Projektliste Prof. Dr. Stefan Weyer

Abstract: The Lower Jurassic is characterized by multiple intervals of organic-rich sediment deposition likely related to high carbon injection into the atmosphere (CO2 and/or CH4) and associated environmental changes, such as global warming, sea level changes, accelerated hydrological cycles and continental weathering. These changes resulted in perturbations in the carbon cycle which are recorded as C isotope excursions (CIE) of organic (Corg) and/or inorganic (Ccarb) carbon in black shale or carbonate archives. Even though such CIE are recorded in basins worldwide, it is still poorly understood to what extent regional or global driving forces resulted in enhanced deposition of organic matter and if (or to which degree) this correlates with an areal expansion of seafloor anoxia at global scales.
Here we propose to investigate the coupled Mo-U isotope (+ trace element) signatures of black shales from three different basins of the European epicontinental seaways (EES), i.e. the Cheshire basin, that was recently intersected by the Prees-2 drilling (and earlier by Mochras Farm) and the Northwest German Basin (sampled by the Schandelah drill core). We focus on selected time intervals, such as e.g. the Triassic-Jurassic- (T-J) boundary incl. the lower Hettangian and the Pliensbachian-Toarcian (Pl-To) boundary incl. the T-OAE), both of which were characterized by significantly negative CIEs, global warming and a first- or second order mass extinction, respectively. For the latter time interval we also consider the Southwest German Basin (sampled by SEPIA Metzingen drill cores). The Mo and U isotope signatures in black shales are both depending on local depositional conditions, such as water mass mixing time scales, water column H2S and metal sources. Their combined use, however, together with concentrations and ratios of redox-sensitive trace element and TOC, provides extremely powerful means to characterize water-circulation timescales within and connection between the different investigated basins.
In a complementary approach, we furthermore propose to investigate the U isotope signatures of selected Lower Jurassic shallow-marine carbonates from Morocco and Portugal. We want to focus here in particular on the Pl-To and T-OAE and consider earlier findings for the T-J boundary. Marine carbonates are ideal archives for the U isotope composition of paleo-seawater that can be used to estimate the areal expansion of seafloor anoxia at global scales. With these complementary investigations, we expect to unravel to what extent the observed Lower Jurassic CIE and enhanced black shale formation in the EES, were driven by global or regional environmental changes. These findings will be ground-breaking for subsequent investigations about the linkage of carbon cycle perturbations and seafloor anoxia expansion in deep time and for projections to the future.

Abstract: Enrichment, transport and crystallisation of metals in Sn-W deposits results from a complex combination of melt- and fluid-driven processes in highly evolved magmatic-hydrothermal systems. However, the exact conditions and controlling parameters in the various stages of ore formation are not yet well constrained. In addition to structural and classical geochemical and mineralogical investigations, the stable isotope fractionation of the metals may provide important information on crucial changes during metal transport that finally result in metal deposition, as their fractionation critically depends on changes in their bonding environment. Thus, provided that the relationship between metal bonding changes and resulting isotopes fractionation is well understood, or even calibrated, isotopic signature may fingerprint the conditions of metal transport and crystallisation.
Here, we propose a complementary approach, combining experiments and case studies (1) to better understand and calibrate the fractionation of Li isotopes between melt-fluid and Li-micas in well-designed laboratory experiments and (2) to apply the Li and Sn isotope proxy to two already well-characterized, but chemically and structurally distinct granitic Sn-W-Li deposits (Sadisdorf, Erzgebirge and Argemela, Portugal). For Li isotopes (mostly) Li micas and for Sn isotopes cassiterite will be analysed, both in situ with femtosecond laser-ablation (LA-) MC-ICP-MS. We expect complementary information from these two isotope systems: Li isotopes are expected to be sensitive to the nature of the fluid and to fractionate during melt-fluid exsolution (first and second boiling) or between to fluids (vapour-brine), which will be experimentally calibrated. Sn isotopes are expected to mostly fractionate as a result of a redox change (from Sn2+ to Sn4+), during vapour-brine separation or during the crystallization of cassiterite (as indicated by several studies of the literature).
The experimental and analytical set up for the investigations planed in this study are already established in Hannover and application of the Li and Sn isotope systems to their host minerals in Sn-W-Li deposits is expected to provide very valuable information on the conditions of metal transport and crystallization. If time allows we plan to establish in situ W isotope analyses of wolframite with LA-MC-ICP-MS, validated with high-precision solution double spike analyses of the same wolframite crystals (in Cologne). As W isotope fractionation during the crystallization of wolframite, which is frequently associated with cassiterites in hydrothermal veins, is not controlled by a redox process, the isotope compositions of W may allow us to constrain the role of decreasing temperatures during the metallogenic evolution.

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