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The pyrite-type FeO2 and FeO2H were synthesized at the pressure-temperature conditions relevant to Earth’s deep lower mantle. Through the water-iron reaction, the pyrite-phase is a good candidate to explain the chemical heterogeneities and seismological anomalies at the bottom of the mantle. The solid solution of pyrite-type FeO2 and FeO2H, namely the FeO2Hx (0 ​≤ ​x ​≤ ​1), is particularly interesting and introduces puzzling chemical states for both the O and H atoms in the deep mantle. While the role of H in the FeO2–FeO2H system has been primarily investigated, discrepancies remain. In this work, we summarize recent progress on the pyrite-phase, including FeO2, FeO2H, and FeO2Hx, which is critical for understanding the water cycling, redox equilibria, and compositional heterogenicities in the deep lower mantle.

The amount of water stored as OH-defects in nominally anhydrous minerals in the deep mantle is poorly constrained and its direct quantification can only be accessed by the analysis of mantle xenoliths. While the vast majority of xenoliths are peridotites and minor pyroxenites, some very rare xenoliths found in kimberlite pipes display an eclogitic mineral assemblage. We investigated three eclogite xenoliths from the 128 m.y. old Robert Victor kimberlite from South Africa that display an assemblage of garnet and omphacite with two samples showing additional kyanite, suggesting low-pressure gabbroic rock as protolith. Thermobarometry estimations based on Fe-Mg partitioning between garnet and pyroxene gives temperatures of 1100-1250 °C. When projected on the cratonic geotherm (Griffin & O’Reilly 2007) an equilibrium depth of 200-210 km is obtained, confirming that these rocks come from the lithosphere-asthenosphere boundary. Therefore these fragments might be key witnesses to understand the deep cycling of water in the mantle. This study focuses on the H2O quantification in the three rock-forming minerals using Fourier transform infrared spectroscopy (FTIR). Omphacite contains 50-250 ppm H2O, kyanite contains 40-60 ppm H2O and garnet of only one eclogite contains 40 ppm H2O. Garnet and omphacite with the highest OH content are enriched in Ca.The use of advanced mapping and profiling techniques enabled the investigation of the spatial repartition of the OH component in these minerals. High-resolution mapping (5.6 µm) of kyanite reveals diffusive gain of OH at the rim of the crystal that is interpreted as hydration during interaction with the kimberlitic melt. The OH plateau in the core of kyanite must therefore have been acquired previously, suggesting that this is residual OH that has been transported by subduction to the lithosphere-asthenosphere boundary by a once hydrated gabbroic protolith. Our results have implications for the retention of hydrogen over long timescale at the lithosphere-asthenosphere boundary and suggest that the deep cycling of water has been running since Archean times. Griffin, W. L., & O'Reilly, S. Y. (2007). Cratonic lithospheric mantle: is anything subducted?. Episodes, 30(1), 43-53.

The late Miocene provides the chance to assess the changing boundary conditions on a warmer world than present. While the climate variability is well understood for the oceanographic records, the water availability and dynamics in terrestrial environment in the densely populated East Asian remains enigmatic. Little is known about the precipitation response to the Antarctic ice-sheets during this time interval. To understand this critical relationship between low- and high-latitude climates, we use a new indicator based on the carbonate variability in Red Earth on the northern Chinese Loess Plateau to reconstruct water availability throughout the interval (7.5–6.9 Ma). Our high-resolution reconstructions show that the carbonate leaching/reprecipitation cycle is dominantly forced by the astronomical parameter obliquity (40-kyr) that is in accord with the Antarctic ice-volume controlled oceanography records at ~7 Ma. Supported by goethite and hematite records in the same site, soil temperatures and precipitations are fully coupled, interpreted as marking the climate pattern of Asian monsoon during the late Miocene. Cyclic correlation between the carbonate variability and the goethite, hematite data, reveal that the obliquity controlled precipitation oscillations were superimposed on a long-term increase of the Asian monsoon, which was synchronous with intensifification of climate cooling, the declining of partial pressure of carbon dioxide, and the growing of Antarctic ice-volume. Combined with the atmospheric- and oceanic-adjustments, we suggest that the cross-equatorial pressure gradient has led to the rise of Asian monsoon.

Subduction-induced deep hydration and water cycling may play significant roles in the modification and destruction of the overriding cratonic lithosphere. Two contrasting modes are generally propos...

Pre-print (óritrýnt handrit), Oceanic crust is a major transport medium of water into the mantle wedge and the convecting mantle. Yet, the water content of the oceanic crust remains uncertain. Active geothermal systems situated at on-land spreading centers provide a unique opportunity to study the hydration of the oceanic crust, with well constrained systems and boreholes reaching depths of >4 km. Here, we present hydrogen isotope data of geothermal fluids and altered basalt for three Icelandic geothermal systems: the meteoric water fed system at Krafla and the seawater fed systems at Reykjanes and Surtsey. The bulk rock δD values of altered and hydrated basalts from these localities, which exhibit significantly higher water contents (up to 8.9 wt.%) than magmatic (non-hydrated) basalts, vary greatly from −125 to −96 at Krafla, from −80 to −46 at Reykjanes and from −78 to −46 at Surtsey. The corresponding fluids have δD values of −84.1 to −81.1 at Krafla, −23.1 to −14.9 at Reykjanes and +2.1 to +4.3 at Surtsey. Comparison of isotope modeling results to the natural data reveals that hydration of the Icelandic crust and corresponding hydrogen isotopic characteristics are controlled by (1) the isotope composition of the source fluid, (2) isotope fractionation between the aqueous geothermal fluids and the alteration minerals formed, and (3) the type and quantity of alteration minerals formed. These factors in turn depend on the extent of fluid-rock interaction and temperature. Using the same modeling approach and expanding it to datasets available for the oceanic crust, we assessed the hydration state and δD values of the oceanic crust as a function of depth. We show that 1400 to 1650 Tg H2O/yr is added to the igneous oceanic crust upon alteration by seawater and that the upper part (, This project was financially supported by NordVulk, the International Continental Scientific Drilling Program (ICDP) through a grant to the SUSTAIN project, and the Icelandic Research Fund (project number: 163083-051). SAH acknowledges support from the Icelandic Research Fund (project number: 196139-051). HS Orka and Landsvirkjun kindly provided access to the drill cuttings. J. Cullen, T. Larson, R. Ólafsdóttir and Á.E. Sveinbjörnsdóttir are thanked for assistance during sample preparation and data acquisition. BIK is particularly grateful of being part of this project as without the project-related lab work she would have never met her future husband E.W. Marshall IV. We thank four anonymous reviewers for their constructive comments and suggestions to an earlier version of this manuscript. Louis Derry is thanked for careful editorial handling of this study.

Shaking out water's dusty origin Where did Earth's water come from? Lavas erupting on Baffin Island, Canada, tap a part of Earth's mantle isolated from convective mixing. Hallis et al. studied hydrogen isotopes in the lavas that help to “fingerprint” the origin of water from what could be a primordial reservoir. The isotope ratios for the Baffin Island basalt lavas suggest a pre-solar origin of water in Earth, probably delivered by adsorption onto dust grains. Science , this issue p. 795