New Mexico Geological Society Annual Spring Meeting — Abstracts

Geochronology precision is undergoing a revolutionary increase that allows incredible insight into complex geological processes. However, each incremental improvement in precision is not necessarily linked to an incremental improvement in accuracy. For relative rate or superposition questions, high or ultrahigh-precision data that determines differences is a hugely powerful tool, however accuracy between different methods and between laboratories hinders direct comparison and correlations between high precision data sets. The geochronology community is working diligently to overcome accuracy issues and great strides are being made. For instance, the noble gas community will soon circulate a traveling gas standard of known composition to first evaluate lab-to-lab differences and then to take action towards normalization of these differences. This is affectionately known as the “Smoking from the same pipe experiment”. In the mean time, geochronology must go on and a potential short-term solution will be for individual labs to calibrate to known geological boundaries, such as the K-Pg boundary. The New Mexico Geochronology Research Laboratory (NMGRL) has recently installed state of the art Thermo-fisher ARGUS VI multi-collector mass spectrometers that are equipped with low noise amplifiers that are unique to our facility alone. We are applying this technology to a variety of geological problems and the K-Pg boundary is a priority target. In contrast to the recently published Science article of Renne et al. (2013), that yielded normally distributed ⁴⁰Ar/³⁹Ar sanidine ages (66.0 Ma) for K-Pg boundary ashes from Montana, NMGRL data from a Denver basin K-Pg ash reveals substantial scatter in sanidine ages that cannot be detected on older instrumentation. Preliminary data indicate that the NMGRL calibration for the K-Pg could be nearer to 66.2 Ma and this offset has important implications for chronostratigraphy studies such as those from the San Juan Basin, NM. This apparently small difference in absolute time placement of the K-Pg is actually a barrier in deciphering important chronostratigraphy problems such as timing and recovery rates from mass extinctions and auxiliary questions such as the debate surrounding Paleocene dinosaurs. Ultra-high precision measurements also challenges the common view that ⁴⁰Ar/³⁹Ar sanidine geochronology stringently abides by simple argon systematics. We are detecting subtle variations between grains assumed to be equal in age, and the variations call into question the accuracy of lower precision mean ages commonly reported for ash horizons. We do not view this as a negative for the geochronology community, but rather a positive in that we will be capable of resolving geologically important variations once we develop the conceptual tools to fully embrace ultrahigh precision data.

References:

1. Renne, P.R., Deino, A.L., Hilgen, F.J., Kuiper, K.F., Mark, D.F., Mitchell, W.S, III, Morgan, L.E., Mundil, R., and Smit, J., 2013, Time scales of critical events around the Cretaceous-Paleogene boundary: Science, v. 339, p. 684-687.