Genesis and fate of incipient melt in shallow lower mantle

Abstract

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.