The use of electron backscatter diffraction and orientation contrast imaging as tools for sulphide textural studies: example from the Greens Creek deposit (Alaska)

The Greens Creek polymetallic massive sulphide deposit is hosted in a typical polyphase deformed lower greenschist facies orogenic setting. The structure of the host rocks is well constrained, exhibiting a series of three superimposed ductile deformations followed by two brittle episodes. The ore is found both in fold hinges where early-formed depositional features are preserved and in fold limbs where primary features are typically strongly modified or obliterated. Samples from both settings have been investigated using electron backscatter diffraction (EBSD) coupled with forescatter orientation contrast (OC) imaging in order to observe the effects of deformation and lower greenschist facies metamorphism on pyrite. Results suggest that colloform pyrite may preserve information relevant to palaeoenvironment, that apparently simple textures are generally more complex, and that pyrite can deform plastically by dislocation glide and creep processes at lower temperatures and/or strain rates than generally accepted. This analysis indicates that EBSD and OC imaging provide powerful tools for observing textural relationships in pyrite that are not shown by more traditional methods. They should become routine tools for pyrite texture analysis.Editorial handling: D. Lentz     

A new laboratory set-up for measurements of electrical, hydraulic, and osmotic fluxes in clays

If clays are subjected to flows of fluid, electrical charge, chemicals, or heat, in most cases, flows of different types occur simultaneously, even if only one driving force is acting. These are so-called coupled flows. Examples of coupling phenomena are streaming potential and electroosmosis, induced by the flows of fluid and electrical charge, respectively.

Since the 1960s, laboratory devices have been constructed to measure streaming potentials and/or electroosmosis in clays or clayey soils. Due to their mechanical and hydraulic properties, clays are not easy to work with. Consequently, laboratory devices have to deal with various complications. A new design for an experimental set-up is proposed. Contrary to earlier devices, the clay sample is mounted in a flexible wall permeameter, which avoids sidewall leakage caused by the possible swell or shrink of the clay. Gold-coated gauze electrodes completely cover the surfaces of the sample, which are in contact with the solution reservoirs that ensure one-dimensional flow. In addition, the thickness of the sample is monitored during the experiment. The chemical composition of the reservoir fluids is controlled during the experiment. The device is flexible with respect to changing the solutions of both reservoirs independently, applying different hydraulic gradients, and measuring or applying electrical potentials. Finally, it is possible to mount undisturbed clay samples in the set-up, keeping them in situ during the whole experiment.

With this set-up, an extensive program of measurements of coupling phenomena like streaming potentials, electroosmosis, and membrane potentials in a sodium montmorillonite is started. Preliminary results of streaming potential measurements are presented and demonstrate that the build-up of a streaming potential due to a hydraulic gradient is a reproducible process that influences the water flow through the clay, and that the extent of the streaming potential depends on the salt concentration of the permeating solution.     

Calibration of cosmogenic Cl production rates from Ca and K spallation in lava flows from Mt. Etna (38°N, Italy) and Payun Matru (36°S, Argentina)

Published cosmogenic ³⁶Cl production rates from Ca and K spallation differ by almost a factor of 2. In this paper we determine production rates of ³⁶Cl from Ca and K in samples of known age containing little Cl. Ca-rich plagioclases and K-feldspars were separated from a total of 13 samples collected on the surfaces of four basaltic lava flows at Mt. Etna (38°N, Italy) and from a trachyte lava flow at Payun Matru volcano (36°S, Argentina). Eruption ages, determined by independent methods, range between 0.4 and 32 ka. Sample site elevations range between 500 and 2500 m. Corresponding scaling factors were calculated using five different published scaling models, four of which consider paleo-geomagnetic field variations integrated over the exposure durations. The resulting five data sets were then analyzed using a Bayesian statistical model that incorporates the major inherent uncertainties in a consistent way. Spallation production rates from Ca and K, considering all major uncertainties, are 42.2 ± 4.8 atoms ³⁶Cl (g Ca)⁻¹ a⁻¹ and 124.9 ± 8.1 atoms ³⁶Cl (g K)⁻¹ a⁻¹ normalized to sea level and high latitude using the scaling method of Stone (2000). Scaling models that account for paleo-geomagnetic intensity changes yield very similar mean values (at most +4%). If the uncertainties in the independent ages are neglected in the Bayesian model, the calculated element specific production rates would be about 12% higher. Our results are in agreement with previous production rate estimations both for Ca and K if only low Cl (i.e. ?20 ppm) samples are considered.     

On the relationship between cloud–radiation interaction, atmospheric stability and Atlantic tropical cyclones in a variable-resolution climate model

We compare two 28-year simulations performed with two versions of the Global Environmental Multiscale model run in variable-resolution mode. The two versions differ only by small differences in their radiation scheme. The most significant modification introduced is a reduction in the ice effective radius, which is observed to increase absorption of upwelling infrared radiation and increase temperature in the upper troposphere. The resulting change in vertical lapse rate is then observed to drive a resolution-dependent response of convection, which in turn modifies the zonal circulation and induces significant changes in simulated Atlantic tropical cyclone activity. The resulting change in vertical lapse rate and its implication in the context of anthropogenic climate change are discussed.     

Controls on solid-phase inclusion during porphyroblast growth: insights from the Barrovian sequence (Scottish Dalradian)

A series of Barrovian sequence samples ranging from garnet to sillimanite zones were investigated to infer their porphyroblast-forming reactions and mineral inclusion histories. Quartz is overgrown and partly consumed during garnet formation and remains as inclusion-rich layers in porphyroblasts of the garnet zone. Staurolite crystals in the staurolite zone display sharp transitions between inclusion-rich and inclusion-free areas, suggesting two stages of growth with a different role of quartz in each. The inclusion-rich domains formed similarly to those in garnet by simple overgrowth and resorption of matrix minerals, with thermodynamic constraints suggesting that this staurolite-forming reaction required the presence of chloritoid that is now absent from the examined samples. The participation of garnet was limited in staurolite formation, with chloritoid breakdown supplying sufficient material to form the large amounts (c. 25 vol%) of staurolite found in the rock. This reaction produces an excess of SiO₂, which leaves the crystal domain as SiO_2aq and thus caused the formation of the inclusion-free areas in the staurolite and precipitation of quartz in the matrix. In the sillimanite zone, staurolite is consumed forming new garnet. The newly formed garnet has less quartz inclusions than its core due to a proportionally greater consumption of quartz by the second garnet-forming reaction than by the initial, garnet-grade reactions. Textural and thermodynamic data both suggest that inclusions in these porphyroblasts represent leftovers of a preferentially overgrown matrix than co-products of the porphyroblast-forming reaction.     

Noble gases in the Xinjiang (Armanty) iron meteorite––A big object with a short cosmic‐ray exposure age

Abstract– We measured the concentrations and isotopic ratios of the cosmogenic noble gases He, Ne, and Ar in the very large iron meteorite Xinjiang (IIIE). The ³He and ⁴He data indicate that a significant portion of the cosmogenic produced helium has been lost via diffusion or in a recent impact event. High ²²Ne/²¹Ne ratios indicate that contributions to the cosmogenic ²¹Ne from sulfur and/or phosphorous are significant. By combining the measured nuclide concentrations with model calculations for iron meteorites we were able to determine the preatmospheric diameter of Xinjiang to 260–320 cm, which corresponds to a total mass of about 70–135 tons. The cosmic‐ray exposure age of Xinjiang is 62 ± 16 Ma, i.e., relatively short compared to most of the other iron meteorites. With the current database we cannot firmly determine whether Xinjiang experienced a complex irradiation history. The finding of ³He and ⁴He losses might argue for a recent impact event and therefore for a complex exposure.     

Fishing‐induced evolution of growth: concepts,mechanisms and the empirical evidence

The interest in fishing‐induced life‐history evolution has been growing in the last decade, in part because of the increasing number of studies suggesting evolutionary changes in life‐history traits, and the potential ecological and economic consequences these changes may have. Among the traits that could evolve in response to fishing, growth has lately received attention. However, critical reading of the literature on growth evolution in fish reveals conceptual confusion about the nature of ‘growth’ itself as an evolving trait, and about the different ways fishing can affect growth and size‐at‐age of fish, both on ecological and on evolutionary time‐scales. It is important to separate the advantages of being big and the costs of growing to a large size, particularly when studying life‐history evolution. In this review, we explore the selection pressures on growth and the resultant evolution of growth from a mechanistic viewpoint. We define important concepts and outline the processes that must be accounted for before observed phenotypic changes can be ascribed to growth evolution. When listing traits that could be traded‐off with growth rate, we group the mechanisms into those affecting resource acquisition and those governing resource allocation. We summarize potential effects of fishing on traits related to growth and discuss methods for detecting evolution of growth. We also challenge the prevailing expectation that fishing‐induced evolution should always lead to slower growth.     

Secondary Ion Mass Spectrometry Methodology for Isotopic Ratio Measurement of Micro‐Grains in Thin Sections: True Grain Size Estimation and Deconvolution of Inter‐Grain Size Gradients and Intra‐Grain Radial Gradients

In a thin section, grains that were approximately spherical in situ appear circular in cross section, and the distribution of apparent diameters frequently assumed to be their size distribution. Scanning ion imaging by secondary ion mass spectrometry (SIMS) is capable of providing precise (< 1‰) stable isotope ratio measurements of such grains, but, importantly, also registers their rate of evolution in apparent size as they are ablated by the primary beam. By assessing rates of radius change with depth, the described methodology enables the ‘true’ size of grains to be estimated, as well as the distance of the sectioned surface from the original grain centre. Transects in three dimensions are made possible, and this capability enables better identification (and thus separation) of both inter‐grain chemical signatures as a function of grain size, and intra‐grain radial trends. In this example, we highlight the specific application to pyrite (FeS₂) minerals, which are frequently analysed by SIMS to determine their inter‐grain and intra‐grain geochemical variations, particularly in their sulfur stable isotopic ratios (δ³⁴S). Benefits of the new methodology over the Faraday cup ‘spot mode’ are described. Data correction algorithms and precision considerations are discussed.     

Late Holocene to Recent aragonite-cemented transgressive lag deposits in the Abu Dhabi lagoon and intertidal sabkha

Modern cemented intervals (beachrock, firmgrounds to hardgrounds and concretionary layers) form in the lagoon and intertidal sabkha of Abu Dhabi. Seafloor lithification actively occurs in open, current-swept channels in low-lying areas between ooid shoals, in the intertidal zone of the middle lagoon, some centimetres beneath the inner lagoonal seafloor (i.e. within the sediment column) and at the sediment surface the intertidal sabkha. The concept of ‘concretionary sub-hardgrounds’, i.e. laminar cementation of sediments formed within the sediment column beneath the shallow redox boundary, is introduced and discussed. Based on calibrated radiocarbon ages, seafloor lithification commenced during the Middle to Late Holocene (ca 9000 cal yr bp ), and proceeds to the present-day. Lithification occurs in the context of the actualistic relative sea-level rise shifting the coastline landward across the extremely low-angle carbonate ramp. The cemented intervals are interpreted as parasequence boundaries in the sense of ‘marine flooding surfaces’, but in most cases the sedimentary cover overlying the transgressive surface has not yet been deposited. Aragonite, (micritic) calcite and, less commonly, gypsum cements lithify the firmground/hardground intervals. Cements are described and placed into context with their depositional and marine diagenetic environments and characterized by means of scanning electron microscope petrography, cathodoluminescence microscopy and Raman spectroscopy. The morphology of aragonitic cements changes from needle-shaped forms in lithified decapod burrows of the outer lagoon ooidal shoals to complex columnar, lath and platy crystals in the inner lagoon. Precipitation experiments provide first tentative evidence for the parameters that induce changes in aragonite cement morphology. Data shown here shed light on ancient, formerly aragonite-cemented seafloors, now altered to diagenetic calcites, but also document the complexity of highly dynamic near coastal depositional environments.