Geochemistry of the Baie Charrier Basaltic Section, Courbet Peninsula, Kerguelen Archipelago: Implications for the Composition of the Kerguelen Mantle Plume.
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.Science, Faculty ofEarth and Ocean Sciences, Department ofUnreviewedUndergraduat