What can Fe stable isotopes tell us about magmas?

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

The majority of the Earth’s crust is formed by magmas, and understanding their production and differentiation is important to interpret the geologic rock record. A powerful tool to investigate magmatic processes is the distribution of the stable isotopes of the major redox-sensitive element in magmas, Fe. Fe isotope compositions of magmatic rocks exhibit systematic differences, where the heaviest compositions are found in rhyolites and granites. Understanding of these systematics is complicated by a lack of constraints on Fe isotope fractionation among minerals and liquids under magmatic conditions, and basic knowledge about the complementarity of erupted magmas and crystal loads left behind during the course of magmatic differentiation, as well as, the residues remaining after partial melting within the crust and mantle. This Ph.D. dissertation addresses basic questions about the differentiation of magmas from the perspective of Fe stable isotopes, integrated with petrology, by studying igneous rocks and their constituent phases (minerals and glasses) from the Bushveld Complex, South Africa, Thingmuli, Iceland, Pantelleria, Italy, and the Bishop Tuff, USA. The findings are interpreted in terms of available theory, experiments and forward modeling incorporating petrologic constraints.
Mafic cumulate rocks from the Bushveld Complex exhibit resolvable whole rock Fe isotopic variation with modal mineralogy and mineral composition, to a 1st order reflecting partitioning of Fe3+ and Fe2+ between melt and minerals. The isotope composition of mineral phases of Bushveld cumulate rocks is highly variably, and records closure temperatures, modal abundance and inter-mineral textural relations. In contrast, isotope compositions of mafic to intermediate lavas from Thingmuli volcano vary little, while silicic lavas are systematically heavier. Generation of heavy Fe isotope compositions is shown to be strongly affected by the intensive variable governing magmatic differentiation, most importantly, oxygen fugacity, mineral and melt composition and structure governing the distribution of Fe2+ and Fe3+ during differentiation, and strongly favored by low magma Fe contents at the terminal stages of crystallization.
I also determine Fe isotope fractionation factors for minerals and melts in rhyolite systems by measuring the isotopic compositions for minerals and quenched liquids (glasses or fine-grained matrices) in volcanic rocks. This work shows that fractionation factors strongly depend on the abundance of Fe3+ in minerals. Fractionation factors also correlate moderately well with Fe3+/Fetot in the melt, but show better correlation with calculated viscosity of the melt at magmatic conditions, reflecting the important influence of melt structure on Fe isotope fractionation. I illustrate these interdependences and provide new constraints of melt force constants of silicic magmas. These results have direct applicability for explaining the enrichment of heavy Fe isotopes during differentiation and for more quantitative model of the magmatic processes producing enigmatic stable isotope compositions of rhyolitic and granite magmas.
ForlagAarhus Universitets Forlag
Antal sider177
StatusUdgivet - sep. 2017

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