The partial fusion of peridotite
In a study of the origin of basalt magma, peridotite has been partially fused and the composition of the liquid phases determined.Peridotite was chosen because it is thought to be the most likely material of the upper mantle. Geological evidence suggests that the upper mantle is not of basaltic composition and eclogitic mineralogy but that it is of ultrabasic composition. Although under some conditions an ultrabasic mantle may be within the field of eclogite stability, the comparative rarity of high-pressure phases such as garnet in ultrabasic nodules suggests that normal basalts are derived from non-eclogitic regions. The peridotite composition selected for experimental work was that of average peridotite (Nockolds).Charges of the synthetic peridotite were held at constant temperature (±1/2°C) in a non-oxidising atmosphere for one week. After quenching, the glass was extracted and analysed.The glass content varied from 4% at 1250°C to 26% at 1355°C. Its composition ranged from a basalt more or less saturated in normative silica at a temperature of 1310°C (15% fusion) and below, to picrite basalt at higher temperatures. The results confirm the general assumption that basalts can be the product of the partial fusion of peridotite.To date, the experimental work has been under anhydrous conditions at atmospheric pressure.