Deep mantle melting, global water circulation and its implications for the stability of the ocean mass

Abstract

Oceans on Earth are present as a result of dynamic equilibrium between degassing and regassing through the interaction with Earth’s interior. We review mineral physics, geophysical, and geochemical studies related to the global water circulation and conclude that the water content has a peak in the mantle transition zone (MTZ) with a value of 0.1–1 wt% (with large regional variations). When water-rich MTZ materials are transported out of the MTZ, partial melting occurs. Vertical direction of melt migration is determined by the density contrast between the melts and coexisting minerals. Because a density change associated with a phase transformation occurs sharply for a solid but more gradually for a melt, melts formed above the phase transformation depth are generally heavier than solids, whereas melts formed below the transformation depth are lighter than solids. Consequently, hydrous melts formed either above or below the MTZ return to the MTZ, maintaining its high water content. However, the MTZ water content cannot increase without limit. The melt-solid density contrast above the 410 km depends on the temperature. In cooler regions, melting will occur only in the presence of very water-rich materials. Melts produced in these regions have high water content and hence can be buoyant above the 410 km, removing water from the MTZ. Consequently, cooler regions of melting act as a water valve to maintain the water content of the MTZ near its threshold level (~ 0.1–1.0 wt%). Mass-balance considerations explain the observed near-constant sea-level despite large fluctuations over Earth history. Observations suggesting deep-mantle melting are reviewed including the presence of low-velocity anomalies just above and below the MTZ and geochemical evidence for hydrous melts formed in the MTZ. However, the interpretation of long-term sea-level change and the role of deep mantle melting in the global water circulation are non-unique and alternative models are reviewed. Possible future directions of studies on the global water circulation are proposed including geodynamic modeling, mineral physics and observational studies, and studies integrating results from different disciplines.

Hydrous silicate melts and the deep mantle H2O cycle

“…If the transition zone is hydrated, then H 2 O can potentially be released at its upper and lower boundaries as hydrous wadsleyite and ringwoodite transform to relatively anhydrous olivine or bridgmanite + ferropericlase during mantle upwelling or downwelling, respectively, leading to the release of H 2 O and the generation of hydrous silicate melts in these regions (Hirschmann, 2006;Panero et al, 2020). This process is the basis of the "transition zone water filter" model, which describes how dense, hydrous silicate partial melts produced by the dehydration of material upwelling through the upper transition zone boundary (410 km depth) may pond and be reabsorbed into the transition zone, efficiently stripping the upwelling mantle of its incompatible trace elements (Bercovici and Karato, 2003;Karato et al, 2020).…”

A Partial Molten Low‐Velocity Layer Atop the Mantle Transition Zone Beneath the Western Junggar: Implication for the Formation of Subduction‐Induced Sub‐Slab Mantle Plume

“…The low dVp/dVs (−4.4%/−5.6%) from the Event 20111227 endorses that the LVL is mainly caused by partial melt induced by water or other volatiles (e.g., Courtier & Revenaugh, 2007; Jacobsen et al., 2008; G. Li et al., 2017; Ma et al., 2020; Revenaugh & Sipkin, 1994; Song, et al., 2004). The observed LVL can be explained by a small amount of melt completely wetting grain‐boundaries (Karato et al., 2020). The small depth variations of 408–410 km for the 410 suggest the slight contribution of temperature to the LVL (e.g., Katsura et al., 2004).…”

“…Besides the ridge subduction, we try to provide a new possible mechanism to explain the slab window opening, plate melting and break‐off with no need of the continuous expansion of the oceanic ridge. The hydrous melt in the LVL can be buoyant and ascend to the shallower mantle (e.g., Jing & Karato, 2012; Karato et al., 2020). Geodynamic modeling shows that the melt can upwell to the Earth surface and be the origin of intraplate volcanism (J. Yang & Faccenda, 2020).…”

“…Li et al, 2017;Ma et al, 2020;Revenaugh & Sipkin, 1994;Song, et al, 2004). The observed LVL can be explained by a small amount of melt completely wetting grain-boundaries (Karato et al, 2020). The small depth variations of 408-410 km for the 410 suggest the slight contribution of temperature to the LVL (e.g., Katsura et al, 2004).…”

“…Most of these detected LVLs are located at the forward direction of the subducted plate, which improves the knowledge about deep behaviors of plate subduction. The melt in the LVL can be buoyant (e.g., Jing & Karato, 2012; Karato et al., 2020) and related to the origin of intraplate volcanism (J. Yang & Faccenda, 2020). Due to the lack of seismic stations in oceanic areas, the LVLs at the backward direction of the subducted plate are rarely been detected, which hinders the understanding of the related geodynamic processes.…”