Natural variations of the 238U/235U isotope composition: A new paleoredox tracer?

• Applicant: Professor Stefan Weyer, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2008 to 2012
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 63424437

We propose to investigate the fractionation of the 238U/235U isotope ratio in terrestrial near surface environments. The major goal will be to evaluate if observed variations in the U isotope composition can serve as a paleoredox tracer. Within the last decade it has been revealed that mass fractionation in nature, once thought to be confined to light elements, is ubiquitous across the periodic table. It has been theorized that for the very heavy elements, volume-dependent rather than mass-dependent isotope fractionation is likely to be the dominant mechanism and thus responsible for the observed natural variations at per mil levels (e.g. for Tl and Hg). We are the first laboratory world-wide that indeed detected U isotope fractionation, up to a per mil level, during preliminary investigations on natural samples. U is a redox-sensitive element, which is well soluble in oxygenated environments and has a long residence time in modern oceans. In times of reduced oceans its solubility and ocean residence time was likely significantly reduced. Theoretical considerations as well as our own preliminary investigations indicate that such redox processes result in significant isotope fractionation for U. Thus U isotope compositions may serve as a tracer to reconstruct paleoredox conditions. . Other applications of U isotopes may be in the fields of environmental sciences (U mobility) or economic geology (formation of U deposits). Notably, variable U isotope compositions may also affect the precision of U-Pb dating. Aims of this study will be (1) to investigate mechanisms of U isotope fractionation during U-reduction in suboxic to euxinic environments, (2) to investigate U isotope fractionation during hydrothermal activity (3) to search for suitable paleo-seawater recorders of the U isotope composition (4) to go back in time and compare U-isotope variations of ancient with modern environments and (5) to compare information from U isotopes with that of other paleoredox tracers.

• DFG Programme: Research Grants