Chemical Stability of FeOOH at High Pressure and Temperature, and Oxygen Recycling in Early Earth History**

Abstract

Goethite, α‐FeOOH, is a major phase among oxidized iron species, commonly called rust. We studied the behavior of iron (III) oxyhydroxide up to 81 GPa and 2100 K using in situ synchrotron single‐crystal X‐ray diffraction. At high pressure‐temperature conditions FeOOH decomposes forming oxygen‐rich fluid and different mixed valence iron oxides (previously known phases of Fe2O3, Fe3O4, Fe5O7, and novel Fe7O10 and Fe6.32O9). Rust is known to form as a byproduct of anoxygenic prokaryote metabolism that took place massively from about 3.8 billion years (Ga) ago until the Great Oxidation Event (GOE) ∼2.2 Ga ago. Rust was buried on the ocean floor and was transported into the mantle as a consequence of plate tectonics (started ∼2.8 Ga ago). Our results suggest that recycling of rust in Earth's mantle contributes to redox conditions of the early Earth and formation of oxygen‐rich atmosphere.

Stability of a Mixed‐Valence Hydrous Iron‐Rich Oxide: Implications for Water Storage and Dynamics in the Deep Lower Mantle

“…The improvement of the refinement is significant. The structure model of HH1‐phase obtained in our study is in good agreement with that of Fe 6.32 O 9 phase (Koemets et al., 2021). In the hollandite‐like structures (Foo et al., 2006; Miura et al., 2000), Na or Ca cations (occupancies of 1/3 and 1/2, respectively) lie in the channels instead of O, similar to our structure model of the HH1‐phase.…”

Partial Deoxygenation and Dehydration of Ferric Oxyhydroxide in Earth's Subducting Slabs

“…Therefore, as FeOOH is present in hydrous sediments, peridotite, and mid-ocean ridge basalt compositions, it may be thermodynamically stable in some very cold subducting slabs and likely transport water into the deep lower mantle. (Wiethoff et al, 2017); open blue triangles are ε-FeOOH (Yoshino et al, 2019); solid red squares (this study), and open wine circles (Koemets et al, 2021) indicate that ε-FeOOH partially decomposes into Fe oxides, H 2 O, and O 2 . Open carmine triangles (Hu et al, 2016(Hu et al, , 2017, diamonds (Nishi et al, 2017), and circles (Koemets et al, 2021) represent Py-FeO 2 H x (x ≤ 1); solid green and violet lines represent the dehydration boundary of α-FeOOH and the phase boundary between α-FeOOH and ε-FeOOH, respectively (Voigt & Will, 1981).…”

“…Open black triangles are α-FeOOH(Wiethoff et al, 2017); open blue triangles are ε-FeOOH(Yoshino et al, 2019); solid red squares (this study), and open wine circles(Koemets et al, 2021) indicate that ε-FeOOH partially decomposes into Fe oxides, H 2 O, and O 2 . Open carmine triangles(Hu et al, 2016(Hu et al, , 2017, diamonds(Nishi et al, 2017), and circles(Koemets et al, 2021) represent Py-FeO 2 H x (x ≤ 1); solid green and violet lines represent the dehydration boundary of α-FeOOH and the phase boundary between α-FeOOH and ε-FeOOH, respectively(Voigt & Will, 1981). The solid orange line is the dehydration boundary of ε-FeOOH(Yoshino et al, 2019).…”

“…Therefore, as FeOOH is present in hydrous sediments, peridotite, and mid-ocean ridge basalt compositions, it may be thermodynamically stable in some very cold subducting slabs and likely transport water into the deep lower mantle. (Hu et al, 2016(Hu et al, , 2017, diamonds (Nishi et al, 2017), and circles (Koemets et al, 2021) represent Py-FeO 2 H x (x ≤ 1); solid green and violet lines represent the dehydration boundary of α-FeOOH and the phase boundary between α-FeOOH and ε-FeOOH, respectively (Voigt & Will, 1981). The solid orange line is the dehydration boundary of ε-FeOOH (Yoshino et al, 2019).…”

“…Where, the GOE is the first long-term oxygenation event of Earth's atmosphere at ∼2.3-2.5 billion years (Gyr) ago based on the evidence of Earth surface oxygenation (e.g., Canfield, 2004;Farquhar et al, 2000;Guo et al, 2009;Holland, 2002). Furthermore, recent LH-DAC experiments indicated that ε-FeOOH may partially dehydrate to form a new hydrous iron oxide η-Fe 12 O 19 H 2 (H. Chen et al, 2020) or decompose to various iron oxides (Fe 2 O 3 , Fe 5 O 7 , Fe 7 O 10 , and Fe 6.32 O 9 ) and an oxygen-rich fluid in the slabs present at the mid-lower mantle (Koemets et al, 2021), suggesting that FeOOH may become unstable and the oxygen-rich fluid may contribute to the GOE. These results indicate that a sturdy approach is urgently needed to further investigate the thermal stability of FeOOH at the relevant P-T conditions of the mid-lower mantle and to explore how oxygen and water are released from the Fe-O-H system.…”