Quest for low-degree mantle melts

Baker et al. report an experimental study at pressures of 1 GPa (1,000 million Pa) to determine the composition of a silicate melt produced at anhydrous, near-solidus conditions from a lherzolite composition. They obtained a composition at 1,250 °C with -57% Si0₂ and -5.7% Na₂0, which is 'andesitic' in the sense of its low normative diopside (5% ), high normative hypersthene (> 15%) and high normative plagioclase (> 75%, andesine). These results conflict with earlier conclusions that anhydrous melts of fertile lherzolite at 1 GPa are basaltic, with ~15-20% normative diopside, > 10% normative olivine and, at low degrees of melting, nephelinenormative. From our own experience and experimental tests, we suggest the following possible explanations for the observations of Baker et al.