English abstract
Earth's long-term cycling of carbon is regulated from mid-ocean ridges to convergent plate boundaries
by mass transfers involving mantle rocks. Here we examine the conversion of peridotite to listvenite (magne site + quartz rock) during CO2 metasomatism along the basal thrust of the Semail Ophiolite (Fanja, Sultanate of
Oman). At the outcrop scale, this transformation defines reaction zones, from serpentinized peridotites to carbon ated serpentinites and listvenites. Based on a detailed petrological and chemical study, we show that carbonation
progressed through three main stages involving the development of replacive textures ascribed to early stages,
whilst carbonate (± quartz) veining becomes predominant in the last stage. The pervasive replacement of ser pentine by magnesite is characterized by the formation of spheroids, among which two types are identified based
on the composition of their core regions: Fe-core and Mg-core spheroids. Fe zoning is a type feature of matrix
and vein magnesite formed during the onset carbonation (Stage 1). While Fe-rich magnesite is predicted to form
at low fluid XCO2 from a poorly to moderately oxidized protolith, our study evidences that the local non-redox
destabilization of Fe oxides into Fe-rich magnesite is essential to the development of Fe-core spheroids. The
formation of Fe-core spheroids is followed by the pervasive (over-)growth of Mg-rich spheroids and aggregates
(Stage 2) at near-equilibrium conditions in response to increasing fluid XCO2. Furthermore, the compositions of
carbonates indicate that most siderophile transition elements released by the dissolution of primary minerals are
locally trapped in carbonate and oxides during matrix carbonation, while elements with a chalcophile affinity are
the most likely to be leached out of reaction zones.
