ORIGIN OF GEOCHEMICAL HETEROGENEITY IN THE MANTLE PERIDOTITES FROM THE BAY OF ISLANDS OPHIOLITE, NEWFOUNDLAND, CANADA - ION PROBE STUDY OF CLINOPYROXENES

Representative samples from a 6 km thick ophiolitic mantle section wer e selected for ion probe analysis of clinopyroxene (cpx). The purpose was to evaluate the trace element heterogeneity occurring on a kilomet er-scale in the geologically well constrained uppermost mantle beneath the ancient Bay of Islands ophiolite spreading center. The range of R EE patterns in the mantle section can be explained by 7-20% near-fract ional melting of a weakly depleted mantle source followed by varying d egrees of trace element chromatographic exchange between an interstiti al liquid and the refractory residue and/or precipitation of phases fr om the interstitial liquid. Peridotites from the basal mantle section are the least depleted ones. They require little to no overprint by me lt circulation and can be modeled as residues of fractional melting wi th a low (0.1%) residual porosity. Their presence in the mantle sectio n is possibly related to ductile accretion of less depleted peridotite during ophiolite obduction. Peridotites from the central, but mainly uppermost mantle section require stronger interaction with percolating melts. Uppermost peridotites also show the highest spatial variabilit y in the degree of depletion. A preferred, but nonunique model for the ir formation is refertilization of initially more refractory residues due to cpx +/- plagioclase addition from liquids depleted relative to N-MORB. Such depleted liquids are also inferred from wehrlitic rocks o ccurring in the crust-mantle transition zone. They may have originated as melts emerging from mantle columns or mixtures of such melts, i.e. , melts whose compositions were affected by reaction with a large volu me of mantle rocks. Copyright (C) 1998 Elsevier Science Ltd.