Rapid ascent of kimberlites as indicated by coexisting melt and fluid phases in peridotites
Purchase, Megan Dayl
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This thesis involves the investigation of OH- in defect structures within garnets taken from peridotites. The information obtained from this is used to estimate the ascent rate of kimberlites to the surface. The importance of determining this ascent rate involves the resorption rate of diamond, which are inversely proportional to each other, as well as the energy needed for the kimberlitic melt to raise dense xenoliths to the surface. The samples used in this study are ten peridotitic xenoliths from the Bultfontein kimberlite mine, South Africa. The samples range from garnet to spinel peridotites and are either harzburgites or lherzolites. Scanning Electron Microscopy (SEM) was used to determine mineral chemistry which was also used in a geothermobarometric study. Fourier Transform InfraRed (FT-IR) spectroscopy investigated the mentioned defect structures for OH- and other volatiles, which were not present. An optical petrographic study also took place. During the petrographic investigation, serpentine and phlogopite were observed and dissolution of garnet to spinel. Serpentine suggests hydration and the phlogopite shows evidence of an Al-, Ti- and K-rich, hydrous silicate melt. Garnet is unstable when interacting with melt at <40 km depth below the surface and temperatures greater than 850 ºC, forming spinel. Temperatures obtained in this study range from 1145 K–1893 K and pressures range from 0.56 GPa–6.03 GPa for various samples. The variations are owing to different metamorphic grades of the samples. The variation in results on the same sample is due to the effect that different analytical methods have on the accuracy of the geothermobarometry. Using the diffusion rate of OH- from within a defect structure out into the matrix of a garnet grain, the ascent rate for the kimberlite was determined and ranges between 30min to a couple of hours.