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The chemical heterogeneity of lower crustal xenoliths from the Hualalai 1800 Kaupulehu flow

Lecoeuche, Camille 2022. The chemical heterogeneity of lower crustal xenoliths from the Hualalai 1800 Kaupulehu flow. MPhil Thesis, Cardiff University.
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Historically, geochemists have studied radiogenic isotope signatures of mantle-derived basalts, like erupted ocean island basalts (OIBs) or mid-ocean ridge basalts (MORBs), to analyse mantle composition and heterogeneity. Lambart et al.’s (2019) study of the heterogeneity of the depleted upper mantle, found that the isotopic composition of minerals of lower crustal cumulates preserved significantly greater heterogeneity than locally erupted MORBs. Hence the lavas underestimated the true heterogeneity of the original upper mantle source. This project applies Lambart et al.’s (2019) approach to study the heterogeneity of the mantle plume beneath Hualalai, Hawaii. The major element, trace element and isotopic data of lower crustal xenoliths brought to the surface by the 1800 Hualalai eruption were analysed to test whether Hawaiian lower crustal cumulates reveal greater chemical heterogeneity than their corresponding lavas. These samples record primary sample- and crystal-scale compositional variation, revealing greater trace element and isotopic heterogeneity than corresponding basaltic lavas. In line with the theory of Lambart et al. (2019) for cumulates from mid-ocean ridge (MOR) settings, it is hypothesised that this greater heterogeneity is expressed in lower crustal Hawaiian cumulates because the melts from which they formed experience less magma mixing and homogenisation of trace element and isotopic signatures than corresponding lavas. This implies that homogenisation from magma mixing in the crust occurs at both MOR and OIB settings – at least under Hualalai. As a result, new data suggests that previous studies on the Hawaiian mantle plume beneath Hualalai, based on basaltic lavas, may have underestimated its true heterogeneity. These findings, however, show that sampled regions with the most heterogeneous isotopic values do not correspond to regions with the most heterogeneous trace element values, suggesting that the isotopic 87Sr/86Sr heterogeneity is a subtle heterogeneity that is not detectable in the trace element signatures.

Item Type: Thesis (MPhil)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Subjects: Q Science > Q Science (General)
Date of First Compliant Deposit: 15 May 2023
Last Modified: 17 May 2023 10:51

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