Sr–Nd–O isotopic evidence of variable sources of mantle metasomatism in the subcratonic lithospheric mantle beneath the Grib kimberlite, northwestern Russia

Abstract

To provide new insights into the type and extent of mantle metasomatism in the subcratonic lithospheric mantle, we examined the Sr–Nd–O isotopic compositions of orthopyroxene, clinopyroxene, garnet, ilmenite and phlogopite from sheared garnet lherzolite, granular garnet harzburgites and lherzolites and clinopyroxene-phlogopite rocks from the Grib kimberlite in the Arkhangelsk diamond province in northwestern Russia.Clinopyroxene and orthopyroxene from sheared garnet lherzolite initially have a close value of 87Sr/86Sr(t) (~0.7034) and close weak positive εNd. Orthopyroxene and clinopyroxene are in oxygen isotope equilibrium with coexisting olivine. Clinopyroxene from a garnet harzburgite has a low 87Sr/86Sr(t) isotope ratio of 0.70266. Clinopyroxene from granular garnet lherzolites has a relatively narrow variation in 87Sr/86Sr(t) (0.70456–0.70582) and considerably larger variations in εNd (−4.3 − +1.0) isotope ratios. Garnet displays elevated initial 87Sr/86Sr(t) values (0.70540–0.70633). Ilmenite shows a narrow range in 87Sr/86Sr(t) (0.70497–0.70522) coupled with εNd values of +0.4 and +3.5. These isotopic data suggest granular garnet lherzolite of mantle metasomatism took place during the interaction of kimberlite melts with SCLM that contained mica-amphibole-rutile-ilmenite-diopside (MARID)-type metasomes. Clinopyroxenes from clinopyroxene-phlogopite (phlogopite wehrlite) xenoliths display a broader range in 87Sr/86Sr(t) (0.70486–0.70813) that is significantly higher than the kimberlite values and a circa-chondritic εNd (−0.1 − +1.3) with a restricted δ18O range (5.11‰–5.33‰). More radiogenic Sr isotopic composition decoupled from Nd isotopes could have been induced by metasomatic melt/fluid related to a subducted material.The isotopic compositions of mantle minerals preserve Sr–Nd isotopic evidence of pre-kimberlite metasomatic events that were probably due to incomplete reequilibration with ultramafic carbonated melt. Based on mineral pairs Rb–Sr isochrons and a clinopyroxene-based Sm–Nd errochron, these mantle metasomatic events correspond to ~550–600 Ma and ~1200 Ma episodes of magmatic–thermal activity.