Trace and minor elements in sphalerite: A LA-ICPMS study

Sphalerite is an important host mineral for a wide range of minor and trace elements. We have used laser-ablation inductively coupled mass spectroscopy (LA-ICPMS) techniques to investigate the distribution of Ag, As, Bi, Cd, Co, Cu, Fe, Ga, Ge, In, Mn, Mo, Ni, Pb, Sb, Se, Sn and Tl in samples from 26 ore deposits, including specimens with wt.% levels of Mn, Cd, In, Sn and Hg. This technique provides accurate trace element data, confirming that Cd, Co, Ga, Ge, In, Mn, Sn, As and Tl are present in solid solution. The concentrations of most elements vary over several orders of magnitude between deposits and in some cases between single samples from a given deposit. Sphalerite is characterized by a specific range of Cd (typically 0.2-1.0 wt.%) in each deposit. Higher Cd concentrations are rare; spot analyses on samples from skarn at Baisoara (Romania) show up to 13.2 wt.% (Cd²⁺ ↔ Zn²⁺ substitution). The LA-ICPMS technique also allows for identification of other elements, notably Pb, Sb and Bi, mostly as micro-inclusions of minerals carrying those elements, and not as solid solution. Silver may occur both as solid solution and as micro-inclusions. Sphalerite can also incorporate minor amounts of As and Se, and possibly Au (e.g., Magura epithermal Au, Romania). Manganese enrichment (up to ∼4 wt.%) does not appear to enhance incorporation of other elements. Sphalerite from Toyoha (Japan) features superimposed zoning. Indium-sphalerite (up to 6.7 wt.% In) coexists with Sn-sphalerite (up to 2.3 wt.%). Indium concentration correlates with Cu, corroborating coupled (Cu⁺In³⁺) ↔ 2Zn²⁺ substitution. Tin, however, correlates with Ag, suggesting (2Ag⁺Sn⁴⁺) ↔ 3Zn²⁺ coupled substitution. Germanium-bearing sphalerite from Tres Marias (Mexico) contains several hundred ppm Ge, correlating with Fe. We see no evidence of coupled substitution for incorporation of Ge. Accordingly, we postulate that Ge may be present as Ge²⁺ rather than Ge⁴⁺. Trace element concentrations in different deposit types vary because fractionation of a given element into sphalerite is influenced by crystallization temperature, metal source and the amount of sphalerite in the ore. Epithermal and some skarn deposits have higher concentrations of most elements in solid solution. The presence of discrete minerals containing In, Ga, Ge, etc. also contribute to the observed variance in measured concentrations within sphalerite.     

Crude Oil at the Bemidji Site: 25 Years of Monitoring,Modeling, and Understanding

The fate of hydrocarbons in the subsurface near Bemidji, Minnesota, has been investigated by a multidisciplinary group of scientists for over a quarter century. Research at Bemidji has involved extensive investigations of multiphase flow and transport, volatilization, dissolution, geochemical interactions, microbial populations, and biodegradation with the goal of providing an improved understanding of the natural processes limiting the extent of hydrocarbon contamination. A considerable volume of oil remains in the subsurface today despite 30 years of natural attenuation and 5 years of pump‐and‐skim remediation. Studies at Bemidji were among the first to document the importance of anaerobic biodegradation processes for hydrocarbon removal and remediation by natural attenuation. Spatial variability of hydraulic properties was observed to influence subsurface oil and water flow, vapor diffusion, and the progression of biodegradation. Pore‐scale capillary pressure‐saturation hysteresis and the presence of fine‐grained sediments impeded oil flow, causing entrapment and relatively large residual oil saturations. Hydrocarbon attenuation and plume extent was a function of groundwater flow, compound‐specific volatilization, dissolution and biodegradation rates, and availability of electron acceptors. Simulation of hydrocarbon fate and transport affirmed concepts developed from field observations, and provided estimates of field‐scale reaction rates and hydrocarbon mass balance. Long‐term field studies at Bemidji have illustrated that the fate of hydrocarbons evolves with time, and a snap‐shot study of a hydrocarbon plume may not provide information that is of relevance to the long‐term behavior of the plume during natural attenuation.     

A Compilation of New and Published Major and Trace Element Data for NIST SRM 610 and NIST SRM 612 Glass Reference Materials

Microanalytical trace element techniques (such as ion probe or laser ablation ICP-MS) are hampered by a lack of well characterized, homogeneous standards. Two silicate glass reference materials produced by National Institute of Standards and Technology (NIST), NIST SRM 610 and NIST SRM 612, have been shown to be homogeneous and are spiked with up to sixty one trace elements at nominal concentrations of 500 μg g-1 and 50 μg g-1 respectively. These samples (supplied as 3 mm wafers) are equivalent to NIST SRM 611 and NIST SRM 613 respectively (which are supplied as 1 mm wafers) and are becoming more widely used as potential microanalytical reference materials. NIST however, only certifies up to eight elements in these glasses. Here we have compiled concentration data from approximately sixty published works for both glasses, and have produced new analyses from our laboratories. Compilations are presented for the matrix composition of these glasses and for fifty eight trace elements. The trace element data includes all available new and published data, and summaries present the overall average and standard deviation, the range, median, geometric mean and a preferred average (which excludes all data outside ± one standard deviation of the overall average). For the elements which have been certified, there is a good agreement between the compiled averages and the NIST data. This compilation is designed to provide useful new working values for these reference materials.     

Nonaxisymmetric instabilities in self-gravitating disks. II. Linear and quasi-linear analyses

We studied global nonaxisymmetric hydrodynamic instabilities in an extensive collection of hot, self-gravitating polytropic disk systems, systems that covered a wide expanse of the parameter space relevant to protostellar and protoplanetary systems. We examined equilibrium disk models varying three parameters: the ratio of the inner to outer equatorial radii, the ratio of star mass to disk mass, and the rotation law exponent q. We took the polytropic index n=1.5 and examined the exponents q=1.5 and 2, and the transitional one q=1.75. For each of these sets of parameters, we examined models with inner to outer radius ratios from 0.1 to 0.75, and star mass to disk mass ratios from 0 to 10³. We numerically calculated the growth rates and oscillation frequencies of low-order nonaxisymmetric disk modes, modes with azimuthal dependence ∝e ^im? . Low-m modes are found to dominate with the character and strength of instability strongly dependent on disk self-gravity. Representatives of each mode type are examined in detail, and torques and mass transport rates are calculated.     

Mineralogical Zonation of Wall-Rock Alteration in Jiaodong Gold Province, North China

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Geochemical and biological consequences of groundwater augmentation in lakes of west-central Florida (USA)

We studied sediment cores from four Florida (USA) lakes that have received groundwater hydrologic supplements (augmentation) for >30 years to maintain lake stage. Top samples (0–4 cm) from sediment cores taken in Lakes Charles, Saddleback, Little Hobbs, and Crystal had ²²⁶Ra activities of 44.9, 17.5, 7.6, and 8.5 dpm g⁻¹, respectively, about an order of magnitude greater than values in deeper, older deposits. The surface sample from Lake Charles yielded the highest ²²⁶Ra activity yet reported from a Florida lake core. Several lines of evidence suggest that groundwater augmentation is responsible for the high ²²⁶Ra activities in recent sediments: (1) ²²⁶Ra activity in cores increased recently, (2) the Charles, Crystal, and Saddleback cores display ²²⁶Ra/²¹⁰Pb disequilibrium at several shallow depths, suggesting ²²⁶Ra entered the lakes in dissolved form, (3) cores show recent increases in Ca, which, like ²²⁶Ra, is abundant in augmentation groundwater, and (4) greater Sr concentrations are associated with higher ²²⁶Ra activities in recent Charles and Saddleback sediments. Sr concentrations in Eocene limestones of the deep Floridan Aquifer are high relative to Sr concentrations in surficial quartz sands around the lakes. Historical water quality inferences for the lakes were based on diatom assemblages in sediments. Recent alkalization in Lakes Charles, Saddleback, Little Hobbs, and Crystal was inferred from weighted-averaging calibration (WACALIB). The lakes also show recent trophic state increases based on WACALIB-derived estimates for limnetic total P. Although residential and agricultural sources might contribute to increased P loading, P in augmentation waters probably has had significant influence on eutrophication. Dystrophic diatoms were abundant in the early history of Lakes Saddleback, Little Hobbs, and Crystal, which suggests that these lakes contained more tannic waters during the past than at present, perhaps as a consequence of greater inflows from surrounding wetlands. Ionic content of lake waters increased, as indicated by diatom autecological analysis. Recent geochemical and biological changes detected in cores from these lakes probably are a result of deliberate groundwater augmentation, although inputs of groundwater pumped for agricultural and residential development in the watersheds also might have contributed to limnological changes.     

Helium isotopic variability within single diamonds from the Orapa kimberlite pipe

The distribution and isotopic composition of helium has been measured in a suite of well-characterized one-carat diamonds from the Orapa kimberlite, Botswana. Crushing of the diamonds in vacuo indicates that most of the helium is contained by the matrix (generally greater than 90%), rather than by the inclusions. Step-heating experiments, performed on inclusion-free fragments remaining after crushing, indicate that the³He/⁴He ratio is variablewithin individual diamonds. The fragments, as small as 10 mg, were heated in two timed steps, both at 2000°C. In every case, lower³He/⁴He ratios are observed in the first graphitization step (0.05–3 × atmospheric), while the last heating step releases helium with systematically higher³He/⁴He ratio (30–80 × atmospheric). We suggest that this internal isotopic variability is the result of stepwise graphitization: the first heating step initiates graphitization, which nucleates around defects, and the second heating step graphitizes the relatively defect-free regions of the diamond. The³He/⁴He ratio measured, using the partial graphitization technique, differs by up to a factor of 100 within a single specimen. The inclusion-free fragments release small quantities of helium below 2000°C, which suggests that helium release is obtained only by graphitization. The³He contents of the monocrystalline diamonds are relatively constant (at 3 × 10⁻¹³ cm³ STP/gram) and indicate that most of the isotopic variability is due to radiogenic⁴He. The variations in⁴He content are either related to zoning of Th and U in the diamonds (i.e., in-situ decay), to zoning of inherited⁴He, or to implantation of α-particles from a Th and U rich environment (i.e., kimberlite). Because the Orapa diamonds were mined from roughly 40 m depth in the kimberlite, spallation reactions from cosmic ray interactions are not a significant source of³He. However, calculations based on the age of the kimberlite (90 m.y.) and reasonable Th and U abundances suggest that most of the³He in the Orapa diamonds could be produced by⁶Li(n, α)T in the diamond. Although this may not be true of all diamonds, nuclear reactions in the crust and mantle (including spallation reactions at the surface) can explain many of the high³He/⁴He ratios previously reported for diamonds.     

Do conservative solutes migrate at average pore-water velocity?

According to common understanding, the advective velocity of a conservative solute equals the average linear pore-water velocity. Yet direct monitoring indicates that the two velocities may be different in heterogeneous media. For example, at the Camp Dodge, Iowa, site the advective velocity of discrete Cl- plumes was less than one tenth of the average pore-water velocity calculated from Darcy's law using the measured hydraulic gradient, effective porosity, and hydraulic conductivity (K) from large-scale three-dimensional (3D) techniques, e.g., pumping tests. Possibly, this difference reflects the influence of different pore systems, if the K relevant to transient solute flux is influenced more by lower-K heterogeneity than a steady or quasi-steady water flux. To test this idea, tracer tests were conducted under controlled laboratory conditions. Under one-dimensional flow conditions, the advective velocity of discrete conservative solutes equaled the average pore-water velocity determined from volumetric flow rates and Darcy's law. In a larger 3D flow system, however, the same solutes migrated at approximately 65% of the average pore-water velocity. These results, coupled with direct observation of dye tracers and their velocities as they migrated through both homogeneous and heterogeneous sections of the same model, demonstrate that heterogeneity can slow the advective velocity of discrete solute plumes relative to the average pore-water velocity within heterogeneous 3D flow sytems.