Metasomatism in the Ultrahigh-pressure Svartberget Garnet-peridotite (Western Gneiss Region, Norway): Implications for the Transport of Crust-derived Fluids within the Mantle

Garnet-peridotites often contain veins or layers of pyroxenite and eclogite of uncertain origin. We investigate the Svartberget garnet-peridotite from the northernmost ultrahigh-pressure domain in the Western Gneiss Region (WGR) in Norway and show that the observed layering represents a sequence of metasomatic reaction zones developed along a fracture system. From the garnet-peridotite wall-rock to the fractures the following sequential reaction zones are recognized: clinohumite bearing garnet-peridotite, olivine-garnet-websterite, garnet-websterite, orthopyroxene-phlogopite-garnet-websterite, coarse-grained phlogopite-garnet-websterite, phlogopite-garnet-websterite, phlogopite-free garnet-websterite, inclusion-rich garnetite, garnetite, eclogite, retrograde omphacitite and felsic amphibole-pegmatite. The MgO, FeO and CaO contents generally decrease from the pristine peridotite towards the most metasomatized samples, with an associated increase in SiO2 and Al2O3. Concentrations of fluid-mobile elements increase from the most pristine peridotite towards the garnetite, whereas Ni and Cr decrease from ∼700 to ∼10 ppm and ∼2600 to ∼25 ppm, respectively. Changes in mineral mode are accompanied by changes in mineral chemistry. All minerals display decreasing Mg# and Cr content with degree of metasomatism, whereas Na2O concentrations in amphibole, and most notably in clinopyroxene, increase from 0·2 to 3·0 and from 0·2 to 8 wt %, respectively. The trivalent ions Cr and Al display complex intra-granular vein-like or patchy zoning in garnet and pyroxenes that may be characteristic of metasomatized peridotites. Dating by the U-Pb method suggests metamorphic growth of zircon in the garnetite at 397·2 ± 1·2 Ma, formation of leucosomes in host-rock gneiss at 391·2 ± 0·8 Ma, and amphibole-pegmatite in the core of a garnetite vein at 390·1 ± 0·9 Ma. Initial 87Sr/86Sr values calculated at 397 Ma are elevated (∼0·723) in the most pristine peridotites and increase to ∼0·743 in the most metasomatized samples. The initial 87Sr/86Sr values of both the host gneiss and its leucosomes are also elevated (0·734-0·776), which suggests that the leucosomes found in the gneisses are the most likely, now solidified, remnants of the reactive agent that metasomatized the Svartberget peridotite. A scenario is envisaged in which material derived from the country rock gneiss was the source of the metasomatic addition of elements to the peridotites and the gneisses acted as the host for all elements removed from the peridotite. The Svartberget peridotite may provide an important analogue of how felsic, slab-derived material interacts with the overlying mantle wedge peridotite in regions of arc magma generatio

Metasomatism in the Ultrahigh-pressure Svartberget Garnet-peridotite (Western Gneiss Region, Norway): Implications for the Transport of Crust-derived Fluids within the Mantle

Garnet-peridotites often contain veins or layers of pyroxenite and eclogite of uncertain origin. We investigate the Svartberget garnet-peridotite from the northernmost ultrahigh-pressure domain in the Western Gneiss Region (WGR) in Norway and show that the observed layering represents a sequence of metasomatic reaction zones developed along a fracture system. From the garnet-peridotite wall-rock to the fractures the following sequential reaction zones are recognized: clinohumite bearing garnet-peridotite, olivine–garnet-websterite, garnet-websterite, orthopyroxene–phlogopite–garnet-websterite, coarse-grained phlogopite–garnet-websterite, phlogopite–garnet-websterite, phlogopite-free garnet-websterite, inclusion-rich garnetite, garnetite, eclogite, retrograde omphacitite and felsic amphibole-pegmatite. The MgO, FeO and CaO contents generally decrease from the pristine peridotite towards the most metasomatized samples, with an associated increase in SiO2 and Al2O3. Concentrations of fluid-mobile elements increase from the most pristine peridotite towards the garnetite, whereas Ni and Cr decrease from ∼700 to ∼10 ppm and ∼2600 to ∼25 ppm, respectively. Changes in mineral mode are accompanied by changes in mineral chemistry. All minerals display decreasing Mg# and Cr content with degree of metasomatism, whereas Na2O concentrations in amphibole, and most notably in clinopyroxene, increase from 0·2 to 3·0 and from 0·2 to 8 wt %, respectively. The trivalent ions Cr and Al display complex intra-granular vein-like or patchy zoning in garnet and pyroxenes that may be characteristic of metasomatized peridotites. Dating by the U–Pb method suggests metamorphic growth of zircon in the garnetite at 397·2 ± 1·2 Ma, formation of leucosomes in host-rock gneiss at 391·2 ± 0·8 Ma, and amphibole-pegmatite in the core of a garnetite vein at 390·1 ± 0·9 Ma. Initial 87Sr/86Sr values calculated at 397 Ma are elevated (∼0·723) in the most pristine peridotites and increase to ∼0·743 in the most metasomatized samples. The initial 87Sr/86Sr values of both the host gneiss and its leucosomes are also elevated (0·734–0·776), which suggests that the leucosomes found in the gneisses are the most likely, now solidified, remnants of the reactive agent that metasomatized the Svartberget peridotite. A scenario is envisaged in which material derived from the country rock gneiss was the source of the metasomatic addition of elements to the peridotites and the gneisses acted as the host for all elements removed from the peridotite. The Svartberget peridotite may provide an important analogue of how felsic, slab-derived material interacts with the overlying mantle wedge peridotite in regions of arc magma generation

Airborne laser scanning reveals uniform responses of forest structure to moose (Alces alces) across the boreal forest biome

1. The moose Alces alces is the largest herbivore in the boreal forest biome, where it can have dramatic impacts on ecosystem structure and dynamics. Despite the importance of the boreal forest biome in global carbon cycling, the impacts of moose have only been studied in disparate regional exclosure experiments, leading to calls for common analyses across a biome-wide network of moose exclosures. 2. In this study, we use airborne laser scanning (ALS) to analyse forest canopy re-sponses to moose across 100 paired exclosure-control experimental plots dis-tributed across the boreal biome, including sites in the United States (Isle Royale), Canada (Quebec, Newfoundland), Norway, Sweden and Finland. 3. We test the hypotheses that canopy height, vertical complexity and above- ground biomass (AGB) are all reduced by moose and that the impacts vary with moose density, productivity, temperature and pulse disturbances such as logging and insect outbreaks. 4. We find a surprising convergence in forest canopy response to moose. Moose had negative impacts on canopy height, complexity and AGB as expected. The responses of canopy complexity and AGB were consistent across regions and did not vary along environmental gradients. The difference in canopy height be-tween exclosures and open plots was on average 6 cm per year since the start of exclosure treatment (±2.1 SD). This rate increased with temperature, but only when moose density was high. 5. The difference in AGB between moose exclosures and open plots was 0.306 Mg ha−1 year−1 (±0.079). In browsed plots, stand AGB was 32% of that in the exclosures, a difference of 2.09 Mg ha−1. The uniform response allows scaling of the estimate to a biome-wide impact of moose of the loss of 448 (±115) Tg per year, or 224 Tg of carbon. 6. Synthesis: Analysis of ALS data from distributed exclosure experiments identified a largely uniform response of forest canopies to moose across regions, facilitat-ing scaling of moose impacts across the whole biome. This is an important step towards incorporating the effect of the largest boreal herbivore on the carbon cycling of one of the world's largest terrestrial biomes.publishedVersio

Evaluating the importance of metamorphism in the foundering of continental crust

The metamorphic conditions and mechanisms required to induce foundering in deep arc crust are assessed using an example of representative lower crust in SW New Zealand. Composite plutons of Cretaceous monzodiorite and gabbro were emplaced at ~1.2 and 1.8 GPa are parts of the Western Fiordland Orthogneiss (WFO); examples of the plutons are tectonically juxtaposed along a structure that excised ~25 km of crust. The 1.8 GPa Breaksea Orthogneiss includes suitably dense minor components (e.g. eclogite) capable of foundering at peak conditions. As the eclogite facies boundary has a positive dP/dT, cooling from supra-solidus conditions (T > 950 ºC) at high-P should be accompanied by omphacite and garnet growth. However, a high monzodioritic proportion and inefficient metamorphism in the Breaksea Orthogneiss resulted in its positive buoyancy and preservation. Metamorphic inefficiency and compositional relationships in the 1.2 GPa Malaspina Pluton meant it was never likely to have developed densities sufficiently high to founder. These relationships suggest that the deep arc crust must have primarily involved significant igneous accumulation of garnet–clinopyroxene (in proportions >75%). Crustal dismemberment with or without the development of extensional shear zones is proposed to have induced foundering of excised cumulate material at P > 1.2 GPa

Reaction mechanism for the replacement of calcite by dolomite and siderite: Implications for geochemistry, microstructure and porosity evolution during hydrothermal mineralisation

Carbonate reactions are common in mineral deposits due to CO2-rich mineralising fluids. This study presents the first in-depth, integrated analysis of microstructure and microchemistry of fluid-mediated carbonate reaction textures at hydrothermal conditions. In doing so, we describe the mechanisms by which carbonate phases replace one another, and the implications for the evolution of geochemistry, rock microstructures and porosity. The sample from the 1.95 Moz Junction gold deposit, Western Australia, contains calcite derived from carbonation of a metamorphic amphibole—plagioclase assemblage that has further altered to siderite and dolomite. The calcite is porous and contains iron-rich calcite blebs interpreted to have resulted from fluid-mediated replacement of compositionally heterogeneous amphiboles. The siderite is polycrystalline but nucleates topotactically on the calcite. As a result, the boundaries between adjacent grains are low-angle boundaries (<10°), which are geometrically similar to those formed by crystal–plastic deformation and recovery. Growth zoning within individual siderite grains shows that the low-angle boundaries are growth features and not due to deformation. Low-angle boundaries develop due to the propagation of defects at grain faces and zone boundaries and by impingement of grains that nucleated with small misorientations relative to each other during grain growth.The cores of siderite grains are aligned with the twin planes in the parent calcite crystal showing that the reactant Fe entered the crystal along the twin boundaries. Dolomite grains, many of which appear to in-fill space generated by the siderite replacement, also show alignment of cores along the calcite twin planes, suggesting that they did not grow into space but replaced the calcite. Where dolomite is seen directly replacing calcite, it nucleates on the Fe-rich calcite due to the increased compatibility of the Fe-bearing calcite lattice relative to the pure calcite. Both reactions are interpreted as fluid-mediated replacement reactions which use the crystallography and elemental chemistry of the calcite. Experiments of fluid-mediated replacement reactions show that they proceed much faster than diffusion-based reactions. This is important when considering the rates of reactions relative to fluid flow in mineralising systems

Worm control practice against gastro-intestinal parasites in Norwegian sheep and goat flocks

<p>Abstract</p> <p>Background</p> <p>Anthelmintic treatment is the most common way of controlling nematode infections in ruminants. However, several countries have reported anthelmintic resistance (AR), representing a limitation for sustainable small ruminant production. The knowledge regarding worm control management represents a baseline to develop a guideline for preventing AR. The aim of the present study was therefore to improve our knowledge about the worm control practices in small ruminant flocks in Norway.</p> <p>Methods</p> <p>A questionnaire survey regarding worm control practices was performed in small ruminant flocks in Norway. Flocks were selected from the three main areas of small ruminant farming, i.e. the coastal, inland and northern areas. A total of 825 questionnaires, comprising 587 sheep flocks (return rate of 51.3%) and 238 goat flocks (52.6%) were included.</p> <p>Results</p> <p>The results indicated that visual appraisal of individual weight was the most common means of estimating the anthelmintic dose used in sheep (78.6%) and goat (85.1%) flocks. The mean yearly drenching rate in lambs and ewes were 2.5 ± 1.7 and 1.9 ± 1.1, respectively, whereas it was 1.0 (once a year) in goats. However, these figures were higher in sheep in the coastal area with a rate of 3.4 and 2.2 in lambs and ewes, respectively. Benzimidazoles were the predominant anthelmintic class used in sheep flocks (64.9% in 2007), whereas benzimidazoles and macrocyclic lactones were both equally used in dairy goat flocks. In the period of 2005-2007, 46.3% of the sheep flocks never changed the anthelmintic class. The dose and move strategy was practiced in 33.2% of the sheep flocks.</p> <p>Conclusions</p> <p>The present study showed that inaccurate weight calculation gives a risk of under-dosing in over 90% of the sheep and goat flocks in Norway. Taken together with a high treatment frequency in lambs, a lack of anthelmintic class rotation and the common use of a dose-and-move strategy, a real danger for development of anthelmintic resistance (AR) seems to exist in Norwegian sheep and goat flocks. This risk seems particularly high in coastal areas where high treatment frequencies in lambs were recorded.</p

Patterns of ash (Fraxinus excelsior L.) colonization in mountain grasslands: the importance of management practices

International audienceWoody colonization of grasslands is often associated with changes in abiotic or biotic conditions or a combination of both. Widely used as fodder and litter in the past traditional agro-pastoral system, ash (Fraxinus excelsior L.) has now become a colonizing species of mountain grasslands in the French Pyrenees. Its present distribution is dependent on past human activities and it is locally controlled by propagule pressure and abiotic conditions. However, even when all favourable conditions are met, all the potentially colonizable grasslands are not invaded. We hypothesize that management practices should play a crucial role in the control of ash colonization. From empirical field surveys we have compared the botanical composition of a set of grasslands (present and former) differing in management practices and level of ash colonization. We have displayed a kind of successional gradient positively linked to both ash cover and height but not to the age of trees. We have tested the relationships between ash presence in grassland and management types i.e. cutting and/or grazing, management intensity and some grassland communities' features i.e. total and local specific richness and species heterogeneity. Mixed use (cutting and grazing) is negatively linked to ash presence in grassland whereas grazing alone positively. Mixed use and high grazing intensity are directly preventing ash seedlings establishment, when low grazing intensity is allowing ash seedlings establishment indirectly through herbaceous vegetation neglected by livestock. Our results show the existence of a limit between grasslands with and without established ashes corresponding to a threshold in the intensity of use. Under this threshold, when ash is established, the colonization process seems to become irreversible. Ash possesses the ability of compensatory growth and therefore under a high grazing intensity develops a subterranean vegetative reproduction. However the question remains at which stage of seedling development and grazing intensity these strategies could occur

Growth rings show limited evidence for ungulates' potential to suppress shrubs across the Arctic

Global warming has pronounced effects on tundra vegetation, and rising mean temperatures increase plant growth potential across the Arctic biome. Herbivores may counteract the warming impacts by reducing plant growth, but the strength of this effect may depend on prevailing regional climatic conditions. To study how ungulates interact with temperature to influence growth of tundra shrubs across the Arctic tundra biome, we assembled dendroecological data from 20 sites, comprising 1153 individual shrubs and 223 63 annual growth rings. Evidence for ungulates suppressing shrub radial growth was only observed at intermediate summer temperatures (6.5 degrees C-9 degrees C), and even at these temperatures the effect was not strong. Multiple factors, including forage preferences and landscape use by the ungulates, and favourable climatic conditions enabling effective compensatory growth of shrubs, may weaken the effects of ungulates on shrubs, possibly explaining the weakness of observed ungulate effects. Earlier local studies have shown that ungulates may counteract the impacts of warming on tundra shrub growth, but we demonstrate that ungulates' potential to suppress shrub radial growth is not always evident, and may be limited to certain climatic conditions