Isotopic signatures of heavy noble gases in the Earth's mantle contain a major component recycled by subduction. The experimental and field studies reported in the literature show increasing evidence that serpentine minerals can hold large quantities of noble gases, potentially serving as their primary vectors to depth. However, at present, their retention mechanism in these minerals is not fully understood. Additionally, noble gas solubilities from field and experimental studies show large differences in terms of elemental concentrations. Here, we performed crystal chemical modeling to evaluate the incorporation mechanism of noble gases and their solubilities in serpentine minerals along subduction zone geotherms. To this end, we determined the thermal equation of state of xenon using X-ray diffraction and absorption up to 60 GPa and 728 K. In this range, the xenon equation of state is well-adjusted using the Mie-Grüneisen-Debye formalism with relevant fitting parameters. We show that the experimentally observed solubility trend, which follows the order Ne < He < Ar < Kr < Xe, can be explained by the incorporation of noble gases at two distinct crystallographic sites. The light noble gases He and Ne are most likely retained at the van der Waals hydrogen-oxygen bond position between the layers, while the heavy and larger noble gases enter the voids between the six-membered SiO rings. It should be noted that octahedral sites can potentially host xenon, but cannot accommodate argon and krypton. Indeed, this would require unrealistic flexibility of the crystal lattice. Our models extended to mantle wedge conditions predict decreasing solubilities, particularly for light noble gases, in agreement with observations from natural samples. Compared to the noble gas concentrations determined experimentally in serpentine, natural concentrations are much higher and very variable. Our solubility model confirms that equilibrium processes cannot explain these observations. We therefore suggest that the high and variable noble gas concentrations found in natural samples must be due to hybrid hydration processes in ultramafic rocks that involve different degrees of water activities.

中文翻译：

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氙的热状态方程：对蛇纹石矿物中稀有气体掺入及其向深处传输的影响


地幔中重惰性气体的同位素特征包含通过俯冲作用回收的主要成分。文献中报道的实验和现场研究表明，越来越多的证据表明蛇纹石矿物可以容纳大量惰性气体，可能成为它们进入深度的主要载体。然而，目前，它们在这些矿物质中的保留机制尚不完全清楚。此外，现场和实验研究的稀有气体溶解度在元素浓度方面显示出巨大差异。在这里，我们进行了晶体化学建模，以评估稀有气体的掺入机制及其在沿俯冲带地温的蛇纹石矿物中的溶解度。为此，我们使用高达 60 GPa 和 728 K 的 X 射线衍射和吸收确定了氙的热状态方程。在此范围内，使用 Mie-Grüneisen-Debye 形式很好地调整了氙的状态方程：相关拟合参数。我们表明，实验观察到的溶解度趋势遵循 Ne < He < Ar < Kr < Xe 的顺序，可以通过在两个不同的晶体位点掺入惰性气体来解释。轻的稀有气体 He 和 Ne 最有可能保留在层间的范德华氢氧键位置处，而重且较大的稀有气体进入六元 SiO 环之间的空隙。应该指出的是，八面体位点可能容纳氙，但不能容纳氩和氪。事实上，这需要晶格具有不切实际的灵活性。我们的模型扩展到地幔楔条件，预测溶解度会降低，特别是轻惰性气体，这与天然样品的观察结果一致。 与蛇纹岩中实验确定的稀有气体浓度相比，天然浓度要高得多且变化很大。我们的溶解度模型证实平衡过程无法解释这些观察结果。因此，我们认为，在天然样品中发现的高且可变的稀有气体浓度必定是由于超镁铁质岩石中涉及不同程度的水活动的混合水合过程造成的。