The Effect of Degassing of H2O on Crystallization and Oxidation in Highly-Evolved Magmas:Implications for the Origin of Rhyolite Liquids.

Degassing of H2O;Obisidan;Rhyolite;Phase Equilibrium and Decompression Experiments;Geology and Earth Sciences;Science;Geology

Waters, Laura E.Simon, Adam Charles ;

University of Michigan

关键词: Degassing of H2O;    Obisidan;    Rhyolite;    Phase Equilibrium and Decompression Experiments;    Geology and Earth Sciences;    Science;    Geology;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/102378/lewaters_3.pdf?sequence=3&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Rhyolites (>72 wt% SiO2) are the most differentiated magmas on Earth, and they also constitute the most voluminous (1000’s of km3) explosive eruptions on Earth over the last several million years.Understanding their origin is of considerable interest because their formation fundamentally re-constitutes and differentiates continental crust.However, the primary mechanism by which rhyolites form on Earth, whether by crystal fractionation from a cooling intermediate (60-70 wt% SiO2) magma body or by partial melting of the crust, is highly controversial and strongly debated in the literature.A key to understanding the origin of rhyolites is to first obtain information on their temperatures, melt water concentrations, oxidation states, and depths of segregation from their source regions, all of which can be gleaned from thermodynamic analysis of the various mineral phases that crystallize from these highly-evolved melts.A second goal is to determine the driving force for crystallization, whether by cooling in an isobaric magma chamber within the crust, or by the progressive loss of dissolved water (degassing) during magma ascent to the surface.In this thesis, the first study is an electron microprobe investigation of six, phenocryst-poor (<6%) rhyolite samples, each erupted as glassy obsidian, in which their pre-eruptive temperatures (780-940°C), oxidation states, and melt H2O concentrations (2-6 wt%) are all determined.Second, with the pre-eruptive oxidation state known for each sample, a comparison with their post-eruptive oxidation state is made to test the effect of H2O degassing on the oxidation state of these iron-poor magmas.Contrary to what is often argued in the literature, there is no evidence that H2O degassing alters the oxidation state of rhyolites.Third, a series of phase-equilibrium and decompression experiments on three obsidian samples were conducted under H2O fluid-saturated conditions.These experiments permit a test of whether the wide compositional range of plagioclase (≤40 mol% An), which is remarkable given their low overall abundance (< 3%) in the natural obsidian samples, crystallized because of cooling or because of the effect of rapid H2O degassing. Finally, these hydrous phase-equilibrium experiments are combined with those from the literature to calibrate the first plagioclase-liquid hygrometer applicable to rhyolites.