The generation of low-δ18O rhyolites along the Yellowstone hotspot track: Constraints from experiments, oxygen isotopes and thermal models
- Doctoral Thesis
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So-called supereruptions result in the explosive eruption of hundreds of cubic kilometres of viscous, volatile-rich rhyolite magma and represent some of the most hazardous volcanic eruptions on Earth. The two commonly adduced endmember models for the generation of such magmas in the upper crust diverge significantly in the degree to which crustal material is involved in the petrogenesis: partial or bulk crustal melting, or melt extraction from mush zones characterised by differentiation of mantle-derived magmas with limited assimilation of crustal lithologies. Constraining the mechanisms and degree of crustal contribution to these magmas has important implications for understanding the dynamics of these potentially highly explosive systems. The Miocene-to-recent Yellowstone hotspot track features extraordinary volumes of rhyolitic ignimbrites and lava flows, many of which show a strong crustal signature in oxygen isotopes. These low-δ18O rhyolites record interaction with material that was hydrothermally altered by meteoric water, and have become a major argument for crustal melting. However, the processes by which this chemical interaction proceeds are poorly constrained and altered source materials have not been characterised for their chemical composition, mineralogy and melting behaviour. This thesis investigates the processes that generate low-δ18O rhyolite magmas, in order to align them with more general rhyolite formation models and provide new insights into the behaviour of highly-evolved large-scale magmatic systems. In order to document storage conditions of rhyolite magmas in the currently active Yellowstone volcanic feld, zircon from rhyolitic domes and lavas from the Island Park-Mount Jackson series is investigated by U/Pb geochronology, Raman spectroscopy, oxygen isotopic and trace elemental compositions in Chapter 2. Compositional groups are identified that record different stages along a continuous magmatic evolution from trace element-poor rhyolite to extremely fractionated rhyolite with highly enriched zircon trace element compositions. Inclusions of U-Th-REE phases in zircon domains that are dark in cathodoluminescence likely reflect coupled dissolution-reprecipitation of metastable trace element-rich zircon in contact with a fluid phase, whose presence is consistent with rhyolite-MELTS simulations. These findings illustrate that magma reservoirs contain a variety of co-existing magmatic environments, including fluid-saturated transition zones between the magmatic and hydrothermal realms. Low-δ18O rhyolite lavas in Yellowstone have been interpreted as the products of bulk crustal melting of previously deposited and hydrothermally altered rhyolite in the down-dropped caldera roof. Such hydrothermally altered material is sampled by drillcores resulting from a USGS drilling campaign in Yellowstone, which are investigated for major and trace elements, oxygen isotopes and water content in Chapter 3. Rhyolite decreases in δ18O values as a function of increasing temperature with depth but does not change systematically in major and trace element composition. Due to the low water contents in the drillcore samples, water is the most limiting factor during melting. Melting curves modelled by rhyolite-MELTS indicate a maximum of 35 % melt can be created at 850 °C, suggesting that large-scale bulk melting is unrealistic. Low-δ18O rhyolite magmas in Yellowstone more likely result from assimilation of <30 % partially melted altered caldera roof, which is supported by isotopic mass-balance models as well as thermal and volumetric constraints. In the central Snake River Plain, an older segment of the Yellowstone hotspot track, uniformly low-δ18O, metaluminous rhyolitic ignimbrites and lava flows have been inferred to be the result of large-scale partial or bulk melting of pre-existing hydrothermally altered lithologies of the neighbouring Idaho batholith and Challis volcanic field. In Chapter 4, the melting behaviour of these hydrothermally altered source materials is investigated via disequilibrium partial melting experiments at conditions of 750-1000 °C and 1-2 kbar. Partial melt produced in the experiments is metaluminous and inherits the bulk oxygen isotope signature from hydrothermally altered peraluminous source materials independently of the melt fraction, excluding the possibility for selective melting of the most altered lithologies. The generation of low-δ18O rhyolites in the central Snake River Plain can be explained by a two-stage model, whereby magmas assimilate partial melt from pre-existing hydrothermally altered crust during their ascent, prior to assimilation of hydrothermally altered caldera roof in a nested caldera complex. This result links previous models that favour melting of either pre-existing or syn-magmatically altered lithologies. The findings provide new insights into the physical and chemical exchange mechanisms during assimilation processes in magmatic environments as well as into the conditions required for the generation of low-δ18O rhyolites along the Yellowstone hotspot track and of other low-δ18O magmas worldwide. Show more
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ContributorsExaminer: Bachmann, Olivier
Examiner: Ellis, Ben S.
Examiner: Ulmer, Peter
Examiner: Cashman, Katharine V.
Examiner: Holtz, François
Subjectrhyolite; Yellowstone; oxygen isotopes
Organisational unit03958 - Bachmann, Olivier / Bachmann, Olivier
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