Iron-bearing Silicate Research Articles

Predicting rock type and detecting hydrothermal alteration using machine learning and petrophysical properties of the Canadian Malartic ore and host rocks, Pontiac Subprovince, Québec, Canada

The Canadian Malartic deposit is a world class intrusion-related Archean gold deposit hosted in the Pontiac Subprovince, Superior Province, in Québec, Canada. Laboratory petrophysical properties measurements were performed on 824 rock samples collected from the various rock types observed within the ore body and peripheral host rocks. The various rock types present in the Malartic District, mainly meta-sedimentary rocks, felsic-intermediate intrusive rocks and mafic dykes have contrasting grain densities and magnetic susceptibilities. Using support vector machines, it is shown that these two physical properties can be used to predict the rock type of a sample with an average precision and recall rate of 89%. Within the meta-sedimentary rocks class, variations in magnetic susceptibility are due to the changes in mineralogy associated with hydrothermal alteration. These are caused by the destruction of iron-bearing silicate minerals and magnetite in unaltered rocks (10−4 to 10−3 SI) to form pyrite, carbonates, K-feldspar and Fe-depleted hydrothermal biotite in altered rocks (10−5 to 10−4 SI). Within the felsic-intermediate intrusive rocks, grain densities below 2.7 g/cm3 and magnetic susceptibilities in the 10−6 to 10−4 SI range yield the highest probabilities that a rock has been submitted to carbonate and pyrite alteration. However, magnetic susceptibility and grain density of these intrusive rocks are also dependent on their Fe2O3/Al2O3 and TiO2/Al2O3 ratios, which are not related to the hydrothermal alteration footprint, but are rather due to distinct protolith compositions. Within the mafic dykes, grain density is the best indicator of hydrothermal alteration. Unaltered mafic dykes (2.95 to 3.10 g/cm3) are mostly composed of amphibole, whereas altered mafic dykes (2.70 to 2.95 g/cm3) have reduced amphibole contents and higher abundances of carbonates, pyrite, quartz, and biotite alteration. The support vector machine classifier is extended to predict if meta-sedimentary rocks, felsic-intermediate intrusive rocks and mafic dykes have undergone hydrothermal alteration with average F1 scores of 73%, 69% and 93%, respectively. In altered meta-sedimentary rocks, the integration of grain density and magnetic susceptibility allows the identification of altered but unmineralized samples. The classifier is further extended to predict if the gold content of meta-sedimentary rocks is above or below threshold values of 0.01, 0.1 and 1 ppm with average F1 scores of 83%, 80%, and 76%, respectively. Using conceptual models of the rock physical properties at the Malartic District scale, it is shown that ground magnetic surveys are the most promising geophysical tool for early-stage greenfield exploration of this type of deposit. However, depending on the scale at which the surveys are conducted, magnetic susceptibility contrasts between the various investigated rock types can overshadow the specific signatures of hydrothermally altered rocks. This in part explains why past airborne geophysical exploration campaigns for this type of deposit in the Malartic District were inconclusive. Finally, the machine learning process used in this case study can be applied in advanced exploration stages, during which drilling and either subsequent laboratory petrophysical analyses of core samples or downhole geophysical surveys produce large amounts of data that can be used to train prediction models.

Kinetics of iron electrochemical reduction into liquid metal at 1823 K in a molten oxide electrolyte

The goal of this study was the measurement of the kinetic parameters of the electrochemical reduction of iron oxide into liquid iron metal in a molten oxide electrolyte, Al2O3-FeOx-MgO-SiO2, at 1823 K. A large surface anode was used to circumvent the need of a reference electrode and a stepped linear scan voltammetry was applied to an iron-bearing silicate electrolyte over a cell-voltage range of 0.8 V to 1.9 V. In order to extract the kinetic parameters the current response is treated concerning the electronic charge transfer and the ohmic drop due to the electrolyte in the electrochemical cell. The cathode half-reaction identified by this method is the reduction of ferrous iron into its metal state. Derived parameters imply that electrode kinetics follow Tafel behavior. An order of reaction close to unity with transfer coefficients between 0.54 and 0.66 is identified.

Lunar mare TiO2 abundances estimated from UV/Vis reflectance

The visible (400–700 nm) and near-infrared (700–2800 nm) reflectance of the lunar regolith is dominantly controlled by variations in the abundance of plagioclase, iron-bearing silicate minerals, opaque minerals (e.g., ilmenite), and maturation products (e.g., agglutinate glass, radiation-produced rims on soil grains, and Fe-metal). The same materials control reflectance into the near-UV (250–400 nm) with varying degrees of importance. A key difference is that while ilmenite is spectrally neutral in the visible to near-infrared, it exhibits a diagnostic upturn in reflectance in the near-UV, at wavelengths shorter than about 450 nm. The Lunar Reconnaissance Orbiter Wide Angle Camera (WAC) filters were specifically designed to take advantage of this spectral feature to enable more accurate mapping of ilmenite within mare soils than previously possible. Using the reflectance measured at 321 and 415 nm during 62 months of repeated near-global WAC observations, first we found a linear correlation between the TiO2 contents of the lunar soil samples and the 321/415 nm ratio of each sample return site. We then used the coefficients from the linear regression and the near-global WAC multispectral mosaic to derive a new TiO2 map. The average TiO2 content is 3.9 wt% for the 17 major maria. The highest TiO2 values were found in Mare Tranquillitatis (∼12.6 wt%) and Oceanus Procellarum (∼11.6 wt%). Regions contaminated by highland ejecta, lunar swirls, and the low-TiO2 maria (e.g., Mare Frigoris, the northeastern units of Mare Imbrium) exhibit very low TiO2 values (<2 wt%). We find that the Clementine visible to near-infrared based TiO2 maps (Lucey et al., 2000) have systematically higher values relative to the WAC estimates. The Lunar Prospector Gamma-Ray Spectrometer (GRS) TiO2 map is consistent with the WAC TiO2 map, although there are local offsets possibly due to the different depth sensitivities and large pixel scale of the GRS relative to the WAC. We find a wide variation of TiO2 abundances (from 0 to 10 wt%) for early mare volcanism (>2.6 Ga), whereas only medium to high TiO2 values (average = 6.8 wt%, minimum = 4.5 wt%) are found for younger mare units (<2.6 Ga).

Visible to near-infrared MSL/Mastcam multispectral imaging: Initial results from select high-interest science targets within Gale Crater, Mars

The Mastcam CCD cameras on the Mars Science Laboratory Curiosity Rover each use an 8-position filter wheel in acquiring up to 1600 × 1200 pixel images. The filter set includes a broadband near-infrared cutoff filter for RGB Bayer imaging on each camera and 12 narrow-band geology filters distributed between the two cameras, spanning the wavelength range 445–1013 nm. This wavelength region includes the relatively broad charge-transfer and crystal-field absorption bands that are most commonly due to the presence of iron-bearing minerals. To identify such spectral features, sequences of images taken with identical pointings through different filters have been calibrated to relative reflectance using pre-flight calibration coefficients and in-flight measurements of an onboard calibration target. Within the first 1000 sols of the mission, Mastcam observed a spectrally diverse set of materials displaying absorption features consistent with the presence of iron-bearing silicate, iron oxide, and iron sulfate minerals. Dust-coated surfaces as well as soils possess a strong positive reflectance slope in the visible, consistent with the presence of nanophase iron oxides, which have long been considered the dominant visible-wavelength pigmenting agent in weathered martian surface materials. Fresh surfaces, such as tailings produced by the drill tool and the interiors of rocks broken by the rover wheels, are grayer in visible wavelengths than their reddish, dust-coated surfaces but possess reflectance spectra that vary considerably between sites. To understand the mineralogical basis of observed Mastcam reflectance spectra, we focus on a subset of the multispectral data set for which additional constraints on the composition of surface materials are available from other rover instruments, with an emphasis on sample sites for which detailed mineralogy is provided by the results of CheMin X-ray diffraction analyses. We also discuss the results of coordinated observations with the ChemCam instrument, whose passive mode of operation is capable of acquiring reflectance spectra over wavelengths that considerably overlap the range spanned by the Mastcam filter set (Johnson et al. 2016). Materials that show a distinct 430 nm band in ChemCam data also are observed to have a strong near-infrared absorption band in Mastcam spectral data, consistent with the presence of a ferric sulfate mineral. Long-distance Mastcam observations targeted toward the flanks of the Gale crater central mound are in agreement with both ChemCam spectra and orbital results, and in particular exhibit the spectral features of a crystalline hematite layer identified in MRO/CRISM data. Variations observed in Mastcam multi-filter images acquired to date have shown that multispectral observations can discriminate between compositionally different materials within Gale Crater and are in qualitative agreement with mineralogies from measured samples and orbital data.

Application of IR and Raman spectroscopy for the determination of the role of oxygen fugacity in the formation of N–С–О–Н molecules and complexes in the iron-bearing silicate melts at high pressures

Large-scale melting of the Earth’s early mantle under the effect of global impact processes was accompanied by the generation of volatiles, which concentration was mainly controlled by the interaction of main N, C, O, and H gas-forming elements with silicate and metallic melts at low oxygen fugacity (fO2), which predominated during metallic segregation and self-oxidation of magma ocean. The paper considers the application of Raman and IR (infrared) Fourier spectroscopy for revealing the mechanisms of simultaneous dissolution and relative contents of N, C, O, and H in glasses, which represent the quench products of reduced model FeO–Na2O–Al2O3–SiO2 melts after experiments at 4 GPa, 1550°C, and fO2 1.5–3 orders of magnitude below the oxygen fugacity of the iron—wustite buffer equilibrium (fO2(IW)). Such fO2 values correspond to those inferred for the origin and evolution of magma ocean. It was established that the silicate melt contains complexes with N–H bonds (NH3, NH2+, NH2-), N2, H2, and CH4 molecules, as well as oxidized hydrogen species (OH– hydroxyl and molecular water H2O). Spectral characteristics of the glasses indicate significant influence of fO2 on the N–C–O–H proportion in the melt. They are expressed in a sharp decrease of NH2+, NH2-(O–NH2), OH–, H2O, and CH4 and simultaneous increase of NH2-(≡Si–NH2) and NH3 with decreasing fO2. As a result, NH3 molecules become the dominant nitrogen compounds among N–C–H components in the melt at fO2 two orders of magnitude below fO2(IW), whereas molecular СН4 prevails at higher fO2. The noteworthy feature of the redox reactions in the melt is stability of the ОН– groups and molecular water, in spite of the sufficiently low fO2. Our study shows that the composition of reduced magmatic gases transferred to the planet surface has been significantly modified under conditions of self-oxidation of mantle and magma ocean.

Origin of red beds in the Paleoproterozoic Franceville Basin, Gabon, and implications for sandstone-hosted uranium mineralization

Red beds are extensively used as evidence for an oxygenated atmosphere in the Paleoproterozoic. Red beds in the unmetamorphosed, *ca*. 2.15 Ga FA Formation of the Franceville Basin, Gabon were geochemically, petrographically, and mineralogically characterized to constrain the process of their formation and their relationship to atmospheric oxygenation in the early Paleoproterozoic. Petrographic observations indicate that ferric oxides are dispersed in clay filling intergranular pores and along platy cleavage in altered phyllosilicates. Grain-coating hematite is generally rare to absent in most samples suggesting hematite precipitation after sediment deposition and during burial diagenesis. Textures and geochemical data suggest that iron was likely sourced internally by alteration of iron-bearing minerals such as biotite and chlorite in sediments and redistributed in rocks during late diagenesis. Positive correlation between Fe/Mg ratio and δ^56^Fe values of bulk samples suggests mixing relationship with end members being authigenic hematite and iron-bearing silicates (biotite and chlorite). The lack of relationship between Fe^3+^/Fe~T~ ratios and iron isotope compositions suggest that the isotopically heavy iron oxide was already present in the sediments during early diagenesis, and was incorporated into green (reduced) facies that likely replaced red facies during diagenesis and burial. Large range of δ^56^Fe values extending towards positive values, up to +1 permil, is similar to that observed in Archean and Paleoproterozoic iron formations or modern groundwater-derived Fe-oxyhydroxides, suggesting partial oxidation of Fe under mildly oxidizing conditions during early diagenesis. In addition, positive correlation between Cr/Fe ratios and iron isotope values, especially in the fine-grained sandstones and mudstones, is evidence for authigenic Cr enrichment under locally mildly-reducing conditions in a fluvial setting and strongly oxidizing conditions during weathering and riverine transport. Uranium released from the lower, fluvial oxidized sandstones and added to the reduced sandstones and silty mudstones in the upper tidal-deltaic sediments potentially resulted in a uranium mineralization of a sandstone-type in the FA Formation of the Franceville Basin.

Structural characterization of the Misajé granitic pluton (NW Cameroon): constraints from magnetic and field observations

The Misaje granitic pluton, emplaced between 569 and 560 Ma in an amphibolitic and gneissic host rock, comprises four petrographic units namely biotite-hornblende granite (BHG), granodiorite (Gd), biotite granite (BG), and leucocratic granite (LG). Four major tectonic events have been described in the studied area: a D1-early tectonic event, responsible of the E–W flat foliation which has been progressively transposed by a D2 tectonic event. A D2 event has developed heterogeneous simple shear in a dextral transpressive context with moderate to strong dipping NE–SW striking foliation; a D3 tectonic event has lead to a sinistral N–S ductile shear characterized by N- to ENE-striking foliation and E–W strike-slip shear corridors and a D4 tectonic event that developed N–S dextral ductile strike-slip deformation. The magnetic study of the pluton, based on the AMS parameters, reveals the coexistence of both paramagnetic (dominated by iron-bearing silicates; 54 % of sites) and ferromagnetic (due to the occurrence of PSD and MD grains of magnetite or other ferromagnetic minerals; 46 % of sites) behaviors. Magnetic foliation shows best poles at 55/82 for the whole pluton, 95/32 in BHG, and 273/83 in BG, and the magnetic lineation trends are mostly NNE–SSW with best lines at 210/8, 198/19, and 36/3, respectively. The trend of the magnetic lineation in BG indicates an S-shape trajectory, suggesting a sinistral sense of shear motion along discrete E–W corridors situated at the northern and southern ends. Kinematic indicators in BG point to a sinistral sense of shear, suggesting its emplacement during the D3 event. The close relationship between K1 and K3 points to a syn-kinematic emplacement and crystallization of the Misaje granitic pluton during the Pan-African event, and the tectonic evolution of the study area is considered to be coeval with the tectonic evolution of the trans-Saharan Pan-African belt of eastern Nigeria.

Mineralogical evolution of Fe–Si-rich layers at the olivine-water interface during carbonation reactions

Recent studies investigating carbonation of iron-bearing silicates have shown that the rates of these reactions, although formally not depending on oxygen fugacity, are strongly different at different redox states of the system (Saldi et al. 2013; Sissmann et al. 2013). Here we provide a micro- and nanostructural characterization of the olivine/water interface during the carbonation of forsteritic olivine at 150 °C and p CO 2 = 100 bar. When the reaction starts under oxic conditions, the observed temporal sequence of interfacial layers consists of: a hematite/SiO 2(am) assemblage, Fe-rich phyllosilicates with mixed Fe valence and a non-passivating Fe-free amorphous SiO 2 layer, which allows the formation of ferroan magnesite. In contrast, starting at micro-oxic conditions, carbonation rates are much faster, with no real evidence of interfacial layers. Separate deposits of goethite/lepidocrocite in the early stages of the reaction and then formation of magnetite are observed at these conditions, while precipitation of siderite/magnesite proceeds unhindered. The evolution of the redox conditions during the reaction progress controls the sequence of the observed reaction products and the passivating properties of Fe–Si-rich interfacial layers. These findings have important implications for modeling the carbonation of ultramafic rocks under different oxygen fugacity conditions as well as for understanding the technological implications of adding accessory gases to CO 2 in carbon capture and storage mineralization processes involving ultrabasic rocks.

Effect of iron content on the structure and disorder of iron-bearing sodium silicate glasses: A high-resolution 29Si and 17O solid-state NMR study

Despite its geochemical importance and implications for the properties of natural magmatic melts, understanding the detailed structure of iron-bearing silicate glasses remains among the outstanding problems in geochemistry. This is mainly because solid-state NMR techniques, one of the most versatile experimental methods to probe the structure of oxide glasses, cannot be fully utilized for exploring the structural details of iron-bearing glasses as the unpaired electrons in Fe induce strong local magnetic fields that mask the original spectroscopic features (i.e., paramagnetic effect). Here, we report high-resolution 29Si and 17O solid-state NMR spectra of iron-bearing sodium silicate glasses (Na2O–Fe2O3–SiO2, Fe3+/ΣFe=0.89±0.04, thus containing both ferric and ferrous iron) with varying XFe2O3 [=Fe2O3/(Na2O+Fe2O3)], containing up to 22.9wt% Fe2O3. This compositional series involves Fe–Na substitution at constant SiO2 contents of 66.7mol% in the glasses. For both nuclides, the NMR spectra exhibit a decrease in the signal intensities and an increase in the peak widths with increasing iron concentration partly because of the paramagnetic effect. Despite the intrinsic difficulties that result from the pronounced paramagnetic effect, the 29Si and 17O NMR results yield structural details regarding the effect of iron content on Q speciation, spatial distribution of iron, and the extent of polymerization in the iron-bearing silicate glasses. The 29Si NMR spectra show an apparent increase in highly polymerized Q species with increasing XFe2O3, suggesting an increase in the degree of melt polymerization. The 17O 3QMAS NMR spectra exhibit well-resolved non-bridging oxygen (NBO, Na–O–Si) and bridging oxygen (BO, Si–O–Si) peaks with varying iron concentration. By replacing Na2O with Fe2O3 (and thus with increasing iron content), the fraction of Na–O–Si decreases. Quantitative consideration of this effect confirms that the degree of polymerization is likely to increase with iron content and that Fe3+ is predominantly a network-former. The 17O NMR spectra suggest a moderate degree of preferential partitioning of iron between NBO and BO clusters. The present results bear strong promise for studying iron-bearing silicate glasses using solid-state NMR techniques, constraining the effect of iron content on the degree of polymerization. The observed changes in atomic structures of iron-bearing sodium silicate glasses will be helpful for unraveling atomic origins of the properties of natural silicate melts.

Electrical conductivity of iron-bearing silicate glasses and melts. Implications for the mechanisms of iron redox reactions

The electrical conductivity of a series of glasses and melts of the system SiO2–CaO–MgO–M2O–“FeO” (M=Li and Na) and of a borosilicate has been measured from room temperature to about 1820K. For samples with predominantly reduced iron, the conductivity increases markedly upon addition of Na+ and still more of Li+, which is consistent with the increasing order Mg2+, Na+, Li+ order of cation mobility. For the oxidized samples the conductivity is in contrast almost not affected by the presence of alkali cations, which agrees with the low mobility of alkali cations that are then serving as charge compensators of tetrahedrally coordinated Al3+. The conductivity is higher for oxidized than for reduced samples. As indicated by polarization electrode phenomena and complementary continuous current measurements, this difference is due to an important contribution of electronic conduction caused by electronic charge transfer between iron species that exists in the oxidized samples. The diffusivities of oxygen and divalent cations were then determined from Eyring relationship and the measured conductivities, respectively and compared with the redox diffusivies determined for the same samples. The good agreement found between both kinds of data confirms the controlling role of divalent cations and of oxygen species in the redox kinetics near the glass transition and at high temperatures, respectively. In addition it illustrates that describing melt properties in an integrated manner is becoming possible.

A model for calculating the viscosity of natural iron-bearing silicate melts over a wide range of temperatures, pressures, oxygen fugacites, and compositions

A new general model that takes into account the pressure and redox state effect is presented to calculate melt viscosities of natural Fe-bearing melts. This new model is applicable to melts that span a wide range of temperatures (from 733 to 1873 K), pressures (0.001–15 kbar), H2O content (from 0 to 12.3 wt%), and compositions (from ultramafic, mafic to silicic melts). The accuracy of the model is calculated to be ±0.23 log units of viscosity, which is within or close to experimental uncertainty. The transport properties, including glass transition temperature and melt fragility, can also be calculated from this model. A spreadsheet to calculate the viscosity is provided in an Electronic Supplement.

Absence of pressure-induced electron spin-state transition of iron in silicate glasses upon compression

Upon compression, silicate melts in the Earth’s interior are expected to be subject to successive structural transitions with multiple densification mechanisms that are distinct from those of their crystalline analogs. Experimental verification of this phenomenon remains a major target of glass-melt studies. Early studies of Fe spin-state transitions in silicate glasses under compression used synchrotron X-ray emission spectroscopy (XES) to develop seemingly irreconcilable interpretations that vary from complete transitions to low spin states at high pressure to a prevalence of high spin states. In an effort to reconcile the controversy, Mao et al. (February–March 2014 issue of American Mineralogist ) showed that the XES data must be properly handled with a correct reference spectrum for each spin state and suggested that the subtle effect of pressure-induced broadening in the spectra be considered. Based on the new analyses, they concluded that no pressure-induced spin-state transitions exist in iron-bearing silicate glasses at high pressure, relevant to Earth’s deep lower mantle. Thanks to several series of experimental studies, the elusive spin state in glasses under compression is being revealed, rendering the spin state of iron among the best understood for glasses at high pressure.

Magnetic properties of the Wenchuan Earthquake Fault Scientific Drilling Project Hole-1 (WFSD-1), Sichuan Province, China

We measured the magnetic susceptibility of the core from the first borehole of the Wenchuan Earthquake (May 12, 2008, Mw 7.9) Fault Scientific Drilling Project (WFSD-1) at 1-cm intervals. The correlations between magnetic susceptibility anomalies and fault rock occurrence are shown by a few fault zones in the WFSD-1 core. The values for the mass and ferromagnetic material magnetic susceptibility for the sample at 589.25-m depth are higher than those for the other samples. All the thermomagnetic curves display a rapid increase in slope after 380°C, and a marked peak occurs at about 510°C in the heating curves. The cooling curves are clearly higher than the heating curves. The saturation magnetization (Ms) shows a significant peak at a depth of 589.25 m, as do the mass magnetic susceptibility and the ferrimagnetic magnetic susceptibility. The mechanism principally responsible for the high magnetic susceptibility at a depth of 589.25 m might be the production of new magnetite from iron-bearing silicates (e.g., chlorite) or clays caused by frictional heating during seismic slip. Therefore, we suggest that the presence of high magnetic susceptibility fault gouges in the same country rock can be considered as an indicator of earthquakes or seismic signatures.

酸化鉄を含むアルカリ土類シリケートスラグの 粘度に及ぼす酸素分圧の影響

酸化鉄を含むアルカリ土類シリケートスラグの 粘度に及ぼす酸素分圧の影響

First-principles studies of spin-state crossovers of iron in perovskite

The discovery of spin crossovers in iron under pressure in ferropericlase and iron-bearing magnesium silicate perovskite, the major mineral phases of the Earth’s lower mantle, ignited in the last eight years intense discussions on the nature of this phenomenon and potential effects on the state and evolution of the Earth’s mantle and of the planet as a whole. The nature of spin crossover in ferropericlase is better understood, and consensus on the interpretation of experimental data has been essentially achieved. However, in perovskite numerous experiments and computations have been carried out, often giving different results or suggesting different interpretations. Perovskite is a much more challenging system to investigate for a couple of reasons: there are two types of iron, ferrous (Fe 2+ ) and ferric (Fe 3+ ), more than one site can be occupied by iron, the A and B perovskite sites (as in ABX 3 ), and experiments do not probe the spin state unambiguously. Calculations are also especially difficult. This type of problem has challenged condensed matter theorists for decades. In addition, novel density functional theory + Hubbard U (DFT+ U ) electronic structure techniques, sometimes previously untested, are being used to address this type of problem. In summary, the challenge is multi-faceted. Here we review a series of calculations that have succeeded in reproducing experimental data and have been essential for their interpretation. They used the recently developed DFT+ Usc method where the Hubbard U is obtained self-consistently by first principles. In particular, the electric field gradients (EFGs) at the nucleus of iron and thus quadrupole splittings (QSs) have been successfully computed, making direct comparisons between measured (via Mossbauer spectroscopy) and computed QSs for various spin states possible. This strategy led to unexpected findings in (Mg 1− x Fe x )SiO 3 and in (Mg 1− x Fe x )(Si 1− x Fe x )O 3 ( x = 0.125): in the pressure range of the lower mantle a) ferrous iron in the A site undergoes a site change, not a spin-state change and b) in ferric iron, spin crossover happens only in the B-site.

비정질 알칼리 규산염 원자구조의 철 함량 효과에 관한 고체 NMR 분광학 연구

철을 포함한 비정질 규산염 용융체의 원자 구조 규명은 지표 환경의 화성활동 및 맨틀 심부의 초저속도층의 속도구조에 이르는 광범위한 지질과정의 미시적인 원인에 대한 단서를 제공한다. 본 연구에서는 철을 포함한 비정질 규산염의 원자 구조 규명에 가장 적합한 고상 핵자기공명분광분석(NMR)을 이용하여 최대 16.07 wt%의 <TEX>$Fe_2O_3$</TEX>가 포함된 비정질 알칼리 규산염(iron-bearing alkali silicate glasses)의 철의 함량 변화가 원자구조에 미치는 영향을 규명하였다. <TEX>$^{29}Si$</TEX> 스핀-격자 완화시간(<TEX>$T_1$</TEX>)을 측정한 결과, 철의 함량에 따라 스핀-격자 완화시간이 짧아지는데 이는 철이 가지고 있는 홀전자(unpaired electron)와 핵 스핀(nuclear spin)간의 상호작용으로부터 기인한다. <TEX>$^{29}Si$</TEX> MAS NMR 실험 결과, 철이 포함되지 않은 시료의 경우 <TEX>$Q^2$</TEX>, <TEX>$Q^3$</TEX> 그리고 <TEX>$Q^4$</TEX>의 환경을 지시하는 피크가 분리됨에 반하여, 철이 포함된 시료의 경우 NMR 신호의 급격한 감소와 피크 폭이 넓어짐으로써 각각의 규소 환경이 거의 분리되지 않았다. 그러나 철의 함량에 따라 스펙트럼이 넓어지고 화학적 차폐값(chemical shift)이 높아지는 현상을 확인하였는데, 이는 <TEX>$Q^4$</TEX>의 규소 환경을 나타내는 방향으로서 철 주변의 <TEX>$Q^n$</TEX>이 불균질하게 분포하고 있음을 지시한다. <TEX>$^{17}O$</TEX> MAS NMR 실험에서도 철이 포함되지 않은 시료에서는 연결산소(Si-O-Si)와 비연결산소(Na-O-Si)가 부분적으로 분리되지만, 철의 함량이 증가하면서 각각의 산소 환경이 거의 분리되지 않는다. 이러한 연구결과는 고상 핵자기공명분광분석이 철을 포함한 비정질 규산염의 상세한 구조 연구에 효과적인 도구임을 지시한다. The study on the atomic structure of iron-bearing silicate glasses has significant geological implications for both diverse igneous processes on Earth surface and ultra-low velocity zones at the core-mantle boundary. Here, we report experimental results on the effect of iron content on the atomic structure in iron-bearing alkali silicate glasses (<TEX>$Na_2O-Fe_2O_3-SiO_2$</TEX> glasses, up to 16.07 wt% <TEX>$Fe_2O_3$</TEX>) using <TEX>$^{29}Si$</TEX> and <TEX>$^{17}O$</TEX> solid-state NMR spectroscopy. <TEX>$^{29}Si$</TEX> spin-lattice (<TEX>$T_1$</TEX>) relaxation time for the glasses decreases with increasing iron content due to an enhanced interaction between nuclear spin and unpaired electron in iron. <TEX>$^{29}Si$</TEX> MAS NMR spectra for the glasses show a decrease in signal intensity and an increase in peak width with increasing iron content. However, the heterogeneous peak broa-dening in <TEX>$^{29}Si$</TEX> MAS NMR spectra suggests the heterogeneous distribution of <TEX>$Q^n$</TEX> species around iron in iron-bearing silicate glasses. While nonbridging oxygen (<TEX>$Na-O-Si$</TEX>) and bridging oxygen (Si-O-Si) peaks are partially resolved in <TEX>$^{17}O$</TEX> MAS NMR spectrum for iron-free silicate glass, it is difficult to distinguish the oxygen clusters in iron-bearing silicate glass. The Lorentzian peak shape for <TEX>$^{29}Si$</TEX> and <TEX>$^{17}O$</TEX> MAS NMR spectra may reflect life-time broadening due to spin-electron interaction. These results demonstrate that solid-state NMR can be an effective probe of the detailed structure in iron-bearing silicate glasses.

Experimental Study of the Stability of a Dolomite + Coesite Assemblage in Contact With Peridotite: Implications for Sediment-Mantle Interaction and Diamond Formation During Subduction

Carbonates carried by oceanic platescin the form of sediment or alteration products of basaltscare introduced into the mantle by subduction.The high-pressure^high-temperature stability of carbonates in a closed system has been constrained by a number of studies, but the effects of the interactions between subducted carbonates and the surrounding mantle on carbonate stability are poorly known. These interactions may, however, influence the stability depth of the subducted material, the composition of the interaction zone, and the rate of carbon transfer from the slab to the mantle. To determine the exchange mechanisms at the interface between a subducted dolomiteþ coesite assemblage and the overlying mantle, we performed experiments at 6 GPa and 9008C and 11008C in a multi-anvil apparatus. In a first series of experiments, we studied the reaction and migration processes operating along the interface between a cylinder of dolomiteþ coesite and a cylinder of garnet peridotite. In a second series of experiments, homogeneous mixtures of dolomiteþ coesite and garnet peridotite were equilibrated at high pressure and high temperature to characterize the phase relations as a function of the (dolomiteþ coesite)/peridotite ratio.We show that the destabilization temperature of a subducted dolomiteþ coesite assemblage is less than 9008C at 6 GPa when in contact with a garnet lherzolite or a harzburgite. A reaction zone composed mainly of clinopyroxene þmagnesite is produced at the interface between dolomiteþ coesite and peridotite. Carboncin the form of a fluid or in a carbonatitic meltcalso infiltrates the garnet peridotite to form magnesite and clinopyroxene. Moreover, graphite was observed in two experiments. We believe that it was produced by a redox reaction at the interface between dolomiteþ coesite and iron-bearing silicates in the peridotite. As our experimental conditions are in the stability field of diamond, this suggests a potential mechanism for diamond crystallization from subducted dolomiteþ coesite in contact with the mantle wedge.

Kinematic evolution of the eastern Tethyan Himalaya: constraints from magnetic fabric and structural properties of the Triassic flysch in SE Tibet

Abstract Anisotropy of magnetic susceptibility (AMS) combined with structural analysis are used in this work with the aim to characterize the tectonic evolution of the Triassic flysch within the eastern Tethyan Himalaya Thrust Belt in SE Tibet. The attitude of the magnetic foliation and lineation are concordant with the planar and linear structures of tectonic origin defined by the preferred orientation of the iron-bearing silicates. Two different tectonic domains can be defined: (a) the southern domain is controlled by the Eohimalayan tectonic foliation (S1) recorded in the magnetic foliation which trends east–west and dips to the north; (b) the northern domain is dominated by the Neohimalayan magnetic foliation with WNW–ESE strike and dips to the south opposite to the vergence of the main structures. A slightly prolate magnetic ellipsoid has been found in between the two domains recording the intersection of S1 and the subtle development of the S2 tectonic foliation. Hinterland propagation of the deformation lead to the Great Counter backthrust generation, pointed out by the SSW steeply plunging magnetic lineation. Furthermore different orientations of magnetic foliation may indicate an Early Miocene c. 20° clockwise vertical-axis rotation.

Tungsten in Archean banded iron formation of the former Sherman and Adams mines, Ontario: possible mineralization processes

Anomalous tungsten occurs in Archean banded iron formation from the previous Sherman and Adams mines, Ontario. In Sherman samples from three open pits and one drill core site, with one exception, anomalous tungsten occurred only in the Sherman North pit. Adams mine samples from three pits carried anomalous tungsten in samples of chlorite-rich silicate facies. Statistical evaluation showed that ≥10 ppm tungsten was anomalous with respect to the entire data set. The implications of restricted tungsten distribution, as seen in Sherman occurrences, were evaluated with respect to tungsten mineralization processes. Scenarios considered involve syngenetic, metamorphic, and magmatic-hydrothermal processes. Thermal constraints provided by iron-bearing silicate assemblages and oxygen isotopes attest to contact metamorphism as a potential mechanism for tungsten redistribution within iron formation. However, tungsten of magmatic-hydrothermal origin emplaced as an epigenetic component is considered the essential mineralization process.

High-pressure X-ray diffraction and X-ray emission studies on iron-bearing silicate perovskite under high pressures

Iron-bearing silicate perovskite is believed to be the most abundant mineral of the Earth's lower mantle. Recent studies have shown that Fe2+ exists predominantly in the intermediate-spin state with a total spin number of 1 in silicate perovskite in the lower part of the lower mantle. Here we have measured the spin states of iron and the pressure–volume relation in silicate perovskite [(Mg0.6,Fe0.4)SiO3] at pressure conditions relevant to the lowermost mantle using in situ X-ray emission and X-ray diffraction in a diamond cell. Our results showed that the intermediate-spin Fe2+ is stable in the silicate perovskite up to ∼ 125 GPa but starts to transition to the low-spin state at approximately 135 GPa. Concurrent X-ray diffraction measurements showed a decrease of approximately 1% in the unit cell volume in the silicate perovskite [(Mg0.6,Fe0.4)SiO3], which is attributed to the intermediate-spin to the low-spin transition. The transition pressure coincides with the pressure conditions of the lowermost mantle, raising the possibility of the existence of the silicate perovskite phase with the low-spin Fe2+ across the transition from the post-perovskite to the perovskite phases in the bottom of the D″ layer.