An Investigation Into the use of Calcite Fossils as Alpha Thermochronometers
Abstract
Diffusion studies of He in calcite suggest a potential to use carbonates as an alpha thermochronometer (Tc ~ 65°C). These results are all from coarse-grained calcite and dolomite but carbonate mud is a common constituent in many limestones. However, coarse calcite can usually be found in even the most micritic limestones in the form of biotic allochems. We have investigated the suitability of using crinoid columnals and shells of the bivalve Inoceramus for alpha thermochronolgy in limestones. After the death of a crinoid, the high-Mg calcite converts to low-Mg calcite with no loss of optical continuity, and the porosity is filled with syntaxial cement thus becoming permineralized. Bivalves undergo typical diagenesis where the high- Mg calcite converts to low-Mg calcite. Two of four crinoid samples yield reasonable alpha ages: A crinoid from the Thrifty Fm. (Pennsylvanian) in Texas gives an age of 120 Ma (0.7-4.0 ppm U) and one from the Edwardsville Fm. of the Borden Group (Lower Mississippian) in Indiana gives an age of 64 Ma (0.9 ppm U). Each of these results are consistent with the tectonic history of the regions. Crinoid pieces from the New Providence Shale Mbr. of the Borden Fm. (Lower Mississippian) in Kentucky have very low [U] (less than 90 ppb) and give Archean ages. Crinoids from the Hughes Creek Shale (Permian) in Kansas have seemingly robust amounts of U (0.7-2.1 ppm) but this sample also seems to be dominated by common He, yielding Proterozoic ages. Inoceramus from the Burditt Fm. of the Austin Gr. (Upper Cretaceous) in Texas gives an age of 0.5 Ma, suggesting that the closure temperature may be much lower than 65°C. Our preliminary conclusion is that crinoid columnals with more than 500 ppb U may be generally suitable for alpha thermochronology but that the closure temperature of He in Inoceramus shells may be too low for geologic application.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.T11D0460C
- Keywords:
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- 1140 Thermochronology