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

拟议的研究将应用三种最先进的技术，环境扫描电镜x射线能谱仪（ESEM x射线能谱仪），紫外激光烧蚀微探针电感耦合等离子体质谱仪（LAMP-ICP-MS）和流体包裹体萃取色谱法（Extraction-IC），对显生宙蒸发岩中的流体包裹体进行化学分析，试图回答全球海水是否在地质时期改变了化学成分这一重要但尚未解决的问题。迄今为止，对被认为是海洋起源的古代蒸发岩中岩盐晶体大包裹体（102-103微米）中捕获的流体的分析是获得古代海水主要离子化学线索的唯一直接方法（见Holser, 1963; Das et al., 1990; Horita et al., 1991, 1996等）。然而，携带这些大包裹体的岩盐晶体的起源仍然不确定，但它们可能是再结晶或其他二次晶粒生长过程的结果（例如Das et al., 1990, p.323; Land et al., 1995）。这些研究中使用的流体萃取技术不能应用于岩盐纹章中的小包裹体和无疑是原生起源的堆积晶体。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射线能谱和LAMP-ICP-MS）；它们允许对小至20微米的原生包裹体中的流体进行直接化学分析。为了进行比较，将使用整体提取方法（extraction IC）来分析使用环境扫描电镜和激光烧蚀-质谱分析的相同样品的裂口上的大型流体包裹体（200微米）。整体萃取分析的目的是为了交叉检查直接萃取过程所需的大型流体包裹体是否代表了沿晶体生长带存在于较小流体包裹体中的原生海水母盐水，并获得盐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，将分析来自空间分离但同时期的海相蒸发岩的初级雪佛龙和累积岩盐晶体中的小流体包裹体。我们将使用Harvie和Weare（1980）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。我们选择分析泥盆纪、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。在全球范围内，二叠纪和中新世的钾盐蒸发岩含有丰富的MgSO4盐，这种矿物组合不可能由现代海水蒸发形成，一个多世纪以来一直困扰着地质学家。这两组蒸发岩在四个不同的地质时期沉积在不同大陆的沉积盆地中，具有如此不同的初级矿物学特征，我们的结果应该为海水的主要离子化学是否在显生宙时期发生了变化提供一个重要的初步测试。

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.