Masters Degrees (Earth Sciences)

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Browsing Masters Degrees (Earth Sciences) by Author "Burness, Sara"

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    Pyroxene stability within kimberlite magma in the upper mantle : an experimental investigation
    (Stellenbosch : Stellenbosch University, 2015-04) Burness, Sara; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Entrainment and assimilation of xenolithic material during kimberlite ascent is considered to be important in shaping the chemistry of the magma and fuelling magma ascent by driving CO2 exsolution. Previous, but as yet unpublished experimental work from Stellenbosch University has demonstrated that orthopyroxene has a key role in this. Orthopyroxene is a very rare xenocrystic constituent of kimberlite but makes up a considerable fraction of the entrained xenolithic material. The initial study used a natural kimberlite composition (ADF1) doped with a peridotite mineral suite (by weight); 88 % ADF1 5% olivine, 5% orthopyroxene and 2% garnet-spinel intergrowth as a starting composition. The subsequent high PT experiments (1100 to 1300°C and 2.0 to 3.5GPa) established that equilibrium orthopyroxene is stable at 1100°C above 2.5GPa, at 1200°C above 2.5GPa and at 1300°C between 2.0 and 3.5GPa. At lower pressures orthopyroxene is completely digested by the experimental melt by the reaction; Mg2Si2O6 (opx) = Mg2SiO4 (ol) + SiO2 (in liquid). In contrast, clinopyroxene is a common phase in kimberlite and often occurs as more than one generation of crystals. Xenocrystic clinopyroxene is dominated by diopside compositions. However, rare omphacite is sometimes also inherited from an eclogite source. The Omphacite, like orthopyroxene, displays textural evidence of severe disequilibrium and may also contribute to the evolution of kimberlitic melt. Thus, a second study produced experiments on the ADF1 kimberlite material at upper mantle PT conditions (1100 to 1300°C and 2.0 to 4.0GPa) as well as an omphacite doped starting material (ADF1+O). These experiments examine the behaviour of pyroxene in kimberlite magma including the influence this may have on magma buoyancy. Within this PT range omphacitic clinopyroxene breaks-down via complex multipart reactions. At 1100°C and 2.0GPa reaction textures around remnant omphacite suggest that omphacite melts incongruently in a complex reaction similar to: Omp + Melt = Ap + Cr-diop + SiO2-enriched Melt. At 1300°C omphacite melts completely and is perceived to produce peritectic Cr-diopside, calcium-rich olivine, carbonate in the melt as well as enrich the melt in SiO2. The melts produced by both the ADF1+O and ADF1 compositions at 1300°C and 4.0GPa are reduced in SiO2 content and have increased TiO2, Cr2O3, Al2O3, MnO, CaO, K2O and P2O5 compared to their respective starting compositions. However, significantly higher proportions of Ca, Na and Fe observed within the ADF1+O melt is a direct consequence of omphacite melting. The ADF1+O starting composition produced equilibrium orthopyroxene above 1100°C and 4.0GPa as well as at 1300°C above 2.0GPa. At lower pressure the orthopyroxene melts incongruently to form peritectic olivine and more silica-rich melt compositions. This digestion favours CO2 exsolution. The effect of orthopyroxene melting can be seen in the melt compositions produced by the peridotite doped starting material (ADF1+P) of the initial study. At 1300°C and 2.0GPa, ADF1+P produced a siliceous melt (37.0 wt.% SiO2) enriched in Al and alkalis compared to the starting ADF1+P composition. This behaviour is directly attributed to xenocrystic orthopyroxene melting at high temperature. In contrast, at the same PT the original kimberlite (ADF1) composition produces a melt with 28.9 wt.% SiO2 and high Ca and Mg contents. Overall, with an increase in pressure the melts become enriched in alkalis and Al2O3 as a direct result of xenocrystic pyroxene melting. In addition, increased pressure allows for a greater solubility of CO2 within the melt. This results in a lower SiO2 melt content and the increased stabilization of equilibrium silica-rich mineral phases (i.e. olivine and equilibrium orthopyroxene). Within the peridotite doped static system (unpublished) the mineral separates with an average crystal size of 115μm ±10μm were all effectively digested in less than 48hours. Similarly, the omphacite doped experiments consumed the 150μm (±10μm) xenocrysts in under 24 hours. Thus, it is suggested that xenocrystic pyroxene is unstable in these experimental kimberlitic melt compositions and is likely to be efficiently assimilated in less than 24 hours. These experimental melts most likely resemble those of natural systems under upper mantle PT conditions. Therefore, pyroxene melting increases the silica content of the melt which in turn drives CO2 exsolution and ascent.