Si isotope fractionation

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The Si isotope fractionation is the mass-dependent isotope fractionation in the per mil range of the relative ratios of the stable isotopes of silicon . The stable isotopes of silicon are the so-called “non-traditional” stable isotopes (e.g. Mg, Ca, Si, Cr, Fe, Cu, Zn, Ni, Mo, Tl), which have only been developed since Development of the multi-collector ICP-MS became possible ten years ago.

Until ten years ago, Si isotope ratios were very rarely measured using gas mass spectrometry. Today, however, ICP-MS is gaining the upper hand. The isotope ratios 29 Si / 28 Si and 30 Si / 28 Si are measured. The technique of laser ablation has recently been added. If a femtosecond laser is coupled to a multi-collector ICP mass spectrometer, stable Si isotopes can be measured spatially resolved on solids with a resolution of 30 micrometers.

The technique of stable Si isotopes was used in research in the field of geochemistry . z. B. found Ziegler et al. (2005) found a significant shift in the stable isotopes during rock weathering . The biosphere is also fractionated: bananas prefer the light isotope 28 Si in their growth (sacrifice gel et al. 2006).

The study of Si isotopes in sediments from the earliest days of the Earth, the Precambrian , provided particularly interesting data . Archaic Banded iron formations (Engl. Banded Iron Formation "BIF"), these are deposits of the most important iron raw materials, about 2-3 per thousand contain lighter Si as the probable source of the oceanic silicon, hydrothermal solutions (André et al. 2006). It is believed that this was triggered in the hot Archaic Ocean by preferential precipitation of the light Si isotopes from the relatively unfractionated hot solutions. The ocean water is continuously depleted in the light isotopes 28 Si and 29 Si compared to 30 Si. At some point this residual Si solution will also precipitate into the ocean sediments. So in the vicinity of the "light" BIF deposits with heavy Si isotopes would have to be found. However, this has not been the case so far and represents one of the riddles of Si isotope geochemistry.

literature

  • L. André, D. Cardinal, L. Alleman, S. Moorbath: Silicon isotopes in ~ 3.8 Ga West Greenland rocks as clues to the Eoarchaean supracrustal Si cycle. In: Earth and Planetary Science Letters. 2006, 245, 162-173.
  • K. Ziegler, OA Chadwick, A. Brezinski, EF Kelly: Natural variations of d30Si ratios during progressive basalt weathering, Hawaiian Islands. In: Geochimica et Cosmochimica Acta. 2005, 69: 4597-4610.
  • S. Opfergelt et al .: Silicon isotopic fractionation by banana (Musa spp.) Grown in a continuous nutrient flow device. In: Plant Soil. 2006, doi : 10.1007 / s11104-006-9019-1 .