## 1. Introduction

Isotope geochronology was initially suggested as a means to obtain formation ages of minerals and hence the timing of earth processes [*Rutherford*, 1906]. Numerous geochronometers have since been developed to record geological time in various minerals and rock types. Due to the prevalence of K-bearing rocks and minerals at the earth's surface, among the most widely applied geochronometers are the K-Ar system and its variant, the ^{40}Ar/^{39}Ar technique.

The K-Ar technique relies upon the natural radioactive decay of ^{40}K to ^{40}Ar (t_{1/2}: ca. 1250 Ma). Unfortunately, the determination of the daughter/parent ratio by means of a single analytical procedure is impossible; isotopic concentration determinations of both the parent and daughter nuclides are therefore required. Reliable determination of accurate ^{40}Ar concentrations in particular has proven to be a demanding, time-consuming, and low-precision endeavor [*Dalrymple and Lanphere*, 1969; *Lanphere and Dalrymple*, 1966, 2000; *McDougall and Roksandic*, 1974; *McDougall and Wellman*, 2011; *Miiller*, 2006]. The ^{40}Ar/^{39}Ar dating technique [*Merrihue and Turner*, 1966] removes the need for concentration measurements and resolves issues with sample size and heterogeneity by utilizing the neutron activation of ^{39}K to produce ^{39}Ar during irradiation. The attractive feature of this indirect approach is that the age of an unknown sample is not determined by directly measuring its ^{40}Ar and ^{40}K concentrations, but by comparing the sample's ^{40}Ar/^{39}Ar (where ^{39}Ar is a proxy for ^{40}K, via a naturally constant ^{40}K/^{39}K ratio) ratio with that of a mineral standard, which is co-irradiated with the sample of unknown age (fluence monitor). The previously determined ^{40}Ar*/^{40}K (* indicates radiogenic ^{40}Ar) ratio of the standard allows for characterization of the neutron flux, i.e., the production of ^{39}Ar from ^{39}K, during irradiation and the subsequent determination of ages for unknown samples based on measurements of Ar isotopes.

A major limitation of the K-Ar decay system as it is used today is uncertainty in the *true* age (or more accurately the ^{40}Ar/^{40}K ratio) of ^{40}Ar/^{39}Ar mineral standards, which must be determined by some other means. Historically this has been achieved by dating of mineral standards using the conventional K-Ar method [*Dalrymple and Lanphere*, 1969; *Lanphere and Dalrymple*, 2000; *McDougall and Roksandic*, 1974; *McDougall and Wellman*, 2011]. The most commonly used K-Ar concentration measurements for biotite from the Mount Dromedary monzonite (GA1550) were measured in the 1960s [*McDougall and Roksandic*, 1974]; the complete data were only recently published [*McDougall and Wellman*, 2011]. These measurements were extraordinary for their time, but modern technology now allows for more accurate measurements. Further efforts in recent years include calibration with other mineral standards (which ultimately rely upon the accuracy of K-Ar ages) [*Renne et al.*, 1998; *Spell and McDougall*, 2003], the tuning of radioisotopic ages with orbital timescales [e.g., *Channell et al.*, 2010; *Kuiper et al.*, 2008], intercalibration with other mineral phases and geochronometric systems [*Schmitz and Bowring*, 2001], and an optimization model using ^{40}K activity data, K-Ar isotopic data, and ^{40}Ar/^{39}Ar and U-Pb data pairs [*Renne et al.*, 2010]. The aim here is to return to the K-Ar approach in order to improve the accuracy of the ^{40}Ar/^{39}Ar system independent of other chronometers.

Here we focus on the theory and method for making the metrologically traceable measurements of ^{40}Ar concentrations in ^{40}Ar/^{39}Ar mineral standards with an aim of improving the accuracy of the ^{40}Ar/^{39}Ar technique. The construction of a pipette system for similar purposes was reported by *Miiller* [2006]; many of the concepts discussed therein are also examined here. In the interest of full transparency and community involvement, here we report the methods and theory behind a series of forthcoming measurements, along with results from tests on a prototype system. It is the aim of this contribution to fuel scientific discussion regarding the optimal approach for making metrologically traceable concentration measurements of noble gases in geological materials.