Macroscale NTIMS and microscale LA-MC-ICP-MS Re-Os isotopic analysis of molybdenite: Testing spatial restrictions for reliable Re-Os age determinations, and implications for the decoupling of Re and Os within molybdenite

We present a detailed study of Re-Os age determinations for eight natural molybdenite samples of like polytype (2H), spanning a range of age, natural grain size and deposit type. The focus of the study is to critically evaluate the effects of sampling, sample preparation and aliquant size on the accuracy and reproducibility of Re-Os ages for these molybdenite samples. We find that for some molybdenite samples, analysis of small sample aliquants (<20 mg) may not yield accurate or reproducible Re-Os ages, whereas analysis of larger aliquants from the same mineral separate do yield reproducible Re-Os dates. Such an observation is best explained if Re and ¹⁸⁷Os are internally decoupled within molybdenite grains. This finding is supported from spot analyses by laser ablation MC-ICP-MS analyses presented here and is consistent with previously published observations.The degree of decoupling between Re and ¹⁸⁷Os appears to increase both as a function of increasing grain size, and increasing age of molybdenite. From detailed dating of individual molybdenite mineral separates, we provide approximate minimum aliquant amounts required for reproducible Re-Os age dating, as a function of molybdenite age and grain size. Geologically younger, naturally fine-grained molybdenite samples appear to show little Re and ¹⁸⁷Os decoupling, and reproducible ages can be determined from some samples with as little as 1 mg of aliquant. Geologically old, and coarse-grained molybdenite samples may require as much as 40 mg of aliquant from a much larger mineral separate to overcome Re and ¹⁸⁷Os decoupling. The mechanism(s) of Re and ¹⁸⁷Os decoupling within molybdenite is not constrained by this results of this study, but the observation that the degree of decoupling increases with grain size (distance) and age (time/geologic history) may suggest primary diffusive control. Assuming that Re and ¹⁸⁷Os decoupling in molybdenite results primarily from diffusion of ¹⁸⁷Os, apparent diffusion coefficients are calculated (D = x²/t). Estimates of D for Os made in this way range from 2.8 × 10⁻²⁶ to 2.1 × 10⁻²¹ m²/s, which are broadly similar to experimentally derived diffusion coefficients for Os in Fe-sulfide minerals and for Re in molybdenite at temperatures <500°C.     

Geochemical studies in the drainage basin of the Rio Vouga,Portugal. IV. Impact of land use on the hydrogeochemistry of natural waters in the Vouzela region

A hydrogeochemical study of the impact of land use on the composition of natural waters in the Vouzela region in northern Portugal was carried out during the summer of 1983. Water samples were collected from spring and major streams in the area and analyzed for major constituents and some trace elements. Analysis of variance and subsequent pairwise contrast tests demonstrated that waters from agricultural areas are significantly enriched in constiluents such as Na, K, Ca, Mg, Sr, Cl, SO₄, and NO₃. High concentrations of these ions in agricultural areas are the result of both the application of fertilizers and enhanced evaporation by the intensive irrigation of agricultural lands. Constitutents such as H₄SiO₄, HCO₃, F, and probably Li, which are typically related to mineral weathering, were not affected by land use. In waters from forested areas the concentration of most constituents was about 20% higher than in waters from uncultivated areas with mainly a grass cover. This reflects the difference in the evapotranspiration of these two vegetation types. The chemistry of the Rio Zela clearly reflects differences in land use in the Rio Zela valley.     

Habitat partitioning between the xanthid crabsPanopeus herbstii andEurypanopeus depressus on intertidal oyster reefs (Crassostrea virginica) in southeastern North Carolina

The abundances of the xanthid crabsPanopeus herbstii andEurypanopeus depressus were examined relative to surface oyster shell cover, surface oyster cluster volume, subsurface shell content, substrate sand and silt composition, and oyster reef elevation. During August 1986 through July 1987, xanthid crabs were collected monthly from twelve 0.25 m²×15 cm deep quadrats, during low tide, from intertidal oyster reefs in Mill Creek, Pender County, North Carolina, USA, with respective quadrat details recorded. The abundance ofP. herbstii, and to a lesser degreeE. depressus, was positively correlated with surface shell cover. The abundance ofE. depressus, and to a lesser degreeP. herbstii, was positively correlated with surface cluster volume. The majority ofP. herbstii inhabited the subsurface stratum of the oyster reef, whereas the majority ofE. depressus inhabited the cluster stratum. Seasonality (i.e., temperature) appeared to influence the strata habitation of both species, with a higher incidence of cluster habitation during warmer months and a lower incidence during colder months. Crab abundance was not related to other factors examined, such as subsurface shell, substrate sand and silt composition, or elevation within the oyster reef. The analyses show thatP. herbstii andE. depressus have partitioned the intertidal oyster reef habitat, withE. depressus exploiting surface shell clusters andP. herbstii the subsurface stratum.     

Tidal river sediments in the Washington,D.C. area. III. Biological effects associated with sediment contamination

Sediment toxicity and benthic macroinvertebrate community structure were measured as one component of a study cohceived to determine the distribution and effect of sediment contamination in tidal freshwater portions of the Potomac and Anacostia rivers in the Washington, D.C., area. Samples were collected at 15 sites. Analyses included a partial life cycle (28 d) whole sediment test using the amphipod Hyalella azteca (Talitridae) and an assessment of benthic community structure. Survival and growth (as estimated by amphipod length) were experimental endopoints for the toxicity test. Significant mortality was observed in 5 of 10 sites in the lower Anacostia River basin and at the main channel Potomac River site. Sublethal toxicity, as measured by inhibition of amphipod growth, was not observed. Toxicity test results were in general agreement with synoptically measured sediment contaminant concentrations. Porewater total ammonia (NH₃+NH₄ ⁺) appears to be responsible for the toxicity of sediments from the Potomac River, while correlation analysis and simultaneously extracted metals: acid volatile sulfide (SEM∶AVS) results suggest that the toxicity associated with Anacostia River sediments was due to organic compounds. Twenty-eight macroinvertebrate taxa were identified among all sites, with richness varying from 5 to 17 taxa per site. Groups of benthic assemblages identified by group-average cluster analysis exhibited variable agreement with sediment chemical and sediment toxicity results. Integration of toxicological, chemical, and ecological components suggests that adverse environmental effects manifest in the lower Anacostia River benthos result from chemical contamination of sediment.     

Possible effects of pre-existing basement topography on thrust fault ramping

Finite-element models show that one way in which thrust ramps may arise is through the mechanical interaction between basement and overlying sediments. In the simplest case, shear coupling between a planar basement—sediment contact causes the differential stresses in the sediments to die out with depth and distance from the applied load. For such cases, curved thrust faults may result if the strength of the rock is exceeded. Basement topography may also affect the location and shape of ramps by acting as a stress concentrator, by producing a stress shadow and by changing principal stress orientations. Modeling suggests that whether or not these basement topographic features cause ramping will depend on the height and angularity of the feature as well as the rock types that overlie it.Under the assumption of linear elasticity and for given boundary conditions, the Poisson's ratio plays an important role in determining the orientation and magnitude of the principal stresses. Calculations using experimentally measured Poisson's ratios predict that the earliest maximum compressive stress directions should be nearly vertical in the more cratonward portions of thrust belts. However, the stress directions which are inferred to have occurred earliest in this part of thrust belts are nearly horizontal. This suggests that non-elastic or ductile processes have an effect on the propagation of thrust faults.     

Petrographic constraints on models of intergranular pressure solution in quartzose sandstones

Petrographic and SEM observations on 478 samples of six quartzose sandstones provide a data base that can be used to evaluate the role of intergranular pressure solution (IPS) in sandstone diagenesis and to constrain predictive models of the pressure solution process. SEM examination of grain contacts that have experienced pressure solution suggests that IPS occurs at the interior portions of contacts where the greatest stress is concentrated and that granulation of quartz grains at points of contact may contribute to the process. The chemical compaction fabrics that result from IPS suggest that the process is most commonly induced by effective lithostatic stress and that the resulting strain is uniaxial.Numerous geological variables influence IPS. Grain size exerts a fundamental influence, with finer grained samples experiencing more IPS than coarser grained samples. On both local and regional scales, IPS is inhibited by poor sorting, an abundance of ductile grains, the presence of “shallow” cement, slow rates of shallow burial, and overpressured conditions. In contrast, IPS is enhanced by the presence of illite grain coatings, increased maximum burial depth, rapid rates of shallow burial, longer times spent at great depths, higher temperatures, and high volumes and rates of fluid flow.Silica budgets indicate that some of the analyzed sandstones approximate mass balance whereas others have exported silica. Calculations of fluid flow requirements indicate that advanced stages of IPS are favored by high volume, high velocity fluid flow. Such flow can occur as a result of uplift of basin margins, which is typical of foreland and intractonic basins.     

Reactivated strike–slip faults: examples from north Cornwall, UK

Several strike–slip faults at Crackington Haven, UK show evidence of right-lateral movement with tip cracks and dilatational jogs, which have been reactivated by left-lateral strike–slip movement. Evidence for reactivation includes two slickenside striae on a single fault surface, two groups of tip cracks with different orientations and very low displacement gradients or negative (left-lateral) displacements at fault tips.

Evidence for the relative age of the two strike–slip movements is (1) the first formed tip cracks associated with right-lateral slip are deformed, whereas the tip cracks formed during left-lateral slip show no deformation; (2) some of the tip cracks associated with right-lateral movement show left-lateral reactivation; and (3) left-lateral displacement is commonly recorded at the tips of dominantly right-lateral faults.

The orientation of the tip cracks to the main fault is 30–70° clockwise for right-lateral slip, and 20–40° counter-clockwise for left-lateral slip. The structure formed by this process of strike–slip reactivation is termed a “tree structure” because it is similar to a tree with branches. The angular difference between these two groups of tip cracks could be interpreted as due to different stress distribution (e.g., transtensional/transpressional, near-field or far-field stress), different fracture modes or fractures utilizing pre-existing planes of weakness.

Most of the d–x profiles have similar patterns, which show low or negative displacement at the segment fault tips. Although the d–x profiles are complicated by fault segments and reactivation, they provide clear evidence for reactivation. Profiles that experienced two opposite slip movements show various shapes depending on the amount of displacement and the slip sequence. For a larger slip followed by a smaller slip with opposite sense, the profile would be expected to record very low or reverse displacement at fault tips due to late-stage tip propagation. Whereas for a smaller slip followed by larger slip with opposite sense, the d–x profile would be flatter with no reverse displacement at the tips. Reactivation also decreases the ratio of d_max/L since for an original right-lateral fault, left lateral reactivation will reduce the net displacement (d_max) along a fault and increase the fault length (L).

Finally we compare Crackington Haven faults with these in the Atacama system of northern Chile. The Salar Grande Fault (SGF) formed as a left-lateral fault with large displacement in its central region. Later right-lateral reactivation is preserved at the fault tips and at the smaller sub-parallel Cerro Chuculay Fault. These faults resemble those seen at Crackington Haven.     

Patterns in seasonal abundance,growth and biomass of the atlantic silverside,Menidia menidia,in a New England estuary

Patterns in seasonal abundance (no. per m² surface area), growth and biomass (g per m² surface area) of an annual fish, the Atlantic silverside, Menidia menidia (L.) were investigated in a marsh and more seaward bay region of Essex Bay, Massachusetts from August 1976 to May 1978 using a quantitative beach seining technique. Silverside abundance varied greatly by season and year class during the study period. Abundance was high in 1976 but winter mortality (99%) left an adult density of only .01 per m² surface area in the marsh during spring 1977. Resultant 1977 year class density in the marsh was 1.88 per m² by late fall 1977 but winter mortality again produced an adult density of .01 per m² in spring 1978. Abundance was generally higher in the marsh than in the bay region especially during spring and late fall when catches in the bay were negligible. Based on catch rate comparisons, the summer and fall juvenile abundance of the 1976 year class was much higher than the juvenile abundance of the 1977 year class. Coincidentally, mean lengths and condition of the abundant 1976 year class in the late fall were significantly lower than those of the 1977 year class, suggesting density dependent population regulation. In both years, juveniles grew rapidly and reached full adult size by November when an offshore movement to deeper waters outside Essex Bay occurred. Biomass peaked in the marsh region in late fall 1977 at 7.8 g per m² wet weight. Winter mortality was size selective, favoring larger individuals. The annual life history design of M. menidia including an offshore winter movement and high winter mortality suggests that silversides represent an important pathway of energy flow from marsh to offshore trophic systems.