Stable isotope composition of magmatic and deuteric carbonate phases in hypabyssal kimberlite, Lac de Gras field, Northwest Territories, Canada

Abstract

The oxygen and carbon isotope compositions of carbonates from thirteen, Late Cretaceous to Paleogene hypabyssal intrusions from the Lac de Gras kimberlite field indicate that the carbonates are well-preserved and have a complex paragenetic formation history. Kimberlites at ten localities studied are calcite-bearing based on the presence of phenocrysts, microphenocrysts and/or millimeter-scale, calcite-only segregations of Sr-rich calcite. The calcite-bearing kimberlites also contain large (cm-scale), Sr-poor calcite + serpentine segregations that are texturally distinct from the smaller, Sr-rich calcite-only segregations. Kimberlites at three other localities studied are dolomite-bearing based on the presence of calcite + dolomite segregations, some of which preserve complex, oscillatory and banded textures of calcite, dolomite, and magnesian calcite. δ 18O values for whole-rock calcite in calcite-bearing kimberlite and for micro-samples of calcite from individual segregations in calcite- and dolomite-bearing kimberlite generally vary from 6 to 9‰, consistent with their formation at magmatic temperatures (e.g., > 750 °C) from a kimberlite melt. In contrast, higher δ 18O values of 9 to 14‰ characterize micro-samples of calcite from calcite + serpentine segregations in calcite-bearing kimberlite, as well as whole-rock dolomite and micro-samples of dolomite from segregations in dolomite-bearing kimberlite. These data are consistent with formation from deuteric fluids (late-stage, magmatic) at sub-solidus temperatures (e.g., 500 to 100 °C). These higher δ 18O values for whole-rock calcite also correlate with increased intensity of forsterite alteration, increased abundance of calcite + serpentine segregations, textural overprint of primary groundmass minerals by anhedral calcite and serpentine, and development of atoll-spinels, all interpreted as a result of deuteric fluids. In contrast, whole-rock calcite in dolomite-bearing kimberlite has notably lower δ 18O values of 1.5 to 5.5‰ interpreted to have formed from, or been overprinted by, locally-derived, high CO 2/H 2O deuteric fluids at 500 to 100 °C. δ 13C values of the various types of calcite and dolomite (− 8 to − 3 ‰) have no systematic variation with δ 18O values and, therefore, there is no isotopic record of volatile degassing from these hypabyssal bodies.