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Title: The differentiation of mantle-derived magmas from beneath Grenada, Lesser Antilles
Author: Stamper , Charlotte Catherine
ISNI:       0000 0004 5920 928X
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2016
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Primitive melts produced beneath island arc volcanoes are rarely erupted at the surface in their original form, instead charting a huge variety of evolved compositions and testifying to the influence of intracrustal processing during magmatic ascent. The study of cumulates (coarse-grained igneous rocks) that sample directly from magma storage regions offers a chance to glimpse a 'snapshot' of this magmatic evolution. This thesis combines major and trace element analysis of mineral compositions in plutonic xenoliths with experimental petrology to explore the differentiation of mantle-derived magmas beneath volcanic island of Grenada, Lesser Antilles. The observed diversity in cumulate assemblage and texture is caused by variability in parental melt composition and post-cumulus interaction with hydrous evolved melts. The whole plutonic suite is produced in a narrow pressure window (P = 0.2 - O.S GPa) at f'V 8S0 - lOS0°C, tracing a shallow (depth :SlSkm) section of a vertically extensive volcanic system. All but the most primitive Grenadan lavas contain a significant proportion of antecryst material derived from unconsolidated cumulate piles, and hornblende only appears as a true phenocryst in the most evolved andesites. Experimentally determined phase relations of a Grenadan picrite demonstrate that the geochemically and petrologically distinct M- and C-series lavas can be generated from a common pieri tic source; the observed dichotomy results from differentiation in the uppermost mantlellower crust (f'V 1.4 - 1.8 GPa) and upper crust (f'V 0.2 GPa) respectively and the effect of pressure on the temperature of olivine and clinopyroxene saturation. Numerical modelling shows that hydrous M-series picrites are in equilibrium with a garnet lherzolite at a depth of f'V 70 km (2.3 GPa). These results are consistent with aspects of primitive melt composition that indicate the site of magma generation beneath Grenada is significantly deeper than its northerly neighbouring island of St Vincent.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available