Rapid ascent of kimberlites as indicated by coexisting melt and fluid phases in peridotites

[摘要] This thesis involves the investigation of OH- in defect structures within garnets takenfrom peridotites. The information obtained from this is used to estimate the ascent rateof kimberlites to the surface. The importance of determining this ascent rate involves theresorption rate of diamond, which are inversely proportional to each other, as well asthe energy needed for the kimberlitic melt to raise dense xenoliths to the surface. Thesamples used in this study are ten peridotitic xenoliths from the Bultfontein kimberlitemine, South Africa. The samples range from garnet to spinel peridotites and are eitherharzburgites or lherzolites. Scanning Electron Microscopy (SEM) was used to determinemineral chemistry which was also used in a geothermobarometric study. FourierTransform InfraRed (FT-IR) spectroscopy investigated the mentioned defect structuresfor OH- and other volatiles, which were not present. An optical petrographic study alsotook place. During the petrographic investigation, serpentine and phlogopite wereobserved and dissolution of garnet to spinel. Serpentine suggests hydration and thephlogopite shows evidence of an Al-, Ti- and K-rich, hydrous silicate melt. Garnet isunstable when interacting with melt at <40 km depth below the surface andtemperatures greater than 850 ºC, forming spinel. Temperatures obtained in this studyrange from 1145 K�?893 K and pressures range from 0.56 GPa�?.03 GPa for varioussamples. The variations are owing to different metamorphic grades of the samples. Thevariation in results on the same sample is due to the effect that different analyticalmethods have on the accuracy of the geothermobarometry. Using the diffusion rate ofOH- from within a defect structure out into the matrix of a garnet grain, the ascent ratefor the kimberlite was determined and ranges between 30min to a couple of hours.

[发布机构] University of the Free State