First Principles Calibration of 40 Ar Abundances in 40 Ar / 39 Ar Mineral Neutron Fluence Monitors: Methodology and Preliminary Results

• Published in: Geostandards and geoanalytical research 2023-03, Vol.47 (1), p.91-104
• Main Authors: Morgan, Leah E., Davidheiser‐Kroll, Brett, Kuiper, Klaudia F., Mark, Darren F., McLean, Noah M., Wijbrans, Jan R.
• Format: Article
• Language: English
• ISSN: 1639-4488; 1751-908X
• DOI: 10.1111/ggr.12464

The accuracy and traceability of geochronometers are of vital importance to questions asked by many Earth scientists. The widely applied 40 Ar/ 39 Ar geochronometer relies on the co‐irradiation of samples with neutron fluence monitors (reference materials) of known ages; the ages and uncertainties of these monitors are critical to our ability to apply this chronometer. Previously, first principles, astronomical and optimisation calibrations have been made. The first principles method for determining the age of monitor minerals is the K‐Ar method, which involves measurement of their 40 K and 40 Ar* abundances. The AQuA (Absolute Quantities of Argon) pipette system, which emits calibrated quantities of 40 Ar* via the ideal gas law, was used to calibrate the sensitivity of the system across a range of source pressures and estimate 40 Ar* abundances in neutron fluence monitors. These 40 Ar abundances were combined with existing 40 K abundance data for these monitors. Ages for HD‐B1 and MD2 (GA1550) biotite fluence monitors were calculated and combined with intercalibration data for HD‐B1 and Fish Canyon sanidine (FCs) to determine ages for FCs. Current results do not have the targeted accuracy when compared with previous calibrations; however, we show how the extensive methodology development presented here can be used towards making reliable future measurements.