Petrogenesis of Mid-ocean Ridge Basalts

The liquidus and near-liquidus crystallization of primitive, mid-ocean ridge basalt (DSDP3-18-7-1) has been studied experimentally from 0-15 kbar. Olivine is the liquidus phase at low pressure (<12 kbar) and is joined by plagioclase (An₇₈) and clinopyroxene at lower temperature. Calcium-rich clinopyrozene becomes the liquidus phase at ≥ 12 kbar and there is no liquidus or near-liquidus field for orthopyroxene. Orthopyroxene addition experiments confirm that the liquid composition is not close to saturation with orthopyroxene. The low-pressure experiments show that DSP3-18-7-1 could be a derivative liquid by olivine franctionation from a more olivine-rich parent. Olivine addition experiments show that a picritic composition (83 per cent DSDP3-18-7-1 + 17 per cent olivine) is saturated with olivine + orthopyroxene at ~20 kbar, 1430ºC.