Compositional and thermal state of the upper mantle beneath the Bering Sea basalt province: evidence from the Chukchi Peninsula of Russia

Abstract

Nephelinites and basanites of the Enmelen volcanic field, Chukchi Peninsula, Russia, contain upper mantle xenoliths of relatively calcium- and magnesium-rich spinel lherzolites, pyroxenites, and megacrysts. The phase assemblages of the lherzolites require equilibration near 1.5 GPa, and calculated equilibration temperatures for most inclusions are in the range 850–1030 °C. These temperatures are similar to those calculated for lherzolite inclusions from other Bering Sea localities (Nunivak Island and Seward Peninsula) and are higher than temperatures expected for likely conductive geotherms beneath these volcanic fields. The relatively high temperatures may be the result of magma intrusion into the mantle lithosphere and consequent perturbation of the geotherm shortly before entrainment of the xenoliths in basalt. Two Enmelen lherzolites equilibrated at higher temperatures (1230–1240 °C) and provide further evidence for heating due to intrusive magmas. Some spinel lherzolite inclusions have flat rare earth element patterns and major and trace element abundances close to that of the bulk silicate earth. Based on the occurrence of similar fertile peridotites at Nunivak Island and Seward Peninsula, near-primitive mantle compositions appear to be common in the upper mantle beneath the Bering Sea. These peridotites may represent recent additions to the mantle lithosphere from mantle plumes related to the volcanism. Other Enmelen inclusions are relatively light rare earth element-enriched group I lherzolites metasomatized by a silicate melt, group II pyroxenites precipitated from a variety of melts, and augite megacrysts with convex-upward rare earth element patterns consistent with precipitation from the host basalts at high pressures.