K-Ca ages of authigenic sediments: examples from Paleozoic glauconite and applications to low-temperature thermochronometry

K-Ca ages of Cambrian glauconites from the Llano uplift, central Texas, were determined in order to re-evaluate the ability of the K-Ca system to constrain the timing of deposition of sedimentary packages. All of the K-Ca ages presented here were found to be younger than their stratigraphic ages. In addition to being too young, the K-Ca ages are also highly variable, ranging in age from Silurian to Permian. The oldest subset of glauconite ages are in agreement with previously published Rb-Sr ages from the same outcrop and provide further evidence for there having been a post-depositional thermal or recrystallization event that reset both the Rb-Sr and K-Ca systems. The range of younger glauconite K-Ca ages is similar to the distribution of available apatite fission track ages for the Llano basement. K-Ca ages are interpreted as thermochronologic data reflecting partial retention of Ca in thermally fluctuating basin conditions. Estimates of the closure temperature of Ca in glauconite are found to be 60-90 degrees C for cooling rates of similar to 0.5-1 degrees C/My. The K-Ca system is potentially useful as a low-temperature thermochronometer with closure temperatures <100 degrees C for glauconite.