Oxygen thermobarometry measurements on spinel peridotite rocks indicate that the oxygen fugacity at the top of the upper mantle falls within ±2 log units of the fayalite-magnetite-quartz (FMQ) oxygen buffer. Measurements on garnet peridotites from cratonic lithosphere reveal a general decrease in fo2 with depth, which appears to result principally from the effect of pressure on the controlling Fe3+/Fe2+ equilibria. Modeling of experimental data indicates that at approximately 8 GPa, mantle fo2 will be 5 log units below FMQ and at a level where Ni-Fe metal becomes stable. Fe-Ni alloy and an Fe2O3-garnet component will be formed as a result of the disproportionation of FeO, which is experimentally demonstrated through observations of high Fe3+/ΣFe ratios in minerals in equilibrium with metallic Fe. In the lower mantle, the favorable coupled substitution of Al and Fe3+ into (Fe,Mg)SiO3 perovskite results in very high perovskite Fe3+/ΣFe ratios in equilibrium with metallic Fe. As a result, the lower mantle sh...

The Redox State of Earth's Mantle

### Introduction

Earth’s carbon, derived from planetesimals in the 1 AU region during accretion of the Solar System, still retains similarities to carbon found in meteorites (Marty et al. 2013) even after 4.57 billion years of geological processing. The range in isotopic composition of carbon on Earth versus meteorites is nearly identical and, for both, diamond is a common, if volumetrically minor, carbon mineral (Haggerty 1999). Diamond is one of the three native carbon minerals on Earth (the other two being graphite and lonsdaleite). It can crystallize throughout the mantle below about 150 km and can occur metastably in the crust. Diamond is a rare mineral, occurring at the part-per-billion level even within the most diamondiferous volcanic host rock although some rare eclogites have been known to contain 10–15% diamond. As a trace mineral it is unevenly distributed and, except for occurrences in metamorphosed crustal rocks, it is a xenocrystic phase within the series of volcanic rocks (kimberlites, lamproites, ultramafic lamprohyres), which bring it to the surface and host it. The occurrence of diamond on Earth’s surface results from its unique resistance to alteration/dissolution and the sometimes accidental circumstances of its sampling by the volcanic host rock. Diamonds are usually the chief minerals left from their depth of formation, because intact diamondiferous mantle xenoliths are rare.

Diamond has been intensively studied over the last 40 years to provide extraordinary information on our planet’s interior. For example, from the study of its inclusions, diamond is recognized as the only material sampling the “very deep” mantle to depths exceeding 800 km (Harte et al. 1999; McCammon 2001; Stachel and Harris 2009; Harte 2010) although most crystals (~95%) derive from shallower depths (150 to 250 km). Diamonds are less useful in determining carbon fluxes on Earth because they provide only a small, …

Diamonds and the Geology of Mantle Carbon

Earth is unique among the terrestrial planets in our solar system in having a fluid envelope that fosters life. The secrets behind Earth’s habitable climate are well-tuned cycles of carbon (C) and other volatiles. While on time-scales of ten to thousands of years the chemistry of fluids in the atmosphere, hydrosphere, and biosphere is dictated by fluxes of carbon between near surface reservoirs, over hundreds of millions to billions of years it is maintained by chemical interactions of carbon between Earth’s interior, more specifically the mantle, and the exosphere (Berner 1999). This is because of the fact that the estimated total mass of C in the mantle is greater than that observed in the exosphere (Sleep and Zahnle 2001; Dasgupta and Hirschmann 2010) and the average residence time of carbon in the mantle is between 1 and 4 Ga (Sleep and Zahnle 2001; Dasgupta and Hirschmann 2006). But how did Earth’s mantle attain and maintain the inventory of mantle carbon over geologic time? And is the residence time of carbon in the mantle, as constrained by the present-day fluxes, a true reflection of carbon ingassing and outgassing rates throughout Earth’s history? Also, when in the planet’s history did its mantle carbon inventory become established and how did it change through geologic time? The answers to these questions are important because of carbon’s importance in a number of fields of Earth sciences, such as the thermal history of Earth [e.g., trace-volume carbonated melt may extract highly incompatible heat-producing elements from great depths (Dalou et al. 2009; Dasgupta et al. 2009b; Grassi et al. 2012)], the internal differentiation of the mantle and core [carbon influences element partitioning in both carbonate-silicate (e.g., Blundy and Dalton 2000; Dasgupta et al. 2009b; Dalou et al. 2009; Grassi et …

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Ingassing, Storage, and Outgassing of Terrestrial Carbon through Geologic Time

The temperature at the 410-km discontinuity is re-evaluated by comparing the depth of the discontinuity with the olivine–wadsleyite transition pressure obtained using in situ X-ray diffraction experiments by Katsura et al. (2004a) and equation of state (EoS) of MgO by Tange et al. (2009) (Tange scale) and Matsui et al. (2000) . The newly estimated temperature is 1830 ± 48 K, 70 K higher than that by our previous estimation. The EoSes of the major mantle minerals (olivine, wadsleyite, ringwoodite and perovskite) are also recalculated using the Tange scale. The adiabatic temperature gradient is calculated using the thermal expansion coefficient obtained from these EoSes. The adiabatic temperature gradient gradually decreases with increasing depth without a phase transition, and abruptly increases in association with phase transitions. The adiabatic temperature gradients are found to be 04–0.5 and 0.3 K/km in the upper and lower parts of the mantle, respectively. The temperatures at a depth of 200 km, the bottom of the mantle transition zone, the top of the lower mantle and a depth of 2700 km are found to be 1720 ± 40, 2010 ± 40, 1980 ± 40, and 2730 ± 50 K. The mantle potential temperature is found to be 1610 ± 35 K.

Adiabatic temperature profile in the mantle

Carbonatites are familiar to students of petrology as rare igneous rocks formed predominantly of carbonate, whose only modern expression is a single active volcano that erupts strongly alkaline carbonate lavas with no direct match in Earth’s geological record (see Lengai movie in the electronic version of this chapter or on the MSA RiMG website). Based on their Sr-Nd-Pb isotopic data, stable isotopic compositions, noble gases, and experimental phase equilibria, they are derived from the mantle, showing almost no sign of contamination by the crust.

As liquids, carbonate melts have remarkable physical properties, which set them apart from the alkaline silicate melts with which they are often temporally associated. They show very high solubilities of many elements considered rare in silicate magmas, and they have the highest known melt capacities for dissolving water and other volatile species like halogens at crustal pressures. They are highly efficient transport agents of carbon from the mantle to the crust, remaining mobile over extraordinary ranges of temperature, and their very low viscosity should enhance connectivity along grain boundaries in the mantle where they are implicated in geochemical enrichment processes related to metasomatism.

Most carbonatites have unambiguous origins in the mantle and the limit to their depth is not known, but the likelihood that they may exist in the lower mantle (Kaminsky et al. 2009, 2012; Stoppa et al. 2009) needs to be appraised since they may exert a fundamental control on the mobility and long-term storage of deep carbon in Earth. Ultimately the stability of carbonate melt is an extension of the stability of carbonate minerals (Hazen et al. 2013a,b) subject critically to the mantle oxidation state (Luth 1993; Frost and McCammon 2008); carbonate-melts have also been predicted in the oceanic low-velocity zone and deep mantle (Hauri et al. 1993 …

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Carbonate melts and carbonatites

Magnesium carbonate MgCO3 (magnesite) was experimentally studied at pressures of 12–84 GPa and temperatures between 1,600 and 3,300 K. We applied the high-pressure technique using a multianvil press and a diamond anvil cell with laser heating. The phase relations and melting of magnesite were investigated by means of Raman and time-resolved multi-wavelength spectroscopy. Magnesite is found to melt congruently within the entire studied pressure range at temperatures of 2,100–2,650 K. At temperatures above 2,700 K, we observed decomposition of magnesite with formation of MgO and a carbon phase (diamond). Our results demonstrate that at high pressures, the magnesium carbonate melt can exist at a wide range of thermodynamic conditions.

Melting and decomposition of MgCO 3 at pressures up to 84 GPa

Peridotite–eclogite–carbonatite systems at 7.0–8.5 GPa: concentration barrier of diamond nucleation and syngenesis of its silicate and carbonate inclusions

Yu. A. Litvin

Papers

Melting and decomposition of MgCO 3 at pressures up to 84 GPa

Peridotite–eclogite–carbonatite systems at 7.0–8.5 GPa: concentration barrier of diamond nucleation and syngenesis of its silicate and carbonate inclusions

The physicochemical conditions of diamond formation in the mantle matter: experimental studies

Diamond syntheses in multicomponent carbonate–carbon melts of natural chemistry: elementary processes and properties

The first synthesis of diamond in sulfide-carbon systems: The role of sulfides in diamond genesis