CHEMICAL TRENDS IN ABYSSAL PERIDOTITES - REFERTILIZATION OF DEPLETED SUBOCEANIC MANTLE

It is suggested that the chemical trends in abyssal peridotites result dominantly from two processes. The first process is fractional melting of the suboceanic mantle (Johnson et al., 1990) to the point at which clinopyroxene (CPX) is essentially eliminated from the residual mantle. The residue at this stage is depleted harzburgite with either no CPX or <2% CPX that has very low abundances of Na2O (0.01 wt %), Ti (300 ppm), Zr (0.1 ppm) and Ce (0.02 x chondrites). The composite liquids produced during melting are comparable to primitive mid-ocean ridge basalt glasses (1.5-2.6% Na2O, 5500-7500 ppm Ti, 50-80 ppm Zr). The second process is low-pressure (<5 kbar) refertilization of the depleted harzburgite with the addition of up to 10% depleted basaltic liquids; following recrystallization, mantle peridotites at this stage are CPX-bearing harzburgites and lherzolites. The chemical trends in abyssal peridotites are suggested to result dominantly from these refertilization processes rather than from partial melting. The rejection of fractional melting as the dominant process controlling the chemical trends in abyssal peridotites is based principally on the Na2O versus MgO variations in abyssal peridotites; the Na2O abundances in abyssal peridotites at 42% Mgo (0.055 wt %) are 30 times higher than the Na2O abundances (0.002 wt %) in residual mantle formed by fractional melting of primitive upper mantle.