U-Pb and U-series dating of silica: Growth rates in natural systems and the behaviour of uranium and its daughter isotopes during experimental silica aging
Final Report Abstract
The detailed analysis of silica indurations and silicified wood sheds new light on the potential applications and limitations of U-series and U-Pb geochronology of opal and chalcedony. U-series dating of Late Pleistocene opal from weathering profiles in the Yilgarn Craton (Western Australia) substantiate a climate control of silica precipitation rates. U-Pb case studies on Miocene silica from the Siebengebirge (Rhenish Massif) and Lesvos (Northeast Aegean) confirm that hydrous silica is capable to record near-surface, low temperature hydrological events, such as periods of intensified weathering, or hydrothermal activity bound to fault systems. U-Pb systematics of silica allows for drawing inferences on the interplay between climate and denudation, as well as on the timing of major tectonic events that have not yet been accessible with any other geochronological method. Despite reliable age information recorded in simple growth sequences, many silica samples bear evidence that properties had been altered since the initial silica deposition. Textural indications include the discordant replacement of opal growth zones by chalcedony, or the patchy intermingling of distinct silica phases on the µm-scale. Disturbance and secondary mobility is obvious for isotopic data with presence of unsupported radiogenic lead and excessively heterogeneous U-Pb ages. However, hazards to the U-Pb system might happen more hideous. Partially open system behaviour may be encountered in silicified wood with high amounts of relict organic material. In addition, natural and experimental data suggest that uranium and decay products can be leached from opal during fluid interaction without necessarily inducing detectable structural changes.
Publications
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(2013). Rates of natural silica precipitation through time. Mineralogical Magazine 77(5), 1742
Merle R., Nemchin A., Simons S., Tomaschek F. & Geisler T.
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(2013). U-Pb dating of silcretes in the Siebengebirge (Rhenish Massif, Germany). DMG-GV Tübingen 2013 Conference Abstracts, 312
Tomaschek F., Nemchin A.A., Merle R. & Geisler T.
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(2014). U-Pb dating of opaline silica (Siebengebirge, Rhenish Massif). DMG 92nd Annual Meeting Jena 2014 Programme, 353
Tomaschek F., Nemchin A.A., Geisler T., Heuser A. & Merle R.
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(2015). Replacement of wood opal by chalcedony: Implications for U-Pb geochronology. Goldschmidt Abstracts 2015, 3145
Tomaschek F., Heuser A., Nemchin A.A. & Geisler T.
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(2015). U-Pb geochronology of hydrous silica (Siebengebirge, Germany). Geophysical Research Abstracts 17, EGU2015-12377
Tomaschek F., Nemchin A., Geisler T., Heuser A. & Merle R.
