THE recent discovery that CO2 at upper mantle pressures has a dramatic effect on the melting temperature of peridotite (because the silicate assemblage becomes carbonated), and on the composition of near-solid us liquid, has petrological and geophysical implications that have been explored extensively. The influence of H2O has received only passing reference, probably because it was tacitly assumed that this would modify the carbonate-dominated melting relationships significantly. I describe here construction of a schematic diagram for the system peridotite–CO2–H2O which shows that the carbonate can persist even in the presence of vapours with high H2O/CO2 Figure 1 shows that starting with peridotite–H2O at high pressures, a small addition of CO2 is sufficient for the generation of carbonate. This leads to the unexpected conclusion that many of the applications deduced for the melting of carbonated peridotite may remain valid with vapours enriched in H2O/CO2 In particular, the interstitial liquid in the upper part of the seismic low-velocity zone may be carbonatitic even with H2O present as a component.
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WYLLIE, P. Peridotite–CO2–H2O, and carbonatitic liquids in the upper asthenosphere. Nature 266, 45–47 (1977). https://doi.org/10.1038/266045a0