Hydration and Dehydration in Earth's Interior

Abstract

Hydrogen and deuterium isotopic evidence indicates that the source of terrestrial water was mostly meteorites, with additional influx from nebula gas during accretion. There are two Earth models, with large (7–12 ocean masses) and small (1–4 ocean masses) water budgets that can explain the geochemical, cosmochemical, and geological observations. Geophysical and mineral physics data indicate that the upper and lower mantles are generally dry, whereas the mantle transition zone is wetter, with heterogeneous water distribution. Subducting slabs are a source of water influx, and there are three major sites of deep dehydration: the base of the upper mantle, and the top and bottom of the lower mantle in addition to slabs in the shallow upper mantle. Hydrated regions surround these dehydration sites. The core may be a hidden reservoir of hydrogen under the large water budget model. ▪ Earth is a water planet. Where and when was water delivered, and how much? How does water circulate in Earth? This review looks at the current answers to these fundamental questions. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Global water distribution in the mantle transition zone from a seismic isotropic velocity model and mineral physics modeling

“…According to our water distribution results and the water contents and water distributions of the upper mantle (Peslier et al, 2017;Ohtani, 2021), together with the information on deep mantle melting (Karato et al, 2020) and plate tectonic history (Brown et al, 2020), we propose a water circulation model in Figure 7.…”

“…Hydrous regions in the upper mantle are near subduction zones, where slabs transported 0.03-0.3 wt% water into the MTZ (Suetsugu et al, 2010;Peslier et al, 2017;Ohtani, 2021). Because plate tectonics began in the early Paleoproterozoic (2.2 Ga) (Brown et al, 2020), the subduction of oceanic plates may have been going on for hundreds of millions of years.…”

Mantle mineralogy limits to rocky planet water inventories

“…Ultimately, waterworld propensity depends on the interplay of total water budgets, water redistribution between interior and surface reservoirs, and the theoretical capacities of each reservoir. Total water budgets depend on not only the potential size of the mantle reservoir, but also the amount of water in planetary building blocks, and the ability of the growing planet to ingas nebular hydrogen, which depend in turn on the planetary formation time-and spacescales (Sharp 2017;Ohtani 2021, and references therein)note that late delivery of water by comets is not expected to contribute a significant amount of water, at least in the solar system (Morbidelli et al 2000). Water redistribution in the stagnant lid regime is controlled by volcanic outgassing and is tied to the thermal history of the planet (e.g., Ortenzi et al 2020).…”

Enhanced Visibility of Subduction Slabs by the Formation of Dense Hydrous Phase A

“…Serpentine is a dominant hydrous mineral in the slab, and it undergoes a phase transformation to form a phase A‐bearing aggregate around ∼5 GPa, which further transports water into the deep mantle (e.g., Inoue et al., 2009; Irifune et al., 1998). The previous experimental studies show that phase A transforms to form further high‐pressure hydrous minerals (dense hydrous magnesium silicates: DHMS) around the mantle transition zone, and the water transportation continues into deep mantle (e.g., Komabayashi, 2004; Nishi et al., 2017; Ohtani, 2021; Xu et al., 2019).…”