In pursuit of the K-40 branching ratios: K-Ca and Ar-39-Ar-40 dating of gem silicates

This paper presents combined Ar-39-Ar-40 and K-Ca analyses on gem quality, K-rich silicates: a muscovite and a sanidine, both from pegmatites. We atte mpted to directly determine the K-40 branching ratio by comparing the radio genic ingrowth of the daughter isotopes Ca-40* and Ar-40*. Sub-microgram am ounts of Ca were analysed for isotopic composition with an external reprodu cibility of 0.03%. Ca analytics involved a Ca-43/Ca-48 double spike, miniat urised ion-exchange columns with a 1 ng Ca blank, and a Ta2O5 based loading reagent to allow for sub-microgram analyses. The sanidine gave clearly con flicting Ar-39-Ar-40 (461 +/- 6 Ma, 2 sigma, including uncertainty on the a ge monitor) and K-Ca (477 +/- 2 Ma) ages. The reason of the age discordance may be due to diffusive Ar losses; a low-temperature recrystallization is ruled out by the disordered Si-Al distribution. The muscovite sample gave n early coincident Ar-39-Ar-40 and K-Ca ages applying currently recommended d ecay constants. Thus, analytically, this sample fits the requirements for a direct determination of the K-40 branching ratio, which we calculate as B- Ar=0.1067, B-Ca=0.8933. These values are different from those generally use d in geochronology, and closer to the values used in the physics community. It is clear, however, that the branching ratio proposed here needs to be v erified by further investigation on supposedly ideal samples. The precision with which we are able to resolve the individual ages also reveals complic ations of the geological history even in gem quality silicates. (C) 2000 El sevier Science B.V. All rights reserved.