Secular Variations in Seawater Chemistry Over the Past 600 MY: A Study of Fluid Inclusions in Marine Evaporites Using Three Analytical Techniques

小行星9725740 拟议的研究将采用三个国家的最先进的技术，环境扫描电镜X射线能谱仪（ESEM X射线EDS），紫外激光烧蚀微探针-电感耦合等离子体质谱（LAMP-ICP-MS），流体包裹体提取-离子色谱法（浸提-IC）化学分析流体包裹体在中生代的辉长岩，试图回答一个重要的，但尚未解决的问题，全球海水是否改变了化学成分，地质年代 迄今为止，分析被认为是海洋起源的古石盐中的岩盐晶体中的大包裹体（102 - 103微米）中捕获的流体是获得古海水的主要离子化学的线索的唯一直接方法（参见Holser，1963; Das等人，1990; Horita等人，1991年，1996年;除其他外）。 然而，携带这些大包裹体的岩盐晶体的来源仍然不确定，但它们可能是重结晶或其他二次晶粒生长过程的结果（例如Das等人，1990，第323页; Land等人，1995年）。 这些研究中使用的流体萃取技术不能应用于石盐人字形和堆积晶体中的小包裹体无疑是原生的。 两种新的分析技术将用于拟议的研究克服了他的缺点（ESEM X射线EDS和LAMP-ICP-MS），它们允许直接化学分析的流体在初级包裹体小至20微米。 为了进行比较，将使用批量浸提方法（浸提IC）分析使用环境SEM和激光消融质谱分析的相同样品的切片上的大流体包裹体（200 μ m）。 批量提取分析的目的是交叉检查直接提取程序所需的大流体包裹体是否代表沿晶体生长带的较小流体包裹体中存在的原生海水母卤水，并获得有关卤水化学的高精度数据，包括微量元素（如Br）。 拟议研究的中心目标是恢复采样的每个地质时期的"全球"海水信号。 为了做到这一点，小流体包裹体的主要人字形和堆积石盐晶体空间分离，但同时代的海洋碳酸盐岩将进行分析。 我们将使用Harvie和Weare（1980年）的水蒸发计算机程序作为正演模拟工具，从盐水分析回溯到母体"海水"成分。 我们选择了泥盆纪、二叠纪、白垩纪和中新世四个时期的蒸发岩沉积物中的岩盐进行分析。 在全球范围内，二叠纪和中新世时代的钾镁矾含有丰富的MgSO4盐，这种矿物组合预计不会从现代海水蒸发中形成，并且困扰了地质学家超过世纪。 我们从这两组具有不同原生矿物沉积在不同大陆的沉积盆地在四个不同的地质时期的结果应该提供一个重要的初步测试，海水的主要离子化学是否已经改变了中生代。

9725740 Lowenstein The proposed study will apply three state-of-the-art techniques, Environmental SEM X-Ray EDS (ESEM X-Ray EDS), UV laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAMP-ICP-MS), and fluid inclusion extraction-ion chromatography (Extraction-IC) to chemically analyze fluid inclusions in Phanerozoic evaporites in an attempt to answer the important but unresolved question of whether global seawater has changed chemical composition over geologic time. To date, the analysis of fluids trapped in large inclusions (102-103 microns) in halite crystals from ancient evaporites thought to be of marine origin has been the only direct method of obtaining clues to the major ion chemistry of ancient seawater (see Holser, 1963; Das et al., 1990; Horita et al., 1991, 1996; among other). However, the origin of the halite crystals carrying these large inclusions remains uncertain but they may be the result of recrystallization or other secondary grain growth processes (e.g. Das et al., 1990, p.323; Land et al., 1995). The fluid extraction techniques used in these studies could not be applied to the small inclusions in halite chevron and cumulate crystals of undoubtedly primary origin. Two new analytical techniques to be used in the proposed study overcome his drawback (ESEM X-Ray EDS and LAMP-ICP-MS); they allow direct chemical analysis of fluids in primary inclusions as small as 20 microns. For comparison, bulk extraction methods (Extraction IC) will be used to analyze large fluid inclusions (200 microns) on splits of the same samples analyzed with the Environmental SEM and by laser ablation-mass spectrometry. The purposes of the bulk extraction analysis are to crosscheck whether large fluid inclusions required for the direct extraction procedure are representative of the primary seawater parent brines present in smaller fluid inclusions along crystal growth bands, and to obtain high precision data on brine chemistry, including minor elements (such as Br). The central objective of the proposed research is the recovery of a "global" seawater signal for each geologic period sampled. In order to do this, small fluid inclusions in primary chevron and cumulate halite crystals from spatially separated but coeval marine evaporites will be analyzed. We will use the water evaporation computer program of Harvie and Weare (1980) as a forward modeling tool to backtrack from the brine analyses to the parent "seawater" compositions. We have chosen to analyze halites in evaporite deposits from four periods, the Devonian, the Permian, the Cretaceous, and the Miocene. Globally, potash evaporites of Permian and Miocene age contain abundant MgSO4 salts, a mineral assemblage not predicted to form from the evaporation of modern seawater and one that has perplexed geologists for over a century. Our results from these two groups of evaporites with such different primary mineralogies deposited in sedimentary basins on different continents during four different geologic periods should provide an important initial test of whether the major ion chemistry of seawater has changed over Phanerozoic time.