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Title: Polymict peridotites and mantle processes
Author: Morfi, Lisa
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2001
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Olivine and orthopyroxene with smaller amounts of garnet dominate the mineralogy of the ‘clastic’ fragments. The matrix assemblage largely consists of phlogopite and ilmenite with a lesser proportion of orthopyroxene and, rarely, garnet. The cores of individual fragmented mineral grains exhibit an extensive range of major-minor element compositions compatible with derivation from various mantle peridotites, eclogites and megacrysts. In contrast, the rims of specific mineral phases often converge on common compositions, which may also be found in matrix phases. Thus the rims appear to be overgrowths around the original fragmental grains and represent crystallisation products from the melt transporting the fragments. The polymict matrix shows similarities of mineral assemblage and composition to the ilmenite-rutile-phlogopite-sulphide (IRPS) metasomatic suite documented in mantle peridotites from Matsoku and elsewhere. The polymict xenoliths therefore provide evidence that the melt type responsible for this metasomatism, is widely distributed though of presumably very small volume. Trace element compositions show more complex patterns of variation than the major-minor elements. Some convergence of rim matrix compositions is seen in some phlogopites, but the orthopyroxenes show a scatter of compositions suggestive of highly localised crystallisation and perhaps partial re-equilibration. In the one case showing matrix garnet, the garnet rim and matrix trace element compositions are similar, and the rims of orthopyroxene and phlogopites also show convergence. The trace element compositions from this particular polymict are therefore considered to be unequivocally indicative of the matrix melt composition. A new set of mineral-melt partition co-efficients derived from the Matsoku suite of xenoliths are used to calculate the trace element composition of the melt which interacted with this polymict peridotite. This melt is similar to that of a kimberlitic melt. Calculated models show that fractionation of small amounts of garnet and larger amounts of orthopyroxene and phlogopite from a melt of megacrystic composition produces a melt similar to that which interacted with the polymicts.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available