Abstract
Refractory, primary liquids arising in various oceanic plate-tectonic settings are characterized by high MgO, SiO (sub 2) and Ca/Na, low TiO (sub 2) and generally low incompatible element abundances relative to primary liquids parental to MORB. It is suggested that the former melts segregate from upper mantle peridotite which has earlier been depleted by extraction of picritic melts which were parental to MORB. A compositional range in the second-stage melts is expected, depending on the extent of previous depletion of the peridotite, the T and P of melt segregation and the possible influence of volatile phases (C-H-O) present during melting. An example of a second-stage melt-magnesian quartz tholeiite-from the upper pillow lavas of the Troodos ophiolite, Cyprus, is described. This composition has appropriate liquidus phases to have segregated from depleted upper mantle at approx 25 km, 1360 degrees C, leaving a harzburgite residue. The experimental studies are applied to interpretation of cooling histories and water contents of specific upper pillow lavas. Magma batches are estimated to have contained 0.5-1.0% H (sub 2) O. Picritic lavas quenched from olivine + liquid at <5 kbar. Magnesian, pyroxene-phyric lavas exhibit intratelluric crystallization at approx 5 kbar, 1270 degrees C. These and other second-stage melts will crystallize extremely refractory minerals. Multi-stage melting reconciles some of the difficulties in relating ophiolite and ocean-floor basalt compositions and is an important process in ocean crust formation.