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

• Published in: Nature (London) Vol. 493; no. 7431; pp. 211 - 215
• Main Authors: Dasgupta, Rajdeep, Mallik, Ananya, Tsuno, Kyusei, Withers, Anthony C., Hirth, Greg, Hirschmann, Marc M.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 10.01.2013; Nature Publishing Group
• Subjects: 704/2151/431; Carbon dioxide; Carbonation; Earth; Environmental aspects; Experiments; Freezing; Freezing point; Humanities and Social Sciences; letter; Mantle; Melting; Melts; Mineralogy; Minerals; multidisciplinary; Peridotite; Ridges; Science; Silicates; Trends; Upper mantle; Upwelling; United States
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature11731

Carbon-dioxide-rich kimberlitic melt explains the low velocity and high electrical conductivity of the mantle asthenosphere and controls the flux of incompatible elements at oceanic ridges. Ocean-ridge silicate magma generation The depth of onset of silicate magma generation is critical to Earth's continuing thermal evolution and the release of volatile elements, such as water and carbon dioxide, to the atmosphere. In experiments designed to constrain the onset of silicate melting in the mantle, Rajdeep Dasgupta et al . studied carbonated peridotites at pressures of 2–5 GPa. Their results suggest that silicate melting of dry peridotite + CO 2 beneath mid-ocean ridges should commence at a depth of about 180 km. But when the effect of mantle water is taken into account, onset of silicate melting becomes as deep as 300 km. The authors suggest that these findings may help to explain oceanic low-velocity zones and the electrical conductivity structure of the mantle. 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 1 , 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 + 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 6 , 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 7 and that of the oceanic upper mantle 5 , 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.