Retrograde carbon sequestration in orogenic complexes: A case study from the Chinese southwestern Tianshan

The interaction between ascending carbonic fluids and rocks at shallow depths in orogenic systems plays a crucial role in carbon distribution and fluxes regulation. Here we present field, microstructural, petrological and geochemical results of carbonic fluid–rock interactions affecting exhumed mafic eclogites from the Chinese southwestern Tianshan ultrahigh-pressure (UHP) metamorphic belt. The mafic eclogites recorded two successive, superimposed, metamorphic–metasomatic stages: graphite precipitation along fractures and veins at eclogite facies (Stage#1) and pervasive rock carbonation (i.e., silicate dissolution and carbonate precipitation) at retrograde amphibolite to greenschist facies (Stage#2). This work focuses on Stage#2 carbonation, which consists of the transformation of Stage#1 graphite-bearing eclogites by carbonate + paragonite (± zoisite) + quartz. Carbon and oxygen isotopic data and Deep Earth Water (DEW) thermodynamic modeling suggest an external metasedimentary source for the Stage#2 carbonation. This deep carbon sequestration event can be referred to retrograde, greenschist-facies conditions at about 10 kbar and 450 °C, and redox conditions equal or more oxidized than the quartz–fayalite–magnetite (QFM) buffer. Modeling results also show that the saturation of carbonic fluid with respect to graphite can boost retrograde carbonation processes. Our findings provide new insights into the reactivity of metastable, exhumed metamafic rocks with ascending carbonic fluids in orogenic systems. We conclude that retrograde, fluid-mediated rock carbonation can significantly impact on carbon fluxes from active collisional belts.