

Carbonation of mantle rocks during mantle exhumation is reported in present-day oceanic settings, both at mid-ocean ridges and ocean-continent transitions (OCTs). However, the hydrothermal conditions of carbonation (i.e., fluid sources, thermal regimes) during mantle exhumation remain poorly constrained. We focus on an exceptionally well-preserved fossil OCT where mantle rocks have been exhumed and carbonated along a detachment fault from underneath the continent to the seafloor along a tectonic Moho. Stable isotope (oxygen and carbon) analyses on calcite indicate that carbonation resulted from the mixing between serpentinization-derived fluids at ~175 °C and seawater. Strontium isotope compositions suggest interactions between seawater and the continental crust prior to carbonation. This shows that carbonation along the tectonic Moho occurs below the continental crust and prior to mantle exhumation at the seafloor during continental breakup.

Magnetotelluric image of a hyper-extended and serpentinized rift system

“…The residual high-salinity fluid is thereby created by partial serpen- tinization, providing a secondary reservoir contained in the cracks and interstices of the embrittled upper mantle beneath the hyperextended crust. Evidence for mobility of the secondary, mantlereacted fluid is found in rock samples of inverted hyper-extended rift system in the Alps and Pyrenees (Pinto et al, 2015;Salardon et al, 2017;Coltat et al, 2021). There, analysis of rock samples shows that highly-saline fluids promote the mobility of mantle elements (like Cr-Ni-V, Fe and Mn) assumed to be fixed by serpentinization.…”