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

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. delta O-18 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 parts per thousand, consistent with their formation at magmatic temperatures (e.g.,> 750 degrees C) from a kimberlite melt. In contrast, higher delta O-18 values of 9 to 14 parts per thousand 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 degrees C. These higher delta O-18 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 delta O-18 values of 1.5 to 5.5 parts per thousand interpreted to have formed from, or been overprinted by, locally-derived, high CO2/H2O deuteric fluids at 500 to 100 C. delta C-13 values of the various types of calcite and dolomite (-8 to -3 parts per thousand) have no systematic variation with delta O-18 values and, therefore, there is no isotopic record of volatile degassing from these hypabyssal bodies. (c) 2007 Elsevier B.V. All rights reserved.