Abstract 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 and 2200 °C 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.

中文翻译：

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在极端温度下蒸发过程中限制镓同位素的行为

摘要 对镓同位素系统学的重新关注源于这样一个事实，即 Ga 具有适度的挥发性，并且被假设在蒸发过程中经历了动力学分馏。在这里，我们展示了来自陆地挥发性耗竭样品的第一个 Ga 同位素数据，包括一组实验加热的流纹质土壤、落尘熔融玻璃和来自澳大利亚散布场的飞溅形式的 tektites（以下称为 australite tektites）。与陆地玄武岩中的 Ga 相比，这些样品中的 Ga 同位素重，并且估计了块状硅酸盐地球 (BSE) 的成分。对于每个样品套件，Ga 的同位素分馏与 Ga 消耗的程度有关，与蒸发引起的同位素分馏一致。流纹岩土壤的快速实验加热至 1600 至 2200 °C 之间的温度导致起始土壤的挥发物损失。基于蒸发的 Ga 分数以及起始土壤和实验样品之间的 Ga 同位素分馏程度，我们计算出分馏因子 (α) 为 0.99891 ± 0.00024。这在我们之前为同一样品套件中的 Zn 同位素计算的分馏因子的不确定性范围内 (0.99879 ± 0.00013)。尽管来自核沉降熔体玻璃的 Ga 同位素数据不太一致，但 Ga 同位素系统学通常与蒸发过程中大约 0.9995-0.9998 的抑制分馏因子一致，这也类似于 Zn 系统学的行为。因此，尽管从激光加热实验中获得的分馏因子和熔体玻璃沉降物不同，但在这两种情况下，Ga 和 Zn 的行为相似，正如这些样品中 δ71Ga 和 δ66Zn 的共变所证明的那样。由于我们不知道撞击地点的位置，因此不知道目标岩石的化学成分，因此更难以约束澳大利亚陨石中 Ga 同位素的行为。然而，基于挥发性更强的 Muong-Nong 型陨石的组成，我们估计 Ga 的蒸发分馏发生在 0.9998 和 0.9987 之间；与来自激光加热实验和核沉降玻璃的数据大体一致。australite tektite 中的 δ71Ga 和 δ66Zn 值之间没有相关性，这可能反映了风化前体材料的遗传同位素异质性以及 tektite 形成过程中不同程度的蒸发损失。与来自地球的玄武岩相比，玄武岩中的镓同位素比率通常在同位素上重。个别玄武岩的 δ71Ga 和 δ66Zn 值不相关，这与激光悬浮实验和核沉降玻璃的数据相反。这表明 δ71Ga 和/或 δ66Zn 值是由月球吸积后的地质过程划分的。个别玄武岩的 δ71Ga 和 δ66Zn 值不相关，这与激光悬浮实验和核沉降玻璃的数据相反。这表明 δ71Ga 和/或 δ66Zn 值是由月球吸积后的地质过程划分的。个别玄武岩的 δ71Ga 和 δ66Zn 值不相关，这与激光悬浮实验和核沉降玻璃的数据相反。这表明 δ71Ga 和/或 δ66Zn 值是由月球吸积后的地质过程划分的。