Solidus reactions in synthetic lherzolite-H_2O-CO_2 from 20–30 kbar, with applications to melting and metasomatism

Abstract

Mixtures of synthetic forsterite, enstatite, diopside and natural dolomite were used to determine solidus reactions for synthetic lherzolite-H_2O (passing through 1125, 1060 and 1005°C, at 10, 20 and 30 kbar, respectively), and dolomite-lherzolite with mixed vapor, two curves not previously located experimentally. The vapor-buffered curve for dolomite-lherzolite has a distinctive pressure minimum, passing down from the CO_2 system near 28 kbar and 1230°C to a minimum near 22 kbar and 1975°C, with decreasing CO_2/H_2O in the vapor, and then up in pressure through a point at 30 kbar and 990°C. These new results, together with published results in other parts of the system, permit complete construction of the three divariant solidus and decarbonation surfaces meeting along the vapor-buffered dolomite-lherzolite solidus curve. The vapor phase composition at the pressure minimum on this curve is about 85 mol% CO_2. The corresponding curve for natural peridotites is complicated by the addition of amphibole. The three surfaces define boundaries between regions where various kinds of metasomatic fluids can exist. Their positions are moved to somewhat lower temperatures for natural peridotites, and the significant vapor-buffered solidus ledge occurs within a narrow pressure interval at about 75 km depth. Rapid changes in melt and vapor phase compositions coexisting with peridotite occur within this interval. Shallower than about 75 km, silicate and not carbonatite magmas coexist with peridotite; vapors may range from CO_2 to H_2O. Deeper than about 75 km, near-solidus melts are carbonatitic, with vapor if present being H_2O-rich; CO_2-rich vapors cannot exist. Rising volatile-rich melts (of moderate temperatures) in equilibrium with peridotite must begin to crystallize and give off vapors near this depth, with compositions changing from H_2O- to CO_2-rich as depth decreases. Melts and vapors separate rapidly from crystals at this and greater depths.