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<p>Geophysical observations have revealed the existence of low-velocity zones that potentially coincide with the ongoing downward flow of mantle material through the globally recognized sharp discontinuity at a depth of 670&#xa0;km. The leading interpretation for these heterogeneities is related to dehydration induced incipient partial melting. Here, we have employed an iterative method to achieve equilibrium between melt pools and undifferentiated pyrolite at geotherm conditions, allowing for a precise determination of partitioning of the major components (SiO<sub>2</sub>-MgO-Al<sub>2</sub>O<sub>3</sub>-FeO-CaO) between the melt and minerals. The melt holds 5.5 (0.4) wt% H<sub>2</sub>O and is relatively mafic compared to its source material, with considerable enrichment in CaO and FeO. For the (shallow) lower mantle in hydrous conditions, the solidus phase is davemaoite, followed by ferropericlase, and bridgmanite becomes the liquidus phase. Density models show that such magma is positively buoyant in the shallow lower mantle. Thus, it is expected to travel upward and possibly freeze in the transition zone. This process implies that, over time, continuous dehydration of the downgoing slab and subsequent hydrous melt extraction from the lower mantle wedge limit the deep water cycle to the upper 670&#xa0;km depth of the mantle.</p>

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Genesis and fate of incipient melt in shallow lower mantle

  • 期刊论文

  • 发表日期：

    2025年9月17日

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  • Siddharth Khan,
  • Denis Andrault,
  • Pierre Condamine,
  • Federica Schiavi,
  • Jean-Luc Devidal,
  • Nathalie Bolfan-Casanova

摘要

Geophysical observations have revealed the existence of low-velocity zones that potentially coincide with the ongoing downward flow of mantle material through the globally recognized sharp discontinuity at a depth of 670 km. The leading interpretation for these heterogeneities is related to dehydration induced incipient partial melting. Here, we have employed an iterative method to achieve equilibrium between melt pools and undifferentiated pyrolite at geotherm conditions, allowing for a precise determination of partitioning of the major components (SiO2-MgO-Al2O3-FeO-CaO) between the melt and minerals. The melt holds 5.5 (0.4) wt% H2O and is relatively mafic compared to its source material, with considerable enrichment in CaO and FeO. For the (shallow) lower mantle in hydrous conditions, the solidus phase is davemaoite, followed by ferropericlase, and bridgmanite becomes the liquidus phase. Density models show that such magma is positively buoyant in the shallow lower mantle. Thus, it is expected to travel upward and possibly freeze in the transition zone. This process implies that, over time, continuous dehydration of the downgoing slab and subsequent hydrous melt extraction from the lower mantle wedge limit the deep water cycle to the upper 670 km depth of the mantle.