1Josh Wimpenny,1Naomi Marks,1 Kim Knight,1Lars Borg,2James Badro,1Rick Ryerson
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.07.006]
1Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
2Université de Paris, Institut de physique du globe de Paris, CNRS, 75005 Paris, France
Renewed interest in gallium isotope systematics has stemmed from the fact that Ga is moderately volatile and is hypothesized to undergo kinetic fractionation during evaporation. Here, we present the first Ga isotope data from terrestrial volatile depleted samples including a suite of experimentally heated rhyolitic soils, fallout melt glass, and splash-form tektites from the Australasian strewn field (hereafter termed australite tektites). The Ga in these samples is isotopically heavy compared to Ga in terrestrial basalts and estimates for the composition of the bulk silicate Earth (BSE). For each sample suite the isotopic fractionation of Ga scales with the degree of Ga depletion, consistent with isotopic fractionation caused by evaporation.
The rapid experimental heating of rhyolitic soil to temperatures ranging between 1600-2200 oC resulted in volatile loss from the starting soil. Based on the fraction of Ga that was evaporated and the degree of Ga isotopic fractionation between starting soil and experimental samples, we calculate a fractionation factor (α) of 0.99891 ± 0.00024. This is within uncertainty of the fractionation factor we previously calculated for Zn isotopes in the same sample suite (0.99879 ± 0.00013). Although Ga isotopic data from nuclear fallout melt glass is less coherent, the Ga isotope systematics are generally consistent with a suppressed fractionation factor of approximately 0.9995-0.9998 during evaporation, which is also similar to the behavior of Zn systematics. Thus, although the fractionation factors obtained from the laser heating experiments and fallout melt glass are different, in both cases Ga and Zn behave similarly, as evidenced by the covariation of δ71Ga and δ66Zn in these samples.
The behavior of Ga isotopes in australite tektites is more difficult to constrain because we do not know the location of the impact site and hence the chemical composition of the target rocks. Nevertheless, based on the composition of more volatile rich Muong-Nong type tektites, we estimate that evaporative fractionation of Ga occurs with an α between 0.9998 and 0.9987; broadly consistent with data from the laser heating experiments and nuclear fallout glass. There is no correlation between δ71Ga and δ66Zn values in australite tektites which is likely to reflect inherited isotopic heterogeneity from weathered precursor material in combination with varying extents of evaporative loss during tektite formation.
Gallium isotope ratios in mare basalts are generally isotopically heavy compared to basalts from Earth. Individual mare basalts have δ71Ga and δ66Zn values that do not correlate, contrary to data from the laser levitation experiments and nuclear fallout glass. This suggests that δ71Ga and/or δ66Zn values were fractionated by geologic processes after the Moon had accreted.