EarthRef.org Reference Database (ERR) -- Mysen & Boettcher 1975

Four natural peridotite nodules ranging from chemically depleted to Fe-rich, alkaline, and calcic have been investigated in the hypersolidus region from 800 degrees to 1250 degrees C with variable activities of H2O, CO2, and H2. The vapor-saturated peridotite solidi are 50-200 degrees C below those previously published. The temperature of the beginning of melting of peridotite decreases markedly with decreasing Mg/(Mg SFe) of the starting material at constant CaO/Al2O3. Conversely, lowering CaO/Al2O3 reduces the temperature at constant Mg/Mg+SFe) of the starting material. Temperature differences between the solidi up to 200 degrees C are observed. All solidi display a temperature minimum reflecting the appearance of garnet. This minimum shifts to lower pressure with decreasing Mg/(Mg + SFe) of the starting material. The temperature of the beginning of melting decreases isobarically as approximately a linear function of the mol fraction of H2O in the vapor (XH2O). The data also show that some CO2 may dissolve in silicate melts formed by partial melting of peridotite. Amphibole (pargasitic hornblende) is a hypersolidus mineral in all compositions, although its P-T stability field depends on bulk rock chemistry. The upper pressure stability of amphibole is marked by the appearance of garnet. The vapor-saturated (H2O) liquidus curve for one peridotite is between 1250 degrees and 1300 degrees C between 10 and 30 kb. Olivine, spinel, and orthopyroxene are either liquidus phases or coexist immediately below the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity in the oceanic upper mantle because the oceanic geotherm passes through the P/T band covering the appearance of garnet in various peridotites. The variable depth to the low-velocity zone is explained by variable aH2O conditions in the upper mantle and possibly also by variation in the composition of the peridotite itself. It is suggested that komatiite in Precambrian terrane could form by direct melting of hydrous peridotite. Such melting requires about 1250 degrees C compared with 1600 degrees C which is required for dry melting. The genesis of kimberlite can be related to partial melting of peridotite under conditions of XH2O = 0.5-0.25 (XCO2 = 0.5-0.75). Such activities of H2O result in melting at depths ranging between 125 and 175 km in the mantle. This range is within the minimum depth generally accepted for the formation of kimberlite.