Flow Field Formation and Compositional Variations of Flood Basalt Eruptions

Vye, Charlotte (2010). Flow Field Formation and Compositional Variations of Flood Basalt Eruptions. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000ed30

Abstract

Characterisation of three lava flow fields from the Columbia River Basalt Group (CRBG), USA, reveals compositional heterogeneity on all scales, from adjacent lava lobes to the intra-crystal scale. Simple flow fields can be traced from vent areas to distal reaches, but those with multiple-layered lobes present a degree of complexity that make lava pathways and eruption sequences difficult to identify. A 3-D model based on field investigation of a single flow field establishes a volcanological context for geochemical sampling. Compositional variation within each flow field suggests that small-scale geochemical heterogeneity is common within and between lobes of these flood basalt lavas. Lobe cores emplaced during the latter stages of an eruption have less evolved compositions and more enriched incompatible elements, suggesting progressive variation of magma composition caused by either recharge or sequential tapping of compositionally zoned magma. Progressive crustal contamination of magma during a single eruption is indicated by Re-Os isotope ratios. Initial 1870s/1880s ratios range from 1.405 in the earlier emplaced lava crust to 0.128 in the latter emplaced lava core. These values are more radiogenic than any known enriched sources (normative mantle -0.127, HIMU -0.15) consistent with previous data that indicate that neither an enriched source nor sub-continental lithospheric mantle can be the source of CRBG magma. Os isotope signatures can be explained by different degrees' of contamination of two isotopically distinct lithologies represented by the North American craton and Jurassic accreted terranes underlying the province. Intra-crystal Sr isotope analyses within a flow field record variable histories in an open magmatic system. Results suggest crystal residences in an increasingly contaminated melt within the crust with several recharge events of less-contaminated more primitive basaltic melt. Assembly of melt and crystal cargo occurred immediately prior to eruption without significant residence times in the final host magma. The combined compositional evidence suggests that an extensive shallow-crustal network of dykes and sills supplied magma to the CRBG eruptions.

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