Deep water

Research efforts from across the geosciences are uncovering how water deep within the Earth affects its fundamental workings.

Nature Geoscience spoke with Dr Qingyang Hu, a high-pressure mineralogist at HPSTAR; Prof. Suzan van der Lee, a geophysicist at Northwestern University; and Prof. Katherine Kelley, a geochemist at the University of Rhode Island about their work and what the future of deep-water research might bring.

Jörg Hermann suggests that as the process of serpentinization leads to clean energy generation, metal separation and carbon sequestration, it could serve as a natural analogue for a sequential economy.

Delving into recent and historical discoveries, Ananya Mallik explains how diamonds track the workings of the deep Earth that are hidden from view.

Seismic images of Earth’s crust and uppermost mantle around the Mariana trench show widespread serpentinization, suggesting that much more water is subducted than previously thought.

Serpentine subducted below the Lesser Antilles volcanic arc supplies water to the arc, controlling the location of seismicity, volcanic productivity and thickness of crust.

Electromagnetic data collected at the northern Hikurangi Margin, New Zealand show that a seamount on the incoming plate allows more water to subduct, compared with normal, unfaulted oceanic lithosphere.

Structures in the upper, overriding plate impact the geometry, hydration state and seismogenic region of subduction zones, according to a 3D seismic structural model of the Nankai subduction zone.

The lithology of the overriding plate plays a critical role in determining fluid transport in subduction zones, according to magnetotelluric imaging of the impact of the dry, mafic Siletzia terrane on fluids in the Cascadia subduction zone, North America.

Slow slip events may cause fluids to drain from the plate boundary into the overlying plate at subduction zones, according to seismic analyses of events recorded in Kanto, Japan.

Stress cycling in subducting crust before and during slow slip events is due to accumulation and release of fluid pressure, according to analysis of small earthquakes in the Hikurangi subduction zone.

Chemical reactions between slab and mantle rocks may lead to brittle failure where deep episodic tremor occurs in subduction zones, according to field and microstructural observations of a shear zone in New Zealand.

The transition between the locked and slowly slipping regions of the southern Cascadia megathrust has a lower porosity than these regions, according to seismic imaging. This suggests that the transition area is ductile, which may limit rupture propogation.

A distinct kink in the slope of the Gutenberg-Richter distribution of deep earthquakes in the northwest Pacific is identified by unsupervised machine learning and could indicate the rim of the metastable olivine wedge.

Fluids released from progressive breakdown of minerals at increasing pressure within a mélange may explain the trace element systematics and stable thallium isotope data of the Kamchatka arc lavas from volcanic front to back arc.

How the subducted oceanic lithosphere provides fluids and melts to flux the subarc mantle source of arc magmas is controversial. Here the authors use Mo and other isotopes to show serpentinites formed in both the forearc mantle and the subducted lithosphere contribute to generating arc magmas.

Lawsonite dehydration and release of oxidizing fluids could play an important role in sub-arc mantle oxidation in subduction zones, suggest measurements of changing oxygen fugacity in zoned garnets from Sifnos, Greece.

Metasedimentary rocks atop the downgoing slab oxidize ascending slab-derived dehydration fluids by removing reduced species, according to petrological analysis of subduction complex metasedimentary rocks and reactive transport modelling.

Carbonation of a serpentinized mantle wedge is associated with volume reduction and fluid over-pressurization which drives brittle fracture and seismicity, according to chemical analyses of carbonate veins from the Sanbagawa belt Japan and thermodynamic calculations.

Veins of aragonite in deep ocean trenches can record discharges of CO₂-rich paleoseawater and suggest that exposed serpentinized forearcs can act as carbon reservoirs, according to residence time calculations based on geochemical analyses of mantle rocks from offshore Japan.

Albite is one of the major constituents in the Earth’s crust. Here, the authors report that under hydrous cold subduction conditions, albite undergoes breakdown into hydrated smectite and other phases, which release alkaline fluids into the mantle wedge.

Migration of water from the slab to the surface during subduction is highlighted as a key process in the formation of continental crust.

The water content of arc magmas in the lower crust can reach up to 20 wt% during crystallization, according to geochemical analyses of minerals from the Kohistan palaeo-arc, Pakistan, underscoring the role of water in porphyry deposits formation.

Oxidation of arc magmas may be a secondary feature, acquired as hydrogen from magmatic water is incorporated into anhydrous minerals in the mantle wedge, according to analyses of orthopyroxenes in mantle xenoliths from an arc setting.

Volatile exsolution and crustal viscosity dictate that the optimum pressure for the growth of an eruptible magma reservoir is 2 kbar in all tectonic settings and crustal compositions, according to thermomechanical modelling.

Arc volcanism emits higher metal fluxes to Earth’s atmosphere than hotspot volcanism. The systems’ unique gas compositions are controlled by magmatic water content and redox state, as shown by a compilation of volcanic gas and aerosol metal data.

Magmatic and tectonic activity at mid-oceanic ridges can give detailed insights into high-temperature hydrothermal circulation of fluids. The authors here present geochemical and geophysical datasets that suggest a hydrothermal system penetrating the upper lithospheric mantle at an ultra-slow spreading mid-oceanic ridge.

Deep hydration of the upper mantle at transform plate boundaries due to seawater infiltration leads to hydrous melting and lithospheric thinning, according to seismic surveys and thermal modelling of the Romanche transform fault.

A hydrated, heterogeneous mantle resulting from magma ocean solidification is shown to be key to the rapid formation of Earth’s habitable surface environment during the Hadean era.

Accounting for experimental data on hydrous peridotites reduces the estimated water recycled into the deep mantle during subduction and suggests sea-level stability over geological time, according to subduction zone thermopetrological modelling.

The determination of the speciation of ions and molecules in supercritical aqueous fluids under pressure is key to understanding their mass transport in the Earth’s interior. Here the authors present a strategy based on ab-initio molecular dynamics to determine the speciation of carbonates in aqueous fluids.

Diamonds can give us clues to the processes regulating deep carbon transport within the Earth. Here, the author discovers evidence from diamond coatings that organic compounds exist at great depth in Earth’s interior, and furthermore, that organic molecules may provide scaffolds for diamond nucleation and growth.

The widespread intraplate volcanism in northeast China and the unusual ‘petit-spot’ volcanoes offshore Japan could have resulted from the interaction of the subducting Pacific slab with a hydrous mantle transition zone.

Two seismic discontinuities in the mantle transition zone beneath the western Pacific represent subducted slab interfaces that could be the slab Moho and partially molten sub-slab asthenosphere, according to an analysis of seismic data.

Phase transitions in the mantle transition zone could affect material and heat exchange between the upper and lower mantle. This Review discusses how compositional heterogeneity affects mantle convection based on seismic observations, plausible mantle compositions and model predictions.

Hydrous conditions extend across the 660 km discontinuity between Earth’s mantle transition zone and lower mantle, according to analysis of a polyphase mineral inclusion in a gem diamond from the Karowe mine, Botswana

Hydrogen isotopes and compositions of melt inclusions in olivine in komatiites indicate a hydrous source produced by recycling of seawater-altered crust into the deep mantle over 3.3 billion years ago.

Under conditions of Earth’s deep lower mantle, hydrogen ions diffuse freely through the FeOOH lattice framework and electrical conductivity increases rapidly, according to electrical conductivity experiments and first-principles simulations.

The Earth’s core may host most of the planet’s water inventory, according to calculations of the partitioning behaviour of water at conditions of core formation.

Based on diamond-anvil cell experiments and cutting-edge secondary ion mass spectrometry analyses, the authors here show that hydrogen may be an important constituent in the Earth’s core and also in the metallic cores of any terrestrial planet or moon having a mass in excess of 10% of the Earth.

Although the presence of ‘light’ elements (such as S, Si, O, C and H) can explain the core’s density deficit, the exact composition of the Earth’s core remains unknown. This Review explores the likely range of outer and inner core compositions and their implications.