Highly refractory peridotites on Macquarie Island and the case for anciently depleted domains in the Earth's mantle

Abstract

Macquarie Island (Southern Ocean) is a fragment of Miocene ocean crust and upper mantle formed at a slow-spreading ridge system, uplifted and currently exposed above sea-level. The crustal rocks on the island have unusually enriched compositions and the strong signature of an enriched source requires low overall degrees of melt depletion in the underlying mantle. Peridotites on the island, however, are highly refractory harzburgites that can be modeled as residues of >20-25% of near-fractional melting from which all the free clinopyroxene was melted out. The peridotites have some of the highest spinel Cr-numbers (0·40-0·49) and lowest orthopyroxene-core Al2O3 concentrations (2·7-3·0 wt %) reported so far for oceanic peridotites. The peridotites were subsequently modified by melt-rock reactions underneath the Miocene ridge system. The refractory character of the peridotites is inconsistent with the slow-spreading ridge setting as well as with the enriched character of the overlying crust, and must indicate a previous depletion event; the peridotites are not the source residue of the overlying ocean crust on Macquarie Island. Osmium isotopic compositions of peridotite samples are very unradiogenic (187Os/188Os = 0·1194-0·1229) compared with normal abyssal peridotites and indicate a long-lived rhenium depletion. Proterozoic rhenium-depletion ages indicate that these rocks have preserved a memory of an old mantle melting event. We argue that the Macquarie Island harzburgites are samples from an anciently depleted refractory mantle reservoir that may be globally important, but that is generally overlooked because of its sterility; that is, its inability to produce basalts. This reservoir may preserve key information about the history of the Earth's mantle as a whole. © The Author 2009. Published by Oxford University Press.