Open-system O-isotope behaviour and trace element enrichment in the sub-Eifel mantle

Abstract

Spinel peridotites from the West Eifel, western Germany exhibit a wide range of δ 18O SMOW values for olivine ( +5.1 to +6.3‰), smaller ranges for pyroxenes ( +5.7 to +6.3‰ for orthopyroxene; +5.6 to +6.4‰ for clinopyroxene) and variable 18O/ 16O fractionations between olivine and pyroxenes ( Δ clinopyroxene-olivine = +0.6 to −0.3‰ and Δ orthopyroxene-olivine = +0.7 to −0.1‰). These features are characteristic of a disequilibrium distribution of O-isotopes between coexising olivine and pyroxenes. Enrichment in 18O is observed primarily in olivine from amphibole and/or phlogopite-bearing peridotites and correlates with degree of LREE/HREE enrichment, indicating that the processes responsible for trace element enrichment (i.e. mantle metasomatism) and O-isotope disequilibrium are linked. The 18O- and incompatible trace element-enriched fluid(s) cannot be generated by intra-mantle fractionation processes as presently understood, and are attributed to recycling of altered oceanic lithosphere and/or pelagic sediments into the mantle via subduction. This is supported by δD SMOW values for associated Eifel amphibole and mica ( −50 to −45‰ respectively) which fall midway between δD values for upper mantle ( −80‰) and seawater (0‰). Coexisting amphibole and mica also have distinctly different δ 18O values ( +7.0 and +5.9‰, respectively) which suggests that they did not originate from the same fluids/melts. This implies that the sub-Eifel mantle has experienced at least two distinct metasomatic episodes.