Peridotite-CO_2-H_2O and the low-velocity zone

Abstract

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 CO_2 exists in the upper mantle indicates that peridotite-CO_2-H_2O would be a better model. The divariant solidus surface for peridodite-CO_2-H_2O 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 H_2O to high CO_2/H_2O. Above about 25 kbar, the vapor phase is buffered by the melting of dolomite-peridotite, with composition ranging from CO_2 to high H_2O/CO_2 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 H_2O/CO_2. Near the buffered curve for the solidus of partly carbonated peridotite there is a temperature maximum on the peridotite-vapor solidus surface. On the CO_2 side of the maximum, above 26 kbar, CO_2/H_2O is greater in liquid than in vapor; on the H_2O side of this maximum, and at all pressures below 26 kbar, CO_2/H_2O 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 CO_2/H_2O, 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 H_2O-rich vapor or without vapor, with the generation of CO_2-rich, alkalic, SiO_2-poor liquid (larnite-normative) that in extreme conditions may be carbonatitic.