In pursuit of the [formula omitted] branching ratios: K-Ca and [formula omitted] dating of gem silicates

Abstract

This paper presents combined 39 Ar– 40 Ar and K–Ca analyses on gem quality, K-rich silicates: a muscovite and a sanidine, both from pegmatites. We attempted to directly determine the 40 K branching ratio by comparing the radiogenic ingrowth of the daughter isotopes 40 Ca* and 40 Ar* . Sub-microgram amounts of Ca were analysed for isotopic composition with an external reproducibility of 0.03%. Ca analytics involved a 43 Ca/ 48 Ca double spike, miniaturised ion-exchange columns with a 1 ng Ca blank, and a Ta 2O 5 based loading reagent to allow for sub-microgram analyses. The sanidine gave clearly conflicting 39 Ar– 40 Ar (461±6 Ma, 2 σ, including uncertainty on the age 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 nearly coincident 39 Ar– 40 Ar and K–Ca ages applying currently recommended decay constants. Thus, analytically, this sample fits the requirements for a direct determination of the 40 K branching ratio, which we calculate as B Ar=0.1067, B Ca=0.8933. These values are different from those generally used 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 verified by further investigation on supposedly ideal samples. The precision with which we are able to resolve the individual ages also reveals complications of the geological history even in gem quality silicates.