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Carbon-dioxide-rich silicate melt in the Earth’s upper mantle

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Abstract

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 ridges1,2, 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 + CO2 beneath ridges commences at 180 km. Accounting for the effect of 50–200 p.p.m. H2O on freezing point depression, the onset of silicate melting for a sub-ridge mantle with 100 p.p.m. CO2 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 deep6, 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 peridotite7 and that of the oceanic upper mantle5, suggest that mantle at depth is CO2-rich but H2O-poor. Finally, carbonated silicate melts restrict the stability of carbonatite in the Earth’s deep upper mantle, and the inventory of carbon, H2O and other highly incompatible elements at ridges becomes controlled by the flux of the former.

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Figure 1: Experimental conditions, phase assemblages, melt CO 2 isopleths and extent of melting as a function of temperature.
Figure 2: Temperature–composition diagram showing evolution of melt composition.
Figure 3: Plot of Δ T as a function of concentration of CO2 in the partial melts, .
Figure 4: Melting regime and mantle flow beneath a mid-oceanic ridge along a 1,350 °C mantle potential temperature ( T p ) adiabat.

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Acknowledgements

We thank C. Ballhaus for reviewing the manuscript. This study received support from the National Science Foundation and a Packard fellowship to R.D.

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Authors

Contributions

R.D. designed the project, performed most of the experiments and sample analyses, and wrote the paper. A.M. conducted the experiments and analyses of carbonated silicate melt fluxed peridotite compositions. K.T. participated in conducting the piston cylinder experiments and analyses of peridotite + CO2 bulk compositions. A.C.W. participated in the multi-anvil experiments. All authors, including G.H. and M.M.H., participated in the discussion and commented on the paper.

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Correspondence to Rajdeep Dasgupta.

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The authors declare no competing financial interests.

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Dasgupta, R., Mallik, A., Tsuno, K. et al. Carbon-dioxide-rich silicate melt in the Earth’s upper mantle. Nature 493, 211–215 (2013). https://doi.org/10.1038/nature11731

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