MIDOCEAN RIDGE MELTING - CONSTRAINTS FROM LITHOSPHERIC XENOLITHS AT OAHU, HAWAII

One plagioclase-spinel lherzolite and four spinel lherzolite xenoliths from Oahu, Hawaii, contain clinopyroxene grains that show homogeneous rare earth element (REE) abundances and smooth REE patterns with syst ematic depletion of light (REE) (LREE). These five xenoliths are mid-o cean ridge basalt (MORB) magma-depleted residues with compositions tha t were not modified by later metasomatism. Trace element systematics o f these xenoliths were used to investigate the melt production rate (d F/dP) within a 90-my-old residual mantle column (RMC). Such rates were calculated as ratios of the difference in extents of depletion (dF) t o the difference in equilibrium pressures (dP) between two xenolith sa mples. The extents of melting were modeled from REE, Sr and Zr abundan ces in clinopyroxene; and equilibrium pressures were inferred from the two-pyroxene geobarometer of Mercier et al. (1984, Contributions to M ineralogy and Petrology, 85, 391-403). Equilibrium pressures range fro m 21 to 7 kbar and extents of melting vary from 2 to 8%. Together, the se data constrain the maximum extent of melting in the garnet lherzoli te stability field to be <2% and a dF/dP of 0.43%/kbar within the stab ility field of spinel lherzolite. Uncertainties in the estimates of eq uilibrium pressure and extent of depletion lead to a slightly broader range of dF/dP values (0.26-0.78%/kbar). These values are significantl y lower than that of similar to 1.2%/kbar suggested by most previous s tudies. With the best estimated mean dF/dP of 0.43%/kbar, only 3.9 km of crust could have been generated by melting lherzolite in the pressu re range of 26-7 kbar. The thickness and composition of the crust that overlies the 90 Ma Oahu RMC require a higher extent of melting. Based on the REE abundances, most samples from the 90 Ma crust and East Pac ific Rise can be explained as pooled melts derived from the lherzoliti c source in a passive melting regime with a maximum extent of melting (F-max) of 30%. This model produces a dF/dP of 4.4%/kbar in the pressu re range of 7-2 kbar and generates an additional 1.8 km of crust. In d etail, the high [Sm/Yb](DM) ratios in most East Pacific Rise samples c annot be explained by melting a lherzolitic source. Instead, they refl ect mixing between two components: (1) the pooled melt derived from lh erzolitic source in a passive melting regime with an F-max of 30% (sim ilar to 80% of the mixture) and (2) melt derived from garnet pyroxenit e by 40% fractional melting (similar to 20% of the mixture). This mode l produces 7.1 km of crust.