HR: 1340h
AN: T53C-1451    [Abstracts]
TI: The Origin of Fibrous Calcite Veins: Aragonite?
AU: Elburg, M A
EM: Marlina.Elburg@UGent.be
AF: Department of Geology and Geophysics, Ghent University, Krijgslaan 281 S8, Ghent, 9000 Belgium
AU: * Bons, P D
EM: paul.bons@uni-tuebingen.de
AF: Institute for Geosciences, Tübingen University, Sigwartstrasse 10, Tübingen, 72076 Germany
AB: Truly fibrous calcite veins occur mainly in carbonaceous shales and are characterised by high length:width ratios of their fibres (>10). Previous studies on their Sr isotopic geochemistry (Elburg et al., 2002: Geol. Soc. London Spec. Publ. 200, 103-118; Hilgers and Sindern, 2005: Geofluids, in press) have shown that some of the material could be derived from the local wall rock. These studies also showed that the veins were always enriched in Sr compared to the calcite in the host rocks. Aragonite can contain significantly more Sr than calcite, while it also tends to have a fibrous crystal habit. It is therefore possible that the fibrous habit of these veins, which now consist of calcite, are a reflection of their initial aragonitic mineralogy, rather than of any special tectonic regime during their formation. This idea was investigated by analysing the major and trace element geochemistry of selected fibrous and non-fibrous calcite veins from Arkaroola (northern Flinders Ranges, Australia). The fibrous vein analysed for major elements contains less than 1% MgCO3, whereas calcite in the host rock, with which it is in Sr isotopic equilibrium, contains 18% MgCO3. Calcite can contain significant Mg, whereas the aragonitic structure cannot accomodate this ion, so this result is consistent with the idea of an original aragonitic mineralogy of the veins. The fibrous veins show an enrichment in the middle rare earth elements (REE) compared to the calcite in the host rock and blocky veins. In a Post-Archean Average Shale normalised diagram, Eu is more strongly enriched compared to its neighbouring elements in the fibrous veins, but not in the host calcite, blocky veins, or in the silicate fraction of the host rock, suggesting more reducing conditions during fibrous vein formation. This data cannot be used as direct evidence for the fibrous veins' aragonitic mineralogy. It does, however, show that significant differences exist between calcite in host rocks, blocky and fibrous calcite veins, and this data should be incorporated in any model explaining the origin of fibrous veins.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1065 Major and trace element geochemistry
DE: 8030 Microstructures
DE: 8045 Role of fluids
DE: 9330 Australia
SC: Tectonophysics [T]
MN: Fall Meeting 2005