Prof. Daniel J. Frost [University of Bayreuth, Germany]

Title: The redox state of Earth's mantle and its development

Language: English presentation

Time: Beijing, 10:00 - 11:30 AM, Monday, April 8, 2024

Place: Conference room A516, HPSTAR (Beijing) 

Host: Yanhao Lin

Abstract:

The redoxstate of the Earth's upper mantle controls the nature of volatile phasesdegassing from the interior and has, therefore, influenced the development ofhabitable surface conditions. One of the most important events in the creationof these conditions was the rapid oxidation of the upper mantle after coreformation. During Earth's accretion, mantle silicates equilibrated withcore-forming metallic iron, which would have imposed a low mantle oxygenfugacity, where H2O, CO and H2 would have dominateddegassing species, which would have resisted any increase in atmosphericoxygen. Evidence from oxybarometry, based on redox-sensitive elements, however, indicates that the oxygen fugacity (fo2) of the upper mantle must have increased by about 5 log units by the time thefirst rocks and minerals were formed, and has since remained relativelyconstant. Degassing volatile species since core formation have, therefore, beendominated by H2O and CO2, the essential reactants forphotosynthesis.Oxidationby H2O has been the main mechanism proposed to explain the increase in mantle redox state in the past. While this almost certainly occurred to someextent, the released H2 would have made it difficult for the mantleto approach its current redox state unless mixing occurred with material thathad been oxidized at or near the surface, which may have been a relatively slowprocess.

Furthermore, such a mechanism raises the question of why Mars, aseemingly more volatile-rich planet compared to Earth, has a more reducedmantle. Analternative oxidation mechanism is based on experimental studies which show that the dominant lower mantle mineral bridgmanite has a high Fe3+/∑Fe ratio when in equilibrium with iron metal. This implies that the equilibrium 3FeO = Fe0 + Fe2O3, involving ferric and ferrous iron components in mineral phases, would have shifted to the right asthe lower mantle formed, resulting in disproportionation of FeO and precipitation of iron metal. Segregation of precipitated iron metal from thecrystallizing lower mantle into the core could have raised the bulk oxygen content of the entire mantle after convective mixing. Animportant question, however, is whether silicate magmas might also go through the same shift in Fe2O3 stability with pressure. If pressure causes the concentration of Fe2O3 components insilicate magmas to increase at lower fo2, then at the base of a deep magma ocean, the melt could have contained high levels of ferric iron in equilibrium with iron metal. Separation of iron metal to the core could then have raised the redox state of the mantle. A magma ocean would therefore simply oxidise if it were sufficiently deep. By examining the Fe3+/∑Fe ratio of silicate melts as a function of pressure and fo2 this magma oceanself-oxidation scenario can be shown to be plausible and to also have implications for the carbon content of the interior.

Publication Summary:

224 peer reviewed articles including 5 in the journal Nature and 6 in Science

Biography of the Speaker:

https://www.profilfelder.uni-bayreuth.de/en/advanced-fields/4_High_pressure-and-high--temperature-research/contributors/2-frost_dan/