How mantle melt composition governs the distribution of low-velocity zones above 410-km depth
- Drs. Longjian Xie and Ho-Kwang Mao

An international research team led by Dr. Longjian Xie and Dr. Ho-Kwang Mao from the Center for High Pressure Science & Technology Advanced Research (HPSTAR) has solved a longstanding mystery in deep Earth science. Their pioneering high-pressure experiments have uncovered the chemical signature of water-rich melts at the base of Earth's upper mantle, finally explaining the enigmatic global-yet-patchy distribution of low-velocity zones (LVZs) above the 410-kilometer discontinuity (D410). The findings will be published in Proceedings of the National Academy of Sciences (PNAS) with the title "Water-rich incipient melt of the deep upper mantle indicates locally-preserved low-velocity zones above 410-km discontinuity."

The 410-km discontinuity marks the boundary between the upper mantle and the transition zone. Global seismic observations detected LVZs above this boundary. These zones exhibit a unique bimodal characteristic: while globally distributed, they form discontinuous patches rather than a continuous layer and show no direct correlation with specific tectonic background (Taulzin et al., 2010). The conventional explanation (Bercovici & Karato, 2003)—that ascending mantle material undergoes dehydration melting due to differing water solubility between wadsleyite (transition zone) and olivine (upper mantle)—couldn't account for this discontinuous pattern.

The team hypothesized that only melts with specific water and iron concentrations would achieve neutral buoyancy and thus are stable near D410. Testing this hypothesis requires overcoming a major technical hurdle: capturing the fleeting high-pressure melts for accurate chemical analysis. The researchers' breakthrough came with their newly developed rapid-quench assembly that is able to quench hydrous melt into glass or small quenched crystals at pressures higher than 10 GPa (Fig. 1). This breakthrough enabled the precise measurement of primitive melt composition at the bottom of upper mantle: 43 mol% H₂O, 9.2 mol% CaO, 30.5 mol% (Mg,Fe)O, 0.2 mol% Al₂O₃, and 17 mol% SiO₂.

"The high water content means only iron-rich melts from specific mantle domains achieve neutral buoyancy," Dr. Xie explained. "Normal mantle melts are too buoyant, quickly rising toward the surface—this explains why LVZs appear in patches rather than a global layer."

Dr. Mao added: "Our findings point to subducted oceanic crust as the likely source of these iron-rich mantle domains. This compositional heterogeneity creates the 'global but spotty' LVZ distribution observed" (Fig. 2).

Fig. 1: High-pressure (15 GPa) sample characterization: (a) Backscattered electron image, (b) Bright-field TEM image, (c) Electron diffraction pattern of hydrous glass, (d) Electron diffraction pattern of quenched crystals. Dia: diamond; Maj: majorite; Wd: wadsleyite.

Fig. 2: Schematic of melt preservation mechanism: Neutrally buoyant melts (red) from iron-rich mantle domains remain stable near 410-km depth

The developed high-pressure quenching technique provides a powerful new tool for studying deep-Earth volatiles, with significant implications for understanding planetary volatile cycles. The determined melt composition not only explains LVZ distribution characteristics but also will advance our knowledge of mantle melt dynamics and chemical heterogeneity.

近日，北京高压科学研究中心（HPSTAR）谢龙剑研究员与毛河光院士领衔的国际合作团队通过自主研发的快速淬火实验技术，精确测量了上地幔底部富水初始熔体的成分，揭示了410公里不连续面（D410）上方低速区（LVLs）的全球分布与局部保存机制。相关成果以《Water-rich incipient melt of the deep upper mantle indicates locally-preserved low-velocity zones above 410-km discontinuity》为题发表于《美国国家科学院院刊》（PNAS）。