The properties of the seismic low-velocity zone are consistent with incipient melting of mantle peridotite. Vapor-absent melting of amphibole-peridotite has been used to model the low-velocity zone, but evidence that CO2 exists in the upper mantle indicates that peridotite-CO2-H2O would be a better model. The divariant solidus surface for peridodite-CO2-H2O is traversed by a series of univariant lines marking the intersections of divariant subsolidus reactions involving dolomite or magnesite, amphibole, or phlogopite (other hydrous minerals are neglected in this treatment), or combinations of these. The vapor phase compositions are buffered to specific values, which limits the range of vapor compositions that can coexist with peridotite at various pressures. Below about 30 kbar, the vapor phase is buffered by the melting of amphibole-peridotite, with composition ranging from H2O to high CO2/H2O. Above about 25 kbar, the vapor phase is buffered by the melting of dolomite-peridotite, with composition ranging from CO2 to high H2O/CO2 at pressures above 30 kbar. The buffered curve for phlogopite-peridotite intersects the dolomite-peridotite curve, generating another line for phlogopite-dolomite-peridotite; the strong buffering capacity of dolomite forces the vapor on this line to high H2O/CO2. Near the buffered curve for the solidus of partly carbonated peridotite there is a temperature maximum on the peridotite-vapor solidus surface. On the CO2 side of the maximum, above 26 kbar, CO2/H2O is greater in liquid than in vapor; on the H2O side of this maximum, and at all pressures below 26 kbar, CO2/H2O is greater in vapor than in liquid. The suboccanic low-velocity zone is caused by incipient melting of amphibole-peridotite in the presence of vapor with high CO2/H2O, with generation of forsterite-normative liquid. The subcontinental low-velocity zone, where present, is probably caused by incipient melting of dolomite-peridotite, or phlogopite-dolomite-peridotite, either with H2O-rich vapor or without vapor, with the generation of CO2-rich, alkalic, SiO2-poor liquid (larnite-normative) that in extreme conditions may be carbonatitic.
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Wyllie, P.J. Peridotite-CO2-H2O and the low-velocity zone. Bull Volcanol 41, 670–683 (1978). https://doi.org/10.1007/BF02597393
- Hydrous Mineral
- Solidus Surface
- Carnegie Inst
- Incipient Melting