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

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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.