Abstract
Mantle minerals undergo dissolution while being transported by kimberlite magma from the depth to the surface of the Earth. In this study, we experimentally investigate morphology of dissolution features developed on chromite crystals in the presence of H2O-rich and CO2-rich fluid and compare them to the features on natural kimberlite-borne chromite macrocrysts from Misery, Beartooth, and Grizzly kimberlites, Ekati Mine, Canada. The experiments were done in a piston-cylinder apparatus at 1,350–1,400 °C and 1 GPa in a synthetic CaO–MgO–SiO2 ± H2O–CO2 system with 0, 5, 13, and 15 wt % H2O, and 5 and 27 wt % CO2. The surface of chromite crystals with experimentally and naturally induced resorption was studied using a field-emission scanning electron microscope (FE-SEM). It showed angular or smooth steplike dissolution features developed in the presence of H2O-rich fluid and highly irregular surfaces with cavities, knobs, and polyhedrons developed in the presence of CO2 fluid. Naturally resorbed chromite grains from volcaniclastic Ekati kimberlites (Misery and Beartooth) show steplike features that are like the products of H2O-bearing experiments, whereas grains from hypabyssal (coherent) Grizzly kimberlite have irregular surfaces with flakes and spikes that do not resemble any of our experimental products. We propose a high H2O:CO2 ratio in kimberlitic fluid and a potential for using chromite morphology for investigating the volatile system within kimberlite magma and its effect on diamond preservation.
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Acknowledgments
We thank Jon Carlson and BHPBilliton Diamonds Inc. for providing kimberlite concentrate for this study. Dan MacDonald is thanked for the help with EMP analyses, Patricia Scallion for the help with SEM work, and Institute for Research in Materials for the access to the FE-SEM funded by Canada Foundation for Innovation. We thank Chris Smith for very helpful review of the earlier version of this manuscript. This research was supported by NSERC of Canada Discovery and RTI grants to YF.
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Fedortchouk, Y., McIsaac, E. (2013). Surface Dissolution Features on Kimberlitic Chromites as Indicators of Magmatic Fluid and Diamond Quality. In: Pearson, D., et al. Proceedings of 10th International Kimberlite Conference. Springer, New Delhi. https://doi.org/10.1007/978-81-322-1170-9_19
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DOI: https://doi.org/10.1007/978-81-322-1170-9_19
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