Collaborative Research: Combined Cathodoluminescence Microscopy and Silicate Microsample Sr Isotopic Studies of Paleogroundwater Hydrology in the Western United States

智力优点：该项目旨在评估大陆火成岩和沉积岩中低T硅酸盐蚀变相的Sr同位素数据作为古地下水水文示踪剂的实用性，特别是跟踪古地下水流量的分布和时间。阴极发光岩相学，物理微量取样，TIMS锶同位素和氧同位素分析将结合起来，以表征第三纪低T（200 C）流体流动事件在美国西部的第三纪火山套房形成的次生冰长石被选中进行研究：在新墨西哥州南部的K-交代的Lemitar凝灰岩和K-交代的第三纪火山岩在亚利桑那州西部的Harcuvar山脉。从迁移流体中沉淀出的冰长石的数据将用于识别流体事件，并了解流体的起源及其路径的几何形状（例如，无论它们是密度驱动的，与封闭盆地卤水的形成有关，还是大气流体驱动的，与早期盆地和山脉伸展期间表面地形的形成有关）。上Lemitar凝灰岩的初步研究表明，交代流体的同位素不同的斑晶长石取代次生冰长石和凝灰岩基质，因此必须来自外部的凝灰岩的来源。锶同位素数据与当地前寒武纪花岗岩源岩和支持模型，其中钾交代流体地形驱动的大气沃茨是一致的。进一步的研究将通过对当地前寒武纪基底岩石、长石矿物的Sr和O同位素分析以及对上勒米塔尔凝灰岩中冰长石、基质和其他原生矿物相的微量样品分析来验证这一假设。流体Sr同位素组成的横向变化，由于岩石/水的相互作用沿着流体路径将进行评估取样断面。计划对Harcuvar山脉进行类似的研究。第三个项目涉及对宾夕法尼亚/二叠纪喷泉组（科罗拉多中部）的长石砂岩表面和井样中的次生胶结物和成岩相进行微取样和分析;目标是调查流体与这些沉积物的相互作用。更广泛的影响：该项目涉及一种新的使用锶同位素数据的古水文调查，如果成功的话，可以在许多地区具有广泛的适用性。该项目还将向贝洛伊特学院的本科生介绍研究方法，这些本科生将在他们的家乡机构和博尔德的科罗拉多大学开展研究活动。该项目计划作为两个机构之间的联合研究活动，向本科生介绍各种实验室技术及其应用。

Intellectual merit: This project aims to evaluate the utility of Sr isotopic data from low-T silicate alteration phases in continental igneous and sedimentary rock as tracers for paleogroundwater hydrology, specifically tracking the distribution and timing of paleogroundwater flow. Cathodoluminescence petrography, physical microsampling, TIMS Sr isotopic, and oxygen isotope analyses will be combined to characterize secondary adularia formed during Tertiary low-T (200 C) fluid flow events in the western U.S. Two Tertiary volcanic suites were selected for study: the K-metasomatized Lemitar Tuff in southern New Mexico and K-metasomatized Tertiary volcanic rocks in the Harcuvar Mountains in western Arizona. The data for adularia precipitated from migrating fluids will be used to identify fluid events, and to understand the origin the fluids and the geometry of their pathways (e.g., whether they were density driven and related to the formation of closed basin brines, or meteoric fluids driven and related to the formation of surface topography during early Basin and Range extension). Preliminary studies of the Upper Lemitar tuff reveal that the metasomatizing fluids were isotopically distinct both from the phenocryst feldspars replaced by secondary adularia and from the tuff matrix, and so must have been derived from sources external to the tuff. Sr isotopic data are consistent with local Precambrian granitic source rocks and support models in which K-metasomatizing fluids were topographically driven meteoric waters. Further studies will test this hypothesis through Sr and O isotopic analyses of local Precambrian basement rocks, evaporitic minerals, and microsample analyses of adularia, matrix and other primary mineral phases within the Upper Lemitar Tuff. Lateral variations in fluid Sr isotopic compositions due to rock/water interactions along fluid pathways will be evaluated by sampling transects. Similar studies are planned for the Harcuvar Mountains. A third project involves microsampling and analysis of secondary cements and diagenetic phases within surface and well samples of arkoses from the Penn/Permian Fountain Formation (central Colorado); the goal is to investigate fluid interactions with these sediments. Broader impacts: This project involves a novel use of Sr isotopic data for paleohydrologic investigations, that if successful, could have wide applicability in many regions. This project will also provide an introduction to research methodology to undergraduates at Beloit College, who will conduct research activities both at their home institution and at the University of Colorado, Boulder. The project is planned as a joint research activity between the two institutions, with undergraduates being introduced to various laboratory techniques and their applications.