Melting behavior of impure limestone under H2O-poor conditions: Implications for the contribution of carbonate-rich sediments to arc magmatic carbon output

Abstract

Sedimentary carbon may be transferred from subducting slabs to the overlying mantle wedge as a component of sediment-derived melts and contribute to carbon output in subduction zones. To investigate the melting behavior of subducting carbonate-rich sediments at shallow depths and evaluate the consequent efficiency of carbon recycling, this study presents the phase relations of H2O-poor impure limestone at 1.5–4.5 GPa and 750–1300 °C. The onset of melting occurs between 750 and 800 °C at 1.5–2.5 GPa, between 950 and 1000 °C at 3.5 GPa, and between 1000 and 1050 °C at 4.5 GPa. The melting experiments yield silicate melts with CO2 contents below 5 wt% and H2O contents below 10 wt%, exhibiting compositional evolution from rhyolitic to dacitic and trachytic as the temperature increases. Combining these results with previous experimental studies on various sediment systems, we conclude that the position of the solidus and melt compositions are influenced by multiple factors, including pressure, bulk composition, and volatile concentrations. Calculations demonstrate that the subduction of carbonate-rich sediment leads to a near-surface recycling efficiency for carbon below 5%, notably lower than the recycling efficiency of water (>75%). This study also indicates that sediment melting in subarc regions is restricted to either hot subduction zones or to subducted slabs at subarc depths exceeding 150 km, which are uncommon among global subduction zones on the modern Earth.However, sediment diapirism may be an efficient way for subducting sedimentary carbon to migrate into source regions of arc magmas. Reaction experiments involving hydrous impure limestone and lherzolite were conducted at 2.5 GPa and 1000 °C, as well as 4.5 GPa and 1100 °C to constrain the melting behavior of carbonate-rich sediment diapirs in the mantle wedge. Silicate melts and a thin websterite reaction layer are generated at 2.5 GPa, while at 4.5 GPa carbonate melts are produced, resulting in the thorough metasomatism of lherzolite. These findings highlight distinct and diverse contributions that carbonate-rich sediment diapirs make to compositional heterogeneity in the mantle wedge at different depths, and also to carbon degassing in magmatic arcs.