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Deep-Earth volatile cycles couple the mantle with near-surface reservoirs.Volatiles are emitted by volcanism and, in particular, from mid-ocean ridges,which are the most prolific source of basaltic volcanism. Estimates of volatileextraction from the asthenosphere beneath ridges typically rely on measurementsof undegassed lavas combined with simple petrogenetic models of the mean degreeof melting. Estimated volatile fluxes have large uncertainties; this is partlydue to a poor understanding of how volatiles are transported by magma in theasthenosphere. Here, we assess the fate of mantle volatiles through numericalsimulations of melting and melt transport at mid-ocean ridges. Our simulationsare based on two-phase, magma/mantle dynamics theory coupled to idealisedthermodynamic model of mantle melting in the presence of water and carbondioxide. We combine simulation results with catalogued observations of allridge segments to estimate a range of likely volatile output from the globalmid-ocean ridge system. We thus predict global MOR crust production of 66-73Gt/yr (22-24 km3/yr) and global volatile output of 52-110 Mt/yr, correspondingto mantle volatile contents of 100--200~ppm. We find that volatile extractionis limited: up to half of deep, volatile-rich melt is not focused to the axisbut is rather deposited along the LAB. As these distal melts crystallise andfractionate, they metasomatise the base of the lithosphere, creatingrheological heterogeneity that could contribute to the seismic signature of theLAB.

Volatiles beneath mid-ocean ridges: deep melting, channelised transport, focusing, and metasomatism