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

智力优势：该项目旨在评估大陆火成岩和沉积岩中低T硅酸盐蚀变相的锶同位素数据作为古地下水水文学示踪剂的有效性，特别是追踪古地下水的分布和时间。阴极发光岩相学、物理显微取样、TIMS-锶同位素和氧同位素分析将结合起来描述美国西部第三纪低T(200℃)流体流动事件中形成的次生火山岩。选择了两个第三纪火山岩套进行研究：新墨西哥州南部的K交代莱米塔凝灰岩和亚利桑那州西部哈尔库瓦山脉的K交代第三纪火山岩。从迁移的流体中沉淀的成虫数据将用于确定流体事件，并了解流体的来源及其路径的几何形状(例如，它们是由密度驱动并与闭合盆地卤水的形成有关，还是由大气流体驱动并与早期盆地和山脉伸展期间地表地形的形成有关)。Lemitar上部凝灰岩的初步研究表明，交代流体与次生榴辉岩交代的斑晶长石和凝灰岩基质在同位素上是不同的，因此一定来自凝灰岩以外的来源。锶同位素数据与当地前寒武纪花岗岩源岩和K-交代流体为地形驱动的大气降水的支撑模式一致。进一步的研究将通过对当地前寒武纪基岩、蒸发矿物的锶和氧同位素分析，以及对上勒米塔尔凝灰岩中的榴辉石、基质和其他原生矿物相的微量样品分析来验证这一假说。流体锶同位素组成的横向变化由流体路径上的岩石/水相互作用引起，将通过取样断面进行评估。类似的研究也计划在哈尔库瓦山脉进行。第三个项目涉及对来自宾夕法尼亚/二叠纪喷泉地层(科罗拉多州中部)的岩石表面和井样品中的次生胶结物和成岩相进行显微取样和分析；目的是调查流体与这些沉积物的相互作用。更广泛的影响：该项目涉及将锶同位素数据用于古水文研究的新用途，如果成功，可能在许多地区具有广泛的适用性。该项目还将向Beloit学院的本科生介绍研究方法，他们将在自己的母校和科罗拉多大学博尔德分校开展研究活动。该项目计划作为两个机构的联合研究活动，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.