Establishing the Magnesium Isotope Signature of Dolomites

• Applicant: Professor Dr. Adrian Immenhauser
• Subject Area: Palaeontology
• Term: from 2011 to 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 189689865

• Final Report Year: 2014

Final Report Abstract

The aim of this project was to explore dolomite Mg isotope ratios. This is clearly significant as previous data on dolomite 26Mg ratios were scarce and scattered in the literature. Particularly, Mg is an important element in the lithosphere, the hydrosphere and the biosphere and is directly linked to the global carbon cycle. The data resulting from this project represent a significant advance in dolomite Mg isotope research. These are the following main outcomes: The Mg isotope signature of dolomites is complex and responds to numerous factors including the chemistry of the precipitating fluid, fluid-solid interaction during precipitation including clay mineral leaching and formation of authigenic clays, precipitation kinetics, stoichiometry of dolomites, fluid temperature and perhaps geomicrobial factors. This implies that simplistic interpretations of past seawater magnesium isotope signatures as based on dolomite δ26Mg are not encouraged. A case study from the recent dolomite formation environment in Abu Dhabi clearly documents that complex natural precipitation environments question simplistic laboratorybased experimental data on ∆26Mgfluid-dol. Specifically, the label “apparent fractionation” is suggested as some dolomites in the Abu Dhabi tidal flats are enriched in 26Mg relative to their host fluid. In contrast to this high level of complexity, the fact that dolomites are exceptionally conservative and homogenize – as opposed to many other isotope systems – under increasing burial temperature is encouraging. This clearly accounts for volumetrically significant dolostone units (“closed” system behaviour) but might not be the case for volumetrically small dolomite unis embedded in non-dolomite lithologies. Based on the present level of knowledge, we see no direct and easy way to use dolomite δ26Mg ratios as fingerprint for the genesis and origin of fossil dolomites but clearly, the potential for future work lies in the combined case-study-based, experimental and numerical approach. The data shown here are a strong argument that geochemical studies, which often include modelling but ignore petrographic boundary conditions and diagenetic pathways, will lead to conclusions that seem sophisticated at first glance but are in fact of very limited geological hindcasting significance when considered in more detail.

Publications

• . Impact of diagenesis and low grade metamorphosis on isotope (δ26Mg, δ13C, δ18O and 87Sr/86Sr) and elemental (Ca, Mg, Mn, Fe and Sr) signatures of Triassic sabkha dolomites 2012, Chemical Geology, 332-333, 45-64
  Geske, A., Zorlu, J., Richter, D.K., Niedermayr, A., Immenhauser, A.
  
• 2014. Magnesium isotope composition of sabkha pore fluids and related (Sub-)Recent stoichiometric dolomites, Abu Dhabi (UAE). Chemical Geology
  Geske, A., Lockier, S., Dietzel, M., Richter, D.K., Buhl, D., Immenhauser, A.
  
• 2014. Shallow burial dolomitization of an Eocene carbonate platform. GeoArabia, 19, 17-54
  Zohdi, A., Seyed, A.M., Reza, M.-M., Asadollah, M., Richter, D.K., Geske, A., Abbas, A.N., Immenhauser, A.
  
• 2014. The magnesium isotope (δ26Mg) signature of dolomites. Geochimica et Cosmochimica Acta
  Geske, A., Goldstein, R.H., Richter, D.K., Buhl, D., Kluge, T., John, C., Immenhauser, A.