EGU2020-15726
https://doi.org/10.5194/egusphere-egu2020-15726
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Hafnium (and Sr-Nd) isotope analysis of mineral dust: from sample digestions to mass spectrometry

Gabor Ujvari1,2, Urs Klötzli1, Monika Horschinegg1, Wencke Wegner3, Dorothee Hippler4, Nathalie Tepe5, Gabriella Kiss6, Anikó Horváth6, and Anders Svensson7
Gabor Ujvari et al.
  • 1Department of Lithospheric Research, University of Vienna, Vienna, Austria (gabor.ujvari@univie.ac.at)
  • 2Institute for Geological and Geochemical Research, Research Centre for Astronomy and Earth Sciences, Budapest, Hungary
  • 3Natural History Museum, Vienna, Austria
  • 4Institute of Applied Geosciences, Graz University of Technology, Graz, Austria
  • 5Department of Environmental Geosciences, University of Vienna, Vienna, Austria
  • 6Isotope Climatology and Environmental Research Centre, Institute for Nuclear Research, Debrecen, Hungary
  • 7Niels Bohr Institute, Ice and Climate Research, University of Copenhagen, Copenhagen, Denmark

Mineral dust in ice cores provides insight into past atmospheric circulation patterns provided that the source(s) of these aerosols can be identified. Isotopes of strontium, neodymium and lead are frequently used for source discrimination in ice cores, while those of hafnium much less so. This is because of the extremely low (1-5 ng) amounts of Hf present in 5-10 mg dust samples usually available for isotopic analyses from the dustiest periods of past glaciations, e.g. the Last Glacial Maximum. The use of 176Hf/177Hf isotopic ratios in dust fingerprinting is crucial in situations when Sr-Nd isotopes are inconclusive in source identification.

The overall Hf budget is dominated by the heavy mineral zircon in silt-sized, wind-blown material, while it is significantly depleted in the finer (<5 µm) fractions and the effects of other minerals (apatite, sphene, monazite, xenotime and clay minerals) become increasingly important. Since the major hosts of Hf are refractory heavy minerals, the complete digestion of dust material is crucial in determining reliable Hf isotope ratios.

Here we introduce a closed vessel ammonium bifluoride (NH4HF2) digestion method (220 °C), which is a fast and low blank (0.5 ng for Sr, 0.2 ng for Nd, and <25 pg for Hf) technique for dust dissolution, prior to column chemistry for combined Hf-Sr-Nd isotope analyses. Repeated measurements of the Hf isotope ratios of USGS geological reference materials (AGV-2, BCR-2 and GSP-2) demonstrate that raw, non fractionation corrected 176Hf/177Hf ratios are accurate within 5-50 ppm, while the JMC-475 fractionation corrected values are accurate to 5-10 ppm, compared to reference values using our ion exchange chemistry setup. This methodology also allows separating Sr and Nd from the same samples, and analysing the 87Sr/86Sr and 143Nd/144Nd isotopic compositions. Here we discuss mass spectrometry issues (including sensitivity) of TIMS and two different MC-ICP-MS instruments, and major limitations on dust sample size for Hf-Sr-Nd isotope analyses. Furthermore, the mineralogical background of Hf isotopic compositions, including zircon depletion effects and clay mineralogy (illite) control will be demonstrated. Hf isotope data obtained from four NorthGRIP ice core samples will be presented.

This study was financially supported by the FWF Austria through a Lise Meitner grant (project nr. M 2503-N29) and the European Regional Development Fund in the project of GINOP-2.3.2.-15-2016-00009 ‘ICER’.

How to cite: Ujvari, G., Klötzli, U., Horschinegg, M., Wegner, W., Hippler, D., Tepe, N., Kiss, G., Horváth, A., and Svensson, A.: Hafnium (and Sr-Nd) isotope analysis of mineral dust: from sample digestions to mass spectrometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15726, https://doi.org/10.5194/egusphere-egu2020-15726, 2020

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