Late-Pleistocene and Holocene remains of Hysterocarpus traski (Tule Perch) from Clear Lake,California, and inferred Holocene temperature fluctuations

The remains of scales of Hysterocarpus traski Gibbons (Tule perch) were found throughout a 27.44-m core from Clear Lake. Most scales occurred between the mud surface and deposits approximately 11,000 years old. Changes in growth rates of the animals were examined by measuring scale annuli and applying an empirically established regression of fish length on scale radius. The data indicate a pattern of accelerating growth rates, reaching a peak between ?4000 and 2800 BP. After ?2800 BP, growth rates decline markedly. Because the growth rates of these animals are essentially dependent on temperature, the changes observed in the patterns of growth probably reflect changes in climate in the northern Coast Range. The general pattern of inferred temperature increase during the early and middle Holocene, ending between ?4000 and 2800 BP, is consistent with evidence from tree-line studies and palynology indicating higher temperatures in parts of the western United States during this period.     

An appreciation of galena concentration using a steady-state flotation model

A general-purpose computer program has been developed for the steady-state modelling of flotation plants. It permits simulation of flotation banks, made up of any number of cells, interconnected if necessary through a regrind unit. Specification of the fresh feed material in terms of the relative amounts and flotation-rate behaviour of its flotable components is necessary so that the program can then calculate the concentration of these components in all the plant streams.The use of the program for plant investigation is demonstrated using data for a lead concentration plant. The model was fitted to steady-state plant data and then tested by comparing the predictions of the model with actual plant measurements at a second feed condition. Full circuit simulation results are also presented to show how steady-state modelling can be used to give information on the likely response of the plant to other changes in feed material and the benefits which might result from rearrangements of the circuit.     

High Purge Volume Sampling—A New Paradigm for Subslab Soil Gas Monitoring

Subslab soil gas sampling and analysis is a common line of evidence for assessing human health risks associated with subsurface vapor intrusion to indoor air for volatile organic compounds; however, conventional subslab sampling methods have generated data that show substantial spatial and temporal variability, which often makes the interpretation difficult. A new method of monitoring has been developed and tested that is based on a concept of integrating samples over a large volume of soil gas extracted from beneath the floor slab of a building to provide a spatially averaged subslab concentration. Regular field screening is also conducted to assess the trend of concentration as a function of the volume removed to provide insight into the spatial distribution of vapors at progressive distances away from the point of extraction. This approach minimizes the risk of failing to identify the areas of elevated soil vapor concentrations that may exist between discrete sample locations, and can provide information covering large buildings with fewer holes drilled through the floor. The new method also involves monitoring the extraction flow rate and transient vacuum response for mathematical analysis to help interpret the vapor concentration data and to support an optimal design for any subslab venting system that may be needed.     

Formation process of pyrite polyframboid based on the heavy-metal analysis by micro-PIXE

Pyrite framboid in sedimentary rocks could be concerned with arsenic contamination in groundwater of acidic environment and has been studied for the formation process of its unique morphology. However, not much has been discussed about the formation process based on the heavy-metal distribution in pyrite framboids and their aggregates. To reveal the formation process of pyrite framboids and their aggregates, mudstone from the Late Cretaceous Hakobuchi Group, central Hokkaido, Japan, are investigated for mode, petrographical, mineralogical, and micro-PIXE (particle-induced X-ray emission) analysis in this study. Spherical and sub-spherical pyrite framboids observed in polyframboids can be divided into two types based on the diameter of framboids (D) and the average diameter of microcrystals (d) within framboids: type 1, ranging from 2 to 9 μm and from 0.4 to 0.9 μm, respectively, and type 2, ranging from 8 to 50 μm and from 0.5 to 1.8 μm, respectively. Based on the quantitative traverse analyses and 2-D elemental mapping results by micro-PIXE, heterogeneities in the concentrations of heavy metals within the analyzed aggregations of pyrite are exhibited. On the basis of the As and Pb zoning patterns, the time range and chronological stages of pyrite-aggregation growth are revealed.     

Controls on porphyroblast size along a regional metamorphic field gradient

Garnet-bearing schists from the Waterville Formation of south-central Maine provide an opportunity to examine the factors governing porphyroblast size over a range of metamorphic grade. Three-dimensional sizes and locations for all garnet porphyroblasts were determined for three samples along the metamorphic field gradient spanning lowest garnet through sillimanite grade, using high-resolution X-ray computed tomography. Comparison of crystal size distributions to previous data sets obtained by stereological methods for the same samples reveals significant differences in mode, mean, and shape of the distributions. Quantitative textural analysis shows that the garnets in each rock crystallized in a diffusion-controlled nucleation and growth regime. In contrast to the typical observation of a correlation between porphyroblast size and position along a metamorphic field gradient, porphyroblast size of the lowest-grade specimen is intermediate between the high- and middle-grade specimens’ sizes. Mean porphyroblast size does not correlate with peak temperatures from garnet-biotite Fe-Mg exchange thermometry, nor is post-crystallization annealing (Ostwald Ripening) required to produce the observed textures, as was previously proposed for these rocks. Robust pseudosection calculations fail to reproduce the observed garnet core compositions for two specimens, suggesting that these calc-pelites experienced metasomatism. For each of these two specimens, Monte Carlo calculations suggest potential pre-metasomatism bulk compositions that replicate garnet core compositions. Pseudosection analyses allow the estimation of the critical temperatures for garnet growth: ∼481, ∼477, and ∼485°C for the lowest-garnet-zone, middle-garnet-zone, and sillimanite-zone specimens, respectively. Porphyroblast size appears to be determined in this case by a combination of the heating rate during garnet crystallization, the critical temperature for the garnet-forming reaction and the kinetics of nucleation. Numerical simulations of thermally accelerated, diffusion-controlled nucleation, and growth for the three samples closely match measured crystal size distributions. These observations and simulations suggest that previous hypotheses linking the garnet size primarily to the temperature at the onset of porphyroblast nucleation can only partially explain the observed textures. Also important in determining porphyroblast size are the heating rate and the distribution of favorable nucleation sites.     

Oxidation of pyrite during extraction of carbonate associated sulfate

The sulfur isotopic composition of carbonate associated sulfate (CAS) has been used to investigate the geochemistry of ancient seawater sulfate. However, few studies have quantified the reliability of δ³⁴S of CAS as a seawater sulfate proxy, especially with respect to later diagenetic overprinting. Pyrite, which typically has depleted δ³⁴S values due to authigenic fractionation associated with bacterial sulfate reduction, is a common constituent of marine sedimentary rocks. The oxidation of pyrite, whether during diagenesis or sample preparation, could thus adversely influence the sulfur isotopic composition of CAS. Here, we report the results of CAS extractions using HCl and acetic acid with samples spiked with varying amounts of pyrite. The results show a very strong linear relationship between the abundance of fine-grained pyrite added to the sample and the resultant abundance and δ³⁴S value of CAS. This data represents the first unequivocal evidence that pyrite is oxidized during the CAS extraction process. Our mixing models indicate that in samples with much less than 1 wt.% pyrite and relatively high δ³⁴S_pyrite values, the isotopic offset imparted by oxidation of pyrite should be much less than ? 4‰. A wealth of literature exists on the oxidation of pyrite by Fe³⁺ and we believe this mechanism drives the oxidation of pyrite during CAS extraction, during which the oxygen used to form sulfate is taken from H₂O, not O₂. Consequently, extracting CAS under anaerobic conditions would only slow, but not halt, the oxidation of pyrite. Future studies of CAS should attempt to quantify pyrite abundance and isotopic composition.     

Fluid sources for the La Guitarra epithermal deposit (Temascaltepec district, Mexico): Volatile and helium isotope analyses in fluid inclusions

The La Guitarra deposit (Temascaltepec district, South-Central Mexico), belongs to the low/intermediate sulfidation epithermal type, has a polymetallic character although it is currently being mined for Ag and Au. The mineralization shows a polyphasic character and formed through several stages and sub-stages (named I, IIA, IIB, IIC, IID, and III). The previous structural, mineralogical, fluid inclusion and stable isotope studies were used to constrain the selection of samples for volatile and helium isotope analyses portrayed in this study. The N₂/Ar overall range obtained from analytical runs on fluid inclusion volatiles, by means of Quadrupole Mass Spectrometry (QMS), is 0 to 2526, and it ranges 0 to 2526 for stage I, 0 to 1264 for stage IIA, 0 to 1369 for stage IIB, 11 to 2401 for stage IIC, 19 to 324 for stage IID, and 0 to 2526 for stage III. These values, combined with the CO₂/CH₄ ratios, and N₂-He-Ar and N₂-CH₄-Ar relationships, suggest the occurrence of fluids from magmatic, crustal, and shallow meteoric sources in the forming epithermal vein deposit. The helium isotope analyses, obtained by means of Noble Gas Mass Spectrometry, display R/Ra average values between 0.5 and 2, pointing to the occurrence of mantle-derived helium that was relatively diluted or “contaminated” by crustal helium. These volatile analyses, when correlated with the stable isotope data from previous works and He isotope data, show the same distribution of data concerning sources for mineralizing fluids, especially those corresponding to magmatic and crustal sources. Thus, the overall geochemical data from mineralizing fluids are revealed as intrinsically consistent when compared to each other.The three main sources for mineralizing fluids (magmatic, and both deep and shallow meteoric fluids) are accountable at any scale, from stages of mineralization down to specific mineral associations. The volatile and helium isotope data obtained in this paper suggest that the precious metal-bearing mineral associations formed after hydrothermal pulses of predominantly oxidized magmatic fluids, and thus it is likely that precious metals were carried by fluids with such origin. Minerals from base-metal sulfide associations record both crustal and magmatic sources for mineralizing fluids, thus suggesting that base metals could be derived from deep leaching of crustal rocks. At the La Guitarra epithermal deposit there is no evidence for an evolution of mineralizing fluids towards any dominant source. Rather than that, volatile analyses in fluid inclusions suggest that this deposit formed as a pulsing hydrothermal system where each pulse or set of pulses accounts for different compositions of mineralizing fluids.The positive correlation between the relative content of magmatic fluids (high N₂/Ar ratios) and H₂S suggests that the necessary sulfur to carry mostly gold as bisulfide complexes came essentially from magmatic sources. Chlorine necessary to carry silver and base metals was found to be abundant in inclusion fluids and although there is no evidence about its source, it is plausible that it may come from magmatic sources as well.     

The effect of iron on montmorillonite stability. (I) Background and thermodynamic considerations

It is envisaged that high-level nuclear waste (HLW) will be disposed of in underground repositories. Many proposed repository designs include steel waste canisters and bentonite backfill. Natural analogues and experimental data indicate that the montmorillonite component of the backfill could react with steel corrosion products to produce non-swelling Fe-rich phyllosilicates such as chamosite, berthierine, or Fe-rich smectite. In K-bearing systems, the alteration of montmorillonite to illite/glauconite could also be envisaged. If montmorillonite were altered to non-swelling minerals, the swelling capacity and self-healing properties of the bentonite backfill could be reduced, thereby diminishing backfill performance. The main aim of this paper was to investigate Fe-rich phyllosilicate mineral stability at the canister-backfill interface using thermodynamic modelling. Estimates of thermodynamic properties were made for Fe-rich clay minerals in order to construct approximate phase-relations for end-member/simplified mineral compositions in logarithmic activity space. Logarithmic activity diagrams (for the system Al₂O₃-FeO-Fe₂O₃-MgO-Na₂O-SiO₂-H₂O) suggest that if pore waters are supersaturated with respect to magnetite in HLW repositories, Fe(II)-rich saponite is the most likely montmorillonite alteration product (if f_O2(g) values are significantly lower than magnetite-hematite equilibrium). Therefore, the alteration of montmorillonite may not be detrimental to nuclear waste repositories that include Fe, as long as the swelling behaviour of the Fe-rich smectite produced is maintained. If f_O2(g) exceeds magnetite-hematite equilibrium, and solutions are saturated with respect to magnetite in HLW repositories, berthierine is likely to be more stable than smectite minerals. The alteration of montmorillonite to berthierine could be detrimental to the performance of HLW repositories.