Serpentinization versus carbonation: geochemical and thermodynamic constraints from an ophiolitic reaction profile

Peridotite carbonation is an efficient process for carbon sequestration in Earth’s carbon cycle. This process is inevitably associated with serpentinization. However, the interplay of the two processes and the fluid-rock reaction details remain elusive. Here we present a ~330-meter-long fluid-peridotite reaction profile with a southward zonation of harzburgite, serpentinite to soapstone-bearing listvenite in the Luobusa ophiolite (Tibet). From harzburgite to listvenite, gradual decreases in whole-rock MgO, SiO₂ and FeO_T as well as nearly constant Al₂O₃ and trace-element patterns suggest a continuous reaction from serpentinization to carbonation. The H₂O⁺ contents were rapidly elevated during serpentinization, and then abruptly dropped once the carbonation initiated as evidenced by a jump in CO₂ contents. Such contrasting volatile behaviors indicate a competition between serpentinization and carbonation, which caused strong variations in H₂ fugacity and redox states in the reaction system and controlled the compositional variations of involved fluids and crystallization of zoned magnesites. Clumped isotopes constrain the carbonation temperatures up to ~192-302 °C. In addition, thermodynamic modelling shows that the mineralogical, chemical and redox variations from serpentinization to carbonation are consistent with those observed in the Luobusa profile. C-O isotopic compositions suggest that the fluids were derived primarily from the mantle and added by those from surface reservoirs. Such CO₂-rich fluids migrated along the trans-lithospheric thrust in Himalaya and reacted with the ophiolite, forming the studied profile. This study shows that the serpentinization-versus-carbonation processes may suppress the capacity of carbon sequestration, and calls for a reevaluation of the sequestrated carbon budget in ophiolite-rich orogens.