FeOOH instability at the lower mantle conditions

TL;DR

This study investigates the stability of goethite (FeOOH) under lower mantle conditions, revealing its decomposition into iron oxides and oxygen-rich fluids at high pressures and temperatures, which may influence Earth's oxygen cycle.

Contribution

It provides the first in situ high-pressure, high-temperature analysis of goethite's behavior, showing its decomposition in the lower mantle and potential role in oxygen release.

Findings

Goethite decomposes into iron oxides at mantle conditions.

Decomposition releases oxygen-rich fluids.

Recycling of rust could impact Earth's oxygen levels.

Abstract

Goethite, {\alpha}-FeOOH, is a major component among oxidized iron species, called rust, which formed as a product of metabolism of anoxygenic prokaryotes (1, 2) inhabiting the Earth from about 3.8 billion years (Gy) ago until the Great Oxidation Event (GOE) of about 2.5 Gy ago. The rust was buried on the ocean floor (1, 2) and had to submerge into the Earth mantle with subducting slabs due to the plate tectonics started about 2.8 Gy ago (3). The fate and the geological role of the rust at the lower mantle high-pressure and high-temperature(HPHT) conditions is unknown. We studied the behavior of goethite up to 82(2) GPa and 2300(100) K using in situ synchrotron single-crystal X-ray diffraction. At these conditions, corresponding to the coldest slabs at the depth of about 1000 km, {\alpha}-FeOOH decomposes to various iron oxides (Fe2O3, Fe5O7, Fe7O10, Fe6.32O9) and an oxygen-rich fluid. Our results suggest that recycling of the rust in the Earth mantle could contribute to oxygen release to the atmosphere and explain the sporadic increase of the oxygen level before the GOE linked to the formation of Large Igneous Provinces(4).