RUI: Raman Spectroscopy and Stable Isotope Investigations of Graphite in Marble

碳是一种独特的元素，构成了我们所知的生命的化学基础。当生物死亡时，它们的遗骸通常被氧化或消耗，从而在地球大气层、水圈和生物圈中循环利用。在适当的条件下，一些有机质被困在堆积的沉积盆地中，从而在地壳和地幔中循环。这种含碳物质有许多重要的后果，例如含有煤、石油和天然气的矿藏。但是，除了经济上重要的化石燃料的积累，地壳岩石中分散的小块有机物的命运可以告诉我们许多关于古代沉积环境、生命历史和在地壳环境中运行的地球化学过程的信息。该项目将研究最终将沉积物中的碳质物质转化为矿物石墨的物理和化学过程。这种转化称为石墨化，涉及碳的重组和其他元素的损失，并在变质过程中对热、压力和变形做出反应。随着我们对这一过程认识的提高，利用碳质物质更好地约束地质事件的温度-压力-变形史，并可能识别原始沉积在古代沉积物中的有机质的差异也将是我们的能力。本研究的目的是阐明变质温度与碳质材料(CM)和石墨的结构状态之间的关系，并将CM向石墨的转化与石墨和方解石之间的碳同位素交换联系起来。该项目将使用激光拉曼光谱来表征低到高品位大理岩中CM和石墨的结晶度，并首次将其与依赖温度的方解石-石墨同位素分馏进行比较。将对拉曼分析技术和结果的重复性、样品内可变性、露头间可变性、制备方法和退变作用进行严格的评估。在中角闪岩-麻粒岩相大理岩中，碳的稳定同位素在方解石和石墨之间的分馏与温度有关，可用作地温计，但在较低温度的岩石中观察到的同位素分馏与理论上预期的分馏不一致。对不同粒度的CM进行同位素和激光拉曼光谱研究，将石墨粗化与同位素交换联系起来，揭示石墨中是否存在进行性同位素分带。将石墨的结构状态与碳同位素交换的程度进行对比，将有助于我们更好地了解同位素交换过程，并可能为解开中低温岩石的热历史提供新的工具。

Carbon is a unique element, forming the chemical basis for life as we know it. When living things die, their remains are often oxidized or consumed and thereby recycled in the earth's atmosphere, hydrosphere, and biosphere. Under appropriate conditions, some organic matter is trapped in accumulating sedimentary basins, and thus, cycled through the earth's crust and mantle. This carbonaceous material has many significant consequences, such as comprising deposits of coal, petroleum and natural gas. But aside from economically important accumulations of fossil fuels, the fate of small, dispersed bits of organic matter in rocks of the earth's crust can tell us much about ancient sedimentary environments, the history of life, and geochemical processes that operate in crustal environments. This project will investigate the physical and chemical processes that ultimately convert carbonaceous material in sediment into the mineral graphite. This conversion, known as graphitization, involves reorganization of the carbon and loss of other elements, and occurs in response to heat, pressure and deformation during metamorphism. As our understanding of this process improves, so will our ability to use carbonaceous material to better constrain the temperature-pressure-deformation history of geological events, and also to possibly recognize differences in the organic matter originally deposited in ancient sediments.The objectives of this research are to clarify the relationship between metamorphic temperature and the structural state of carbonaceous material (CM) and graphite, and to correlate the conversion of CM to graphite with the exchange of carbon isotopes between graphite and calcite. This project will use laser Raman spectroscopy to characterize the crystallinity of CM and graphite in low- to high-grade marble and compare that for the first time with temperature dependent calcite-graphite isotopic fractionations. Raman analytical techniques and results will be critically evaluated for reproducibility, within-sample variability, across-outcrop variability, preparation methods, and retrograde metamorphism. The fractionation of stable isotopes of carbon between calcite and graphite is temperature dependent and useful as a geothermometer in middle amphibolite to granulite facies marble, however, the isotopic fractionations observed in lower temperature rocks do not agree with theoretically expected fractionations. Different size fractions of CM will be studied isotopically and by laser Raman to relate graphite coarsening with isotopic exchange, and will reveal whether or not prograde isotopic zoning exists in graphite. Comparing the structural state of graphite with the extent of the carbon isotope exchange will help us better understand the isotopic exchange processes and may lead to new tools for unraveling the thermal history of moderate- to low-temperature rocks.