Deep mantle storage of the Earth’s missing niobium in late-stage residual melts from a magma ocean     

O. Nebel  W. van Westrenen  M. Wille 《Geochimica et cosmochimica acta》2010,74(15):4392-250

The origin of the observed niobium deficit in the bulk silicate Earth (BSE) compared to chondritic meteorites constitutes a long-standing problem in geochemistry. The deficit requires a large-scale process fractionating niobium from tantalum, and a super-chondritic Nb/Ta reservoir hidden in the deep silicate Earth and/or in the metallic core. The only voluminous super-chondritic Nb/Ta silicate reservoir analysed to date is found in lunar basalts that assimilated highly evolved Fe-rich rocks associated with anorthosites in the lunar crust. These Fe-rich rocks, enriched in incompatible elements, are thought to represent the last fractions of melt remaining at the end of lunar magma ocean crystallization. Here we report high-precision Nb-Ta data for a Fe-rich, late-stage rock suite associated with a terrestrial anorthosite from the Proterozoic Bolangir complex in India. The geochemical characteristics of this rock suite resemble those expected for late-stage residual melts from a terrestrial magma ocean. Samples show extreme, super-chondritic Nb/Ta up to 31.1 and highly elevated Nb concentrations up to 338 ppm. We argue that formation of an early enriched crustal reservoir (EECR) with these characteristics (high Fe, high Nb, superchondritic Nb/Ta) is likely in the course of Hadean late-stage terrestrial magma ocean solidification. Subduction and subsequent permanent deep mantle storage in the D′′ layer of a minor amount (∼0.5% of the BSE mass) of this EECR can readily explain the terrestrial Nb deficit, without the need to invoke core Nb storage. Our model is consistent with short-lived ¹⁴²Nd and long-lived ¹⁷⁶Hf-¹⁴³Nd isotope models for early differentiation of the Earth’s crust. In addition, the inferred Lu/Hf of this EECR implies that this reservoir can also balance the offset of terrestrial Hf isotope ratios compared to the chondritic reservoir. As such, late-stage magma ocean residual melts may constitute the enigmatic parental reservoir of Hadean zircons with low time-integrated Hf isotope compositions.  相似文献   

Deep-sea coral aragonite as a recorder for the neodymium isotopic composition of seawater     

Tina van de Flierdt  Laura F. Robinson 《Geochimica et cosmochimica acta》2010,74(21):6014-6032

Deep-sea corals have been shown to be useful archives of rapid changes in ocean chemistry during the last glacial cycle. Their aragonitic skeleton can be absolutely dated by U-Th data, freeing radiocarbon to be used as a water-mass proxy. For certain species of deep-sea corals, the growth rate allows time resolution that is comparable to ice cores. An additional proxy is needed to exploit this opportunity and turn radiocarbon data into rates of ocean overturning in the past.Neodymium isotopes in seawater can serve as a quasi-conservative water-mass tracer and initial results indicate that deep-sea corals may be reliable archives of seawater Nd isotopes. Here we present a systematic study exploring Nd isotopes as a water-mass proxy in deep-sea coral aragonite. We investigated five different genera of modern deep-sea corals (Caryophyllia, Desmophyllum, Enallopsamia, Flabellum, Lophelia), from global locations covering a large potential range of Nd isotopic compositions. Comparison with ambient seawater measurements yields excellent agreement and suggests that deep-sea corals are reliable archives for seawater Nd isotopes.A parallel study of Nd concentrations in these corals yields distribution coefficients for Nd between seawater and coral aragonite of 1-10, omitting one particular genus (Enallopsamia). The corals and seawater did however not come from exactly the same location, and further investigations are needed to reach robust conclusions on the incorporation of Nd into deep-sea coral aragonite.Lastly, we studied the viability of extracting the Nd isotope signal from fossil deep-sea corals by carrying out stepwise cleaning experiments. Our results show that physical removal of the ferromanganese coating and chemical pre-cleaning have the highest impact on Nd concentrations, but that oxidative/reductive cleaning is also needed to acquire a seawater Nd isotope signal.  相似文献   

Crustal sulfur is required to form magmatic Ni–Cu sulfide deposits: evidence from chalcophile element signatures of Siberian and Deccan Trap basalts     

Reid R. Keays  Peter C. Lightfoot 《Mineralium Deposita》2010,45(3):241-257

Process models for ore formation in magmatic Ni–Cu–platinum group element (PGE) sulfide systems require that S saturation is achieved in a mafic–ultramafic magma. Traditional models explain the achievement of S saturation or sulfide saturation either by the addition of crustal S, by the felsification of the magma by crustal contamination, or by mixing between primitive and evolved magmas. Which process matters most is important to industry-oriented exploration models where crustal S sources are believed to be encouraging features of a metallotect. Studies of the Siberian Trap flood basalts at Noril’sk have demonstrated that chalcophile element depletion is linked to assimilation of silica-rich crust, but it is less clear whether this contaminant contained an appreciable amount of S. At Noril’sk, the Ni–Cu–PGE sulfide deposits are associated with subvolcanic intrusions that were emplaced into Permian and Carboniferous sedimentary sequences rich in shales, marlstones, and evaporites. Similar to the Siberian Trap basalts, the Deccan Trap contains a volumetrically important suite of crustally contaminated tholeiitic basalts. We present new PGE data for samples from a stratigraphic sequence of basalts from the southern Deccan province. Two of the formations in this sequence (the Bushe and Poladpur Formations) have geochemical signatures indicative of a wide degree of crustal contamination of a magma type that gave rise to the stratigraphically higher Ambenali Formation (a product of transitional midocean ridge basalt magmatism). There are no known deposits or occurrences of Ni–Cu–PGE sulfides associated with subvolcanic intrusions in the Deccan province. Despite the fact that the Bushe Formation exhibits a stronger crustal contamination signature than the most contaminated Siberian Trap basalt formations, and the Poladpur lavas are also strongly crustally contaminated, the Bushe and Poladpur basalts are undepleted in Ni, Cu, or PGE. This indicates that the contaminated Deccan Trap lavas did not achieve S saturation. This, in turn, places constraints on the potential of the Deccan Trap in southern India to host significant magmatic sulfide deposits. Conversely, this observation also indicates that an S-rich crustal contaminant is required for the genesis of magmatic Ni–Cu–PGE sulfide deposits.  相似文献   

Numerical estimation of effective diffusion coefficients for charged porous materials based on micro-scale analyses     

Arash Mohajeri  Guillermo A. Narsilio  Peter Pivonka  David W. Smith 《Computers and Geotechnics》2010

In order to describe diffusive transport of solutes through a porous material, estimation of effective diffusion coefficients is required. It has been shown theoretically that in the case of uncharged porous materials the effective diffusion coefficient of solutes is a function of the pore morphology of the material and can be described by the tortuosity (tensor) (Bear, 1988 [1]). Given detailed information of the pore geometry at the micro-scale the tortuosity of different materials can be accurately estimated using homogenization procedures. However, many engineering materials (e.g., clays and shales) are characterized by electrical surface charges on particles of the porous material which strongly affect the (diffusive) transport properties of ions. For these type of materials, estimation of effective diffusion coefficients have been mostly based on phenomenological equations with no link to underlying micro-scale properties of these charged materials although a few recent studies have used alternative methods to obtain the diffusion parameters (Jougnot et al., 2009; Pivonka et al., 2009; Revil and Linde, 2006 2, 3 and 4). In this paper we employ a recently proposed up-scaled Poisson–Nernst–Planck type of equation (PNP) and its micro-scale counterpart to estimate effective ion diffusion coefficients in thin charged membranes. We investigate a variety of different pore geometries together with different surface charges on particles. Here, we show that independent of the charges on particles, a (generalized) tortuosity factor can be identified as function of the pore morphology only using the new PNP model. On the other hand, all electro-static interactions of ions and charges on particles can consistently be captured by the ratio of average concentration to effective intrinsic concentration in the macroscopic PNP equations. Using this formulation allows to consistently take into account electrochemical interactions of ions and charges on particles and so excludes any ambiguity generally encountered in phenomenological equations.  相似文献   

Oxidative pre-treatment in thiosulphate leaching of sulphide gold ores     

D. Feng  J.S.J. van Deventer 《International Journal of Mineral Processing》2010,94(1-2):28-34

A diagnostic leaching showed that partial oxidation of the sulphide minerals in a gold ore was beneficial for thiosulphate leaching of gold. A pre-treatment process with oxidative ammoniacal solution enhanced the thiosulphate leaching of the sulphide ore, while the thiosulphate consumption was substantially reduced. The sulphide minerals partially decomposed in the pre-treatment process, exposing gold to the leach solution. Oxygen input by air bubbling and a longer contact time enhanced the oxidative ammonia pre-treatment process and hence accelerated subsequent thiosulphate leaching of the sulphide ore. Gold extraction in 0.8 M ammonia and 0.1 M thiosulphate solution after 24 h increased from 69% without pre-treatment to 81%, 84%, 90% and 94% respectively after 1, 3, 7 and 22 h pre-treatment. The consumption of sodium thiosulphate was 2.37 kg/t after 24 h leaching without pre-treatment, but was negligible after over 1 h oxidative ammonia pre-treatment. A counter-current leaching process was conducted in the leaching of the sulphide ore. The fresh leachant still gave higher leaching rates in contact with the pre-leached ore, while the pre-used leachant had significantly lower leaching kinetics and overall gold extraction in contact with the fresh ore. This 2-step counter-current leaching process proved that the leachant, other than the passivation, was the determinant factor causing the gold leaching rates to decrease after a certain time of leaching. The findings enable the thiosulphate leaching of high sulphide containing gold ores to be more efficient at lower thiosulphate consumption following the oxidative ammoniacal pre-treatment.  相似文献   

Heat capacity, entropy, and magnetic properties of jarosite-group compounds     

Juraj Majzlan  Peter Glasnák  Robert A. Fisher  Mary Anne White  Michel B. Johnson  Brian Woodfield  Juliana Boerio-Goates 《Physics and Chemistry of Minerals》2010,37(9):635-651

Jarosite phases are common minerals in acidic, sulfate-rich environments. Here, we report heat capacities (C _p) and standard entropies (S°) for a number of jarosite samples. Most samples are close to the nominal composition AFe₃(SO₄)₂(OH)₆, where A = K, Na, Rb, and NH₄. One of the samples has a significant number of defects on the Fe sites and is called the defect jarosite; others are referred to as A-jarosite. The samples, their compositions, and the entropies at T = 298.15 K are:

Sample	Chemical composition	S o/(J mol−1 K−1)
K-jarosite	K0.92(H3O)0.08Fe2.97(SO4)2(OH)5.90(H2O)0.10	427.4 ± 0.7
Na-jarosite	Na0.95(H3O)0.05Fe3.00(SO4)2(OH)6.00	436.4 ± 4.4
Rb-jarosite	RbFe2.98(SO4)2(OH)5.95(H2O)0.05	411.9 ± 4.1
NH4-jarosite	(NH4)0.87(H3O)0.13Fe3.00(SO4)2(OH)6.00	447.2 ± 4.5
Defect jarosite	K0.94(H3O)0.06Fe2.34(SO4)2(OH)4.01(H2O)1.99	412.7 ± 4.1

There are additional configurational entropies of 13.14 and 8.23 J mol⁻¹ K⁻¹ in defect and NH₄-jarosite, respectively. A detailed analysis of the synchrotron X-ray diffraction patterns showed a large anisotropic peak broadening for defect and NH₄-jarosite. The fits to the low-temperature (approx. <12 K) C _p data showed that our samples can be divided into two groups. The first group is populated by the K-, Na-, Rb-, and NH₄-jarosite samples, antiferromagnetic at low temperatures. The second group contains the H₃O-jarosite (studied previously) and the defect jarosite. H₃O- and defect jarosite are spin glasses and their low-T C _p was fit with the expression C _p = γT + ΣB _j T _j , where j = (3, 5, 7, 9). The linear term is typical for spin glasses and the sum represents the lattice contribution to C _p. Surprisingly, the C _p of the K-, Na-, Rb-, and NH₄-jarosite samples, which are usually considered to be antiferromagnetic at low temperatures, also contains a large linear term. This finding suggests that even these phases do not order completely, but have a partial spin-glass character below their Néel transition temperature.  相似文献   

P–V–T equations of state of MgO and thermodynamics     

Peter I. Dorogokupets 《Physics and Chemistry of Minerals》2010,37(9):677-684

A simplest equation within the framework of the Mie-Grüneisen–Einstein approach is considered. Pressure estimation values are presented that are derived by conventional arithmetic and algebraic calculations as a function of temperature and volume. The equation under consideration complies with the Mie-Grüneisen–Debye model at high temperature. Different versions of an equation of state (EoS) of MgO proposed by Speziale et al. (J Geophys Res 106B:515–528, 2001) as a pressure standard at high temperatures are subject to analyses. In the literature, at least four versions of Speziale et al. EoS of MgO are discussed; the discrepancy between them reaching a few GPa at T > 2,000 K and P > 100 GPa. Our analyses of these equations suggest that the volume dependence of the Debye temperature is accepted arbitrarily and does not agree with the definition of the Grüneisen parameter, γ = −(∂lnΘ/∂lnV) _T . Pressure as a function of temperature and volume in the Mie-Grüneisen–Einstein approach or the Gao pressure calculator can be used to estimate true pressure at compression x = V/V ₀ < 1 with the Speziale et al. EoS of MgO.  相似文献   

Mobility of metals in nickel mine spoil materials     

Sophie Raous  Thierry Becquer  Jérémie Garnier  Éder de Souza Martins  Guillaume Echevarria  Thibault Sterckeman 《Applied Geochemistry》2010

An understanding of the biogeochemical behaviour of metals in mine spoil materials is a prerequisite to rehabilitate Ni mining sites. The objective of this study was to characterize the fate of metals in different Ni ore spoil materials as influenced by hydrological conditions and fertilisation practices. In tropical ultramafic complexes, the different stages of lateritic weathering lead to two types of ores, and therefore, to two spoil types. They are mainly either a clay-rich saprolite, so-called “garnierite”, enriched in phyllosilicates, or a limonitic material, enriched in Fe oxides. Lysimeter columns were designed to monitor leaching waters through both spoil materials. The garnieritic spoil released higher concentrations of Mg (mean = 2.25 mg L⁻¹), Ni (0.39 mg L⁻¹) and Cr (1.19 mg L⁻¹) than the limonitic spoil (Mg = 0.5 mg L⁻¹; Ni = 0.03 mg L⁻¹ and Cr = 0.25 mg L⁻¹). Chromium was mainly in an anionic form in leaching solutions. As exchangeable pools of Cr(VI) in limonite (980 mg kg⁻¹ of KH₂PO₄-extractable Cr) are considerable its release in water may still occur in the case of a pH increase. In mixed spoil, metal concentrations were almost as low as in the limonitic one. The effect of mineral-N fertilisation was a strong release of cations (Ni, Mg) into the leachate. Phosphate amendment did not affect the soil solution composition under experimental conditions.  相似文献   

Solar shield: forecasting and mitigating space weather effects on high-voltage power transmission systems     

Antti Pulkkinen  Michael Hesse  Shahid Habib  Luke Van der Zel  Ben Damsky  Fritz Policelli  David Fugate  William Jacobs  Elizabeth Creamer 《Natural Hazards》2010,53(2):333-345

In this paper, central elements of the Solar Shield project, launched to design and establish an experimental system capable of forecasting the space weather effects on high-voltage power transmission system, are described. It will be shown how Sun–Earth system data and models hosted at the Community Coordinated Modeling Center (CCMC) are used to generate two-level magnetohydrodynamics-based forecasts providing 1–2 day and 30–60 min lead-times. The Electric Power Research Institute (EPRI) represents the end-user, the power transmission industry, in the project. EPRI integrates the forecast products to an online display tool providing information about space weather conditions to the member power utilities. EPRI also evaluates the economic impacts of severe storms on power transmission systems. The economic analysis will quantify the economic value of the generated forecasting system. The first version of the two-level forecasting system is currently running in real-time at CCMC. An initial analysis of the system’s capabilities has been completed, and further analysis is being carried out to optimize the performance of the system. Although the initial results are encouraging, definite conclusions about system’s performance can be given only after more extensive analysis, and implementation of an automatic evaluation process using forecasted and observed geomagnetically induced currents from different nodes of the North American power transmission system. The final output of the Solar Shield will be a recommendation for an optimal forecasting system that may be transitioned into space weather operations.  相似文献   

Geochemistry of igneous rocks associated with ultramafic–mafic-hosted Cu (Co, Ni, Au) VMS deposits from the Main Uralian Fault (Southern Urals, Russia)     

Paolo Nimis  Paolo Omenetto  Bernd Buschmann  Peter Jonas  Vladimir A. Simonov 《Mineralogy and Petrology》2010,100(3-4):201-214

Ultramafic–mafic- and ultramafic-hosted Cu (Co, Ni, Au) volcanogenic massive sulfide (VMS) deposits from ophiolite complexes of the Main Uralian Fault, Southern Urals, are associated with island arc-type igneous rocks. Trace element analyses show that these rocks are geochemically analogous to Early Devonian boninitic and island arc tholeiitic rocks found at the base of the adjacent Magnitogorsk volcanic arc system, while they are distinguished both from earlier, pre-subduction volcanic rocks and from later volcanic products that were erupted in progressively more internal arc settings. The correlation between the sulfide host-rocks and the earliest volcanic units of the Magnitogorsk arc suggests a connection between VMS formation and infant subduction-driven intraoceanic magmatism.  相似文献