Documenting the Early Diagenetic History of Articulate Brachiopods Using Transmission Electron Microscopy

使用透射电子显微镜记录关节腕足动物的早期成岩历史

• 批准号: 9628364
• 负责人: Sandra Carlson
• 金额: $ 10.5万
• 依托单位: University of California-Davis
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1996
• 资助国家: 美国
• 起止时间: 1996-09-15 至 1999-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9628364&HistoricalAwards=false
• 关键词: Documenting; Early; Diagenetic; History; Articulate

9628364 Carlson Paleozoic isotopic studies of paleoclimate and paleoceanography rely on the assumption that 18O values are obtained from original, unaltered brachiopod calcite. Recently, the reliability of the standard tests (textural and chemical) used to detect diagenesis in low magnesium calcite has been questioned. It has been suggested that the well-documented Paleozoic trend of depleted 18O with increasing geologic time may indicate post-depositional alteration rather than a primary trend in ocean temperatures. As assessed by the standard tests of optical and scanning electron microscopic images, trace element analyses, and cathodoluminescent microscopy, "pristine" brachiopod calcite often exhibits signs of isotopic alteration. Apparently, fine-scale dissolution/precipitation indicative of early diagenesis will alter the original isotopic composition without affecting the calcitic fabric. Transmission electron microscopy (TEM) is the only technique able to detect fine-scale diagenesis by imaging the crystal ultrastructure at an atomic level. TEM analysis provides both optical images of specimens at a resolution of 0.2 nm and X-ray diffraction patterns. X-ray diffraction is used to determine crystal lattice structures, whereas, TEM images can reveal structural features such as grain and twin boundaries. The purpose of this study is to use TEM to characterize fine-scale diagenesis in fossil brachiopods. A comparison of TEM features of original low magnesium calcite in extant brachiopods with TEM features identified in Plio-Pleistocene relatives of diagenetically altered low magnesium calcite will establish criteria to identify fine-scale diagenesis. Examination of fossil brachiopods should reveal features that characterize diagenetic low magnesium calcite such as increased microporosity, cross-cutting relationships at twin boundaries and dislocation sites, and a higher dislocation density with respect to original low magnesium calcite. These criteria that typify early diag enesis will be compared with standard diagenetic tests and with isotopic values from extant and fossil brachiopods to test the reliability of the standard test to diagnose fine-scale diagenesis necessary to alter the original isotopic composition. The significance of the research described herein is that the proposed TEM examination provides a direct and unique means of identifying fine-scale diagenesis that alters the original isotopic composition without altering the calcitic fabric. The ability to successfully identify diagenetic features that indicate isotopic alteration should have a profound effect on our ability to accurately interpret isotope data from fossil brachiopods.

小行星9628364 古气候和古海洋学的古生代同位素研究依赖于这样的假设， 18 O值是从原始的、未改变的腕足动物方解石中获得的。 最近，用于检测低镁方解石成岩作用的标准测试（结构和化学）的可靠性受到质疑。 有人认为，有据可查的古生代趋势亏损18 O随着地质时间的增加可能表明沉积后的蚀变，而不是在海洋温度的主要趋势。 正如通过光学和扫描电子显微镜图像、微量元素分析和阴极射线显微镜的标准测试所评估的那样，“原始”腕足动物方解石往往表现出同位素蚀变的迹象。 显然，早期成岩作用的细尺度溶解/沉淀指示将改变原来的同位素组成，而不影响方解石织物。 透射电子显微镜（TEM）是唯一能够通过在原子水平上成像晶体超微结构来检测精细尺度成岩作用的技术。 TEM分析提供了分辨率为0.2 nm的样品光学图像和X射线衍射图案。 X射线衍射用于确定晶格结构，而TEM图像可以揭示结构特征，如晶粒和孪晶界。 本研究的目的是利用透射电镜来表征腕足类化石中的细尺度成岩作用。 原始低镁方解石在现存的腕足动物的TEM特征与TEM特征的比较确定在上新世更新世亲属的成岩蚀变低镁方解石将建立标准，以确定精细规模的成岩作用。 化石腕足动物的检查应揭示成岩低镁方解石的特点，如增加的微孔性，在孪晶边界和位错网站的横切关系，以及较高的位错密度相对于原始低镁方解石的功能。 这些标准，典型的早期成岩作用将与标准的成岩试验和同位素值从现存的和化石腕足动物进行比较，以测试的可靠性的标准测试，以诊断精细规模的成岩作用，改变原来的同位素组成。 本文所述的研究的意义在于，所提出的TEM检查提供了一种直接和独特的手段来识别细尺度成岩作用，这种成岩作用改变了原始同位素组成，而不改变方解石组构。 成功地识别成岩特征，表明同位素蚀变的能力应该有深刻的影响，我们的能力，准确地解释同位素数据从化石腕足动物。