In recent years, diffusion has gained increasing recognition as an important process for explaining the lithium (Li) isotope ratios (δLi) of magmatic systems. However, the role of diffusion in explaining the variability of mantle-derived basalts has not yet been investigated in detail. We have measured δLi values in a comprehensive suite of fresh, subglacial Icelandic basalt glasses and observe a wide range of values from 2.4‰ to 7.3‰, the largest range in Li isotope compositions identified at any ocean island. We find that δLi values do not correlate with lithophile element tracers, but do correlate with He/He. One possibility is that the high diffusivities of both Li and He permit fractionation of Li isotope compositions that couple it to He/He, and decouple it from other, slower-diffusing lithophile tracers. In this study, we explore this idea and model diffusive processes in crystal mushes and mantle melt channels, both with and without melt transport. Our modelling indicates that large (>3‰) fractionation of Li isotope compositions from MORB-like values can be generated by diffusion and that the coupling of Li isotope ratios and He/He signatures can occur as a consequence of diffusive interactions within mantle melt channels. This suggests that the Li isotope compositions of melts cannot be used as a straightforward passive tracer of mantle heterogeneity, because they can be fractionated during magmatic processes. In contrast, we find that diffusive fractionation of He/He is small (∼1 Ra) next to the variability in Icelandic basalts (∼8 to ∼35 Ra). Diffusive fractionation of Li isotope ratios should be most pronounced at OIB localities whose mantle melts have a wide range of [Li], such as Hawaii and Iceland. Although a mantle source can contain Li isotope heterogeneities as a result of crustal recycling, we show that a mantle that is homogenous in Li isotope composition can still generate melts that have variable δLi values via diffusion processes.

中文翻译：

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扩散对冰岛玄武岩中锂同位素比的影响


近年来，扩散作为解释岩浆系统锂（Li）同位素比（δLi）的重要过程越来越受到人们的认可。然而，扩散在解释幔源玄武岩变异性方面的作用尚未得到详细研究。我们测量了一套完整的冰下冰岛玄武岩玻璃的δLi值，并观察到从2.4‰到7.3‰的广泛值，这是在任何海洋岛屿上发现的锂同位素组成的最大范围。我们发现 δLi 值与亲石元素示踪剂无关，但与 He/He 相关。一种可能性是，Li 和 He 的高扩散率允许 Li 同位素成分发生分馏，将其与 He/He 耦合，并将其与其他扩散较慢的亲石示踪剂分离。在这项研究中，我们探索了这个想法，并模拟了晶体糊和地幔熔体通道中的扩散过程，无论有没有熔体传输。我们的模型表明，类 MORB 值中 Li 同位素成分的大量（>3‰）分馏可以通过扩散产生，并且 Li 同位素比率和 He/He 特征的耦合可能是地幔熔体通道内扩散相互作用的结果。这表明熔体的锂同位素组成不能用作地幔异质性的直接被动示踪剂，因为它们可以在岩浆过程中被分馏。相比之下，我们发现 He/He 的扩散分馏很小（~1 Ra），仅次于冰岛玄武岩的变异性（~8 至~35 Ra）。 Li同位素比率的扩散分馏在地幔熔体具有广泛[Li]的OIB地点最为明显，例如夏威夷和冰岛。 尽管由于地壳循环，地幔源可能含有锂同位素异质性，但我们表明，锂同位素组成均匀的地幔仍然可以通过扩散过程产生具有可变δLi值的熔体。