The aim of this thesis is to get a better understanding of the composition of a primary melt. Kimberlites are the deepest magmas known to erupt on the Earth's surface, and thus provide us with discrete, direct sampling of the upper mantle. Kimberlites are thought to reach the surface of the earth directly from the source region (150 - 200 km), but in spite of this, they do not correctly represent the source region due to the alteration experienced upon ascent and after emplacement (eg. serpentinisation). Determining the composition of a primary melt that gives rise to kimberlites, can give us important clues on melt generation in the mantle and the composition of the upper mantle. Constraining the true volatile content of these magmas has important implications regarding the eruption dynamics and the ascent ofkimberlite magma from the mantle. We use the petrology and geochemistry of the Igwisi Hills Volcano (IHV), Tanzania, and experiments on a simple peridotitic model system CaO-MgO-Ah03 -Si02 -H20-C02 -K20 (KCMASH-C02) to gain insight into the composition of a primary melt. From the ground mass mineralogy of IHV, there is evidence for magma mixing and stalling which is disrupted by a second pulse of kimberlite magma. This is reflected in the presence of forsterite inclusions in a titanomagnetite host, two different perovskite species with each ex- periencing a different range of oxygen fugacity, the presence of annite (Fe-rich) and phlogopite (Fe-poor), and the two types of NiO diffusion profiles seen in olivines that are next together in the same sample. Phosphorous zoning provides evidence for rapid crystallisation which is likely due to the onset of an eruption. Igwisi olivine is exceptionally wet, with up to 150 ppm (or 245 ppm according to FTIR) H20 found in the cores of several grains. This would imply over 5 wt% H20 in the source region. A change in H speciation is accompanied with variable H20 concen- tration, reflecting the changing conditions that the melt sees during ascent. The homogenous H20 content in olivine from pyroclastic deposits suggests that equilibrium was reached between the water content in the melt and olivine. Experiments in the KCMASH-C02 system illustrate that at isobaric univarience, phlogopite does not take part in the melting reaction. Melting starts between 1000 and 1050 QC, 100 QC lower than the solidus in CMAS-C02 . At 30 kbar and 1400 QC, a melt with a kimberlitic composition is in equilibrium with a carbonatitic melt and demonstrates a kimberlitic melt forming at relatively low pressures in the melting of a K-bearing peridotite.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:559275 |
| Date | January 2011 |
| Creators | Buisman, Iris |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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