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

2016-06-21T11:17:58Z (GMT) by F. Jourdan P. R. Renne
<p>The <sup>40</sup>Ar/<sup>39</sup>Ar dating technique requires the transformation of <sup>39</sup>K into <sup>39</sup>Ar by neutron activation. Neutron activation has undesirable secondary effects such as interfering isotope production, and recoil of <sup>39</sup>Ar and <sup>37</sup>Ar atoms from their (dominant) targets of K and Ca. In most cases, the grains analysed are large enough (>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 <sup>39</sup>Ar. However, <sup>37</sup>Ar loss can affect the ages of Ca-rich minerals via interference corrections on <sup>36</sup>Ar (and, to a lesser extent, <sup>39</sup>Ar), yielding lower <sup>40</sup>Ar*/<sup>39</sup>Ar<sub>K</sub> and, thus, an age spuriously too young. New results focused on <sup>37</sup>Ar 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 <5 µm. We use previous result on sanidine grains to correct for the <sup>39</sup>Ar recoil loss. For the finest fractions, FCp and Hb3gr apparent ages are younger than the <sup>39</sup>Ar 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 <sup>37</sup>Ar-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><sub>0</sub> 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 <sup>37</sup>Ar loss is not well understood, but might be related to the radiation damage caused to the mineral during irradiation. <em>x</em><sub>0</sub> (<sup>39</sup>Ar) and <em>x</em><sub>0</sub> (<sup>37</sup>Ar) values obtained in this study, along with crystal dimensions, can be used for correcting <sup>40</sup>Ar/<sup>39</sup>Ar ages from <sup>39</sup>Ar and <sup>37</sup>Ar recoil loss. We also discuss the relevance of our results to vacuum-encapsulation studies and isotopic redistribution in fine-grained minerals. </p>