Mercury mobility at the Carson River Superfund Site, west-central Nevada, USA: Interpretation of mercury speciation data in mill tailings, soils, and sediments

The Carson River Superfund Site in west-central Nevada is an area of Hg-contaminated soil, sediment, water, air, and biola resulting from the amalgamation milling of Ag-Au ores of the Comstock lode worked approximately a century ago. In order to develop an understanding of the behavior, transport, and fate of Hg at this site, a technique was developed to estimate the proportions of total, elemental, exchangeable, organic, and sulfide Hg in soils, sediments, and tailings.Results of this analysis performed on active Carson River sediments indicate that Hg is selectively dissolved out of Hg-Au amalgam particles and subsequently adsorbed to fine-grained sediments which are then deposited in downstream, low-energy reaches of the Carson River and Labontan Reservoir. In the relatively more-reducing environment of the reservoir Hg appears to be converted, in large part, to relatively-insoluble HgS.The original elemental form of Hg released to the environment is the chemical form which is still dominant in most highly-contaminated soils, sediments, and tailings. Deeper, more-reducing soil horizons, however, appear to fix a significant portion of the Hg as HgS, analogous to the Lahontan Reservoir example described above. This fixation as HgS is documented to be largely limited to higher-sulfur areas where sulfide minerals from the Comstock ores increase the total sulfur concentrations of contaminated soils, sediments, and tailings.     

High- and low-Cr chromitite and dunite in a Tibetan ophiolite: evolution from mature subduction system to incipient forearc in the Neo-Tethyan Ocean

The microstructures, major- and trace-element compositions of minerals and electron backscattered diffraction (EBSD) maps of high- and low-Cr# [spinel Cr# = Cr³⁺/(Cr³⁺ + Al³⁺)] chromitites and dunites from the Zedang ophiolite in the Yarlung Zangbo Suture (South Tibet) have been used to reveal their genesis and the related geodynamic processes in the Neo-Tethyan Ocean. The high-Cr# (0.77–0.80) chromitites (with or without diopside exsolution) have chromite compositions consistent with initial crystallization by interaction between boninitic magmas, harzburgite and reaction-produced magmas in a shallow, mature mantle wedge. Some high-Cr# chromitites show crystal-plastic deformation and grain growth on previous chromite relics that have exsolved needles of diopside. These features are similar to those of the Luobusa high-Cr# chromitites, possibly recycled from the deep upper mantle in a mature subduction system. In contrast, mineralogical, chemical and EBSD features of the Zedang low-Cr# (0.49–0.67) chromitites and dunites and the silicate inclusions in chromite indicate that they formed by rapid interaction between forearc basaltic magmas (MORB-like but with rare subduction input) and the Zedang harzburgites in a dynamically extended, incipient forearc lithosphere. The evidence implies that the high-Cr# chromitites were produced or emplaced in an earlier mature arc (possibly Jurassic), while the low-Cr# associations formed in an incipient forearc during the initiation of a new episode of Neo-Tethyan subduction at ~130–120 Ma. This two-episode subduction model can provide a new explanation for the coexistence of high- and low-Cr# chromitites in the same volume of ophiolitic mantle.     

A subsidiary fast-diffusing substitution mechanism of Al in forsterite investigated using diffusion experiments under controlled thermodynamic conditions

Diffusion of Al in synthetic forsterite was studied at atmospheric pressure from 1100 to 1500 °C in air along [100] with activities of SiO₂, MgO and Al₂O₃ (a_SiO2, a_MgO and a_Al2O3) buffered. At low a_SiO2, the buffer was forsterite + spinel + periclase (fo + sp + per) at all temperatures, while at high a_SiO2 and subsolidus conditions a variety of three-phase assemblages containing forsterite and two other phases from spinel, cordierite, protoenstatite or sapphirine were used at 1100–1350 °C. Experiments at high a_SiO2 and 1400 °C used forsterite + protoenstatite + melt (fo + en + melt), and at 1500 °C, fo + melt. The resulting diffusion profiles were analysed by LA–ICP–MS in scanning mode. Diffusion profiles in the high a_SiO2 experiments were generally several hundred microns in length, but diffusion at low a_SiO2 was three orders of magnitude slower than in high a_SiO2 experiments carried out at the same temperature, producing short profiles only a few microns in length and close to the spatial resolution of the analytical method. Interface concentrations of Al in the forsterite, obtained by extrapolating the diffusion profiles to the crystal/buffer interface, were only a fraction of those expected at equilibrium, and varied among the differing buffer assemblages according to (a_Al2O3)^1/2 and (a_SiO2)^3/4, pointing to the substitution of Al in forsterite by an octahedral-site, vacancy-coupled (OSVC) component with the stoichiometry Al _4/3 ³⁺ vac_2/3SiO₄, whereas the main substitution expected from previous equilibrium studies would be the coupled substitution of 2 Al for Mg + Si, giving the stoichiometry MgAl₂O₄. It is proposed that this latter substitution is not seen on the length scales of the present experiments because it requires replacement of Si by Al on tetrahedral sites, and is accordingly rate-limited by the slow diffusivity of Si. Instead, diffusion of Al by the OSVC mechanism is relatively fast, and at high a_SiO2, even faster than Fe–Mg interdiffusion.     

Earth Science Education in Kenya

The University of Nairobi is currently the only University in Kenya which offers degree level courses in the earth sciences. The Department was founded in 1961, became part of an autonomous University in 1970, awarded its first degrees in 1972 and awarded its first Geology degrees in 1978. The Department currently offers a four-year B.Sc. course - delivered as part of a course unit system - and an M.Sc. programme in Geology. Students are admitted to the University after eight years of primary and four years of secondary schooling. Graduates find employment either in government departments or in the private sector. The Department currently has 15 academic staff who are involved in a variety of research projects. Currently, there are collaborative links with Universities in Germany, Sweden and South Africa.     

Consistent patterns of the size distribution of thermokarst lakes

The purpose of this work is to study empirically the patterns of size distribution of thermokarst lakes within lacustrine thermokarst plains. Investigations were performed at 16 sites with various geomorphological, geocryological, and physical geographical conditions (Kolyma Lowland, Western Siberia, Lena River valley, Alaska). The accordance of the distribution area with the lognormal and exponential laws, and the accordance of the average diameter distribution with the normal law have been tested; the tested laws of distribution resulted from previous investigations. The results have shown that the lognormal law of distribution of thermokarst lake areas is valid for the vast majority of cases, and the other types of distribution are inconsistent with empirical data. This evidence favors the development pattern for lacustrine thermokarst plains, when thermokarst processes started simultaneously and the rate of lake growth was proportional to the density of heat loss through the side surface.     

Sources of fluids and material for gold and antimony mineralization in the Adychanskii ore region (East Yakutia,Russia)

The peculiarities of fluid inclusions; the O and C isotope composition of host rocks, vein minerals, and inclusions; and the S and Pb isotope composition of sulfides allowed us to distinguish two groups of fluids with a similar temperature, salinity, and source of the aqueous part produced upon metagenesis and mobilized during collisional events. Quartz-A precipitates from the CO₂–H₂O hydrocarbonate–Na fluid with a salinity of 7–10 wt % eq. NaCl at a depth of ～6 km (290–340°C, 1550 bar). Regeneration of quartz (quartz-C), precipitation of quartz-B, and quartz-AB with carbonate and chlorite occurred at a depth from 3.5 to 1.5 km (250–380°C, 1250–900–350 bar) from CO₂–CH₄–N hydrous sulfate–hydrocarbonate Na–Mg fluids with Cl^–, Ca, and K and a salinity of 5–10 wt % eq. NaCl, and a wide variety of impurities. The localization of veins in sinistral shear dislocations and strong heterogeneity in the P–T conditions allow us to explain the formation of fluid-2 by the postcollisional events.     

Morphotectonic controls of groundwater flow regime and relating environmental impacts in Northwest Sinai,Egypt

The frequent appearance of some hydro-environmental hazard features, such as waterlogging and soil salinization along the susceptible zones at Northwest Sinai area (NWSA), has put serious challenges and obstacles for a correct and efficient land use planning of this region, for several decades. Although previous studies have shown that the whole region of Northern Sinai is greatly affected by the tectonic movements associated with the Syrian Arc folding system (SAS), NWSA is barren of any obvious surficial structures. The current work aims to investigate the effect of subsurface tectonic features on the hydrogeologic regime of NWSA.Hydrogeological and remote sensing data were integrated with ground geophysical gravity and magnetic measurements, using the geographic information system. Data integration asserts the role played by buried tectonic features not only in governing the landforms of the upper water-bearing quaternary formations but also in controlling their flow regime.Two major subsurface structures were identified through interpreting the geophysical measurements. A buried dome-like structure, dominating the central part of the mapped area, coincides with the radial flow pattern observed on the water table map. At the southwestern corner of the study area, an elevated groundwater level, caused by continuous groundwater accumulation at the discharge boundary, is superimposing a subsurface block-faulted depression. The waterlogging features (saturation of the soil by groundwater and inundation of local depressions due to rising of water table) dominating the discharge lowlands of NWSA support the conclusion that a buried block-faulted structure exerts a strong influence on the thickness and groundwater flow regime of the shallow quaternary aquifer.     

Deep earth electrical conductivity-depth profiles around African geomagnetic equator using solar quiet currents

Electrical conductivity structure of the Earth’s deep interior has been successfully mapped out down to approximately 1500 km around the geomagnetic dip equatorial regions of Africa using solar quiet-day ionospheric currents. Spherical harmonic analysis (SHA) was employed in separating the internal and external field contributions to the solar quiet variations. Transfer function was used for each of the external and internal pairs to compute the conductivity-depth profile for the region. Calculated average electrical conductivity values were evidently higher than obtained in other parts of the world farther away from the geomagnetic equator. Sq current vortex foci are observed very close to the geomagnetic equator. Depth of penetration was greatly enhanced. Stations on latitudes less than 1° from the geomagnetic equator show higher electrical conductivity when compared with that situated more than 4° away from it at various corresponding depths. Evidence of discontinuities in the earth layers were also noted at some depths. Highly conductive layers were delineated around 400 km depth and beyond 1200 km.     

Age and sources of matter for the Kedrovskoe gold deposit,Northern Transbaikal tegion,Republic of Buryatia: Geochronological and isotopic geochemical constraints

The paper presents new geochronological and isotopic geochemical data on gold mineralization of the Kedrovskoe deposit. The deposit is located in the northeastern part of the Transbaikal metallogenic province, Russia’s largest. The Early Permian age (273 ± 4 Ma) of mineralization based on the results of Rb–Sr study of metasomatic rocks is correlated with the age of the final phases of Hercynian magmatism in the Baikal–Muya Foldbelt. The Sr, Nd, and Pb isotopic geochemical characteristics of mineralization show that the host rocks are involved in the formation of the latter. It has been established that ore lead was supplied to the hydrothermal system of the deposit mainly from a geochemical reservoir represented by the Neoproterozoic juvenile continental crust of the Baikal–Muya Foldbelt.     

Reconstructing the Cryogenian–Ediacaran evolution of the Porongos fold and thrust belt,Southern Brasiliano Orogen,based on Zircon U–Pb–Hf–O isotopes

The Neoproterozoic geotectonic triad of the Brasiliano Orogen is reconstructed in southern Brazil from studies focused on the Porongos fold and thrust belt. We integrate field geology with isotopic studies of zircon U–Pb SHRIMP and Lu–Hf–O laser determinations in seven metasedimentary and three metavolcanic rock samples. The results indicate that the Porongos palaeo-basin was derived from mixed sources (3200–550 Ma), with major contributions from Rhyacian (2170 Ma) and Ediacaran (608 Ma) sources. Minor contributions from Archaean to Tonian sources are also registered. The maximum depositional age of the Porongos palaeo-basin is established by the age range of 650–550 Ma with T_DM model ages between 2.5 and 1.3 Ga. The reworked signature (ε_Hf values = ?34 to ?4) and the characteristic crustal magma reservoirs (δ¹⁸O ≥5.3 ‰) indicate that these sediments are equivalent to Neoproterozoic granites of the Dom Feliciano Belt. The episodic depositional history started in the Cryogenian (650 Ma) and lasted until the Ediacaran (most likely 570 Ma). A magmatic event of Tonian age is recorded in rhyodacite samples interleaved with the metasedimentary rocks and dated at 773, 801, and 809 Ma. The crustal evolution of the Sul-Riograndense Shield included mountain building, folding and thrusting and flexural subsidence in the foreland. An orogenic triad is revealed as the Pelotas Batholith, the Porongos fold and thrust belt and the Camaquã Basin, all part of the Dom Feliciano Belt.