Petrogenesis of peralkaline granite dykes of the Straumsvola complex,western Dronning Maud Land,Antarctica

Peralkaline syenite and granite dykes cut the Straumsvola nepheline syenite pluton in Western Dronning Maud Land, Antarctica. The average peralkalinity index (PI?=?molecular Al/[Na?+?K]) of the dykes is 1.20 (n?=?29) and manifests itself in the presence of the Zr silicates eudialyte, dalyite and vlasovite, and the Na–Ti silicate, narsarsukite. The dykes appear to have intruded during slow cooling of the nepheline syenite pluton, and the petrogenetic relationship of the dykes and the pluton cannot be related to closed-system processes at low pressure, given the thermal divide that exists between silica-undersaturated and oversaturated magmas. Major and trace element variations in the dykes are consistent with a combination of fractional crystallization of parental peralkaline magma of quartz trachyte composition, and internal mineral segregation prior to final solidification. The distribution of accessory minerals is consistent with late-stage crystallization of isolated melt pockets. The dykes give an Rb–Sr isochron age of 171?±?4.4 Ma, with variable initial ⁸⁷Sr/⁸⁶Sr ratio (0.7075?±?0.0032), and have an average ε _Nd of ? 12.0. Quartz phenocrysts have δ¹⁸O values of 8.4–9.2‰, which are generally in O-isotope equilibrium with bulk rock. Differences in the δ¹⁸O values of quartz and aegirine (average Δ_{quartz?aegirine} = 3.5‰) suggest aegirine formation temperatures around 500 °C, lower than expected for a felsic magma, but consistent with poikilitic aegirine that indicates subsolidus growth. The negative ε _Nd (< ? 10) and magma δ¹⁸O values averaging 8.6‰ (assuming Δ_quartz?magma = 0.6‰) are inconsistent with a magma produced by closed-system fractional crystallization of a mantle-derived magma. By contrast, the nepheline syenite magma had mantle-like δ¹⁸O values and much less negative ε _Nd (average ??3.1, n?=?3). The country rock has similar δ¹⁸O values to the granite dykes (average 8.0‰, n?=?108); this means that models for the petrogenesis of the granites by assimilation are unfeasible, unless an unexposed high-δ¹⁸O contaminant is invoked. Instead, it is proposed that the peralkaline syenite and granite dykes formed by partial melting of alkali-metasomatised gneiss that surrounds the nepheline syenite, followed by fractional crystallization.     

Determining water requirements for Black Box (Eucalyptus largiflorens) floodplain woodlands of high conservation value using drip-irrigation

Black Box (Eucalyptus largiflorens F. Muell.), is a keystone tree species of lowland semi-arid floodplain ecosystems in south-eastern Australia. E. largiflorens woodlands are of high conservation value and threatened by climate change-induced drought and irrigation water diversions due to their location on upper floodplain areas where flood frequency has declined. Water requirements of E. largiflorens have not been well quantified using empirical data. Accordingly, knowledge gaps exist in relation to volumes of environmental water required to maintain and improve ecological condition for disconnected floodplain woodlands. To further assist conservation and water resource management, we tested the use of drip irrigation to provide a variety of water regimes to experimental plots in order to monitor tree responses. Water was provided via irrigation delivery across four regimes representing known volumes of water, referred to as an environmental water provision, applied over a 22-week period for two Austral summers. Benefits to trees were identified by measuring transpiration and plant water status using sap flow sensors and a Scholander pressure chamber, respectively. Results indicate that volumes of 0.3, 0.4, 0.7 and 0.8 ML increased transpiration and improved plant water status in comparison to a control, with delivery recommended to commence early autumn. Greater volumes (1.4 ML), substantially increased transpiration and improved water status, especially when delivered at a rate of ~25 mm week⁻¹ compared to a monthly 'burst' which broadly represented natural, sporadic summer rainfall in the region. For an environmental watering provision of 25 mm week⁻¹, ~178 ha of E. largiflorens woodland can be watered with a 1 GL environmental water allocation. The study methods presented are relevant worldwide and our results further the collective understanding of the benefits environmental water provides to E. largiflorens.     

Contaminant transport in a fracture with spatially variable aperture in the presence of monodisperse and polydisperse colloids

A quasi-three-dimensional particle tracking model is developed to characterize the spatial and temporal effects of advection, molecular diffusion, Taylor dispersion, fracture wall deposition, matrix diffusion, and co-transport processes on two discrete plumes (suspended monodisperse or polydisperse colloids and dissolved contaminants) flowing through a variable aperture fracture situated in a porous medium. Contaminants travel by advection and diffusion and may sorb onto fracture walls and colloid particles, as well as diffuse into and sorb onto the surrounding porous rock matrix. A kinetic isotherm describes contaminant sorption onto colloids and sorbed contaminants assume the unique transport properties of colloids. Sorption of the contaminants that have diffused into the matrix is governed by a first-order kinetic reaction. Colloids travel by advection and diffusion and may attach onto fracture walls; however, they do not penetrate the rock matrix. A probabilistic form of the Boltzmann law describes filtration of both colloids and contaminants on fracture walls. Ensemble-averaged breakthrough curves of many fracture realizations are used to compare arrival times of colloid and contaminant plumes at the fracture outlet. Results show that the presence of colloids enhances contaminant transport (decreased residence times) while matrix diffusion and sorption onto fracture walls retard the transport of contaminants. Model simulations with the polydisperse colloids show increased effects of co-transport processes.     

Geochemistry and hydrology of perched groundwater springs: assessing elevated uranium concentrations at Pigeon Spring relative to nearby Pigeon Mine,Arizona (USA)

The processes that affect water chemistry as the water flows from recharge areas through breccia-pipe uranium deposits in the Grand Canyon region of the southwestern United States are not well understood. Pigeon Spring had elevated uranium in 1982 (44 μg/L), compared to other perched springs (2.7–18 μg/L), prior to mining operations at the nearby Pigeon Mine. Perched groundwater springs in an area around the Pigeon Mine were sampled between 2009 and 2015 and compared with material from the Pigeon Mine to better understand the geochemistry and hydrology of the area. Two general groups of perched groundwater springs were identified from this study; one group is characterized by calcium sulfate type water, low uranium activity ratio ²³⁴U/²³⁸U (UAR) values, and a mixture of water with some component of modern water, and the other group by calcium-magnesium sulfate type water, higher UAR values, and radiocarbon ages indicating recharge on the order of several thousand years ago. Multivariate statistical principal components analysis of Pigeon Mine and spring samples indicate Cu, Pb, As, Mn, and Cd concentrations distinguished mining-related leachates from perched groundwater springs. The groundwater potentiometric surface indicates that perched groundwater at Pigeon Mine would likely flow toward the northwest away from Pigeon Spring. The geochemical analysis of the water, sediment and rock samples collected from the Snake Gulch area indicate that the elevated uranium at Pigeon Spring is likely related to a natural source of uranium upgradient from the spring and not likely related to the Pigeon Mine.     

An updated method for estimating landslide‐event magnitude

Summary statistics derived from the frequency–area distribution (FAD) of inventories of triggered landslides allows for direct comparison of landslides triggered by one event (e.g. earthquake, rainstorm) with another. Such comparisons are vital to understand links between the landslide‐event and the environmental characteristics of the area affected. This could lead to methods for rapid estimation of landslide‐event magnitude, which in turn could lead to estimates of the total triggered landslide area. Previous studies proposed that the FAD of landslides follows an inverse power‐law, which provides the basis to model the size distribution of landslides and to estimate landslide‐event magnitude (mLS), which quantifies the severity of the event. In this study, we use a much larger collection of earthquake‐induced landslide (EQIL) inventories (n=45) than previous studies to show that size distributions are much more variable than previously assumed. We present an updated model and propose a method for estimating mLS and its uncertainty that better fits the observations and is more reproducible, robust, and consistent than existing methods. We validate our model by computing mLS for all of the inventories in our dataset and comparing that with the total landslide areas of the inventories. We show that our method is able to estimate the total landslide area of the events in this larger inventory dataset more successfully than the existing methods. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Detailed mineralogy and petrology of highly shocked poikilitic shergottite Northwest Africa 6342

Northwest Africa (NWA) 6342 is an intermediate, poikilitic shergottite, found in Algeria in 2010. It is comprised of two distinct petrographic areas; poikilitic domains with rounded Mg‐rich olivine chadacrysts enclosed by large low‐Ca pyroxene oikocrysts, and a nonpoikilitic domain mainly comprised of subhedral olivine and vesicular recrystallized plagioclase. Oxygen fugacity conditions become more oxidizing during crystallization from the poikilitic to the nonpoikilitic domain (QFM?3.0 to QFM?2.2). As such, it is likely that NWA 6342 experienced a two‐stage (polybaric) crystallization history similar to that of the enriched poikilitic shergottites. NWA 6342 also experienced relatively high levels of shock metamorphism in comparison to most other poikilitic shergottites as evidenced by the fine‐grained recrystallization texture in olivine, as well as melting and subsequent crystallization of plagioclase. The recrystallization of plagioclase requires an extended period of postshock thermal metamorphism for NWA 6342 and similarly shocked intermediate poikilitic shergottites NWA 4797 and Grove Mountains 99027 most likely due to launch from Mars. The similarities in petrology, chemistry, and shock features between these three meteorites indicate that they have similar crystallization and shock histories; possibly originating from the same source area on Mars.     

Beyond the “fit”: introducing climate forecasts among organic farmers in Georgia (United States)

Organic farmers are a prime clientele for climate services by virtue of their social profile and vulnerability of produce to climate extremes. The study draws on an online survey and in-depth interviews with organic farmers in Georgia (United States). It shows that organic farmers access and act on climate information in ways that reflect their emphasis on diversified and flexible systems. They favor a pluralistic knowledge base that integrates scientific expertise with place-based experience and intuitive understandings. Their management style combines information at multiple temporal scales and draws on a range of technical and social resources. Translating climate forecasts into usable science for organic farming requires attention to the identities, commitments, and relationships that define the organic farming community.     

The robustness of the Zr-in-rutile and Ti-in-zircon thermometers during high-temperature metamorphism (Ivrea-Verbano Zone, northern Italy)

This study investigates the behaviour of the Zr-in-rutile and Ti-in-zircon thermometers in granulite facies metapelites from the Ivrea-Verbano Zone lower crustal section. U–Pb ages of zircon constrain the timing of regional amphibolite–granulite facies metamorphism to 316 ± 3 Ma and record zircon recrystallisation and resetting of U–Pb ages at 276 ± 4 Ma and 258 ± 3 Ma. Zr-in-rutile thermometry records peak contact metamorphic temperatures related to intrusion of mafic magmatic rocks and gives peak temperatures between 900–930 °C and 1,000–1,020 °C that are consistent with the geological settings of the samples. Ti-in-zircon temperatures of 700–800 °C and 810–870 °C record growth or re-equilibration of zircon after cooling from peak temperatures. Ti-in-quartz thermometry for one sample records both peak and retrograde temperatures. Some rutiles in all samples record resetting of Zr-in-rutile temperatures at ~750–800 °C. Electron microprobe profiles across individual rutiles demonstrate that Zr expulsion occurred by recrystallisation rather than by diffusive exchange. Exsolution of small needles of baddelyite or zircon from rutile is an important method of Zr redistribution, but results in no net Zr loss from the grain. The demonstration that Zr-in-rutile thermometry can robustly record peak temperatures that are not recorded by any other thermometer emphasises the relevance of this technique to investigating the evolution of high-grade metamorphic terranes, such as those that characterise the lower crust.