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

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Wang, Baoliang, Moynier, Frederic, Jackson, Matthew G., Huang, Fang, Hu, Xia, Ari Halldórsson, Sæmundur, Dai, Wei, Devos, Gabriel
• Other Authors: Institut de Physique du Globe de Paris (IPGP (UMR_7154)), Institut national des sciences de l'Univers (INSU - CNRS)-Université de La Réunion (UR)-Institut de Physique du Globe de Paris (IPG Paris)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2023
• Subjects: Rb isotopes; Hekla volcano; Magmatic evolution; MORB; Earth's mantle; [SDU]Sciences of the Universe [physics]; Hekla; Iceland
• Online Access: https://insu.hal.science/insu-04155683; https://doi.org/10.1016/j.gca.2023.05.021

International audience 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 85 Rb) and a long-term reproducibility of 0.03‰ on δ 87 Rb (the permil deviation of the 87 Rb/ 85 Rb 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 δ 87 Rb 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 ...