Clinopyroxene microtextures reveal incompletely extracted melts in abyssalperidotites

Textural evidence is interpreted to suggest that in regions where upwelling rates of the mantle are slow to very slow, a small amount (similar to2%) o f melt was present when plagioclase-free abyssal peridotites entered the co nductive regime at the base of the oceanic lithosphere. Upon crystallizatio n, this melt appears to have been undersaturated in orthopyroxene, but prec ipitated clinopyroxene, Al-rich and Ti-poor spinel, and sulfides. Furthermo re, the primary clinopyroxene grains have rare earth element patterns typic al of residues of fractional melting, suggesting that the interstitial liqu ids were incremental partial melts rather than having mid-oceanic-ridge bas alt compositions.