Chemical element imaging for speleothem geochemistry: Application to a uranium-bearing corallite with aragonite diagenesis to opal (Eastern Siberia, Russia)

Abstract : Two complementary image-based microanalysis methods, digital autoradiography of radioactive elements and X-ray fluorescence microspectroscopy (micro-XRF), were used to study the geochemistry of a speleothem from Eastern Siberia. Our objective was to show the interest of these micro-chemical methods to investigate the distribution of U and some other elements as Ca, Sr, and Si in speleothems. The sample studiedwas a corallite concretion that formed slowly by the precipitation of minerals from moisture that had condensed underground. Polished sections of the sample revealed alternating laminas of gray calcite and white aragonite. The concretion, which is older than 400 Kyr (U/Th-TIMS), showed no signs of detrital contamination 230Th/232Th>10,000). Digital autoradiography and XRF analyses indicated exceptionally high uranium contents, ranging from 3-10 to more than 1300 μg∙g−1 in different areas of the sample. Element maps for calcium(Ca), uranium(U), strontium (Sr) and silicon (Si) showed inverse correlations between Ca and U and between Ca and Si, but there is a strong correlation between Si and U. Under the optical microscope, the low Ca appears as corrosion voids, which were found to contain uranium-enriched amorphous silica replacing areas of aragonite and, in some cases, calcite. This postgenetic amorphous and uraniferous silica is composed of two kinds of opal: opal A formed by large microspheres of 30-40 μm in diameter and a gray opal CT with irregular lepispheres. This late opal, dated around 385-412 Kyr, is probably due to a special volcanic event formed by uraniferous glass ashes easily soluble bymeteoric waters, as suggested by the presence of volcanic suite in the fine fraction of the opal. Opal precipitated after speleothemcrystallization by evaporation or cryogenic supersaturation. The study of this exceptional concretion shows the value of direct chemical imaging methods, as they easily revealed the presence and distribution of U in the concretion, and allowed an unusual case of opal diagenesis in a carbonate speleothem to be described. Given the value of speleothems as geo-chronometers and recorders of environmental changes at different time scales, from season to isotopic stage, thismethodology opens newperspectives in understanding the geochemical system and consequently the validity of radiometric datings, U/Th and U/Pb, especially in speleothem samples.
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Submitted on : Tuesday, January 24, 2012 - 12:19:08 PM
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Guillaume Devès, Anne-Sophie Perroux, Thomas Bacquart, Cyril Plaisir, Jérome Rose, et al.. Chemical element imaging for speleothem geochemistry: Application to a uranium-bearing corallite with aragonite diagenesis to opal (Eastern Siberia, Russia). Chemical Geology, Elsevier, 2012, 294-295, p190-202. 〈10.1016/j.chemgeo.2011.12.003〉. 〈halsde-00662504〉

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