Ferric/ferrous ratio in silicate melts: a new model for 1 atm data with special emphasis on the effects of melt composition

The effect of MgO and total FeO on ferric/ferrous ratio in model multicomponent silicate melts was investigated experimentally in the temperature range 1300–1500 °C at 1 atm total pressure in air. We demonstrate that the addition of these weak network modifier cations results in an increase of Fe³⁺/Fe²⁺ ratio in both mafic and silicic melts. Based on present and published experimental data, a new empirical equation is proposed to predict the ferric/ferrous ratio as a function of oxygen fugacity, temperature and melt composition. In contrast to previous equations, the compositional effect of melts on the Fe³⁺/Fe²⁺ ratio is not only modeled by the sum of the molar fraction of the individual oxide components. Additional interactions terms have also been incorporated. The main advantage of the proposed model is its applicability for a wide compositional range. However, its application to felsic melts (>?68 wt% SiO₂) is not recommended. Other advantages of this equation and differences when compared with previous models are discussed.     

Activities and volatilities of trace components in silicate melts: a novel use of metal–silicate partitioning data

Ian Carmichael spent 45 years thinking about and working on the activities of components in silicate melts and their use to estimate physicochemical conditions at eruption and in the source regions of igneous rocks. These interests, principally in major components such as SiO₂, led us to think about possible ways of determining the complementary activity coefficients of trace components in silicate melts. While investigating the conditions of accretion and differentiation of the Earth, a number of authors have determined the partitioning of trace elements such as Co, Ni, Mo and W between liquid Fe metal and liquid silicate. These data have the potential to provide activity information for a large number of trace components in silicate melts. In order to turn the partitioning measurements into activities, however, we need to know the activity coefficient of FeO, γ_FeO in the silicate. We obtained γ_FeO as a function of melt composition by fitting a simple model to 83 experimental data for which the authors had measured the FeO content of the silicate melt in equilibrium with metal (Fe-bearing alloy) at known f_O2. The compositional dependence of γ_FeO is weak, but, when calculated in the system Diopside–Anorthite–Forsterite, it decreases towards the Forsterite apex. A similar approach for Ni, for which twice as many data are available, leads to similar composition dependence of activity coefficient and confirms the suggestion that γ_NiO/γ_FeO is almost constant over a wide range of silicate melt composition. The activity coefficients for FeO were used in conjunction with measured Mo and W partitioning between Fe-rich metal and silicate melt to estimate activity coefficients for trace MoO₂ and WO₃ dissolved in silicate melt. When combined with data on Mo- and W-saturated silicate melts a strong dependence of activity coefficient is observed. Calculated in the system Diopside–Anorthite–Forsterite, both MoO₂ and WO₃ exhibit similar behaviour to FeO and NiO in that activity coefficients decrease as Forsterite content increases. The effect is much larger for Mo and W, however, γ_MoO2 and γ_WO3 varying by factors of 20 and nearly 100, respectively, in this system. In order to illustrate the potential applications of the metal–silicate partitioning approach to determine the activity coefficients of volatile elements, we used it to determine activity coefficients of PbO, CuO_0.5 and InO_1.5 in a silica-saturated melt at 1,650 °C. We find values of 0.22, 3.5 and 0.02, respectively, indicating a strong dependence on cation charge. The value for CuO_0.5 is in excellent agreement with experimental data of Holzheid and Lodders (Geochim Cosmochim Acta 65:1933–1951, 2001), which shows that the method is viable. When combined with thermodynamic data on the gas species, we find that Pb is the most volatile of the 3 elements under ‘normal’ terrestrial conditions of oxygen fugacity but that In should become the most volatile under strongly reducing conditions such as those of the solar nebula. The oxygen fugacity dependence of volatility has implications for the high relative abundance of In in silicate Earth. We conclude that metal–silicate partitioning experiments are a viable means for determining activities of trace components in silicate melts and are particularly useful if the metal of the element is unstable or volatile at igneous temperatures.     

Kinematic effects of velocity fluctuations for the dark-halo population in a ΛCDM model

It is known that ΛCDM cosmological models predict too many dark halos compared to the observed numbers. This excess is derived from the virialized mass in the Local Supercluster and its vicinity. Taking into account cosmological velocity fluctuations during the formation of the dark halo population makes it possible to eliminate this remaining contradiction in the ΛCDM model. Based on Press-Schechter formalism, a model describing the formation of the dark halo population is developed, taking into account kinematic effects in the dark matter. A quantitative explanation of the virialized-mass deficit in the local Universe is obtained in this model.     

Carbon isotopic composition of alkenes in paleosol with PY-GC-IRMS: Implications for paleovegetation changes on the Loess Plateau, China

The carbon isotopic composition of alkenes from loess-paleosol sequences in China Loess was measured by pyrolysis /gas chromatography /isotopic ratio mass spectrometry. Thermochemolysis products of the insoluble residues were characterized using GC/MS, an…     

Evolution of melts of the Changbaishan Tianchi volcano (China–North Korea) as a model of ore–magmatic system formation: Data from melt inclusions studies

The composition and evolution of the melts of trachytes from the volcano were studied based on examining the inclusions of mineral-forming media by means of X-ray and ion microanalysis. A correlation was shown between the degree of enrichment of these melts in rare elements and the processes of magmatic differentiation. It was found that trachytes of the volcano were generated in highly differentiated alkaline melts enriched in Hf, Nb, Zr, Ta, U, Th, Rb, Y, and REEs under 1020–1060°C. The evolution of melts was determined by the processes of crystal fractionation. The main volatile components in the melts are water, fluorine, and chlorine with the concentrations of 0.1–0.5, 0.2–0.5, and 0.2–0.3 wt %, respectively. The melt crystallization was accompanied by degassing caused by the decrease in the outer pressure. The low concentrations of water and fluorine represent the melt composition by these components exclusively at the time of the melt movement towards the Earth’s surface just before the eruption.     

A new correction model for C ages in aquifers with complex geochemistry – Application to the Neogene Aquifer,Belgium

The objective of this paper is to build a general correction model that takes into account all the different radiocarbon-dilution reactions and resolving the processes that are geochemically “aging” the groundwater in the Neogene Aquifer. For this, δ¹³C and radiocarbon in groundwater are investigated with their relationship to other chemical components in groundwater. The δ¹³C values in the Neogene Aquifer are influenced by various geochemical reactions like calcite dissolution, oxidation of organic matter and methanogenesis. Calcite dissolution and CH₄ production increase δ¹³C while the oxidation of organic matter decreases δ¹³C in the groundwater. The reactions that modify δ¹³C also influence the ¹⁴C activity. Due to the complex geochemical environment, existing correction models are not applicable to this situation. A correction model for initial ¹⁴C activity is formulated in which the different C sources that influence ¹⁴C activity are taken into account. It is observed that recent dissolved organic matter plays an important role in redox reactions. The corrected ¹⁴C ages lie between −0.792 and 6.425 ka representing the maximum age. If a part of the organic matter that oxidises is fossil, the determined age will represent an overestimated age.     

Assessing the ratio of archaeol to caldarchaeol as a salinity proxy in highland lakes on the northeastern Qinghai–Tibetan Plateau

The ratio of archaeol to caldarchaeol (the ACE index) has been proposed recently as an index for paleosalinity reconstruction and is based principally on archaeal core lipids (CLs) from coastal salt pans (Turich, C., Freeman, K.H., 2011. Archaeal lipids record paleosalinity in hypersaline systems. Organic Geochemistry 42, 1147–1157). We have examined possible relationships between salinity and ACE in both CLs and intact polar lipids (IPLs) from suspended particulate matter (SPM) and surface sediments of lakes and surrounding soils on the northeastern Qinghai–Tibetan Plateau. Our results showed that ACE values were positively correlated with salinity in all samples; however, CL ACE values were systematically higher than IPL ACE values, probably due to different degradation kinetics of intact polar (IP) archaeol and IP caldarchaeol. On the other hand, surface sediment ACE values from both CLs and IPLs were lower than SPM ACE values, probably due to enhanced production of caldarchaeol relative to archaeol in the sediment. Our results demonstrate that the ACE proxy reflects changes in salinity in diverse environments on the Qinghai–Tibetan Plateau, which is promising for paleosalinity reconstruction; however, caution should be used when applying the salinity proxy before we have a better understanding of degradation kinetics of archaeal IPLs and in situ production of caldarchaeol and archaeol in sediments.     

Analysis of human risks due to dam-break floods—part 1: a new model based on Bayesian networks

Dam breaks have catastrophic consequences for human lives. This paper presents a new human risk analysis model (HURAM) using Bayesian networks for estimating human risks due to dam-break floods. A Bayesian network is constructed according to a logic structure of loss-of-life mechanisms. The nodes (parameters) and the arcs (inter-relationships) of the network are quantified with historical data, existing models and physical analyses. A dataset of 343 dam-failure cases with records of fatality is compiled for this purpose. Comparison between two existing models and the new model is made to test the new model. Finally, sensitivity analysis is conducted to identify the important parameters that lead to loss of life. The new model is able to take into account a large number of important parameters and their inter-relationships in a systematic structure; include the uncertainties of these parameters and their inter-relationships; incorporate information derived from physical analysis, empirical models and historical data; and update the predictions when information in specific cases is available. The application of this model to the study of human risks in a specific dam-break case is presented in a companion paper.     

The Ca-Eskola component in eclogitic clinopyroxene as a function of pressure, temperature and bulk composition: an experimental study to 15 GPa with possible implications for the formation of oriented SiO2-inclusions in omphacite

Experiments have been conducted in the P-T range 2.5–15 GPa and 850–1,500°C using bulk compositions in the systems SiO₂–TiO₂–Al₂O₃–Fe₂O₃–FeO–MnO–MgO–CaO–Na₂O–K₂O–P₂O₅ and SiO₂–TiO₂–Al₂O₃–MgO–CaO–Na₂O to investigate the Ca-Eskola (CaEs Ca_0.5□_0.5AlSi₂O₆) content of clinopyroxene in eclogitic assemblages containing garnet + clinopyroxene + SiO₂ ± TiO₂ ± kyanite as a function of P, T, and bulk composition. The results show that CaEs_ss in clinopyroxene increases with increasing T and is strongly bulk composition dependent whereby high CaEs-contents are favoured by bulk compositions with high normative anorthite and low diopside contents. In this study, a maximum of 18 mol% CaEs_ss was found at 6 GPa and 1,350°C in a kyanite-eclogite assemblage garnet + clinopyroxene + kyanite + rutile + coesite. By comparison, no significant increase in CaEs_ss with increasing P could be observed. If the formation of oriented SiO₂-rods frequently observed in eclogititc clinopyroxenes is due to the retrogressive breakdown of a CaEs-component then these textures are a cooling rather than a decompression phenomenon and are most likely to be found in kyanite-bearing eclogites cooled from temperatures ≥750°C. The presence of clinopyroxene with approx. 4 mol% CaEs_ss in an experiment conducted at 2.5 GPa/850°C confirms earlier suggestions based on field data that vacancy-rich clinopyroxenes are not necessarily restricted to ultrahigh pressure metamorphic conditions. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Application of a 3D photorealistic model for the geological analysis of the Permian carbonates (Khuff Formation) in Saudi Arabia

Three dimensional (3D) photorealistic models of geological outcrops have the potential to enhance the teaching of earth sciences by providing scale models in a virtual reality environment. These models can be run on low-cost desktop computers. Photorealistic models for geological outcrops are a digital illustration of outcrop photographs with either a point cloud representation or Triangular Irregular Network (TIN) mesh of the outcrop surface. The level of detail for these models is dependent on the target resolutions (physical and optical) that were used during data acquisition. In addition, the technique in which the data is rendered as a digital model affects the level of detail that can be observed by the geologists. A colored point cloud representation is suitable for large-scale features, but fine details are lost when the geologist zooms in to view the model close up. In contrast, a photorealistic model that is constructed from photographs draped onto a triangle mesh surface derived from Light Detection and Ranging (LiDAR) point clouds provides a level of detail that is restricted only by the resolution of the photographs.     

Interaction of peridotite with Ca-rich carbonatite melt at 3.1 and 6.5 GPa: Implication for merwinite formation in upper mantle,and for the metasomatic origin of sublithospheric diamonds with Ca-rich suite of inclusions

We performed an experimental study, designed to reproduce the formation of an unusual merwinite?+?olivine-bearing mantle assemblage recently described as a part of a Ca-rich suite of inclusions in sublithospheric diamonds, through the interaction of peridotite with an alkali-rich Ca-carbonatite melt, derived from deeply subducted oceanic crust. In the first set of experiments, we studied the reaction between powdered Mg-silicates, olivine and orthopyroxene, and a model Ca-carbonate melt (molar Na:K:Ca?=?1:1:2), in a homogeneous mixture, at 3.1 and 6.5 GPa. In these equilibration experiments, we observed the formation of a merwinite?+?olivine-bearing assemblage at 3.1 GPa and 1200 °C and at 6.5 GPa and 1300–1400 °C. The melts coexisting with this assemblage have a low Si and high Ca content (Ca#?=?molar 100?×?Ca/(Ca?+?Mg)?>?0.57). In the second set of experiments, we investigated reaction rims produced by interaction of the same Ca-carbonate melt (molar Na:K:Ca?=?1:1:2) with Mg-silicate, olivine and orthopyroxene, single crystals at 3.1 GPa and 1300 °C and at 6.5 GPa and 1400 °C. The interaction of the Ca-carbonate melt with olivine leads to merwinite formation through the expected reaction: 2Mg₂SiO₄ (olivine)?+?6CaCO₃ (liquid)?=?Ca₃MgSi₂O₈ (merwinite)?+?3CaMg(CO₃)₂ (liquid). Thus, our experiments confirm the idea that merwinite in the upper mantle may originate via interaction of peridotite with Ca-rich carbonatite melt, and that diamonds hosting merwinite may have a metasomatic origin. It is remarkable that the interaction of the Ca-carbonate melt with orthopyroxene crystals does not produce merwinite both at 3.1 and 6.5 GPa. This indicates that olivine grain boundaries are preferable for merwinite formation in the upper mantle.     

Phase relations and melting of carbonated peridotite between 10 and 20 GPa: a proxy for alkali- and CO<Subscript>2</Subscript>-rich silicate melts in the deep mantle

We determined the melting phase relations, melt compositions, and melting reactions of carbonated peridotite on two carbonate-bearing peridotite compositions (ACP: alkali-rich peridotite + 5.0 wt % CO₂ and PERC: fertile peridotite + 2.5 wt % CO₂) at 10–20 GPa and 1,500–2,100 °C and constrain isopleths of the CO₂ contents in the silicate melts in the deep mantle. At 10–20 GPa, near-solidus (ACP: 1,400–1,630 °C) carbonatitic melts with < 10 wt % SiO₂ and > 40 wt % CO₂ gradually change to carbonated silicate melts with > 25 wt % SiO₂ and < 25 wt % CO₂ between 1,480 and 1,670 °C in the presence of residual majorite garnet, olivine/wadsleyite, and clinoenstatite/clinopyroxene. With increasing degrees of melting, the melt composition changes to an alkali- and CO₂-rich silicate melt (Mg# = 83.7–91.6; ~ 26–36 wt % MgO; ~ 24–43 wt % SiO₂; ~ 4–13 wt % CaO; ~ 0.6–3.1 wt % Na₂O; and ~ 0.5–3.2 wt % K₂O; ~ 6.4–38.4 wt % CO₂). The temperature of the first appearance of CO₂-rich silicate melt at 10–20 GPa is ~ 440–470 °C lower than the solidus of volatile-free peridotite. Garnet + wadsleyite + clinoenstatite + carbonatitic melt controls initial carbonated silicate melting at a pressure < 15 GPa, whereas garnet + wadsleyite/ringwoodite + carbonatitic melt dominates at pressure > 15 GPa. Similar to hydrous peridotite, majorite garnet is a liquidus phase in carbonated peridotites (ACP and PERC) at 10–20 GPa. The liquidus is likely to be at ~ 2,050 °C or higher at pressures of the present study, which gives a melting interval of more than 670 °C in carbonated peridotite systems. Alkali-rich carbonated silicate melts may thus be produced through partial melting of carbonated peridotite to 20 GPa at near mantle adiabat or even at plume temperature. These alkali- and CO₂-rich silicate melts can percolate upward and may react with volatile-rich materials accumulate at the top of transition zone near 410-km depth. If these refertilized domains migrate upward and convect out of the zone of metal saturation, CO₂ and H₂O flux melting can take place and kimberlite parental magmas can be generated. These mechanisms might be important for mantle dynamics and are potentially effective metasomatic processes in the deep mantle.     

The O’okiep Copper District, Namaqualand, South Africa: a review of the geology with emphasis on the petrogenesis of the cupriferous Koperberg Suite

The O’okiep Copper District is the oldest formal mining area in South Africa. Between 1852 and 2002, the 2,500 km² area yielded two million tons of copper from 32 mines ranging in ore tonnages from 140,000 to 37 million tons. This paper summarizes the calendar of events from the formation of the first primitive crust 1,700–2,000?Ma ago to early Cambrian times ~500?Ma ago, with particular emphasis on the Namaquan (Grenville) Orogeny, notably: the O’okiepian Episode (1,180–1,210?Ma ago) of alpine-type folding, regional granite plutonism, and granulite facies metamorphism and the Klondikean Episode (1,020–1,040?Ma ago) of open and tight folding and the intrusion of the Rietberg Granite and the Koperberg Suite. Almost all of the copper in the O’okiep District occurs in the Koperberg Suite, of which there are 1,700 small bodies that constitute 0.7% of the outcrop area. The suite comprises jotunite, anorthosite, biotite diorite, and hypersthenic rocks ranging from leuconorite to hypersthenite, and it is one of only two world examples of economic copper mineralization in rocks of the anorthosite–charnockite kindred; the second example is Caraiba, Brazil. High I _Sr and low ε _Nd (for a 1,030 Ma intrusion age), and high μ ₂ of 10.1, for Koperberg rock-types indicate a crustal progenitor for the suite, and the presence of jotunite suggests a (subducted) crustal source at ca. 40–50 km depth. The magmatic sulphide paragenesis in the Koperberg Suite is chalcopyrite?+?pyrrhotite (Narrap-type ore) that, in a number of ore-bodies, has been inverted under upper amphibolite facies conditions to bornite?+?Ti-free magnetite (Carolusberg-type ore). Meteoric fluids resulted in supergene Cu enrichment in Koperberg bodies to ~500?m below the pre-Nama peneplane, and lower greenschist facies metamorphism 500–570?Ma ago is reflected by inter alia Hoits-type ore bearing second-generation bornite?+?chalcopyrite(±?covellite?±?chalcocite).     

Silicate melts at magmatic temperatures: in-situ structure determination to 1651°C and effect of temperature and bulk composition on the mixing behavior of structural units

The abundance of coexisting structural units in K-, Na-, and Li-silicate melts and glasses from 25° to 1654°C has been determined with in-situ micro-Raman spectroscopy. From these data an equilibrium constant, K_x, for the disproportionation reaction among the structural units coexisting in the melts, Si₂O₅(2Q³)SiO₃(Q²)+SiO₂(Q⁴), was calculated (K_x is the equilibrium constant derived by using mol fractions rather than activities of the structural units). From ln K_x vs l/T relationships the enthalpy (H_x) for the disproportionation reaction is in the range of-30 to 30 kJ/mol with systematic compositional dependence. In the potassium and sodium systems, where the disproportionation reaction shifts to the right with increasing temperature, the H_x increases with silica content (M/Si decreases, M=Na, K). For melts and supercooled liquids of composition Li₂O·2SiO₂ (Li/Si=1), the H_x is indistinguishable from 0. By decreasing the Li/Si to 0.667 (composition LS3) and beyond (e.g., LS4), the disproportionation reaction shifts to the left as the temperature is increased. For a given ratio of M/Si (M=K, Na, Li), there is a positive, near linear correlation between the H_x and the Z/r² of the metal cation. The slope of the H_x vs Z/r² regression lines increases as the system becomes more silica rich (i.e., M/Si is decreased). Activity coefficients for the individual structural units, _i, were calculated from the structural data combined with liquidus phase relations. These coefficients are linear functions of their mol fraction of the form _i=a lnX _i+b, where a is between 0.6 and 0.87, and X _i is the mol fraction of the unit. The value of the intercept, b, is near 0. The relationship between activity coefficients and abundance of individual structural units is not affected by temperature or the electronic properties of the alkali metal. The activity of the structural units, however, depend on their concentration, type of metal cation, and on temperature.     

Water-table and discharge changes associated with the 2016–2017 seismic sequence in central Italy: hydrogeological data and a conceptual model for fractured carbonate aquifers

A seismic sequence in central Italy from August 2016 to January 2017 affected groundwater dynamics in fractured carbonate aquifers. Changes in spring discharge, water-table position, and streamflow were recorded for several months following nine Mw 5.0–6.5 seismic events. Data from 22 measurement sites, located within 100 km of the epicentral zones, were analyzed. The intensity of the induced changes were correlated with seismic magnitude and distance to epicenters. The additional post-seismic discharge from rivers and springs was found to be higher than 9 m³/s, totaling more than 0.1 km³ of groundwater release over 6 months. This huge and unexpected contribution increased streamflow in narrow mountainous valleys to previously unmeasured peak values. Analogously to the L’Aquila 2009 post-earthquake phenomenon, these hydrogeological changes might reflect an increase of bulk hydraulic conductivity at the aquifer scale, which would increase hydraulic heads in the discharge zones and lower them in some recharge areas. The observed changes may also be partly due to other mechanisms, such as shaking and/or squeezing effects related to intense subsidence in the core of the affected area, where effects had maximum extent, or breaching of hydraulic barriers.

     

Environmental controls on the δ18O composition of freshwater calcite and aragonite in a temperate,lowland river system: significance for palaeoclimatic studies

The oxygen and carbon isotopic composition of surficial carbonates is a key technique for reconstructing past environments and climates. The understanding of modern isotopic systems is, however, a vital first step before applying these techniques to fossil examples. In this study the δ¹⁸O of aragonite shells from three different freshwater mollusc species and the δ¹⁸O of tufa stromatalites are analysed and compared to the modern temperature and isotopic regime of the river system in which they form (the river Gipping, Sproughton, southeast England). In all cases this range of carbonates appear to form in isotopic equilibrium with the δ¹⁸O of modern waters at water temperatures experienced during the summer months. It is, therefore, likely that in Quaternary interglacial deposits of the UK and western Europe the δ¹⁸O of fossil freshwater molluscs and tufa stromatolite carbonates will provide an indication of past summer temperature regimes and not mean annual temperatures, as has been previously suggested. The paper concludes by discussing the implications of this study for the isotopic analysis of interglacial deposits in Britain and western Europe.