Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle

Frost, Daniel J.; Liebske, Christian; Langenhorst, F.; McCammon, Catherine; Trønnes, Reidar G.; Rubie, D. C.

doi:10.1038/nature02413

Cited by 536 publications

(413 citation statements)

References 27 publications

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“…Given the totality of these observations, we suggest that the resolution of conflicting fO 2 indicators is that in the deep mantle (Ͼ300 km) the fO 2 is low and the elevated abundance of Fe 3ϩ is a consequence of crystallographic constraints imposed by high-pressure host phases. Upon decompression and stabilization of upper-mantle minerals with abundant Fe 2ϩ sites, the Fe 3ϩ and Fe 0 react in the reverse direction of the Fe 2ϩ disproportionation proposed by (21) 2Fe 3ϩ ϩ Fe 0 3 3Fe 2ϩ , and, if all Fe 0 is consumed, the fO 2 rises to the observed values of the upper mantle. In the present case, this reaction took place outside of chromite but the low fO 2 was preserved within massive chromite because of the abundant reduced phases within a highly restricted, refractory, chemical environment.…”

Section: Terrestrial Nitrides and Single Crystals Tio2 II As Inclusiomentioning

confidence: 99%

“…Another perplexing aspect of this problem is the very low fO 2 conditions of these materials, including SiC (12), although the latter is not incorporated in this particular specimen. Recent experimental work has shown that, under conditions of the lower mantle (21,22) and even deep upper mantle (23), ferrous ion disproportionates into ferric ion ϩ Fe metal. Thus, it may be that only the outermost shell of Earth [perhaps as thin as Ϸ250 km (23)] is strongly oxidized and that it should be expected that rocks/ minerals brought to shallow levels from great depth could arrive highly reduced if protected during their last few hundred kilometers of travel.…”

Section: Terrestrial Nitrides and Single Crystals Tio2 II As Inclusiomentioning

confidence: 99%

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High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite

Dobrzhinetskaya

Wirth

Yang³

et al. 2009

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

140

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The deepest rocks known from within Earth are fragments of normal mantle (Ϸ400 km) and metamorphosed sediments (Ϸ350 km), both found exhumed in continental collision terranes. Here, we report fragments of a highly reduced deep mantle environment from at least 300 km, perhaps very much more, extracted from chromite of a Tibetan ophiolite. The sample consists, in part, of diamond, coesite-after-stishovite, the high-pressure form of TiO 2, native iron, high-pressure nitrides with a deep mantle isotopic signature, and associated SiC. This appears to be a natural example of the recently discovered disproportionation of Fe 2؉ at very high pressure and consequent low oxygen fugacity (fO2) in deep Earth. Encapsulation within chromitite enclosed within upwelling solid mantle rock appears to be the only vehicle capable of transporting these phases and preserving their low-fO 2 environment at the very high temperatures of oceanic spreading centers.boron nitride ͉ coesite after stishovite ͉ Luobasa chromitite ͉ TiO2 II ͉ titanium nitride-osbornite U ntil recently, the deepest rock samples recovered from Earth's interior were xenoliths carried to the surface in explosive eruptions of kimberlite and related rocks, with maximum depth of Ϸ300 km (1). However, the advent of microstructural analysis of rocks from continental collision zones has established exhumation from still greater depths [peridotites from Ͼ300-400 km (2-5) and metamorphosed sediments from Ϸ350 km (6)]. More recently, a small fraction of diamonds from kimberlitic rocks has been found to carry inclusions that strongly suggest even greater depths, perhaps 1,700 km (7), but rocks that previously incorporated those diamonds at depth have not been recognized. We report here discovery of unexpected mineral assemblages from an ocean-spreading center environment, representing another window into Earth's deep interior.The discovery consists of very high-pressure and highly reduced phases extracted from chromitite within the mantle portion of an only lightly altered (e.g., small amounts of serpentine) fossil cross-section of oceanic crust and uppermost mantle (an ophiolite) in Tibet. Initial description of this material (8) documented diamonds and coesite-after-stishovite, establishing a minimum depth of origin of Ϸ300 km. This surprising and curious discovery was strengthened by observation of coesite and diopside lamellae within chromite collected from the same outcrop (9) and, very importantly, discovery of diamonds and indicators of very low oxygen fugacity from similar chromitite in a second ophiolite in the Polar Ural Mountains (10), establishing that the Tibetan occurrence is not unique. This discovery strongly enhances the conclusion of ref. 8 that this occurrence of very high-pressure minerals is not the result of meteorite impact. Thus, a subset of ophiolites (currently of unknown abundance) contains high-pressure and reduced phases, despite the overwhelming evidence that the ophiolites themselves formed at oceanic spreading centers under oxidizing conditions...

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Section: Terrestrial Nitrides and Single Crystals Tio2 II As Inclusiomentioning

confidence: 99%

Section: Terrestrial Nitrides and Single Crystals Tio2 II As Inclusiomentioning

confidence: 99%

High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite

Dobrzhinetskaya

Wirth

Yang³

et al. 2009

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

140

View full text Add to dashboard Cite

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“…Depending on speciation, this mechanism may also explain why some moderately volatile elements, like K, do not appear to show strong isotopic fractionation between terrestrial and lunar materials. Oxidation of the Earth's mantle could have been triggered by sequestration of the impactor's core into the terrestrial core, or through the sudden increase in size of the Earth from accretion of the Moon-forming impactor, increasing generation of Mg-rich perovskite at the base of a terrestrial magma ocean, forcing disproportionation of ferrous iron into ferric iron plus metal [88,89]. On the other hand, new high-pressure elemental partitioning experiments have shown that the Earth's core could have been formed under rather oxidizing conditions [62].…”

Section: (B) Volatile Loss During the Earth-moon Forming Giant Impact?mentioning

confidence: 99%

Evaporative fractionation of volatile stable isotopes and their bearing on the origin of the Moon

Day

Moynier

2014

Phil. Trans. R. Soc. A.

108

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The Moon is depleted in volatile elements relative to the Earth and Mars. Low abundances of volatile elements, fractionated stable isotope ratios of S, Cl, K and Zn, high μ ( 238 U/ 204 Pb) and long-term Rb/Sr depletion are distinguishing features of the Moon, relative to the Earth. These geochemical characteristics indicate both inheritance of volatile-depleted materials that formed the Moon and planets and subsequent evaporative loss of volatile elements that occurred during lunar formation and differentiation. Models of volatile loss through localized eruptive degassing are not consistent with the available S, Cl, Zn and K isotopes and abundance data for the Moon. The most probable cause of volatile depletion is global-scale evaporation resulting from a giant impact or a magma ocean phase where inefficient volatile loss during magmatic convection led to the present distribution of volatile elements within mantle and crustal reservoirs. Problems exist for models of planetary volatile depletion following giant impact. Most critically, in this model, the volatile loss requires preferential delivery and retention of late-accreted volatiles to the Earth compared with the Moon. Different proportions of late-accreted mass are computed to explain present-day distributions of volatile and moderately volatile elements (e.g. Pb, Zn; 5 to >10%) relative to highly siderophile elements (approx. 0.5%) for the Earth. Models of early magma ocean phases may be more effective in explaining the volatile loss. Basaltic materials (e.g. eucrites and angrites) from highly differentiated airless asteroids are volatile-depleted, like the Moon, whereas the Earth and Mars have proportionally greater volatile contents. Parent-body size and the existence of early atmospheres are therefore likely to represent fundamental controls on planetary volatile retention or loss.

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“…Integral to most 'homogeneous' accretion models is the concept that changes in metal-silicate partition coefficients at high pressures and temperatures may negate the necessity for changing f O 2 during core formation (Li & Agee 1996;Chabot & Agee 2003;Wade & Wood 2005). In addition, however, several fractionation processes involving Fe, O and Si have been identified, which could have changed the oxidation state of the mantle, even if the redox state of accreting material remained essentially constant (Javoy 1995;Frost et al 2004;Rubie et al 2004;Galimov 2005). It is, therefore, important to determine the effectiveness of these processes and the conditions under which they may have operated.…”

Section: Introductionmentioning

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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Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle

Cited by 536 publications

References 27 publications

High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite

High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite

Evaporative fractionation of volatile stable isotopes and their bearing on the origin of the Moon

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

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