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.     

Tectonic subsidence history and thermal evolution of the Orange Basin

The Orange Basin offshore southwest Africa appears to represent a classical example of continental rifting and break up associated with large-scale, transient volcanism. The presence of lower crustal bodies of high seismic velocities indicates that large volumes of igneous crust formed as a consequence of lithospheric extension.     

Identifying Environmental and Natural Resource Management Conflict Potential Using Participatory Mapping

Methods have been proposed for identifying land use conflict potential using participatory mapping data and models. In a case study from Finland, we extend conflict mapping research by evaluating the capacity for participatory mapping to identify conflict for land uses that include mining, tourism development, commercial forestry, recreation, and nature protection. We evaluated two conflict models using reference sites where conflict was expected and assessed whether conflict potential was influenced by participant social group (resident, visitor, holiday home owner). The conflict models correctly identified the locations of current and proposed mining projects and major tourism locations (ski areas) in the region, while conflict for commercial forestry and reindeer herding was spatially distributed. Preferences for land use by social group were more similar than different across the study region. Identification of conflict potential using participatory mapping can provide a useful planning diagnostic but would benefit from additional research for validation.