The Redox State of Earth's Mantle

Frost, Daniel J.; McCammon, Catherine

doi:10.1146/annurev.earth.36.031207.124322

Cited by 919 publications

(564 citation statements)

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“…No Pt was present in these experiments, and thus Pt had no interference with this redox reaction. The redox partner for Fe ðIIIÞ formation could eventually be Fe ð0Þ as observed for high-pressure silicates such as perovskite (9 …”

Section: Discussionmentioning

confidence: 93%

“…Several studies have suggested that magnesium carbonate (magnesite) could become the main host for C at depth at the expense of calcite and dolomite (6)(7)(8). Other models proposed that carbonates in equilibrium with peridotite would become reduced into diamonds at lower mantle conditions (9). However, as observed in the upper mantle, heterogeneities of the lower mantle redox state probably exist: For instance, subduction zones are made of more oxidized materials (10) that could survive on long time scales.…”

mentioning

confidence: 99%

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New host for carbon in the deep Earth

Boulard

Gloter

Corgne

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

125

140

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The global geochemical carbon cycle involves exchanges between the Earth's interior and the surface. Carbon is recycled into the mantle via subduction mainly as carbonates and is released to the atmosphere via volcanism mostly as CO 2 . The stability of carbonates versus decarbonation and melting is therefore of great interest for understanding the global carbon cycle. For all these reasons, the thermodynamic properties and phase diagrams of these minerals are needed up to core mantle boundary conditions. However, the nature of C-bearing minerals at these conditions remains unclear. Here we show the existence of a new Mg-Fe carbon-bearing compound at depths greater than 1,800 km. Its structure, based on three-membered rings of corner-sharing ðCO 4 Þ 4− tetrahedra, is in close agreement with predictions by first principles quantum calculations [Oganov AR, et al. (2008)

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Section: Discussionmentioning

confidence: 93%

mentioning

confidence: 99%

New host for carbon in the deep Earth

Boulard

Gloter

Corgne

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

125

140

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“…Along a normal upper mantle geotherm, increasing pressure would favor formation of NH 3 . Frost and McCammon (2008) have shown that oxygen fugacities prevailing in the upper part of the upper mantle, i.e., in spinel lherzolites, are between ∆FMQ -2 and +2 log units, whereas they gradually decrease with depth attaining values of -3 to -4 log units at 4 to 6 GPa below the cratonic lithosphere, and may attain -5 log units at 8 GPa. It would thus appear that in the lower part of the upper mantle NH 3 /NH + 4 predominates in H-N-O fluids, and therefore may sta-bilize the NH 4 -component in clinopyroxene relative to N 2 plus water.…”

Section: Implications For Nitrogen and Hydrogen Storage In The Earth'mentioning

confidence: 99%

Ammonium-bearing clinopyroxene: A potential nitrogen reservoir in the Earth's mantle

et al. 2010

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In the pseudobinary system CaMgSi 2 O 6 − (NH 4 )M 3+ Si 2 O 6 , with M = Cr or Al, and NH 4 OH in excess, multi-anvil experiments at 9.5 to 12.8 GPa, 725 to 750 • C produced NH 4 -bearing diopside. Incorporation mainly follows the coupled substitution (Ca. Ammonium was identified and quantified by IR spectroscopy. In Cr-bearing diopside we found maximum concentrations in the range of 500 to 1000 ppm of NH 4 .The storage capacity of mantle clinopyroxenes for ammonium turns them to potential candidates for the nitrogen reservoir in the Earth's upper mantle, and this mechanism also contributes to its water budget. While nitrogen is transported into the mantle via cold slabs through NH 4 inherited from sedimentary material, and stored in K-bearing minerals and successor high-pressure phases, nitrogen output from the mantle is through degassing of N 2 . A probable mechanism for that is that nitrogen is kept as NH 4 in clinopyroxene in the Earth's mid and lower mantle, whereas in the upper part, it is lost due to oxidation to molecular nitrogen. It is most likely that clinopyroxene plays a major role in the long-time, large-scale nitrogen cycle between surface and deep mantle of the Earth.

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“…In fact, as will be seen, mantle f O 2 is not constant with depth. Even if the Fe 3C / P Fe or bulk oxygen content of the mantle is constant, the f O 2 will be much lower in the deeper mantle as a result of crystal chemical effects (Ballhaus 1995;Gudmundsson & Wood 1995;Woodland & Koch 2003;Frost & McCammon 2008). The rise in the redox state of the mantle since core formation, therefore, is more accurately reflected in the Fe 3C / P Fe of the upper mantle (or better still the O/Fe ratio), which is in the range of 2-3 per cent in the present-day mantle (Canil & O'Neill 1996), but would have been much lower than this when in equilibrium with metallic Fe.…”

Section: The Evolution Of Mantle F Omentioning

confidence: 99%

“…The precipitation of Ni-Fe alloy raises the Fe 3C / P Fe ratio of the assemblage, which will cause the f O 2 to remain close to the Ni-precipitation curve, although the volume change of equation (3.1) will continue to lower the f O 2 to some extent with pressure, causing more metal to precipitate and to become increasingly Fe rich. By 14 GPa, a typical upper mantle fertile peridotite should have exsolved 0.1-0.2 wt% metal and the f O 2 will be wIW (Frost & McCammon 2008). The f O 2 in the upper mantle will therefore drop by at least 3 log units between depths of 60 and 250 km, even if the bulk oxygen content remains constant.…”

Section: The Redox State Of the Present-day Mantlementioning

confidence: 99%

The redox state of the mantle during and just after core formation

Frost

Mann

Asahara

et al. 2008

Phil. Trans. R. Soc. A.

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Siderophile elements are depleted in the Earth's mantle, relative to chondritic meteorites, as a result of equilibration with core-forming Fe-rich metal. Measurements of metal-silicate partition coefficients show that mantle depletions of slightly siderophile elements (e.g. Cr, V ) must have occurred at more reducing conditions than those inferred from the current mantle FeO content. This implies that the oxidation state (i.e. FeO content) of the mantle increased with time as accretion proceeded. The oxygen fugacity of the present-day upper mantle is several orders of magnitude higher than the level imposed by equilibrium with core-forming Fe metal. This results from an increase in the Fe 2 O 3 content of the mantle that probably occurred in the first 1 Ga of the Earth's history. Here we explore fractionation mechanisms that could have caused mantle FeO and Fe 2 O 3 contents to increase while the oxidation state of accreting material remained constant (homogeneous accretion). Using measured metal-silicate partition coefficients for O and Si, we have modelled core-mantle equilibration in a magma ocean that became progressively deeper as accretion proceeded. The model indicates that the mantle would have become gradually oxidized as a result of Si entering the core. However, the increase in mantle FeO content and oxygen fugacity is limited by the fact that O also partitions into the core at high temperatures, which lowers the FeO content of the mantle. (Mg,Fe)(Al,Si)O 3 perovskite, the dominant lower mantle mineral, has a strong affinity for Fe 2 O 3 even in the presence of metallic Fe. As the upper mantle would have been poor in Fe 2 O 3 during core formation, FeO would have disproportionated to produce Fe 2 O 3 (in perovskite) and Fe metal. Loss of some disproportionated Fe metal to the core would have enriched the remaining mantle in Fe 2 O 3 and, if the entire mantle was then homogenized, the oxygen fugacity of the upper mantle would have been raised to its present-day level.

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The Redox State of Earth's Mantle

Cited by 919 publications

References 116 publications

New host for carbon in the deep Earth

New host for carbon in the deep Earth

Ammonium-bearing clinopyroxene: A potential nitrogen reservoir in the Earth's mantle

The redox state of the mantle during and just after core formation

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