Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
4700786 | Chemical Geology | 2007 | 20 Pages |
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.δ18O 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 δ18O 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 δ18O 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 δ18O values of 1.5 to 5.5‰ interpreted to have formed from, or been overprinted by, locally-derived, high CO2/H2O deuteric fluids at 500 to 100 °C. δ13C values of the various types of calcite and dolomite (− 8 to − 3 ‰) have no systematic variation with δ18O values and, therefore, there is no isotopic record of volatile degassing from these hypabyssal bodies.