High Melt Fraction Induced by the Dehydration Melting Near the 410‐km Discontinuity

Abstract The water‐rich mantle transition zone, coupled with the low water storage capacity of the upper mantle, suggests that dehydration melting occurs just above the mantle transition zone near the 410‐km discontinuity. However, the melt fraction produced by the dehydration melting process remains poorly constrained. Here, we investigated the melt fractions of the peridotite +1 wt.% H 2 O system, which represents a water‐rich mantle transition zone, at 13 GPa and 1,800–2,100 K by high‐pressure experiments combined with mass balance calculations. The results show that the melt fraction increases with rising temperature and reaches about 10.1 vol.% under the 410‐km discontinuity conditions, exceeding the estimates required to explain the observed seismic velocity reduction. Because of the low density, most melts formed near the 410‐km discontinuity may migrate upward rapidly to shallow regions. They may accumulate near the lithosphere‐asthenosphere boundary, contributing to the rheological weakening of the asthenosphere.