| Until now, thermochronology has been used extensively on high uranium minerals such as apatites and zircons, which are common in granites and other rocks derived from them. It would be advantageous to extend this list to include limestones and shales. To overcome the low uranium content in calcite, large calcite fossils such as crinoids and Inoceramus were analyzed. Step-heating experiments of He in biotic calcite gave a closure temperature (Tc) of ∼70°C; this agrees well with the Tc of He in abiotic calcite (60 to 80°C).; Twenty-five crinoid and thirteen Inoceramus samples were analyzed. Most of the fossil crinoids gave reasonable thermal ages, if they had been heated since their deposition. Analyses of a modern crinoid revealed on average over 1 million years worth of 4He, which may suggest the presence of common helium in crinoids. Most of the Inoceramus samples gave reasonable ages reflecting a documented thermal event. |
