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Granulometric and lithologic control on apatite and zircon concentrations in Alpine fluvial sediment

Detrital heavy mineral compositions are controlled by many factors such as mineral fertility in the source rocks and hydraulic sorting. Quantifying and understanding the resulting bias is crucial especially for the correct interpretation of single-grain analyses such as apatite or zircon geochronology in provenance studies. In this study, an inter- and intrasample comparison of apatite and zircon concentrations is conducted on modern Alpine fluvial sands from five mono-lithological catchments draining granitoid, ophiolitic, metamorphic and sedimentary sources. The distribution of these minerals was quantified and compared within narrow grain size windows of each sample using point counting in strewn slides, XRF analysis of P2O5 and Zr as proxies for apatite and zircon, respectively and modelling based on the size shift. In addition, those results have been compared with other complementary monolitholigical catchments from the Alps. While in line with published fertility values in the Alps, the apatite and zircon concentrations vary over three orders of magnitude. The intra-sample comparison shows highest zircon and apatite concentrations in the finer grain size fractions (63-250 µm), which is expected from the settling-equivalence principle. Furthermore, the apatite and zircon concentrations derived from point counting correlate well with those estimated from P2O5 and Zr concentrations through XRF analysis. However, the XRF analysis also reveals a significant amount of P2O5 and Zr contained in the grain sizes smaller than 63 µm. This is especially important, since many single-grain provenance studies do not consider the silt and clay fractions.


Daniela Krieg1, Laura Stutenbecker1, Ariane Djahansouzi1, Christoph Glotzbach2
1Technical University of Darmstadt, Institute of Applied Geosciences, Schnittspahnstraße 9, 64287 Darmstadt; 2University of Tübingen, Department of Geosciences, Schnarrenbergstraße 94-96, 72076 Tübingen
GeoKarlsruhe 2021