上海高压科学研究中心Longjian Xie团队近日研究了深部上地幔富水的早期熔融表明，在410公里的不连续面上方局部保存了低速带。2025年6月5日，
地震低速层（LVL）通常归因于含水硅酸盐熔体，在全球范围内被检测到，但表现出横向不连续性。对地幔过渡带（MTZ）含水量和上地幔溶解度极限的地球物理和实验室研究表明，这些层可能是通过410公里不连续性附近的全球脱水熔融形成的（D410）。一个关键的假设是，熔体在全球范围内形成，但只有在熔体稳定性允许保留的情况下才能保存。然而，在地幔压力下将熔体淬灭成玻璃或细粒晶体的挑战阻碍了熔体成分的精确测定，加剧了关于LVL零星分布机制的争论。
研究组开发了一种快速淬火高压电池组件，可以在>10 GPa的压力下合成含水玻璃或细粒淬火晶体，从而能够对初始熔体成分进行高精度分析。13GPa下的实验表明，410熔体含有43mol%的H2O、9.2mol%的CaO、30.5mol%的（Mg，Fe）O、0.2mol%的Al2O3和17mol%的SiO2。
熔体的高含水量要求富铁以实现中性浮力，这只能来自富铁的异质性（Fe#=100Fe/（Mg+Fe）摩尔；Fe#>18）。相比之下，来自正常MTZ材料（Fe#<18）的熔体保持浮力并向上迁移，从而阻碍了稳定层的形成。研究组得出结论，全球脱水熔融会产生含水熔体，但只有富铁的异质性才能使熔体保持，从而调和了广泛的LVL检测及其横向不连续性的共存。
附：英文原文
Title: Water-rich incipient melt of the deep upper mantle indicates locally preserved low-velocity zones above the 410 km discontinuity
Author: Xie, Longjian, Katsura, Tomoo, Miyajima, Nobuyoshi, Qu, Tongzhang, Mao, Ho-Kwang
Issue&Volume: 2025-6-5
Abstract: Seismic low-velocity layers (LVLs), frequently attributed to hydrous-silicate melts, are detected globally but exhibit lateral discontinuities. Geophysical and laboratory studies of water content in the mantle transition zone (MTZ) and upper mantle solubility limits suggest these layers likely form through global dehydration melting near the 410 km discontinuity (D410). A key hypothesis posits that melts form globally but are preserved only where melt stability permits retention. However, challenges in quenching melts into glass or fine-grained crystals at mantle pressures have precluded precise determination of melt composition, fueling debates over the mechanisms governing LVLs’ sporadic distribution. Here, we developed a fast-quenching high-pressure cell assembly to synthesize hydrous glasses or fine-grained quench crystals at pressures >10 GPa, enabling high-precision analysis of incipient melt composition. Experiments at 13 GPa reveal that the 410 melt contains 43 mol% H2O, 9.2 mol% CaO, 30.5 mol% (Mg, Fe)O, 0.2 mol% Al2O3, and 17 mol% SiO2. The melt’s high water content necessitates Fe enrichment to achieve neutral buoyancy, which can only be sourced from Fe-rich heterogeneities (Fe# = 100Fe/(Mg+Fe) in mole; Fe# >18) within the MTZ. In contrast, melts derived from normal MTZ material (Fe# <18) remain buoyant and migrate upward, precluding stable layer formation. We conclude that global dehydration melting generates hydrous melts, but only Fe-rich heterogeneities enable melt retention, reconciling the coexistence of widespread LVL detections and their lateral discontinuities.
DOI: 10.1073/pnas.2500017122
Source: https://www.pnas.org/doi/abs/10.1073/pnas.2500017122