Metal saturation in the upper mantle

Rohrbach, Arno; Ballhaus, Chris; Golla‐Schindler, Ute; Ulmer, Peter; Kamenetsky, Vadim S.; Kuzmin, D. V.

doi:10.1038/nature06183

Cited by 269 publications

(145 citation statements)

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“…In the upper mantle, the oxygen fugacity (fO 2 ) varies from one to five log units below the fayalitemagnetite-quartz (FMQ) buffer, with a trend of a decrease with depth (6,(12)(13)(14)(15). At a depth of ∼250 km, mantle is reported to become metal saturated (16,17), which holds true for all mantle regions below, including the transition zone and lower mantle. The subduction of the oxidized crustal material occurs to depths greater than 600 km (4)(5)(6).…”

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confidence: 95%

Mantle–slab interaction and redox mechanism of diamond formation

Palyanov

Bataleva

Sokol

et al. 2013

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

176

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Subduction tectonics imposes an important role in the evolution of the interior of the Earth and its global carbon cycle; however, the mechanism of the mantle-slab interaction remains unclear. Here, we demonstrate the results of high-pressure redox-gradient experiments on the interactions between Mg-Ca-carbonate and metallic iron, modeling the processes at the mantle-slab boundary; thereby, we present mechanisms of diamond formation both ahead of and behind the redox front. It is determined that, at oxidized conditions, a low-temperature Ca-rich carbonate melt is generated. This melt acts as both the carbon source and crystallization medium for diamond, whereas at reduced conditions, diamond crystallizes only from the Fe-C melt. The redox mechanism revealed in this study is used to explain the contrasting heterogeneity of natural diamonds, as seen in the composition of inclusions, carbon isotopic composition, and nitrogen impurity content.carbonate-iron interaction | high-pressure experiment | mantle mineralogy | deep carbon cycle S ubduction of crustal material plays an important role in the global carbon cycle (1-6). Depending on oxygen fugacity and pressure-temperature (P-T) conditions, carbon exists in the Earth's interior in the form of carbides, diamond, graphite, hydrocarbons, carbonates, and CO 2 (7-11). In the upper mantle, the oxygen fugacity (fO 2 ) varies from one to five log units below the fayalitemagnetite-quartz (FMQ) buffer, with a trend of a decrease with depth (6,(12)(13)(14)(15). At a depth of ∼250 km, mantle is reported to become metal saturated (16, 17), which holds true for all mantle regions below, including the transition zone and lower mantle. The subduction of the oxidized crustal material occurs to depths greater than 600 km (4-6). The main carbon-bearing minerals of the subducted materials are carbonates, which are thermodynamically stable up to P-T conditions of the lower mantle (10,11,18). As evidenced by the compositions of inclusions in diamond, which vary from strongly reduced, e.g., metallic iron and carbides (19-23), to oxidized, e.g., carbonates and CO 2 (6,20,(24)(25)(26)(27)(28), carbonates may be involved in the reactions with reduced deep-seated rocks, including Fe 0 -bearing species (29-31). A scale of these reactions is determined mainly by the capacity of subducted carbonate-bearing domains. An important consequence of such an interaction is that it can produce diamond. However, studies on diamond synthesis via the reactions between oxidized and reduced phases are limited (32-35).To understand the mechanisms of the interaction of carbonbearing oxidized-and reduced-mineral assemblages, we performed high-pressure experiments with an iron-carbonate system; an approach was used that enabled the creation of an oxygen fugacity gradient in the capsules (Materials and Methods and SI Materials and Methods). Results and DiscussionThe experimental results and the phase compositions are given in Table 1 and Table S1, respectively. At temperatures of 1,000 and 1,100°C, the iron-car...

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confidence: 95%

Mantle–slab interaction and redox mechanism of diamond formation

Palyanov

Bataleva

Sokol

et al. 2013

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

176

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“…Zhao et al, 2015) could continuously advect peridotite that is oxidising enough to enable redox melting at 250 km depth. Alternatively, low-density methane-rich fluids continually rising from the deep upper mantle and transition zone could be oxidised at the depth of metal saturation, providing a continuous supply of reduced carbon from which carbonated melt could be generated near the base of thick lithospheres (Rohrbach et al, 2007;Frost and McCammon, 2008;Rohrbach and Schmidt, 2011) (Fig. 4).…”

Section: Accepted M Manuscriptmentioning

confidence: 99%

Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

Aulbach

Massuyeau

Gaillard

2017

Lithos

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“…Previous work suggests that roughly 1 wt.% metallic iron is produced in the crust portion of subducted slabs as a result of disproportionation of ferrous ion in pyroxene and garnet beyond 250 km depth (26,27) and in bridgmanite when the slab enters the lower mantle (25). The carbon content in the deep mantle slabs may range from 320 ppm to 620 ppm, according to the estimated 2.4-4.8 × 10 13 g annual input of carbon to the mantle past the arc (34).…”

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confidence: 99%

“…Carbonates in the crustal portion of the slab may melt at the mantle wedge or in the transition zone and return to shallow depths (23,24). On the other hand, the slabs that sank beyond the depth of 250 km are expected to contain metallic iron as a result of stabilization of ferric iron in pyroxene, garnet, or bridgmanite (25)(26)(27), plus elemental carbon or carbide through the reduction of carbonates by the metallic iron (28). To assess whether an iron−carbon mixture Significance Nearly three decades ago, seismologists discovered peculiarly dense and slow patches just above Earth's core−mantle boundary (CMB), known as the ultralow velocity zones (ULVZs).…”

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confidence: 99%

Origins of ultralow velocity zones through slab-derived metallic melt

Liu

Hrubiak

et al. 2016

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

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Understanding the ultralow velocity zones (ULVZs) places constraints on the chemical composition and thermal structure of deep Earth and provides critical information on the dynamics of largescale mantle convection, but their origin has remained enigmatic for decades. Recent studies suggest that metallic iron and carbon are produced in subducted slabs when they sink beyond a depth of 250 km. Here we show that the eutectic melting curve of the iron−carbon system crosses the current geotherm near Earth's core−mantle boundary, suggesting that dense metallic melt may form in the lowermost mantle. If concentrated into isolated patches, such melt could produce the seismically observed density and velocity features of ULVZs. Depending on the wetting behavior of the metallic melt, the resultant ULVZs may be short-lived domains that are replenished or regenerated through subduction, or long-lasting regions containing both metallic and silicate melts. Slab-derived metallic melt may produce another type of ULVZ that escapes core sequestration by reacting with the mantle to form iron-rich postbridgmanite or ferropericlase. The hypotheses connect peculiar features near Earth's core−mantle boundary to subduction of the oceanic lithosphere through the deep carbon cycle.core mantle boundary | iron−carbon melt | subduction | deep carbon cycle | diffuse scattering U ltralow velocity zones (ULVZs) occur as isolated patches near the core−mantle boundary (CMB) and are generally associated with the large low shear velocity provinces (LLSVPs) (1, 2). The nonubiquitous distribution of ULVZs gives evidence for thermal and/or chemical heterogeneities at the base of the mantle (3, 4). The density excess of ULVZs likely arises from iron enrichment (5-7), whereas the velocity anomalies may indicate partial melting (3,4,8,9) or iron enrichment (5-7). Elucidating the origin of ULVZs is therefore important for understanding the thermal and chemical state of the CMB, which, in turn, holds a key to unraveling the evolution history and dynamics of deep Earth.Given uncertainties in the melting behavior of mantle rocks (10), elastic properties of relevant phases (11, 12), and iron partitioning between them (13, 14), the origin of ULVZs remains enigmatic. Partial melt of silicate composition has been widely considered as the origin of ULVZs because the presence of partial melt reduces shear wave velocity (Vs) effectively, and partial melt was found to be denser than coexisting solids at deep mantle conditions (e.g., refs. 4, 13, 15, and 16). Models involving silicate partial melt face several challenges. First, the solidus temperatures of silicate compositions happen to fall into the ±500 K uncertainty margin of the CMB temperature. Consequently, nonubiquitous partial melting of a silicate composition critically depends on thermal structure of the lowermost mantle, and the presence of chemically distinct components is often invoked to explain the occurrence of patchy melts near the CMB. Given the controversy over the mantle solidus (3,(8)(9)(10) and...

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Metal saturation in the upper mantle

Cited by 269 publications

References 28 publications

Mantle–slab interaction and redox mechanism of diamond formation

Mantle–slab interaction and redox mechanism of diamond formation

Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

Origins of ultralow velocity zones through slab-derived metallic melt

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