Distribution of incompatible trace elements between the constituents of spinel peridotite xenoliths: ICP-MS data from the East African rift

Abstract

To constrain the geochemical models of the lithospheric mantle, we have carried out a detailed study of the distribution of incompatible trace elements between the various constituents of spinel peridotites. Predominant and accessory minerals were separated in 12 mantle xenoliths from Mega (East African Rift, Sidamo region, SE Ethiopia). The samples range in composition from cpx-rich lherzolites to refractory harzburgites and are devoid of modal metasomatism, except for minor amount of apatite in some of them. Their trace element concentration encompasses almost the whole range reported in the literature for basalt-born xenoliths. Mineral separates (ol, opx, cpx, spinel and apatite) and their leachates were analyzed by ICP-MS, for rare earth elements (REE) and several incompatible trace elements (Rb, Sr, Ba, Zr, Hf, Nb, Ta, Th, U, and Ti). Spinel surfaces were investigated by SEM and EPMA to determine the composition of the attached micro-phases. Mass-balance inversion shows that the trace element composition of whole rocks is controlled by five distinct components: 1). the silicate minerals which account for the total HREE abundance, and 50–90% of LREE, Sr, and Zr-Hf, in the apatite-free peridotites; 2). the mineral-hosted fluid inclusions which play a significant role for Rb (20–25%), and to a lesser degree for the other LILEs; 3). a pervasive grain-boundary component selectively enriched in highly incompatible elements, which contributes 25–90% of the whole-rock budget for Ba, Th and U, and 10–50% for Nb and LREE, in the apatite-free samples; 4). thin reaction layers (<10 μm thick) coating the surfaces of spinel grains and mainly composed of Ti-oxides and phlogopite. They are the predominant repository of Nb-Ta (45–60%) and Rb-Ba (30–80%) in all the studied xenoliths; 5). apatite which largely predominates the budget of Th, U, Sr and LREE (25–75%) in the samples containing this mineral. Compared to the other peridotite constituents, fluid-derived inclusions in minerals provide minor contribution to the trace element budget of whole rocks. However, they strongly affect inter-mineral trace element partitioning. The latter strongly deviates from experimental distribution coefficients for the most incompatible elements and tends to one for elements such as Rb (±Ba, Th and U). This indicates that the different rock-forming minerals contain similar amounts of homogeneous inclusions. Fluid-derived inclusions may be responsible for the discrepancies that have been observed between inter-mineral trace-element partitioning derived from in situ and bulk mineral analyses.