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

9628486里根从减少火山灾害的角度来看，重要的是要了解产生硅质岩浆并使其积累成大量岩浆所需的时间长度。从岩浆中分离出的矿物的放射性测年可以直接提供关于矿物相历史的信息，而这反过来又可以提供关于岩浆历史的信息。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，完成对火山口湖和Katmai-Novarupta火山中心流纹岩生成和积累速率的正在进行的研究。IS项目的大约一半将集中在火山口湖岩浆系统上。将分析与顶峰喷发有关的安山岩和流纹岩的熔岩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山岩浆系统上，该系统在1912年喷出流纹岩、英安岩、安山岩和岩屑特弗拉，以及流纹岩圆顶。为了确定1912年三个岩浆的停留时间，并更好地了解它们之间的成因关系，将从1912年喷发的三种岩浆类型中分离出全岩和矿物，并对一些伴生的熔岩进行U、Th和Ra核素丰度和Ba值的分析。为了正确地确定这里研究的硅质岩浆演化的时间尺度，将开发数值技术来模拟岩浆、悬浮矿物和晶体-液体熔体中的U系列和Th同位素值。这些技术将被设计用于计算晶体分馏、岩浆混合、老围岩的同化(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、岩石学以及从其他研究获得的全岩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.