Reports: AC8
46126-AC8 New Analytical Advancements In K-Ca Geochronology and Applications to Sedimentary Dating
K-Ca ages of Cambrian glauconites from the Llano uplift, central Texas, and Permian K-rich evaporites from the Delaware Basin, New Mexico, were determined in order to re-evaluate the ability of the K-Ca system to constrain the timing of deposition of sedimentary packages. All but one 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 in the case of glauconites and from Permian to Cretaceous in the case of the evaporites. 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 postdepositional thermal or recrystallization event that reset both the Rb-Sr and K-Ca systems. The range of younger glauconite K-Ca ages are not seen in the Rb-Sr data, but are 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 75 - 90 °C for cooling rates of 0.3 - 1 °C/My. Sylvites and langbeinites from the Delaware Basin give K-Ca ages ranging from the timing of primary mineralization (~ 250 Ma) to 100 Ma. The random distribution and wide spread of K-Ca ages is unlike previously published K-Ar and Ar-Ar from the same potash zone, which cluster distinctively and apparently record discrete geologic events. K-Ca ages argue for ongoing recrystallization of the K-salts throughout most of the Mesozoic. The lack of any ages younger than ~ 94 Ma implies that a significant tectonic or paleoenvironmental change occurred at that time, stabilizing the salts and effectively closing even the most sensitive K-Ca system. The K-Ca system is potentially useful as a low temperature thermochronometer with closure temperatures <100 0C for glauconite.
