The Effect of Degassing of H2O on Crystallization and Oxidation in Subduction-Zone Magmas: Implications for the Origin of Andesite in Continental Arcs.

Abstract

The volcanic arcs above subduction zones provide a window into the magmatic processes that create continental crust. The lavas erupted at arcs may span the compositional range from basalt to rhyolite, but the dominant lava types are andesite and dacite, which are intermediate in SiO2 content (57-69 wt%). In this thesis, two common hypotheses for the origin of intermediate magmas in the Mexican volcanic arc are examined: (1) mixing between basalt/basaltic andesite and dacite/rhyolite to form andesite, and (2) crystal fractionation from andesite in the upper crust to form dacite. In addition, the effect of degassing on the oxidation state of arc magmas is evaluated. In Chapter 2, the phenocryst assemblages in seven crystal-poor (< 5%) andesite and dacite samples are examined. A wide compositional range for plagioclase and pyroxene in each sample is found, along with reverse zoning. Both are features that have previously been interpreted as evidence of mixing between disparate magma types. Through application of Fe-Ti oxide thermometry and plagioclase-liquid hygrometry, it is shown that the crystal compositions and textures are best explained by volatile degassing during magma ascent rather than magma mixing. In Chapter 3, the effect of extensive degassing on the oxidation state of intermediate magmas is evaluated. Pre-eruptive Fe2+ concentrations were determined for 12 andesite and dacite magmas that initially contained 8-3 wt% H2O, and were compared to their post-eruptive Fe2+ concentrations after extensive degassing; there is no difference within analytical error, nor is there any evidence of a systematic variation in Fe3+/FeT ratios with differentiation. The results show that the relatively oxidized character of subduction-zone magmas is inherited from their mantle source. In Chapter 4, the eruptive history of the large stratovolcano, Volcán Sanganguey, and its surrounding peripheral vents is examined. Andesite and dacite are the most voluminous types, followed by alkaline basalts. Ar geochronology, whole-rock compositions, and an assessment of pre-eruptive magmatic water concentrations and temperatures are used to show that the andesites cannot be explained by magma mixing between the basalt and dacite, and that the dacites cannot be derived from the andesite by crystal fractionation in the upper crust.