CROSS CALIBRATION OF K-AR STANDARD MINERALS USING AN UNSPIKED AR MEASUREMENT TECHNIQUE

Abstract

We have developed an analytical procedure to intercalibrate the 40Ar content of 40Ar/39Ar and 40K/40Ar standard minerals. This procedure is based on an unspiked volumetric standardisation method developed for K-Ar dating. Argon extracted from independent aliquots of minerals are compared to atmospheric argon doses taken repeatedly from a finite reservoir, leading to a progressive depletion of this reservoir through time. Depletion lines relative to each standard, giving Ar amount versus number of doses, are then fitted by a least mean squares regression; confidence intervals are determined on a common basis for all standards by using the jacknife method. The procedure has been applied to four standards, namely MMhb-1, LP-6, HD-B1, and GL-O. For MMhb-1, LP-6 and HD-B1, the results were compared to their most widely used 40Ar* per gram published values. Concerning HD-B1, a recent published 40Ar* per gram content, called HD-B1new, was also considered. It appears that intercalibrated 40Ar* per gram contents of MMhb-1, LP-6 and HD-B1old are self-consistent to within less than 0.06% and fully concordant with the most generally accepted values, HD-B1new is compatible within confidence intervals, but GL-O presents a discrepancy greater than 1%. This lead us to propose to adjust its recommended 40Ar* per gram content from (6.68+/-0.06).1014 to (6.585+/-0.016).1014. Using the published K content of 6.56 wt.% for GL-O, this would shift the age of GL-O from its recommended value (95.0+/-1.1 Ma) to 93.6+/-0.9 Ma. Conversely, considering GL-O true would result in shifting the Ar content of MMhb-1, LP-6, HD-B1 by ~1%. The procedure may be applied to the assessment of confidence intervals of Ar content of standard minerals in a statistically better way than from interlaboratory variance.