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

We have developed an analytical procedure to intercalibrate the Ar-40 content of Ar-40/Ar-39 and K-40/Ar-40 Standard minerals. This procedur e 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 finit e reservoir, leading to a progressive depletion of this reservoir thro ugh time. Depletion lines relative to each standard, giving Ar amount versus number of doses, are then fitted by a least mean squares regres sion; confidence intervals are determined on a common basis for all st andards by using the jacknife method. The procedure has been applied t o four standards, namely MMhb-1, LP-6, HD-B1, and GL-O. For MMhb-1, LP -6 and HD-BI, the results were compared to their most widely used Ar-4 0 per gram published values. Concerning HD-B2, a recentpublished Ar per gram content, called HD-B1(new) was also considered. It appears that intercalibrated Ar-40 per, gram contents of MMhb-1, LP-6 and HD- B1(old) are self-consistent to within less than 0.06% and fully concor dant with the most generally accepted values, HD-B1(new) is compatible within confidence intervals, but GL-O presents a discrepancy greater than 1%. This lead us to propose to adjust its recommended Ar-40 per gram content from (6.68 +/- 0.06) 10(14) to (6.585 +/- 0.016) 10(14). 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 approximate to 1%. The proced ure may be applied to the assessment of confidence intervals of Ar con tent of standard minerals in a statistically better way than from inte rlaboratory variance. (C) 1998 Elsevier Science B.V. All rights reserv ed.