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The onset of melting in the Earth's upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exosphere, and geochemical and geophysical properties of the mantle. Although carbonatitic melt could be stable 250 km or less beneath mid-oceanic ridges, owing to the small fraction (∼0.03 wt%) its effects on the mantle properties are unclear. Geophysical measurements, however, suggest that melts of greater volume may be present at ∼200 km (refs 3-5) but large melt fractions are thought to be restricted to shallower depths. Here we present experiments on carbonated peridotites over 2-5 GPa that constrain the location and the slope of the onset of silicate melting in the mantle. We find that the pressure-temperature slope of carbonated silicate melting is steeper than the solidus of volatile-free peridotite and that silicate melting of dry peridotite + CO(2) beneath ridges commences at ∼180 km. Accounting for the effect of 50-200 p.p.m. H(2)O on freezing point depression, the onset of silicate melting for a sub-ridge mantle with ∼100 p.p.m. CO(2) becomes as deep as ∼220-300 km. We suggest that, on a global scale, carbonated silicate melt generation at a redox front ∼250-200 km deep, with destabilization of metal and majorite in the upwelling mantle, explains the oceanic low-velocity zone and the electrical conductivity structure of the mantle. In locally oxidized domains, deeper carbonated silicate melt may contribute to the seismic X-discontinuity. Furthermore, our results, along with the electrical conductivity of molten carbonated peridotite and that of the oceanic upper mantle, suggest that mantle at depth is CO(2)-rich but H(2)O-poor. Finally, carbonated silicate melts restrict the stability of carbonatite in the Earth's deep upper mantle, and the inventory of carbon, H(2)O and other highly incompatible elements at ridges becomes controlled by the flux of the former.

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High pressure-temperature experiments suggest late delivery of life-essential elements to Earth by a large differentiated body.

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We present phase equilibria experiments on a depleted peridotite (Mg# 92) fluxed with variable proportions of a slab‐derived rhyolitic melt (with 9.4 wt.% H2O, 5 wt.% CO2), envisaging an interaction that could occur during formation of continents by imbrication of slabs/accretion of subarc mantles. Experiments were performed with 5 wt.% (Bulk 2) and 10 wt.% (Bulk 1) melt at 950–1175°C and 2–4 GPa using a piston‐cylinder and a multi‐anvil apparatus, to test the hypothesis that volatile‐bearing mineral‐phases produced during craton formation can cause reduction in aggregate shear‐wave velocities (VS) at mid‐lithospheric depths beneath continents. In addition to the presence of olivine, orthopyroxene, clinopyroxene, and garnet/spinel, phlogopite (Bulk 1: 3–7.6 wt.%; Bulk 2: 2.6–5 wt.%) at 2–4 GPa, and amphibole (Bulk 1: 3–9 wt.%; Bulk 2: 2–6 wt.%) at 2–3 GPa (≤1050°C) are also present. Magnesite (Bulk 1: ∼1 wt.% and Bulk 2: ∼0.6 wt.%) is present at 2–4 GPa (<1000°C at 3 and < 1050°C at 4 GPa) and its thermal breakdown coincides with the visual appearance of trace‐melt. However, an extremely small fraction of melt is inferred at all experiments based on the knowledge of fluid‐saturated peridotite solidus and the difference between bulk H2O and total H2O stored in the hydrous phases. Calculated mineral end‐member volume‐proportions were used to calculate VS of the resulting assemblage at experimental conditions and along representative continental geotherms (surface heat flow of 40–50 mWm−2). We note that reactive crystallization of phlogopite ± amphibole by infiltration of 3–10% slab‐derived hydrous‐silicic melt can cause up to 6% reduction in VS and that the estimated reduction in VS increases with increasing melt:rock ratio. The presence of phlogopite limits amphibole‐stability, making phlogopite a more likely candidate for MLDs at >100 km depth.