Combining in situ isotopic, trace element and textural analyses of quartz from four magmatic-hydrothermal ore deposits

This study couples in situ ¹⁶O, ¹⁷O and ¹⁸O isotope and in situ trace element analyses to investigate and characterize the geochemical and textural complexity of magmatic-hydrothermal quartz crystals. Euhedral quartz crystals contemporaneous with mineralization were obtained from four magmatic-hydrothermal ore deposits: El Indio Au–Ag–Cu deposit; Summitville Au–Ag–Cu deposit; North Parkes Cu–Au deposit and Kingsgate quartz-Mo–Bi–W deposit. The internal features of the crystals were imaged using cathodoluminescence and qualitative electron microprobe maps. Quantitative isotopic data were collected in situ using 157 nm laser ablation inductively coupled plasma mass spectrometry (for 40 trace elements in quartz) and sensitive high-resolution ion microprobe (for 3 isotopes in quartz). Imaging revealed fine oscillatory zoning, sector zoning, complex “macromosaic” textures and hidden xenocrystic cores. In situ oxygen isotope analyses revealed a δ¹⁸O range of up to 12.4 ± 0.3 ‰ in a single crystal—the largest isotopic range ever ascribed to oscillatory zonation in quartz. Some of these crystals contain a heavier δ¹⁸O signature than expected by existing models. While sector-zoned crystals exhibited strong trace element variations between faces, no evidence for anisotropic isotope fractionation was found. We found: (1) isotopic heterogeneity in hydrothermal quartz crystals is common and precludes provenance analysis (e.g., δD–δ¹⁸O) using bulk analytical techniques, (2) the trace element signature of quartz is not an effective pathfinder toward noble metal mineralization and (3) in three of the four samples, both textural and isotopic data indicate non-equilibrium deposition of quartz.     

Helical probe tests in residual soils of the Appalachian Piedmont

Helical probe tests (HPT) are a quick and economical means for manual field testing of soils to depths of 1.5 m with readings taken at 0.15-m intervals in only 10 min. The equipment is lightweight (only 2 kg) and thus amenable to deploy on initial site reconnaissance explorations, shallow pavement projects, earth retention walls, and/or compaction of fills. Although suitable for use in a variety of geomaterials: sands, silts, clays, and mixed soils, the specific application to residual fine sandy silts and silty fine sands of the Appalachian Piedmont and Blue Ridge geologic provinces is shown here. Existing relationships for converting the measured HPT torque reading to equivalent cone penetration testing (CPT) tip resistances are reviewed, as well as other trends.     

Mathematical models for petroleum-forming processes: carbon isotope fractionation

A mathematical model has been developed in which carbon isotope fractionation during thermal cracking of n-paraffins can be simulated. The model has been calibrated based on data from laboratory cracking experiments carried out on n-octadecane. Relative rate constants for cleavage of C¹²-C¹², C¹²-C¹³ and C¹³-C¹³ bonds agree with the experimental values obtained by other workers.Application of this model to the process of petroleum formation gives good agreement with some existing experimental data, but suggests that a review of our understanding of isotope fractionation during thermal cracking may be necessary. The relative importance of the degree to which the organic material has been cracked and of the type of the organic material in influencing δC¹³ values is discussed.The present model predicts that cracking of n-paraffin distributions having initial odd or even carbon number predominances can induce isotopic inhomogeneity among the homologs of the resulting distribution. The model exhibits some deficiencies in explaining or predicting the δC¹³ values of ethane and propane in relation to methane in gases and of oils and associated methane. Explanations for these discrepancies may lie in the simplicity of our mathematical model, in our assumption of initial isotopic homogeneity within molecules and in our use of only n-paraffins as the source molecules for the cracking reactions.     

Radium uptake during barite recrystallization at 23 ± 2 °C as a function of solution composition: An experimental Ba and Ra tracer study

High-purity synthetic barite powder was added to pure water or aqueous solutions of soluble salts (BaCl₂, Na₂SO₄, NaCl and NaHCO₃) at 23 ± 2 °C and atmospheric pressure. After a short pre-equilibration time (4 h) the suspensions were spiked either with ¹³³Ba or ²²⁶Ra and reacted under constant agitation during 120-406 days. The pH values ranged from 4 to 8 and solid to liquid (S/L) ratios varied from 0.01 to 5 g/l. The uptake of the radiotracers by barite was monitored through repeated sampling of the aqueous solutions and radiometric analysis. For both ¹³³Ba and ²²⁶Ra, our data consistently showed a continuous, slow decrease of radioactivity in the aqueous phase.Mass balance calculations indicated that the removal of ¹³³Ba activity from aqueous solution cannot be explained by surface adsorption only, as it largely exceeded the 100% monolayer coverage limit. This result was a strong argument in favor of recrystallization (driven by a dissolution-precipitation mechanism) as the main uptake mechanism. Because complete isotopic equilibration between aqueous solution and barite was approached or even reached in some experiments, we concluded that during the reaction all or substantial fractions of the initial solid had been replaced by newly formed barite.The ¹³³Ba data could be successfully fitted assuming constant recrystallization rates and homogeneous distribution of the tracer into the newly formed barite. An alternative model based on partial equilibrium of ¹³³Ba with the mineral surface (without internal isotopic equilibration of the solid) could not reproduce the measured activity data, unless multistage recrystallization kinetics was assumed. Calculated recrystallization rates in the salt solutions ranged from 2.8 × 10⁻¹¹ to 1.9 × 10⁻¹⁰ mol m⁻² s⁻¹ (2.4-16 μmol m⁻² d⁻¹), with no specific trend related to solution composition. For the suspensions prepared in pure water, significantly higher rates (∼5.7 × 10⁻¹⁰ mol m⁻² s⁻¹ or ∼49 μmol m⁻² d⁻¹) were determined.Radium uptake by barite was determined by monitoring the decrease of ²²⁶Ra activity in the aqueous solution with alpha spectrometry, after filtration of the suspensions and sintering. The evaluation of the Ra uptake experiments, in conjunction with the recrystallization data, consistently indicated formation of non-ideal solid solutions, with moderately high Margules parameters (W_AB = 3720-6200 J/mol, a₀ = 1.5-2.5). These parameters are significantly larger than an estimated value from the literature (W_AB = 1240 J/mol, a₀ = 0.5).In conclusion, our results confirm that radium forms solid solutions with barite at fast kinetic rates and in complete thermodynamic equilibrium with the aqueous solutions. Moreover, this study provides quantitative thermodynamic data that can be used for the calculation of radium concentration limits in environmentally relevant systems, such as radioactive waste repositories and uranium mill tailings.     

Fast magnetoacoustic solitary waves in dense magnetoplasmas in a cylindrical geometry

Nonlinear properties of the quantum magnetoacoustic wave are studied in electron-ion magnetoplasmas. In this regard, cylindrical Korteweg deVries (CKdV) equation is derived for small amplitude perturbations. The solution of the planar KdV equation is obtained using the tanh method and is subsequently used as an initial profile to solve the CKdV equation. It is found that the system under consideration admits compressive solitary structures. Finally, it is found that the amplitude as well as the width of the nonplanar magnetosonic solitary structure increases with the increase in the magnetic field whereas a decrease is observed with the increase in number density of the system. The present study may be beneficial to understand the nonlinear wave propagation in nonplanar geometries in dense plasmas.     

Solar Flare Prediction Using Advanced Feature Extraction, Machine Learning, and Feature Selection

Novel machine-learning and feature-selection algorithms have been developed to study: i) the flare-prediction-capability of magnetic feature (MF) properties generated by the recently developed Solar Monitor Active Region Tracker (SMART); ii) SMART’s MF properties that are most significantly related to flare occurrence. Spatiotemporal association algorithms are developed to associate MFs with flares from April 1996 to December 2010 in order to differentiate flaring and non-flaring MFs and enable the application of machine-learning and feature-selection algorithms. A machine-learning algorithm is applied to the associated datasets to determine the flare-prediction-capability of all 21 SMART MF properties. The prediction performance is assessed using standard forecast-verification measures and compared with the prediction measures of one of the standard technologies for flare-prediction that is also based on machine-learning: Automated Solar Activity Prediction (ASAP). The comparison shows that the combination of SMART MFs with machine-learning has the potential to achieve more accurate flare-prediction than ASAP. Feature-selection algorithms are then applied to determine the MF properties that are most related to flare occurrence. It is found that a reduced set of six MF properties can achieve a similar degree of prediction accuracy as the full set of 21 SMART MF properties.