Using 238 U-Series Disequilibrium to Constrain Time-Scales of Rhyolite Generation

点击翻译按钮获取中文摘要

9628486 Reagan It is important from a volcanic hazards mitigation point of view to understand the length of time needed to generate silicic magmas and for them to accumulate into significant volumes. Radiometric dating of mineral separates from magmas can directly provide information about the history of mineral phases, which in turn can provide information about the history of the magmas. The requested funding will be used to complete ongoing studies of the rates of rhyolite generation and accumulation for the Crater Lake and Mount Katmai-Novarupta volcanic centers by analyzing whole volcanic rocks and mineral separates for U-series disequilibrium and Th isotope ratios. About half of the is project will focus on the Crater Lake magmatic system. Andesitic and rhyolitic tephras associated with the climactic eruption will be analyzed for U and Th concentrations and isotopic compositions. The rest of the effort of this project will focus on the Mount Katmai-Novarupta magmatic system, which ejected rhyolite, dacite, andesite, and lithic tephra, and a rhyolite dome in 1912. To determine the residence times of the three 1912 magmas and to formulate a better understanding of their genetic interrelationships, whole rocks and mineral separates from the three magma types erupted in 1912 and some associated lavas will be analyzed for U, Th, and Ra nuclide abundances and Ba concentrations. To properly determine the time-scales of evolution for the silicic magmas studied here, numerical techniques will be developed to model U-series and Th isotope values in magmas, suspended minerals, and crystal-liquid mushes. These techniques will be designed to acccount for crystal fractionation, magma mixing, assimilation of old country rock (including incomplete dissolution of crystals), and assimilation of crystal fractionation products from the same magmatic system, with parameters adjusted as appropriate based on U-series data, petrography, and whole rock and phase composition data available from other stud ies.