压力诱导含NaCl冰VII脱盐：富水行星中的组分过滤器
Pressure-induced desalting of NaCl-bearing ice VII: A compositional filter in water-rich planets
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中文总结 AI 辅助
研究发现含NaCl冰VII可在特定热力学路径下形成，并在5.1 GPa减压时排出盐分，其密度特性影响富水行星的化学交换，揭示路径依赖的盐捕获机制。
中文摘要 AI 辅助
氯化钠是地外海洋的主要成分，但通常被认为会被排除在富水行星的高压冰幔之外。在此，我们展示了浓缩NaCl溶液在压缩前被淬火至非晶态时，会结晶形成亚稳态的含NaCl冰VII相。同步辐射X射线衍射和中子衍射揭示了膨胀且高度无序的冰VII晶格，且未检测到结晶NaCl。从头计算通过将水分子替换为Na$^+$和Cl$^-$离子重现了晶格膨胀，并表明结构响应随盐浓度增加而显著增强。在300 K下减压时，该相在$5.1 \pm 0.3$ GPa处排出结晶NaCl，且再压缩时不会重新形成，表明其形成受热力学路径控制。尽管晶胞体积增大了8.5%，含NaCl冰VII的密度接近纯冰VII，且明显低于冰和石盐的等效机械混合物。这些性质表明，含盐冰可能通过深对流地幔输运，但在约5 GPa边界处释放盐分，而密度更大的产物可能抑制进一步的向上输运。因此，依赖路径的盐捕获可能为岩石内部与地下海洋之间的化学交换提供一种先前未被认识的控制机制。
英文摘要
Sodium chloride is a major component of extraterrestrial oceans, but it is generally assumed to be excluded from the high-pressure ice mantles of water-rich planets. Here we show that a concentrated NaCl solution, when quenched in an amorphous state before compression, crystallises in a metastable NaCl-bearing ice VII phase. Synchrotron x-ray diffraction and neutron diffraction reveals an expanded and strongly disordered ice VII lattice with no detectable crystalline NaCl. Ab initio calculations reproduce lattice expansion by substituting water molecules for Na$^+$ and Cl$^-$ ions and show that the structural response increases strongly with salt concentration. Upon decompression at 300 K, the phase expels crystalline NaCl at $5.1 \pm 0.3$ GPa and does not reform upon recompression, demonstrating that its formation is controlled by the thermodynamic pathway. Despite an 8.5% larger unit-cell volume, NaCl-bearing ice VII has a density close to that of pure ice VII and is substantially less dense than an equivalent mechanical mixture of ice and halite. These properties suggest that salty ice may be transported through deep convecting mantles but release salt near the 5 GPa boundary, where the denser products could inhibit further upward transport. Pathway-dependent salt trapping may therefore provide a previously unrecognised control on chemical exchange between rocky interiors and subsurface oceans.
发表机构
- École Polytechnique Fédérale de Lausanne (EPFL)（洛桑联邦理工学院）
- Sorbonne Université（索邦大学）
- Paul Scherrer Institute (PSI)（保罗谢尔研究所）
- Université Paris Cité（巴黎西岱大学）
- Nantes Université（南特大学）
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