Constraints on the composition, source and petrogenesis of plagioclase-bearing mantle peridotite

Abstract

Although plagioclase-bearing mantle rocks are rare as xenoliths in intraplate alkalic basalts they are relatively common in ophiolite and abyssal peridotites. In most settings plagioclase peridotites provide insights into understanding melt depletion and refertilisation in the shallow upper mantle. Using a compilation of 1075 bulk rock and mineral (feldspar, clinopyroxene and spinel) analyses the geochemistry and mineralogy of plagioclase peridotite from intraplate and ophiolite, alpine and abyssal (referred to here as “tectonic”) settings have been characterised and these data have been used to discuss the origin of plagioclase peridotite. Bulk rock samples of plagioclase peridotite from intraplate settings and a sub-set of the clinopyroxenes contained within them have chondrite normalised rare earth element (REE) patterns showing light REE enrichment. A second subset of clinopyroxenes in intraplate settings shows LREE depletion. The intraplate setting samples contain plagioclase with (relative to tectonic setting data) lower anorthite content and mafic minerals that have comparatively higher Mg# numbers. Element abundances in, and modelling of, intraplate plagioclase peridotite are interpreted to reflect lower degrees of partial melting and melt extraction than those prevailing during the evolution of plagioclase peridotites formed in tectonic settings. The chemical variation observed in intraplate plagioclase peridotite can be produced by up to 8% partial melting and melt extraction. The bulk rock patterns and LREE enriched clinopyroxenes from intraplate settings can be modelled by the addition of up to 5% LREE-enriched alkalic, ocean island basalt (OIB)-like melt. The LREE depleted clinopyroxenes may be refertilised by a normal mid ocean ridge basalt (N-MORB)-like melt. The geochemistry of a number of intraplate setting bulk rock samples also requires the addition of up to 0.2% carbonatite melt. Tectonic plagioclase peridotite trace element patterns can be explained by up to 25% partial melting and melt extraction followed by the addition of up to 8% N-MORB-like melt. The bulk rock median values for mantle normalised elements in intraplate plagioclase peridotite are systematically enriched relative to spinel facies subcontinental lithospheric mantle and for tectonic plagioclase peridotite are within error of or more depleted than spinel facies, depleted, mid ocean ridge mantle. These similarities and differences are best explained by higher degrees of partial melting and melt extraction during the formation of the tectonic plagioclase peridotite and refertilisation by enriched (OIB-like) melts in the case of intraplate plagioclase peridotite. Melt refertilisation was a significant and essential process in the formation of the majority of plagioclase peridotites considered in this study.