Neutron-induced ³⁷Ar recoil ejection in Ca-rich minerals and implications for ⁴⁰Ar/³⁹Ar dating

The ⁴⁰Ar/³⁹Ar dating technique requires the transformation of ³⁹K into ³⁹Ar by neutron activation. Neutron activation has undesirable secondary effects such as interfering isotope production, and recoil of ³⁹Ar and ³⁷Ar atoms from their (dominant) targets of K and Ca. In most cases, the grains analysed are large enough (&gt;50 μm) such that the amount of target atoms ejected from the grains is small and has a negligible effect on the ages obtained. However, increasing needs to date fine-grained rocks requires constraining, and in some cases correcting for, the effect of nuclear recoil. Previous quantitative studies of recoil loss focus mostly on ³⁹Ar. However, ³⁷Ar loss can affect the ages of Ca-rich minerals via interference corrections on ³⁶Ar (and, to a lesser extent, ³⁹Ar), yielding lower ⁴⁰Ar*/³⁹Ar_K and, thus, an age spuriously too young. New results focused on ³⁷Ar recoil by measuring the apparent age of multi-grain populations of Ca-rich minerals including Fish Canyon plagioclase (FCp) and Hb3gr hornblende, with discrete sizes ranging from 210 to &lt;5 µm. We use previous result on sanidine grains to correct for the ³⁹Ar recoil loss. For the finest fractions, FCp and Hb3gr apparent ages are younger than the ³⁹Ar recoil-corrected ages expected for these minerals, with a maximum deviation of −40% (FCp) and −21% (Hb3gr) reached for grains below 5 μm. We calculate ³⁷Ar-depletion values ranging from approximately 30 to 91% and from approximately 28 to 98% for plagioclase and hornblende, respectively. This results in <em>x</em>₀ values (mean thickness of the partial depletion layer) of 3.3±0.4 μm (2σ; FCp) and 3.6±1.4 μm (Hb3gr), significantly higher than suggested by current models. The reason for the substantial ³⁷Ar loss is not well understood, but might be related to the radiation damage caused to the mineral during irradiation. <em>x</em>₀ (³⁹Ar) and <em>x</em>₀ (³⁷Ar) values obtained in this study, along with crystal dimensions, can be used for correcting ⁴⁰Ar/³⁹Ar ages from ³⁹Ar and ³⁷Ar recoil loss. We also discuss the relevance of our results to vacuum-encapsulation studies and isotopic redistribution in fine-grained minerals.