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

碳酸盐-在沉积时，存档数据记录了地球环境表面状况的简要快照。所有碳酸盐档案都容易受到沉积后蚀变的影响。挑战是将有意义的环境信号与后来的成岩或变质蚀变所改变的环境信号分开，从而可靠地评估提取的替代数据作为其古环境指标的完整性和稳健性。在早-晚期成岩路径的背景下，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)成岩作用。在许多情况下，识别流体缓冲的成岩作用与岩石缓冲的成岩作用的困难在于，特别是确定区分这些根本不同的环境的普遍适用标准，以及在岩石缓冲的条件下后来叠加流体缓冲的蚀变的情况下。更复杂的是，观察到碳酸盐岩对一种元素(即氧气)可能表现出流体缓冲行为，而对另一种元素(即碳、镁、钙或锶)表现出岩石缓冲行为。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微温数据相结合是否可以作为两种(流体缓冲与岩石缓冲)蚀变模式中的一种的指纹。这项研究将在德国西部莱茵地块北部的Steltenberg采石场进行，在那里将调查一组复杂的泥盆纪碳酸盐，这些碳酸盐经历了埋藏和陨石成岩作用，作为上述考虑的试金石。这个泥盆纪的“天然实验室”在某种意义上是理想的，因为它可以比较流体缓冲和叠印岩石的根本不同类型的成岩作用和构造叠加，这样似乎就证明了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，我们将在详细的初步岩石学评估的基础上，使用碳和氧同位素值、放射性成因的锶、钙和镁同位素值以及显微测温来重建流体古温度和流体化学，并将这些数据置于地层/时间背景下，采用U-Pb碳酸盐测年。这项工作将使我们能够将所有这些岩石学和地球化学数据放在流体性质的背景下，并为开放(流体缓冲)和封闭(岩石缓冲)系统终端成员及其相应的中间阶段建立成岩作用标准。

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.