Oxygen isotope constraints on the origin of high-Cr garnets from kimberlites
The association between diamonds and high-Cr, sub-calcic garnets from kimberlites suggests that garnets have experienced high pressure (and presumably, high temperature) conditions in the subcontinental lithosphere mantle (SCLM). The oxygen isotope compositions of these garnets are generally reported to be lower than average values of mantle olivines (~5.2‰)—opposite the sense of oxygen isotope fractionation expected at high-temperature equilibrium, and to correlate negatively with their Cr-contents. These observations were interpreted to indicate the sub-calcic garnets either are derived from melting Cr-enriched but ^(18)O-depleted source in the SCLM, or have experienced cryptic metasomatism by high-Cr, high-Mg#, but ^(18)O-depleted fluids/melts in the SCLM or during their ascent to the surface. We investigate the possibility that oxygen isotope characteristics of these garnets instead reflect an analytical artifact that reduces the measured δ^(18)O of garnets in proportion to its Cr content—a so-called 'Cr-effect'. Mixtures of garnet standards (UWG-2) plus various amounts of pure Cr-metal were measured for δ^(18)O-values by infrared-laser fluorination technique (ILFT). Our results show that oxygen isotope compositions of sample mixtures vary systematically as a function of Cr-content and O_(2)-yield. Mass-balance analyses indicate that the correlation between the measured δ^(18)O-value and Cr-content is an analytical artifact mostly due to isotopic fractionation between the extracted-O_(2) and chromium oxyfluorides left as residues in the sample chamber during the ILFT analyses, which amounts to a decrease in the measured δ^(18)O values of ~0.064‰ for the increase of every wt.% Cr_(2)O_(3) present in garnets. After applying our experimentally-calibrated correction factor to garnet samples in garnet peridotite xenoliths from South Africa, our study reveals an equilibrium oxygen isotope fractionation between the Cr-poor garnet and coexisting olivine (i.e., garnet≥olivine) with a fractionation factor of ~0.28‰. More importantly, high-Cr garnets (Cr_(2)O_(3)>5%), after correction for the 'Cr-effect', are actually enriched, rather than depleted, in 18O compared with low-Cr garnets, suggesting the involvement of ^(18)O-enriched crustal materials during their formation. We hypothesize a fluid-flow model using depthdependent Cr partitioning. This model simultaneously explains relatively high-δ^(18)O-values and Cr and Mg enrichment in garnets, implying that the reduced C\H\O fluids that are responsible for the formation of diamonds in both eclogite and peridotite xenoliths in kimberlites, are also responsible for transporting Cr to oxidized locations where the Cr partition coefficient between garnet and fluids is high.
Additional Information© 2011 Elsevier B.V. Received 28 March 2011; Received in revised form 12 September 2011; Accepted 16 September 2011; Available online 20 November 2011. ZW would like to dedicate this paper to Ms. Nami Kitchen who taught ZW most skills on ILFT. ZW also wants to thank Prof. John W. Valley for providing UWG-2 standard, Gerry Olack for his help to build laser-fluorination line at Yale university, James Eckert for electron-microscope analysis, Marc Hirschman, Dmitri Ionov, Roberta L. Rudnick for helpful discussions, and Danny Rye and Karl Turekian for inspirations in the coffee hours and encouragement during this research. This paper benefits greatly from the constructive review of Prof. John Valley and two anonymous reviewers and informative suggestions by Dr. R.W. Carlson. This study is funded by Yale startup fund to ZW. LDEO Contribution No. 7499; IPGP Contribution No. 3223.
Supplemental Material - mmc1.pdf