Melting of a Hydrous Mantle: I. Phase Relations of Natural Peridotite at High Pressures and Temperatures with Controlled Activities of Water, Carbon Dioxide, and Hydrogen
Four natural peridotite nodules ranging from chemically depleted to Fe-rich, alkaline and calcic (SiO 2 = 43.7–45.7 wt. per cent, A1 2 O 3 = 1.6O–8.21 wt. per cent, CaO = 0.70–8.12 wt. per cent, ∑ alk = 0.10–0.90 wt. per cent and Mg/(Mg+Fe 2+ ) = 0.94–0.85) have been investigated in the hypersolidus region from 800° to 1250°C with variable activities of H 2 O, CO 2 , and H 2 . The vapor-saturated peridotite solidi are 50–200°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/Al 2 O 3 . Conversely, lowering CaO/Al 2 O 3 reduces the temperature at constant Mg/(Mg+∑Fe) of the starting material. Temperature differences between the solidi up to 200°C are observed. All solidi display a temperature minimum reflecting the appearance of garnet. This minimum shifts to lower pressure with decreasing Mg/(Mg + ∑Fe) of the starting material. The temperature of the beginning of melting decreases isobarically as approximately a linear function of the mol fraction of H 2 O in the vapor ( X H 2 O v ). The data also show that some CO 2 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 (H 2 O) liquidus curve for one peridotite is between 1250° and 1300°C between 10 and 30 kb. Olivine, spinel, and orthopyroxene are either liquidus phases or co-exist 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 aHjo conditions in the upper mantle and possibly also by variations 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°C compared with 1600°C which is required for dry melting. The genesis of kimberlite can be related to partial melting of peridotite under conditions of X H 2 O v = 0.5–0.25 ( X CO 2v = 0.5–0.75). Such activities of H 2 O 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.