HO-induced sedimentary carbon migration from subducting slabs to the forearc mantle

Carbon in sedimentary carbonates dominates the global carbon input flux in subduction zones, the fate of which makes an impact on the global carbon cycle. At forearc depths, ∼32% of subducting water is released through slab dehydration and may greatly promote sedimentary carbon migration to the forearc mantle. However, it is controversial that considering the infiltration of external aqueous fluids, whether extremely limited or a significant portion of sedimentary carbon is liberated from subducting slabs in the forearc region. To explore to what extent hydrous fluids could facilitate carbon migration at forearc depths, hydrous carbonate-dominated sediment (1.14 wt.% HO)-harzburgite reaction (layered) experiments have been performed at 1.5 GPa and 600–1000°C with various durations. For comparison, an anhydrous sediment-harzburgite reaction experiment was conducted to investigate the role of water on carbon migration. In hydrous experiments under subsolidus conditions (600–900°C), (1) a reaction zone comprised of clinopyroxene+dolomite forms at the sediment-harzburgite interface due to the metasomatic reaction; (2) the Ca# (100×Ca/[Ca+Mg+Fe], in molar) of calcite in the sediment layer drastically deceases when approaching the reaction zone; (3) newly formed dolomite and pargasite occur in the upper harzburgite layer. The above phenomena were not observed in the anhydrous experiment. Under a supersolidus condition (1000°C), a reaction zone composed of olivine+clinopyroxene+pargasite+CO formed as a result of hydrous carbonate melt-harzburgite interaction. The experiments demonstrate that aqueous fluids could significantly promote the chemical reaction and component exchange between sediments and mantle peridotite, and also induce subducting sedimentary carbon migration to the forearc mantle. It is estimated roughly that globally, ∼50% of subducting sedimentary carbon may be released at forearc depths. The carbon and water would be stabilized as carbonates (e.g., dolomite) and hydrous minerals (e.g., pargasite) in the forearc mantle, implying that the forearc mantle may be an important carbon reservoir. Our study explains the fate of a portion of carbon that is not returned to the atmosphere through arc volcanism.