Improvements Needed in the 40Ar/39Ar Study of Geomagnetic Excursion Chronology

Abstract:

Our knowledge of the existence and frequency of brief geomagnetic polarity. excursions only increases with time. Precise and accurate ⁴⁰Ar/³⁹Ar ages will be required to document this, because 25 or more excursions may have occurred within the Brunhes Epoch (780ky) separated in time by as little as 10ky. Excursions are and will dominantly be discovered in mafic, low K₂O rocks. Improvements in the analytical protocol to ⁴⁰Ar/³⁹Ar date low K₂O, “young”, and thus low ⁴⁰Ar_rad rocks are required. While conventional K/Ar dating “worked”, the assumption of perfect atmospheric equilibration is flawed. In particular, using a measured isochron intercept (±2s) to embrace an atmospheric intercept assumption turns a ⁴⁰Ar/³⁹Ar diffusive extraction into a series of “K/Ar-lite” experiments. The near ubiquitous excess ⁴⁰Ar exhibited in final steps of “matrix” or “groundmass” fractions from whole-rock experiments (no glass, crystals) suggests equilibration with the atmosphere is not achieved. Removing magnetic sample splits (glass?) thought subject to poor argon retention, and crystals subject to ⁴⁰Ar inheritance are routinely done without documenting different isochrons. Short 15 to 20 minute irradiation times effectively eliminate recoil and dramatically minimize isotopic corrections, and the assumption of equivalence in Ar isotope recoil behavior. Assuming no pressure dependency and constancy of mass discrimination value ignores knowledge from other gas mass spectroscopy (O, H, He, Ne). Dynamic mass spectroscopy in stable isotopic analysis allows routine per mil and 0.1 per mil ratios to be measured. Maintaining more than daily bracketing air pipette measurements at differing pressures, and controlling the range of pressures from each diffusive step will approximate this dynamic precision. Experiments will be discussed that exhibit aspects of ⁴⁰Ar/³⁹Ar dating protocols with which precision and accuracy can be improved.