Sanidine in obsidian and its applicability in neutron flux monitor development for 40Ar/39Ar dating

Abstract

The neutron flux monitor is a critical part of the process of ⁴⁰Ar/³⁹Ar dating. Sanidine in tuffaceous rocks is the best choice for this role, but the currently used standards are mainly cenozoic due to the difficulty in preserving the tuff. Here, we report a new protocol to prepare a neutron flux monitor from the sanidine contained within obsidian, a rock which has undergone rapid eruption and cooling on the surface. The results show that the sanidine in Yunshan obsidian has a homogenous K (7.95 ± 0.18 wt%) and Ca (0.15 ± 0.01 wt%) content, and has no significant intragrain differences in the major elements. The ⁴⁰Ar/³⁹Ar ages range from 93.29 ± 0.46 Ma (2 se) to 93.49 ± 0.29 Ma (2 se) calculated from the step heating method and are 93.74 ± 0.31 Ma (2 sd) at the single grain level. The variations in zircon SIMS U–Pb ages (93.80 ± 0.58 Ma, 2 sd) and the sanidine ⁴⁰Ar/³⁹Ar ages did not exceed analytical uncertainties. We propose the development of older (>∼65 Ma) neutron flux monitor standards from obsidian instead of tuff, as it is readily available in the field and a large amount of neutron flux monitor can be prepared from it for daily use.