Die vier Hauptisotope von Dolomit (C, O, Mg, Ca): Schließung der Kalzium Isotopen (d44/42Ca) Lücke
Mineralogie, Petrologie und Geochemie
Zusammenfassung der Projektergebnisse
The present Project aimed to close the calcium δ44Ca/42Ca isotope "gap" in the fourfold (Ca, Mg, C and O) set of main elements and their isotopes characterising dolomite archives. The motivation was that, with reference to dolomite/dolostone, previous work has predominantly focussed on either conventional isotope systems or Mg isotopes. In the context of this project, we made use of two exceedingly complex natural laboratories displaying long‐term geological, tectonic, magmatic, diagenetic, contact‐metamorphic and structural overprint of marine dolostone archives. The case settings are situated in Western Germany (Steltenberg Quarry, North Rhine Westphalia) and in Northern Italy (Dolomite Mountains, Latemar isolated atoll and Predazzo intrusion). By this, we intend to put the calcium isotope archive (and related Mg, C, O isotopes, radiogenic strontium, petrographic evidence, and cation ordering degree etc.) of early marine diagenetic dolostones exposed to a wide range of diagenetic to contact metamorphic overprint to the test. We realise that these settings represent end members in terms of their geological complexity. That said, if the geochemical (and petrographic) inventory of these extremely complex settings can be understood, then the evidence gained here can be applied, with all due caution, to more simple case examples elsewhere. In order to understand proxy data, we applied principal component analysis. The main results are as follows: Depending on the isotope system tested, the geochemical data analysed can either be interpreted in the context of ambient seawater hydrogeochemistry or diagenetic to metamorphic fluid mixing. Added complexity lies in the fact that fluids at depth might have a marine origin. The case example from North Rhine Westphalia documents that geochemical proxy data, placed into their petrographic, paleotemperature, and local to over‐regional context, significantly increase the ability to extract quantitative information from ancient carbonate rock archives. In particular, (U‐Pb) age control of diagenetic events and well‐calibrated temperature and fluid inclusion data are relevant. This geological foundation is required to interpret proxy data in their wider context. The case example from Northern Italy demonstrates the threshold limits of proxy archives. Under comparably mild hydrothermal overprint (50 to > 100 oC), Ca and Mg isotope data preserve reasonably conservative values. Prograde contact metamorphic overprint forming dolomite and dedolomite marbles re‐equilibrates the geochemical values. This notion represents a warning to those studying (recrystallised = metamorphosed) platform dolostones in Earth's deep time as archives of their marine environments. The value of these proxy data lies in their bearing on diagenetic to metamorphic pathways and fluid fingerprinting. The present work highlights the potential and pitfalls of dolomitised to metamorphosed archives to act as complex archives of their depositional and diagenetic‐to‐metamorphic palaeoenvironments. Depending on the region and time interval considered, but particularly with reference to Proterozoic rocks, this type of overprinted carbonates might be the only available archives.
Projektbezogene Publikationen (Auswahl)
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Towards a better understanding of the geochemical proxy record of complex carbonate archives. Geochimica et Cosmochimica Acta, 376, 68-99.
Mueller, M.; Walter, B.F.; Giebel, R.J.; Beranoaguirre, A.; Swart, P.K.; Lu, C.; Riechelmann, S. & Immenhauser, A.
