UBC Undergraduate Research
Geochemistry of the Baie Charrier Basaltic Section, Courbet Peninsula, Kerguelen Archipelago: Implications for the Composition of the Kerguelen Mantle Plume. Hanano, Diane
The Baie Charrier basaltic section is located on the northern Courbet Peninsula on the Kerguelen Archipelago. The archipelago represents the emergent part of the Northern Kerguelen Plateau, part of the Kerguelen large igneous province in the southern Indian Ocean. The archipelago formed through volcanism (40 Ma to recent) related to the Kerguelen mantle plume. This study presents a detailed petrographic and geochemical analysis of basalts from the Baie Charrier section. The results of this study will compliment the 1000 m Mt. Crozier section located in the center of the Courbet Peninsula. Mineral compositions of olivine and clinopyroxene phenocrysts by electron microprobe analysis were obtained for phenocryst zoning profiles and mineral-melt equilibria. Major and trace element concentrations by XRF and high-resolution ICP-MS, as well as Sr, Nd, and Pb isotopic compositions by TIMS and multi-collector ICP-MS define the chemical and isotopic characteristics of the Baie Charrier section. Based on petrographic and geochemical criteria, the Baie Charrier section is subdivided into four distinct units. The Baie Charrier basalts are primarily olivine-phyric, and sometimes contain phenocrysts of plagioclase or clinopyroxene. Mineral chemistry reveals normally zoned phenocrysts, with olivine core compositions ranging from Fo86-70 and clinopyroxene core Mg# ranging from 0.88 to 0.79. The Baie Charrier basalts are mildly alkalic and possess relatively high MgO contents; Units A and C are comprised of high-MgO (8-10 wt.%) basalts, and sample 240 of unit D is a picrite (>16 wt.%). CaO depletion at ~6 wt.% MgO, coupled with a continuously increasing Al2O3 content, reflect fractionation driven by significant clinopyroxene crystallization. Trace element depletion in Ni, Cr, and Sc with decreasing MgO reflect the relative compatibility of these elements in olivine and clinopyroxene phenocrysts. Incompatible element abundance relationships are used to define a relative incompatibility index in order of increasing compatibility: Th<Ce, Nb, Rb<Ba<Zr<Sr. The Baie Charrier basalts are enriched in highly incompatible elements and light rare earth elements. The units of the Baie Charrier section are also isotopically distinct; Unit C possesses some of the highest 143Nd/144Nd, lowest 87Sr/86Sr, and lowest Pb isotopic ratios, while Unit D has the highest Pb isotopic ratios of the section. The small-scale petrographic and geochemical variation observed in the Baie Charrier section is interpreted to reflect temporal changes in volcanism derived from a heterogeneous source region. Mineral-melt equilibria constrain the maximum MgO content for magmas without accumulated phenocrysts to be between 8-10 wt.% MgO. The similarities between the trace element and isotopic compositions of the Baie Charrier section and the Mt. Crozier section suggests that both are derived from the same source, which supports the interpretation of the Courbet Peninsula as a single volcanic unit. However, the abundance of olivine-phyric basalts at Baie Charrier, and their absence at Mt. Crozier, suggest that the eruptive center of this volcano may not coincide with the present geographic center of the Courbet Peninsula. Lastly, the Baie Charrier basalts, with their highly radiogenic Pb compositions, contain a strong signature of an enriched plume-derived component, and provide additional constraints on the source composition of the Kerguelen mantle plume.
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