Fossils as Alpha Thermochronometers

Calcite is a ubiquitous mineral in sedimentary rocks and often present in significant amounts in rocks that are devoid of high-K and high-U minerals used in thermochronology. Often, the coarsest calcite available in a limestone or shale will be calcite fossils. We have investigated the utility of fragments of crinoid columnals and shells of Inoceramus sp.

Based on step-heating of He-soaked samples, the closure temperature of four crinoids and four Inoceramus is in the range 85 to 50°C.

He/(U,Th,Sm) of Inoceramus from upper Cretaceous rocks of Colorado, Wyoming, New Mexico, Kansas, Texas, and Alaska suggest this fossil has the potential for dating post-depositional thermal history but U uptake is a potential complicating factor. Samples from the high Rockies in Colorado and Wyoming give ages near the stratigraphic age, consistent with regional stratigraphy. Samples from the Great Plains of Kansas, Texas, and Colorado give ages mostly younger than 5 Ma, not consistent with regional stratigraphy; these very young ages may reflect a young fluid-flow event in the Great Plains adding U to the system.

Crinoid ages from Paleozoic rocks of Ohio, Kentucky, Kansas, and Illinois mostly give ages in excess of the stratrigraphic age. A modern crinoid gives an apparent age of 2 Ma. We interpret these too-old ages to be due to the effect of common He and very low alpha production rates (eU ~ 0.1 ppm). A sample of crinoid from the Lake Valley Fm from the Silver City Range in SW NM give a geologically reasonable age of 17 Ma; our hypothesis for the difference between the NM sample and the mid-continent sample is that the NM sample was heated to temperature sufficient to expel common He. Crinoid samples from the Grand Canyon will be analysed to test this hypothesis.