Flux melting of subducting carbonated sediments: An experimental study

Abstract

Subduction zones play a critical role in the global carbon cycle by regulating carbon exchange between the Earth's surface and interior. Processes that are known to release carbon from the slab, including metamorphic decarbonation and carbonate dissolution, cannot explain the high CO₂ flux in magmatic arcs. Slab melting is the least considered mechanism for carbon mobilization at subarc depths based on the high solidus temperatures of carbonated lithologies, which were experimentally determined under dry or H₂O-absent conditions. Subducted sediments are major carbon carriers, however, their melting behaviour with excess H₂O remains largely unexplored. Here, we perform fluid-present melting, high-pressure experiments at 750–1100 °C and 2.5–4 GPa using starting compositions similar to global average subducted sediments to determine the solidus, melting relations and carbonate stability fields. The onset of melting is between 750 and 800 °C at 2.5 GPa and between 850 and 900 °C at 4 GPa. Dolomite melts out on or close to the solidus, whereas crystalline aragonite persists >150 °C above the solidus. Flux melting of carbonated sediment at moderately hot subduction zones is examined to be feasible in the framework of the previously constructed dehydration history of the underlying serpentinites, providing a pathway to transfer carbon from the slab to the subarc mantle. However, complete breakdown of refractory aragonite requires at least 50 °C higher than that predicted for the hottest slab P‒T paths. Thus, even in the presence of H₂O, partial subducted carbon may survive the melting event occurring at shallow regions and reach considerable mantle depths.