Elizabeth McIssac

Kimberlite magmas entrain mantle minerals, including chromites, ilmenites, garnets and diamonds, during their ascent to the Earth’s surface.  Kimberlite magma partially dissolves these minerals during their ascent producing characteristic surface features.  Previous studies established that diamond surface features can be used to infer the chemical composition of magmatic fluid.  Similarly, an examination of more complex mantle minerals, such as chromites and ilmenites, could provide additional information on the chemical composition of fluid systems in magmas.  The purpose of this study was to experimentally investigate the dissolution of chromites and ilmenites in melts with C-O-H fluid.  Natural mineral grains were manually rounded and placed in a synthetic mixture of Ca-Mg-Si-C-H-O with 0, 5, 13, 15, and 31% (w/w) H₂O, and 0, 5, and 27% (w/w) CO₂.  The surface features produced under these conditions were  compared to the surface features on naturally-occurring minerals recovered from kimberlites.  High pressure experiments were conducted using a piston-cylinder apparatus (1350-1400^oC and 1 Gpa).  Scanning electron microscopy was used to evaluate the surface features on the resulting product. Angular step-like dissolution surfaces common among natural kimberlitic chromites were only observed in the presence of an H₂O-rich fluid phase. The reaction between chromite and dry melt resulted in the smoothing of chromite surfaces. Chromite dissolution in CO₂-rich melts produced rounded and polyhedral relief features, which are not typical for natural kimberlite-hosted chromite grains.  Ilmentite underwent rapid dissolution under our experimental conditions.  In H₂O-rich fluid, ilmenite adopted “pyramidal” surface features previously described as the most common surface feature for naturally-occurring kimberlitic ilmenites.  A comparison to natural minerals recovered from Misery and Grizzly kimberlites (NWT, Canada) revealed that Misery chromites and ilmenites possess features similar to those produced experimentally in the presence of H₂O-rich fluid.  This statement is supported by recent data comparing the surface features of  diamonds in H₂O rich fluids to those of Misery diamonds. The surface features of Grizzly chromites were unlike any observed in our experiments and warrant further investigation.  This study supports the hypothesis that xenocrystic minerals surfaces can provide information about the composition of volatiles in mantle-derived magmas.  These results suggest that kimberlites that contain chromites that show angular step-like surfaces indicate the presence of an H₂O-rich fluid during emplacement, and may be associated with high quality diamonds.

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Pages: 118
Supervisor: Yana Fedortchouk