Petrology of mantle wedge lithosphere: New data on supra-subduction zone peridotite xenoliths from the andesitic Avacha volcano, Kamchatka

Abstract

Peridotite xenoliths in arc volcanoes are very rare, usually small and remain poorly studied. Much of the earlier work focused on peridotites affected by re-crystallization, metasomatism and veining that took place shortly before the eruption of the host magmas; such lithologies may not be widespread in the mantle wedge. This study reports petrographic, major and trace element data for 17 large, fresh peridotite xenoliths from the active Avacha volcano and discusses the origin of supra-subduction zone lithospheric mantle, in particular the role and characteristics of partial melting and metasomatism. The xenoliths are spinel harzburgites containing interstitial cpx (1·5-3%) and amphibole (≤1%). Nearly all are medium- to coarse-grained with protogranular to granoblastic microstructures; some have fine-grained domains and thin cross-cutting veins of secondary opx and olivine. Core-rim zoning and unmixing of cpx and spinel in coarse opx indicate long-term cooling to ≤900-1000°C; Cr#Sp and Al and Cr in opx are correlated with equilibration temperatures. The peridotites are highly refractory, with ≥44% MgO and very low Al2O3 and CaO (0·4-0·9%), TiO2 (≤0.01%), Na2O (≤0.03%), K2O and P2O5 (below detection) and REE in whole-rocks, ≤2·1% Al2O3 in opx and ≤0·1-0·3% Na2O in cpx. Comparisons of Mg, Al and Fe contents with melting experiments indicate 28-35% melt extraction at ≤1 to 2 GPa, in line with the absence of primary cpx and high Mg#Ol (0·907-0·918) and Cr#Sp (0·53-0·65). Bulk-rock Al2O3 is a more robust melt extraction index than Cr#Sp, Mg#Ol and Mg#WR. Forearc harzburgites and certain xenoliths from the Western Pacific share many of these characteristics with the Avacha suite and may have similar origins. A distinctive feature of the Avacha harzburgites is a combination of variable but commonly high modal opx (18-30%) with very low modal cpx. At a given olivine or MgO content, they have higher opx and SiO2, and lower cpx (as well as Al2O3 and CaO) than typical refractory peridotite xenoliths in continental basalts. These features may indicate fluid fluxing during melting in the mantle wedge. Alternatively, they could have been produced after partial melting by selective metasomatic enrichment in SiO2 by fluids to transform some olivine into opx, although the latter mechanism is hard to reconcile with the very low alkalis and REE contents and the absence of silica correlation with fluid-mobile elements. Bulk-rock enrichments in silica and opx are unrelated to the presence or abundance of late-stage, fine-grained materials and are due to an ancient event rather than recent re-crystallization and veining. © The Author 2010. Published by Oxford University Press.