Redox-induced lower mantle density contrast and effect on mantle structure and primitive oxygen

Gu, Tingting; Li, Mingming; McCammon, Catherine; Lee, Kanani K. M.

doi:10.1038/ngeo2772

Cited by 24 publications

(29 citation statements)

References 45 publications

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“…For Mg# = Mg/(Mg + Fe) = 90 Bdg representative of the mantle, differences in oxidation state of iron result in a density difference up to ~0.3% (Fig. 4 ), far less than the 1.5–2% redox-induced density contrast required to rapidly separate oxidized materials from reduced materials in the early history of the Earth 20 . Moreover, the spin-transition-induced density increase makes the density contrast of Bdg with different Fe 3+ /ΣFe ratios sharply fade away below the mid-mantle depth (Fig.…”

Section: Discussionmentioning

confidence: 99%

“…The effects of Fe 2+ and Fe 3+ on incompressibility of Bdg are thought to be opposite 19 . The density contrast between Fe 2+ - and Fe 3+ -dominant Bdg may result in separation of oxidized and reduced materials through mantle convection and leave imprints in geochemical and isotopic compositions 20 . However, in many experimental studies on Bdg, Fe 3+ /∑Fe was not characterized.…”

Section: Introductionmentioning

confidence: 99%

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Valence and spin states of iron are invisible in Earth’s lower mantle

et al. 2018

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Heterogeneity in Earth’s mantle is a record of chemical and dynamic processes over Earth’s history. The geophysical signatures of heterogeneity can only be interpreted with quantitative constraints on effects of major elements such as iron on physical properties including density, compressibility, and electrical conductivity. However, deconvolution of the effects of multiple valence and spin states of iron in bridgmanite (Bdg), the most abundant mineral in the lower mantle, has been challenging. Here we show through a study of a ferric-iron-only (Mg0.46Fe3+0.53)(Si0.49Fe3+0.51)O3 Bdg that Fe3+ in the octahedral site undergoes a spin transition between 43 and 53 GPa at 300 K. The resolved effects of the spin transition on density, bulk sound velocity, and electrical conductivity are smaller than previous estimations, consistent with the smooth depth profiles from geophysical observations. For likely mantle compositions, the valence state of iron has minor effects on density and sound velocities relative to major cation composition.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Valence and spin states of iron are invisible in Earth’s lower mantle

et al. 2018

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“…5. An astrobiological implication: That the wet and rocky bodies in the solar system smaller than Earth probably have little or no bridgmanite (perovskite) and, therefore, have more reduced mantles may explain the preponderance of methane on some of the moons of Saturn (Wood et al , 2006 ; Bouquet et al , 2015 ; Dorofeeva, 2016 ; Girard et al , 2016; Gu et al , 2016 ), possibly Europa (Zolotov and Kargel, 2009 ) and even early Mars (Wadhwa, 2001 ; Edwards and Ehlmann, 2015 ; Hu et al , 2015 ; Wong et al , 2017b ). The challenge then is to consider the availability of electron acceptors on these other worlds ( e.g.…”

Section: Résumé and Conclusionmentioning

confidence: 99%

Methane: Fuel or Exhaust at the Emergence of Life?

Russell

Nitschke

2017

Astrobiology

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As many of the methanogens first encountered at hydrothermal vents were thermophilic to hyperthermophilic and comprised one of the lower roots of the evolutionary tree, it has been assumed that methanogenesis was one of the earliest, if not the earliest, pathway to life. It being well known that hydrothermal springs associated with serpentinization also bore abiotic methane, it had been further assumed that emergent biochemistry merely adopted and quickened this supposed serpentinization reaction. Yet, recent hydrothermal experiments simulating serpentinization have failed to generate methane so far, thus casting doubt on this assumption. The idea that the inverse view is worthy of debate, that is, that methanotrophy was the earlier, is stymied by the “fact” that methanotrophy itself has been termed “reverse methanogenesis,” so allotting the methanogens the founding pedigree. Thus, attempting to suggest instead that methanogenesis might be termed reverse methanotrophy would require “unlearning”—a challenge to the subconscious! Here we re-examine the “impossibility” of methanotrophy predating methanogenesis as in what we have termed the “denitrifying methanotrophic acetogenic pathway.” Advantages offered by such thinking are that methane would not only be a fuel but also a ready source of reduced carbon to combine with formate or carbon monoxide—available in hydrothermal fluids—to generate acetate, a target molecule of the first autotrophs. And the nitrate/nitrite required for the putative oxidation of methane with activated NO would also be a ready source of fixed nitrogen for amination reactions. Theoretical conditions for such a putative pathway would be met in a hydrothermal green rust-bearing exhalative pile and associated chimneys subject to proton and electron counter gradients. This hypothesis could be put to test in a high-pressure hydrothermal reaction chamber in which a cool carbonate/nitrate/nitrite-bearing early acidulous ocean simulant is juxtaposed across a precipitate membrane to an alkaline solution of hydrogen and methane. Key Words: Green rust—Methanotrophy—Nitrate reduction—Emergence of life. Astrobiology 17, 1053–1066.

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“…The formation of materials strongly enhanced in bridgmanite has been indeed linked to primordial processes such as magma‐ocean crystallization [ Elkins‐Tanton , ], since Mg‐rich bridgmanite is the magma‐ocean liquidus phase over a wide pressure range [ de Koker et al ., ; Tateno et al ., ], but a mechanism for subsequent Fe‐enrichment remains elusive. Other viable (primordial) candidates for DDD composition involve foundered CaSiO 3 ‐rich protocrust [ Kawai and Tsuchiya , ] as well as anomalously reduced bridgmanitic materials [ Gu et al ., ].…”

Section: Comparison Of Model Predictions With Seismic Constraintsmentioning

confidence: 99%

Compositional layering within the large low shear‐wave velocity provinces in the lower mantle

Ballmer

Schumacher

Lekić

et al. 2016

Geochem Geophys Geosyst

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The large low shear‐wave velocity provinces (LLSVP) are thermochemical anomalies in the deep Earth's mantle, thousands of km wide and ∼1800 km high. This study explores the hypothesis that the LLSVPs are compositionally subdivided into two domains: a primordial bottom domain near the core‐mantle boundary and a basaltic shallow domain that extends from 1100 to 2300 km depth. This hypothesis reconciles published observations in that it predicts that the two domains have different physical properties (bulk‐sound versus shear‐wave speed versus density anomalies), the transition in seismic velocities separating them is abrupt, and both domains remain seismically distinct from the ambient mantle. We here report underside reflections from the top of the LLSVP shallow domain, supporting a compositional origin. By exploring a suite of two‐dimensional geodynamic models, we constrain the conditions under which well‐separated “double‐layered” piles with realistic geometry can persist for billions of years. Results show that long‐term separation requires density differences of ∼100 kg/m3 between LLSVP materials, providing a constraint for origin and composition. The models further predict short‐lived “secondary” plumelets to rise from LLSVP roofs and to entrain basaltic material that has evolved in the lower mantle. Long‐lived, vigorous “primary” plumes instead rise from LLSVP margins and entrain a mix of materials, including small fractions of primordial material. These predictions are consistent with the locations of hot spots relative to LLSVPs, and address the geochemical and geochronological record of (oceanic) hot spot volcanism. The study of large‐scale heterogeneity within LLSVPs has important implications for our understanding of the evolution and composition of the mantle.

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Redox-induced lower mantle density contrast and effect on mantle structure and primitive oxygen

Cited by 24 publications

References 45 publications

Valence and spin states of iron are invisible in Earth’s lower mantle

Valence and spin states of iron are invisible in Earth’s lower mantle

Methane: Fuel or Exhaust at the Emergence of Life?

Compositional layering within the large low shear‐wave velocity provinces in the lower mantle

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