Assessing the significance of dolomitized limestone archives: Application and calibration of the Magnesium isotope proxy

大约70%的地球地质历史记录在碳酸盐岩档案中，这些档案不同程度地见证了早期到晚期成岩沉积后白云石化作用。一些作者认为，尽管存在成岩-近变质叠印，但古环境模式在这些岩石中被相当好地保存了下来，另一些人则认为是开放系统的地球化学蚀变和代理数据的部分或全部叠印。为了解决这些相互矛盾的问题，我们建议分别研究(i)灰岩-白云岩过渡带和（ii）成岩稳定的最早成岩白云岩的岩石学和地球化学性质。关键的观察结果是，灰岩-白云岩过渡带的白云岩表现出一套侧向地层模式，通常在几厘米到几十米的尺度上。这里提出的研究将分别探讨哪些模式是继承自石灰岩前体，哪些是在白云化过程中形成的。通过这种方法，我们希望对白云化灰岩的代理数据有一个批判性的、更系统的和定量的认识。目标代理集将包括delta13C， delta18O， 87Sr/86Sr以及所有岩石学相关相的主微量元素浓度模式，但主要重点是delta26Mg同位素。镁是白云岩中的主要元素，镁同位素比值（delta26Mgdol）的研究是一个很有前途的研究领域，对显生宙的研究越来越深入，但对前寒武纪岩石的研究却很少。目的是通过应用三种古温度代用指标（流体包裹体、碳酸盐团块同位素、三氧同位素分析），将岩石学和地球化学发现置于流体化学和流体温度背景下。每种方法都有优点和缺点，但综合起来，它们代表了一种比单一代理应用更复杂的方法来研究古代海洋环境和成岩流体温度。在这里，在DFG FOR 1644 CHARON和实地研究的背景下进行的实验工作将是工具性的，并有助于评估古流体温度数据。为此，在三个主要研究区域安排了明确的灰岩-白云岩过渡特征的野外工作：(i)西班牙东部Maestrat盆地Benicassim地区的Aptian暴露，（ii）英国中部德比郡台地石炭纪（晚Visean）露头，以及（iii）印度南部2.7-2.6 Ga的Dharwar超群太古代碳酸盐岩。额外的样品材料由世界各地的合作者提供。这项研究有可能为我们认识显生宙和前寒武纪地球系统演化提供一个关键的环节。

Approximately 70% of Earths geological history is recorded in carbonate archives that have seen, to a variable degree, early to late diagenetic post-depositional dolomitization. Whereas some authors claim that despite diagenetic to anchimetamorphic overprint, palaeo-environmental patterns are reasonably well preserved in these rocks, others argue for open system geochemical alteration and partial to full overprint of proxy data. To address these conflicting issues, we propose to investigate petrographic and geochemical properties in (i) limestone-to-dolostone transition zones and in (ii) diagenetically stabilized earliest diagenetic dolomites, respectively. The key observation is that dolomites in limestone-to-dolostone transition zones exhibit a set of lateral to stratigraphic patterns, typically at the scales of a few decimetres to a few tens-of-meters. Research proposed here will investigate which of those patterns are inherited from the limestone precursor and which formed during dolomitization, respectively. By this we intend to gain a critical and more systematic and quantitative understanding on proxy data from dolomitized limestones. Targeted proxy sets will include delta13C, delta18O, 87Sr/86Sr and main and trace elemental concentration patterns of all petrographically relevant phases but the main focus is on delta26Mg isotopes. Magnesium is a major element in dolomite and the study of magnesium isotope ratios (delta26Mgdol) is a promising research field and one that is increasingly well understood for the Phanerozoic world but less explored for Precambrian rocks. The aim is to place petrographic and geochemical findings in a fluid chemistry and fluid temperature context by applying three palaeo-temperature proxies (fluid inclusions, carbonate clumped isotopes, triple oxygen isotope analysis). Each of these approaches has advantages and shortcomings but in combination, they represent a far more sophisticated approach to ancient marine environmental and diagenetic fluid temperatures then the application of a single proxy. Here, experimental work performed in the context of DFG FOR 1644 CHARON and field studies will be instrumental and aid in assessing palaeo-fluid temperature data. In this effect, fieldwork in three main study areas characterized by well-defined limestone-to-dolostone transitions has been arranged: (i) Aptian exposures in the Benicassim area, Maestrat Basin, E Spain, (ii) Carboniferous (Late Visean) outcrops of the Derbyshire Platform (Midlands, UK), and (iii) the Archean carbonates of the Dharwar Supergroup (2.7-2.6 Ga, southern India). Additional sample material is provided by collaborators worldwide. This study has a high potential to provide a critical link in our understanding of Phanerozoic and Precambrian Earth system evolution.