Determination of the decay-constant of ⁸⁷Rb by laboratory accumulation of ⁸⁷Sr

The decay-constant of the geochronometer ⁸⁷Rb (λ ₈₇) has been measured numerous times over the course of nearly a century, yet consensus over an accurate determination has remained elusive. This has resulted in persistent uncertainty, with different laboratories using different values. We have re-determined λ ₈₇ by laboratory accumulation of ⁸⁷Sr in a batch of purified RbClO ₄ over a ca. 30year time span. Daughter ⁸⁷Sr was extracted by RbClO ₄ precipitation and Sr-selective extraction chromatography and measured by isotope dilution thermal ionization mass spectrometry. Sr isotopic compositions are extremely radiogenic, making fractionation correction using a single spike highly imprecise. A double-spike enriched in ⁸⁴Sr and ⁸⁶Sr was prepared in conjunction with this experiment, and used to measure ⁸⁷Sr as well as to correct for instrumental fractionation during mass spectrometer measurements. The isotopic ratios of the double-spike were calibrated against Sr standard SRM-987 using the NIST certified isotopic ratios of that standard: ⁸⁴Sr/ ⁸⁶Sr=0.056 55±0.000 14; ⁸⁷Sr/ ⁸⁶Sr=0.710 34±0.000 26; ⁸⁸Sr/ ⁸⁶Sr=8.378 61±0.003 25 ( ⁸⁶Sr/ ⁸⁸Sr~0.119352). ⁸⁷Rb/ ⁸⁵Rb of SRM-984 was measured with the Sr double-spike by ICP-MS and determined to be=0.386353±0.000004, and this value was used to correct Sr measurements for ⁸⁷Rb interference.Individual measurements of the decay-constant with the double-spike are very precise, but the measurements scatter asymmetrically outside of estimated analytical errors toward high values of λ ₈₇. Our best estimate for λ ₈₇ is 1.3968-0.0018+0.0027×10-11a-1(T:49624-95+65Ma), based on the median and absolute deviation of the median after rejection of two outliers (normalizing Sr ratios to ⁸⁶Sr/ ⁸⁸Sr=0.1194 yields a value for λ ₈₇ of 1.3971×10 ^-11a ^-1). The value measured here is more precise than other recent estimates based on radiometry and comparison with U-Pb ages and it is consistent with those values within the uncertainties of the measurements. We propose that the value reported here be used for Rb-Sr age determinations in place of all previous values.