Coupled Cycling of Carbon and Water in the Form of Hydrous Carbonatitic Liquids in the Subarc Region

Abstract

The melting behavior of subducted ophicarbonate is experimentally investigated under conditions relevant for slabs at subarc depths (2.5–6 GPa and 650–1000°C). Hydrous carbonatitic liquids appear between 900 and 950°C at 2.5 GPa, between 800 and 850°C at 4–5 GPa, and at ∼800°C over 6 GPa. Water significantly depresses the thermal stability of carbonate, which drives the formation of hydrous carbonatitic liquid at low temperatures realistic for most subduction zones. Ophicarbonate fully dehydrates prior to wet melting. From an experimental point of view, the melting of ophicarbonate is essentially equal to the fluid–present melting of magnesite–bearing wehrlite lithology (i.e., meta–ophicarbonate). In the natural subduction process, the melting of meta–ophicarbonate occurs only when external water is available assuming dehydration fluid by ophicarbonate has been efficiently expelled at shallower depths. In the framework of the previously constructed dehydration history of subducting slab, the flux melting of meta–ophicarbonate at the slab–mantle interface is examined to be feasible under majority of geothermal conditions, even in cold subduction zones. Thus, hydrous carbonatitic liquid acts as a pervasive agent to transfer carbon from the slab surface to the mantle wedge at shallow depths beneath arcs. In contrast, ophicarbonate within the slab–serpentinized mantle just undergoes dehydration, and the vast majority of carbon retained in these lithologies is likely transported deeper into the mantle.