Towards an improved understanding of open- versus closed-system diagenesis in carbonate research

碳酸盐档案数据记录了它们沉积时地球表面环境的简要快照。所有碳酸盐档案都易受沉积后蚀变的影响。面临的挑战是将一个有意义的环境信号从一个被后来的成岩或变质蚀变修改，从而可靠地评估提取的代理数据的完整性和鲁棒性作为其古环境的指标。在早期到晚期的成岩路径中，碳酸盐相可能经历岩石缓冲（=保存环境信号）或流体缓冲（叠加环境信号）的成岩作用。在许多情况下，困难在于识别流体与岩石缓冲成岩作用，特别是在定义普遍适用的标准来区分这些根本不同的环境，以及在流体缓冲蚀变后来在岩石缓冲条件下叠加的情况下。额外的复杂性来自于观察到碳酸盐岩可能对一种元素显示出流体缓冲行为（即，氧气）和另一种岩石缓冲行为（即，碳、镁、钙或锶）。该项目将调查岩相学和地球化学分析与来自流体包裹体的显微测温数据的结合是否可以作为两种（流体与岩石缓冲）蚀变模式之一的指纹。该研究将在德国西部莱茵地块北方的施特尔滕贝格采石场进行，在那里，一套复杂的泥盆纪碳酸盐岩经历了埋藏和大气成岩作用，将作为上述考虑因素的试金石进行研究。这个泥盆纪的“天然实验室”是理想的，因为它允许比较流体缓冲和叠印岩石的根本不同类型的成岩和构造叠印，以及似乎证明岩石缓冲成岩作用和保存的环境数据。为了验证我们的假设，我们将建立在一个详细的初步岩相评估重建流体古温度和流体化学使用碳和氧同位素值，放射性锶，钙，镁同位素值，和微热，并将这些数据到地层/时间背景下采用U-Pb碳酸盐定年。这项工作将使我们能够把所有这些岩石学和地球化学数据在流体性质的背景下，并建立标准的成岩作用的开放（流体缓冲）和封闭（岩石缓冲）系统端员和相应的中间阶段。

Carbonate-archive data record, at the time of their deposition, a brief snapshot of the ambient Earth’s surface conditions. All carbonate archives are susceptible to post-depositional alteration. The challenge is to separate a meaningful environmental signal from one that is modified by later diagenetic or metamorphic alteration, and hence to reliably assess the integrity and robustness of the extracted proxy data as indicators of their paleo-environment. In the context of early- to late-diagenetic pathways, carbonate phases may undergo rock-buffered (=preserving environmental signals) or fluid-buffered (overprinting environmental signals) diagenesis. In many cases, the difficulty lies in recognizing fluid- versus rock-buffered diagenesis, particularly in the definition of generally applicable criteria to separate these fundamentally different environments and in cases where fluid-buffered alteration is later overprinted under rock-buffered conditions. Additional complexity arises from the observation that a carbonate rock may display fluid-buffered behavior for one element (i.e., oxygen) and rock-buffered behavior for another (i.e., carbon, magnesium, calcium, or strontium).This project will investigate if the combination of petrographic and geochemical analysis with microthermometrical data derived from fluid inclusions can serve as a fingerprint for either of the two (fluid- versus rock-buffered) alteration modes. The study will be performed in the Steltenberg quarry in the northern Rhenish Massif in western Germany, where a complex set of Devonian carbonates that has undergone burial- and meteoric diagenesis will be investigated as a litmus test for the above considerations. This Devonian `natural laboratory’ is ideal in the sense that it allows comparing fundamentally different types of diagenetic and tectonic overprint of fluid-buffered and overprinted rocks, and such that seem to evidence rock-buffered diagenesis and preserved environmental data. To test our hypothesis, we will build on a detailed preliminary petrographic assessment to reconstruct fluid paleo-temperatures and fluid chemistry using carbon and oxygen isotope values, radiogenic strontium, calcium, and magnesium isotope values, and microthermometry, and place these data into a stratigraphic/temporal context employing U-Pb carbonate dating. This work will allow us to place all this petrographical and geochemical data in a fluid property context and to establish criteria for diagenesis for the open (fluid-buffered) and the closed (rock-buffered) system end-members and their corresponding intermediate stages.