MELTING IN THE OCEANIC UPPER MANTLE - AN ION MICROPROBE STUDY OF DIOPSIDES IN ABYSSAL PERIDOTITES

A systematic study of rare earth and other trace elements in discrete diopsides from residual abyssal peridotites sampled from 5000 km of ocean ridge demonstrates that they are the residues of variable degrees of melting in the garnet and spinel peridotite fields. Further, the data clearly demonstrate that the peridotites are the residues of near-fractional melting, not batch melting, and that typical abyssal basalt can evolve from aggregated fractional melts. Ion microprobe analyses of diopsides in abyssal peridotites from fracture zones along the America-Antarctica and Southwest Indian ridges reveal ubiquitous extreme fractionation of rare earth elements (REE) ([Ce/Yb]n=0.002-0.05); depletion of Ti (300-1600 ppm), Zr (0.1-10 ppm), and Sr (0.1-10 ppm); and fractionation of Zr relative to Ti (Ti/Zr=250-4000). -from Authors