Composite xenoliths from Spitsbergen: evidence of the circulation of MORB-related melts within the upper mantle

Abstract

AbstractThe Sverrefjell Quaternary volcano in Spitsbergen contains composite xenoliths showing lherzolite rocks cross-cut by websterite veins. These two rock types are characterized by similar major element compositions of olivines, orthopyroxenes, clinopyroxenes and spinels, as well as similar trace element composition for clinopyroxene. The clinopyroxenes of both rock types mostly display upwards convex or spoon-shaped REE (rare earth elements) patterns with a systematic enrichment in La over Ce (CeN/YbN 0.72–1.32; SmN/YbN 0.86–1.93 and LaN/CeN 1.27–1.93), except for one sample (SV-69) in which clinopyroxenes show a pattern characterized by low LREE compare to HREE (CeN/YbN 0.33–0.35). Metasomatic processes appear to be the most reasonable origin to form the lherzolite–websterite associations. We therefore propose that the Spitsbergen mantle has undergone at least two events: (1) a sub-alkaline (tholeiitic) metasomatism followed by (2) an alkaline metasomatic event.