Rapid ascent of kimberlites as indicated by coexisting melt and fluid phases in peridotites
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Date
2013-07
Authors
Purchase, Megan Dayl
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
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.
Description
Keywords
Dissertation (M.Sc. (Geology))--University of the Free State, 2013, Kimberlite, Diamonds, Kimberlite -- Inclusions