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

9628486 Reagan从减轻火山灾害的角度来看，了解产生硅岩浆并使其大量积聚所需的时间长度非常重要。岩浆矿物分离物的放射性定年可以直接提供矿物相历史的信息，而矿物相的历史又可以提供岩浆历史的信息。所申请的资金将用于通过分析整个火山岩和矿物分离物的u系列不平衡和Th同位素比率，完成正在进行的火山口湖和Katmai-Novarupta火山中心流纹岩生成和积聚速率的研究。大约一半的项目将集中在火山口湖岩浆系统上。与气候喷发有关的安山岩和流纹岩火山岩将分析U和Th的浓度和同位素组成。该项目的其余工作将集中在Katmai-Novarupta火山岩浆系统上，该岩浆系统喷出流纹岩、英安岩、安山岩和岩屑岩，并在1912年形成流纹岩圆顶。为了确定1912年三种岩浆的停留时间，并更好地了解它们的成因相互关系，将对1912年喷发的三种岩浆类型的整个岩石和矿物分离物以及一些伴生熔岩进行U， Th和Ra核素丰度和Ba浓度的分析。为了正确确定这里所研究的硅质岩浆的演化时间尺度，将开发数值技术来模拟岩浆、悬浮矿物和晶体-液体糊状物中的u系列和Th同位素值。这些技术将被设计用于考虑晶体分馏、岩浆混合、古老岩石的同化（包括晶体的不完全溶解）和来自同一岩浆系统的晶体分馏产物的同化，并根据u系列数据、岩石学以及其他研究中可用的整个岩石和相组成数据适当调整参数。

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.