On the role of Earth's lithospheric mantle in global volatile cycles

Abstract

In whole-Earth models of volatile cycles the outflux from our planet's convecting mantle to its surface via volcanoes, including those essential for life (e.g. C, H2O, S, F, Cl), is traditionally balanced by the influx from subducted slabs of oceanic lithosphere. Here we explore the additional modulating effect of the rigid lithospheric mantle, which forms the large laterally-continuous layer that separates Earth's convecting mantle and crust. Thermal gradients in the lithospheric mantle essentially control residence times but vary over time, and also large spatial scales, and give this reservoir a unique ability to both sequester and release volatiles. Here, we use multi-mode surface wave tomography to quantify the mass of the lithospheric mantle and reveal that this is similar beneath the world's cratons and oceans (3.5×1022kg) and a factor of 2 larger than continental off-craton regions (1.4×1022kg).By combining our estimates of mass with published volatile contents of mantle phases we demonstrate that over ‘deep time’ Earth's lithospheric mantle has sequestered significant amounts of S≈H2O>C>F>Cl. The largest volatile repository resides below the cores of continents (cratonic mantle), and contains >45% of this reservoir's inventory of H2O (7±2×1018kg), S (8±7×1018kg), C (6±3×1018kg) and F (2±1×1018kg). Approximately 44% of the global cratonic mass is located beneath North America/West Greenland and Eurasia, and so we predict these regions will dominate the volatile inventory.Although global off-craton continental mantle contains <34% of the total lithospheric mantle budget of C, H2O, S, F and Cl, the geothermal gradients of this region are more susceptible to perturbation than beneath the cratons, e.g. during formation of Large Igneous Provinces and continental break-up, and residence times of sequestered volatiles will be shorter. Due to its large volume, the oceanic mantle is also an important repository for C, H2O and S (1 to 8×1018kg), and to a lesser extent Cl and F (0.1×1018kg). While the volatile element budgets of the lithospheric mantle are less than the underlying convecting mantle (1018 to 1020kg), the physical location of this reservoir – together with its ability to store and remobilise volatiles over short and long timescales – make it an important consideration in models of whole-Earth volatile cycles that attempt to balance in- and out-gassing fluxes.