Characterising modal metasomatic processes in young continental lithospheric mantle: a microsampling isotopic and trace element study on xenoliths from the Middle Atlas Mountains, Morocco

Abstract

Clinopyroxene is a major host for lithophile elements in the mantle lithosphere, and therefore it is critical whether we are to understand the constraints that this mineral puts on mantle evolution and melt generation. This study presents a detailed in situ trace element and Sr isotope study of clinopyroxene, amphibole and melt from two spinel lherzolites from the Middle Atlas Mountains, Morocco. The results show that there is limited, but discernable, Sr isotopic variation between clinopyroxene crystals within these xenoliths [87Sr/86Sr ranging from 0.703416 (±11 2SE) to 0.703681 (±12 2SE)]. Trace element patterns show similar interelement fractionation with LREE enrichment, but there is a considerable range in terms of elemental concentration (e.g. over 100 ppm in Sr concentrations). Observed modal clinopyroxene is far more abundant than that predicted from estimates of melt depletion. This along with isotope and trace element variability found in these xenoliths supports a multistage metasomatic process in which clinopyroxene and amphibole are recent secondary additions to the lithospheric mantle. Elemental systematics indicate that the metasomatic mineral assemblage has most recently equilibrated with a carbonatitic melt prior to inclusion in the host basalt. The clinopyroxene from this study is typical of global off-craton clinopyroxene in terms of Sr isotope composition, suggesting that the majority of clinopyroxene in off-craton settings may have a recent metasomatic origin. These findings indicate that caution is required when using peridotite xenoliths to estimate the degree of elemental enrichment in the subcontinental lithosphere.