Origin of Geochemical Heterogeneity in the Mantle Peridotites from the Bay of Islands Ophiolite, Newfoundland, Canada: Ion Probe Study of Clinopyroxenes

Abstract

Representative samples from a 6 km thick ophiolitic mantle section were selected for ion probe analysis of clinopyroxene (cpx). The purpose was to evaluate the trace element heterogeneity occurring on a kilometer-scale in the geologically well constrained uppermost mantle beneath the ancient Bay of Islands ophiolite spreading center. The range of REE patterns in the mantle section can be explained by 7–20% near-fractional melting of a weakly depleted mantle source followed by varying degrees of trace element chromatographic exchange between an interstitial liquid and the refractory residue and/or precipitation of phases from the interstitial liquid. Peridotites from the basal mantle section are the least depleted ones. They require little to no overprint by melt circulation and can be modeled as residues of fractional melting with a low (0.1%) residual porosity. Their presence in the mantle section 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 variability in the degree of depletion. A preferred, but nonunique model for their 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 occurring 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 volume of mantle rocks.