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

评估白云石化石灰岩档案的重要性：镁同位素代理的应用和校准

• 批准号: 319222926
• 负责人: Professor Dr. Adrian Immenhauser
• 金额: --
• 依托单位: Institut für Geologie, Mineralogie und Geophysik
• 依托单位国家: 德国
• 项目类别: Research Units
• 财政年份: 2016
• 资助国家: 德国
• 起止时间: 2015-12-31 至 2020-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/319222926?language=en
• 关键词: Assessing; significance; dolomitized; limestone; archives

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.

大约70%的地球地质历史记录在碳酸盐档案中，这些档案在不同程度上见证了早期到晚期的成岩沉积后石化。而一些作者声称，尽管成岩近变质叠印，古环境模式是合理的保存在这些岩石，其他人认为开放系统的地球化学蚀变和部分完全叠印的代理数据。为了解决这些相互矛盾的问题，我们建议调查岩相学和地球化学性质（一）石灰岩白云岩过渡带和（ii）成岩稳定最早的成岩花岗岩，分别。关键的观察结果是，石灰岩到白云岩过渡带中的灰岩表现出一套横向地层模式，通常在几分米到几十米的尺度上。本文提出的研究将分别调查这些模式中哪些是从石灰岩前体继承的，哪些是在灰岩化过程中形成的。通过这一点，我们打算获得一个关键的和更系统的和定量的了解替代数据从石化石灰岩。目标代理集将包括δ 13 C、δ 18 O、87 Sr/86 Sr以及所有岩相学相关阶段的主要和微量元素浓度模式，但主要关注δ 26 Mg同位素。镁是白云岩中的主要元素，镁同位素比值（δ 26Mgdol）的研究是一个很有前途的研究领域，对中生代世界的了解越来越多，但对前寒武纪岩石的研究较少。目的是通过应用三个古温度代用指标（流体包裹体、碳酸盐聚集同位素、三重氧同位素分析），将岩相学和地球化学发现置于流体化学和流体温度背景中。这些方法都有优点和缺点，但结合起来，它们代表了一个更复杂的方法，古海洋环境和成岩流体温度，然后应用一个单一的代理。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有助于评估古流体温度数据。为此，安排了三个主要研究区域的实地工作，这些区域具有明确的石灰岩至白云岩过渡特征：（i）西班牙东部马埃斯特拉特盆地贝尼卡西姆地区的阿普第阶暴露，（ii）德比郡石炭纪（晚维西安）露头（英国中部），以及（iii）达尔瓦尔超群（2.7 - 2.6 Ga，印度南部）的太古代碳酸盐岩。其他样本材料由世界各地的合作者提供。这项研究有很高的潜力，提供了一个关键的环节，在我们的理解中的中生代和前寒武纪地球系统的演化。