Direct measurement of recoil effects on 40Ar/39Ar standards
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Advances in the 40Ar/39Ar method using new techniques have led to considerable efforts to improve the accuracy of the calibration of interlaboratory standards. However, the accuracy of the method ultimately relies on the measurement of 40Ar*/39ArK ratios on standards that have been calibrated with the K–Ar method. Usually a 40Ar/39Ar total gas age is assumed to equate to a K–Ar age, but this assumes that there is zero loss of 39Ar due to recoil. Traditional 40Ar/39Ar total gas ages are Ar retention ages and are not strictly comparable to K–Ar ages. Efforts to estimate the importance of this effect on standards have relied on indirect evidence for 39Ar recoil. We report direct measurements of 39Ar recoil for primary and secondary standards using the vacuum-encapsulation technique and show that adjustments to some standard ages may be needed. Revised ages corrected for recoil are given for hornblendes MMhb-1 and Hb3gr, biotites GA1550 and FCT-3 and sanidines FCT-2 and TCR-2. The results show that, in most cases, recoil loss exceeds that which would be expected from grain size and geometry. Internal defects within mineral grains are likely the dominant control on the fraction of recoiled 39Ar lost from these standards.