When water meets iron at Earth’s core–mantle boundary
Abstract: Hydrous minerals in subducted crust can transport large amounts of water into Earth’s deep mantle. Our laboratory experiments revealed the surprising pressure-induced chemistry that, when water meets iron at the core–mantle boundary, they react to form an interlayer with an extremely oxygen-rich form of iron, iron dioxide, together with iron hydride.
Hydrogen in the layer will escape upon further heating and rise to the crust, sustaining the water cycle. With water supplied by the subducting slabs meeting the nearly inexhaustible iron source in the core, an oxygen-rich layer would cumulate and thicken, leading to major global consequences in our planet. The seismic signature of the D″ layer may echo the chemical complexity of this layer.
Over the course of geological time, the enormous oxygen reservoir accumulating between the mantle and core may have eventually reached a critical eruption point. Very large-scale oxygen eruptions could possibly cause major activities in the mantle convection and leave evidence such as the rifting of supercontinents and the Great Oxidation Event.
INTRODUCTION
Among all of the global boundaries on the planet, the interface between Earth’s core and mantle stands out as having the greatest contrast in chemical composition and physical properties [1]. The enigmatic signature from seismic observations of the D″ layer on top of the core–mantle boundary (CMB) [2] has long eluded satisfactory explanation.
In the present work, we conducted additional experiments in the key Fe-O-H ternary system, presented the mechanism for generating widespread oxygen-rich patches consisting of the Py-phase and other iron oxides and hydrides at the base of the mantle, and proposed far-reaching geophysical, geochemical and geodynamic consequences based on the new observations.
The Py-phase was previously synthesized at the P-T conditions of the deep lower mantle (DLM >1800-km depth) by oxidizing hematite (Fe2O3) or dehydrogenating goethite (FeO2H) [3]. However, neither hematite nor goethite is a major mineral in the crust; their abundances are insufficient to form a significant portion of the D″, which is more massive than the entire crust.
Searching for a possible source of much greater magnitude, we found that, if hydrous minerals go down with slabs to reach the subsolidus side of CMB [6–10], the nearly inexhaustible iron reservoir in the core will react with the water released from the hydrous minerals to generate an enormous quantity of the Py-phase in oxygen-rich patches (ORP) above the CMB.
The formation of the ORP leads to a range of extremely important consequences and implications including: the source of seismic complexity in the D″ layer [2], the chemical and geodynamic metastability of the ORP, the Great Oxidation Event [11] and the episodic dispersions of supercontinents [12].
RESULTS
When water meets iron at moderate P-T above 5 GPa, it oxidizes and hydrogenates iron to form wüstite and iron hydride [13,14], namely
3Fe + H2O = FeO (wüstite) + 2FeH. (1)
For simplification, here we refer to wüstite FexO with x = 0.9–0.947 as FeO, and FeHx with x ≤ 1 as FeH. The simplification does not affect our discussion and conclusion. The assemblage FeO + FeH can coexist with excess water or iron under moderate pressures.
![XRD pattern of reaction products of iron and water. Iron powder was compressed in H2O to 96 GPa, heated up to 2200 K for 5 minutes, and quenched to 300 K. The pattern was composed of the Py-phase (a = 4.370(3) Å), the quenchable high-temperature f.c.c. phase [44] of FeH (a = 3.397(4) Å) and excess ice VII. Inset figure is the caked diffraction pattern, showing the coexistence of the Py-phase (dotted reflections) and FeH (continuous reflections).](https://i0.wp.com/oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/nsr/PAP/10.1093_nsr_nwx109/2/m_nwx109fig1.jpeg?ssl=1)
XRD pattern of reaction products of iron and water. Iron powder was compressed in H2O to 96 GPa, heated up to 2200 K for 5 minutes, and quenched to 300 K. The pattern was composed of the Py-phase (a = 4.370(3) Å), the quenchable high-temperature f.c.c. phase [44] of FeH (a = 3.397(4) Å) and excess ice VII. Inset figure is the caked diffraction pattern, showing the coexistence of the Py-phase (dotted reflections) and FeH (continuous reflections).
Again, for simplicity, we neglect the non-stoichiometry of FeH and the hydrogen loss (1 – x)H in the equation.
XRD pattern of reaction product of Fe2O3 and water. The sample was compressed to 110 GPa, heated to 2250 K and quenched to 300 K. py, pyrite structured FeO2Hx. Inset figure is the caked image with dotted Py-phase reflections, scattered ice spots and bright diamond spots.
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