  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 volatile
extraction from the asthenosphere beneath ridges typically rely on measurements
of undegassed lavas combined with simple petrogenetic models of the mean degree
of melting. Estimated volatile fluxes have large uncertainties; this is partly
due to a poor understanding of how volatiles are transported by magma in the
asthenosphere. Here, we assess the fate of mantle volatiles through numerical
simulations of melting and melt transport at mid-ocean ridges. Our simulations
are based on two-phase, magma/mantle dynamics theory coupled to idealised
thermodynamic model of mantle melting in the presence of water and carbon
dioxide. We combine simulation results with catalogued observations of all
ridge segments to estimate a range of likely volatile output from the global
mid-ocean ridge system. We thus predict global MOR crust production of 66-73
Gt/yr (22-24 km3/yr) and global volatile output of 52-110 Mt/yr, corresponding
to mantle volatile contents of 100--200~ppm. We find that volatile extraction
is limited: up to half of deep, volatile-rich melt is not focused to the axis
but is rather deposited along the LAB. As these distal melts crystallise and
fractionate, they metasomatise the base of the lithosphere, creating
rheological heterogeneity that could contribute to the seismic signature of the
LAB.
