Abstract

The solidus of a spinel lherzolite (HK66) was determined under dry conditions through the pressure range 1 atm to 30 kbar. It was found that the solidus comprises three curves corresponding to subsolidus mineral assemblages with cusps at about 11 and 26 kbar. In order to determine the composition of melt coexisting with peridotite, a thin layer of basalt was sandwiched between compressed blocks of powdered peridotite minerals and then was equilibrated with its host at melting temperatures. A time study at 15 kbar and 1300°C (25°C above the solidus) showed that the basalt melt embedded in the peridotite was completely homogenized with the partial melt in the peridotite matrix within 24 hours. The composition of melt formed along the solidus of the peridotite is a quartz tholeiite at 5 kbar, an olivine tholeiite similar to mid-ocean ridge basalt (morb) at 8 and 10 kbar, an alkali-olivine basalt between 15 and 20 kbar, and an alkali picrite above 25 kbar. The role of K2O in the melting was specifically investigated by adding 20 wt.% potassium feldspar to the starting peridotite. The solidus of the K-rich peridotite is 70° and 150°C lower than that of the original peridotite at 20 and 30 kbar, respectively. Its extension may intersect with the normal oceanic geotherm at a pressure between 30 and 50 kbar. The melts are leucite phonolitic at the solidus of the K-rich peridotite and change gradually to the composition of the alkali picrite at the solidus of the original peridotite. A hypothesis of shallow depth origin for morbs is supported by the fact that the composition of melt formed near the solidus at 8 kbar (16 wt.% Al2O3, 11 wt.% CaO, 9 wt.% MgO) plots in the middle of the morbs cluster in the normative projections. The olivine-liquid Fe/Mg partition coefficient, KD = (Fe/Mg)ol/(Fe/Mg)liq is given by KD = 0.30 + 0.002 P (kbar) in the pressure range 1 atm to 35 kbar. Judging from the KD values, most previously reported glass compositions in peridotite melting experiments are significantly modified by the overgrowth effect of residual solids.

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