Rubidium isotopic fractionation during magmatic processes and the composition of the bulk silicate Earth

Rubidium is a moderately volatile element with high incompatibility and fluid mobility. Its stable isotopes have great potential in tracing various geological processes. For example, lunar rocks are isotopically heavier than terrestrial ones, suggesting volatile loss by evaporation during or following the formation of the Moon. However, these studies rely on a poorly constrained estimate for the composition of the Earth’s mantle and a poor understanding of high-temperature processes which may act to fractionate stable Rb isotopes. It is therefore important to precisely characterize different rock types that sample the Earth’s mantle as well as to evaluate the importance of key isotopic fractionation processes. In order to address these issues, we established a high precision analytical method for Rb isotopic measurements using the Nu Sapphire CC-MC-ICP-MS (collision-cell multi-collector inductively coupled plasma mass spectrometer). In addition, we present a series of Rb isotopic data of volcanic rocks from Hekla volcano (Iceland) and MORB (mid-ocean ridge basalt) samples. We show that our method returns a high Rb sensitivity (∼500 V/μg·g−1 for 85Rb) and a long-term reproducibility of 0.03‰ on δ87Rb (the permil deviation of the 87Rb/85Rb ratio from the SRM 984 standard). This method uses a 2 ng/g Rb solution for analyses, allowing us to consume about 10 times less Rb to achieve similar or better precision than previous studies. Using this method, seven geostandards and one synthetic standard return Rb isotopic data consistent with previous work. Twenty-one Hekla volcanic rocks, spanning compositions from basalt to rhyolite, show limited Rb isotopic variation, with δ87Rb values varying from −0.17‰ to −0.07‰, demonstrating that magmatic evolution has an insignificant effect on Rb isotope ratios. A set of MORB samples (n = 15) from different mid-ocean ridges also span a limited Rb isotopic variation, displaying a range similar to the Hekla rock suite (−0.19 to −0.02‰). Combining our new data together with previously reported OIB data gives an average δ87Rb value of −0.12 ± 0.08‰ (2SD, n = 25), representing the current best estimate of the mantle’s isotopic composition. Considering the δ87Rb values of the upper continental crust (−0.14 ± 0.01‰, 2SE, n = 73), as inferred from recent measurements of granites, loess and sediments, and assuming this value represents the whole crust, the revised Rb isotopic composition of the bulk silicate Earth (and by extension the bulk Earth, assuming no Rb partitioned into the core) is −0.13 ± 0.06‰ (2SD).