COMPOSITIONAL TRENDS OF MINERALS IN OCEANIC CUMULATES

Five differences are noted between the compositions of minerals in oceanic cumulates and minerals that crystallize from mid-ocean ridge basalts (MORBs) at 1 atm. These differences are (1) Mg numbers of clinopyroxene (CPX) in oceanic cumulates are often higher (>88) than found in experimental studies of MORBs at 1 atm; (2) Mg numbers of orthopyroxene (OPX) in oceanic cumulates are often higher (>81) than found in experimental studies of MORBs at 1 atm; (3) the low-Ca pyroxene in oceanic cumulates (with some notable exceptions) is OPX rather than (inverted) pigeonite; (4) Na2O contents of CPX in oceanic cumulates are often higher (0.3 to 0.8% Na2O) than in 1-atm experiments or phenocrysts in MORBs (typically 0.15 to 0.35% Na2O); and (5) TiO2 contents of CPX in oceanic cumulates are often higher than in 1-atm experiments. These characteristics are consistent with crystallization of many oceanic gabbros at moderate pressures (3-10 kbar). The Na2O and TiO2 contents of liquids calculated to be in equilibrium with CPXs from oceanic cumulates generally are comparable to the abundances of these oxides in MORBs. The notable exception is the suite of cumulates from Deep Sea Drilling Project site 334, which appears to have crystallized at low pressures from strongly depleted Na2O-poor, TiO2-poor, SiO2-rich basalts. We suggest that such strongly depleted basaltic liquids are a widespread, but minor, component in mid-ocean ridge petrogenesis even though such liquids have never been directly sampled.