Revealing formation mechanism of mysterious melt layers in Earth's mantle - Dr. Longjian Xie

An international team of scientists has uncovered the secrets behind enigmatic melt layers deep within Earth's mantle. Published in Nature Communications, the study led by Dr. Longjian Xie from the Center for High Pressure Science & Technology Advanced Research (HPSTAR) and University College London unveils the dynamic processes behind the formation of “melt doublets” - paired layers of molten rock above Earth’s 410-kilometer mantle discontinuity. The research combines high-pressure experiments and computational modeling to redefine our understanding of deep-Earth melt behavior and its implications for mantle dynamics and water circulation.

Caption: Schematic representation shows low viscosity enables melt doublet formation above the 410-km discontinuity. (a) Water effect on the viscosity of mantle melts. (b) Observed melt layer distribution in the Afar plume region, modified from Thompson et al. (2015).

For decades, geophysical studies have detected mysterious layers of hydrous silicate melt, typically 30-100 km thick, above the 410 km mantle discontinuity. In some regions, scientists observed two distinct layers. The lack of understanding about key melt properties, particularly viscosity under extreme conditions, has hindered explanations for these observations.

The research team combined the synchrotron X ray radiography and high-pressure techniques to measure the viscosity of mantle melts under conditions of the bottom upper mantle (~14 GPa). Their groundbreaking measurements revealed that melt viscosity decreases dramatically - from 96 to just 11.7 mPa·s - as water content increases from 15.5 to 31.8 mol% H₂O. These values are approximately 100 times lower than previous estimates used in geodynamic models.

"This remarkably low viscosity fundamentally changes our understanding of melt behavior in the mantle," said Dr. Longjian Xie, the study's lead author. "Instead of forming a single homogeneous layer through simple advection, this extremely low viscosity creates a dynamic double-layer system as mantle material rises and dehydrates."

"The low viscosity allows the two melt layers to remain distinct, by creating a melt-free gap," explained co-author Prof. Takashi Yoshino from Okayama University, Japan.

"These findings beautifully explain the seismic observations," emphasized co-corresponding author Prof. David Dobson from University College London. "Depending on local conditions - particularly density contrasts and upwelling rates - the layers can either merge into a single thick layer or remain as distinct doublets. This accounts for the variety of structures observed globally."

"This work transforms our view of melt dynamics in the deep mantle. The interplay between low-viscosity melts and continuous dehydration melting could be a key driver of Earth's deep-water cycle and volcanic activity," concluded co-author Prof. Denis Andrault from Laboratoire Magmas et Volcans, Clermont‑Ferrand, France.

近日，由北京高压科学研究中心（HPSTAR）谢龙剑研究员带领的国际研究小组，在地球深部动力学研究领域取得重要进展。该团队通过创新性的高压实验和数值模拟，成功揭示了410公里地幔不连续面附近熔体双层结构的形成机制，为理解地球深部水循环和岩浆活动提供了全新视角。相关研究以《Low Melt Viscosity Enables Melt Doublets Above the 410-km Discontinuity》为题发表于《自然·通讯》（Nature Communications）。