Abstract
Earth’s long-term cycling of carbon is regulated from mid-ocean ridges to convergent margins by mass transfers involving mantle rocks. Here we examine the conversion of peridotite into listvenite (magnesite+quartz) occurring along the basal thrust of the Semail Ophiolite (Fanja, Sultanate of Oman). At the outcrop scale, this transformation defines reaction fronts, from serpentinized peridotites, to carbonated serpentinites and listvenites. Carbonation of peridotites progressed through distinctive stages, involving the generation of carbonate and/or quartz veins concurrently to the pervasive replacement of serpentinized peridotites by carbonates and quartz. The onset of pervasive carbonation reactions is characterized by the formation of Fe-rich magnesite spheroids and aggregates preserved in listvenites, in the vicinity of antitaxial Fe-rich magnesite veins.We document the changes in reaction textures and carbonate compositions from carbonated serpentinites to listvenites, indicating destabilization of Fe-oxides. We propose that Fe-rich cores of magnesite spheroids result from the breakdown of magnetite into Fe-rich magnesite and hematite, and represent the end product of the early carbonation sequence. Pervasive carbonation induces a change to oxidizing conditions as the reaction progresses. We discuss the linkages between the composition of magnesite replacing the serpentine matrix and variations of the reactive fluid composition and redox conditions, and their possible effects on the speciation of volatiles and mobility of economically-valuable metals.