Evidence for fractional condensation and reprocessing at high temperatures in CH chondrites

Abstract— We performed a detailed study of silica‐rich components (SRC) in the paired CH chondrites Acfer 182 and 207. These SRCs appear either as chondrules or fragments, and they contribute <0.1 vol% to the bulk meteorite. They usually contain a silica and a silicate portion. Both portions are, in most cases, cryptocrystalline and have bulk SiO₂‐concentrations between 65 and 85 wt%. The silicate generally has a pyroxene normative composition. The silica often appears as blebs within the silicate matrix or vice versa. If there are no blebs, silica and silicate still form rounded interfaces. The SRCs are depleted in refractory elements like Ca, Al, and Ti relative to CI. A few SRC‐like objects are extremely rich in Mn and show no depletion in refractory elements. We conducted micro‐Raman studies on the silica portions of the SRCs to determine their structure, and we identified several silica phases: α‐quartz, cristobalite, glass, and a yet unidentified polymorph. The silicate portion is glass when the silica is glass and crystalline when the silica is crystalline. The low contents of Al and Ca make an igneous origin of the SRCs very unlikely, and the absence of metal excludes the formation by reduction of pyroxene. We suggest, instead, a fractional condensation origin of the SRCs from a Si‐enriched gas after removal of gaseous Mg by forsterite condensation. Additional evidence for fractional condensation is provided by a unique layered object with olivine in the core, pyroxene and metal at the rim, and silica at the outermost border; these layers record the condensation sequence. Two chondrules were found with several percent of Mn and high Cr, Na, and K contents, providing further evidence for condensation from a fractionated gas. The texture of the SRCs and the occurrence of cristobalite and silica glass, however, require formation by liquid immiscibility at high temperatures, above 1968 K, and subsequent fast cooling. Therefore, we propose a 2‐stage model for the formation of SRCs in CH chondrites: 1) fractional condensation of forsterite, enstatite, and SiO₂‐rich phases; and 2) reheating of SiO₂‐rich components to temperatures above 1968 K followed by rapid cooling. All other phases identified in CH chondrites can be understood within the framework of this model. Thus, the extremely unequilibrated CH chondrites provide a wealth of evidence for fractional condensation processes in the early solar nebula, in metals (Meibom et al. 1999), and in silicates.     

Diurnal precipitation cycle in Austria

A back-trajectory clustering method was developed to identify synoptic patterns associated with heavy precipitation in Austria. Extending this work from daily precipitation sums to accumulation intervals from 12 to 48 hours, an unexpected problem was encountered related to the observation schedule of data. To modify available data to our specific needs, evening precipitation falling between 19 and 21 LT (Local Time) was studied. Hourly precipitation data for the period of 1950 to 2005 from 132 weather stations over Austria were used. It was found that the evening precipitation fraction has its maxima in the central Alps in all seasons, except in autumn when maxima shift more towards the south of the Alps. These results encouraged us to analyse the variation of diurnal precipitation for different seasons in Austria as well. A total of 119 stations with sixteen years (1990?C2005) of data were used for this purpose. Mean hourly precipitation values were calculated, which were then normalised, smoothed and were finally clustered to find regions with homogeneous diurnal cycles. The diurnal cycles of precipitation of the centres exhibit systematic patterns and seasonal cycles, reflecting intensity and timing of convective activity over these regions. The winter season is relatively dry and summer is associated with the highest precipitation. The areas inside the Alps receive higher amplitudes of precipitation in summer. Interesting features such as bimodal distributions, nocturnal maxima and early morning maxima were also observed in many regions.     

运输方式对马氏珠母贝(Pinctada fucata)稚贝存活率、生长、消化及抗氧化性能的影响探究

为优化马氏珠母贝苗种运输技术,提高运输效率,以马氏珠母贝(Pinctada fucata)稚贝为研究对象,分别以有水和无水两种运输方式汽运8 h条件下开展实验,比较分析其在运输前后、运输后恢复阶段的生长、消化和抗氧化性能。结果显示:有水运输后存活率达98%,无水运输97%;在恢复期第14 d,有水运输和无水运输存活率分别达97%、96%;有水运输皮质醇含量由(2 607.61±15.27)μg/L显著降低至(1 412.98±5.09)μg/L,无水运输后显著降低至(2 287.36±10.18)μg/L;在恢复期第7 d和14 d皮质醇含量均呈逐渐上升趋势;有水和无水运输后和恢复期淀粉酶活性不断上升,且有水运输酶活性高于无水运输(P<0.05),最高值为(1.43±0.05) U/mg prot;有水和无水运输后及恢复期超氧化物歧化酶酶活性在不断下降,最低下降至(16.00±0.14) U/mg prot,有水运输超氧化物歧化酶活性均高于无水运输(P<0.05);有水运输碱性磷酸酶、谷草转氨酶和谷丙转氨酶活性均上升,无水运输均下降;在恢复阶段,碱性磷酸酶活性在不断下降,谷草转氨酶和谷丙转氨酶活性在不断上升。有水和无水运输会对马氏珠母贝稚贝产生不同的影响,有水运输方式相对无水运输方式可使马氏珠母贝稚贝在后期阶段具有更好的存活效果和生长状态。     

Employing airborne multispectral digital imagery to map Brazilian pepper infestation in South Texas

A study was conducted in south Texas to determine the feasibility of using airborne multispectral digital imagery for differentiating the invasive plant Brazilian pepper (Schinus terebinthifolius) from other cover types. Imagery obtained in the visible, near-infrared, and mid-infrared regions of the light spectrum and a supervised classification approach were employed to develop thematic maps of two areas infested with Brazilian pepper. Map accuracies ranged from 84.2 to 100% for the Brazilian pepper class. Findings support using airborne multispectral digital imagery as a tool for separating Brazilian pepper from associated land cover types and further encourage exploration of airborne multispectral digital imagery and image processing techniques for developing maps of Brazilian pepper infestation in Texas and abroad.     

Slow cooling versus episodic fluid injections: Deciphering the Caledonian orogeny in Vestvågøy,Lofoten islands,Norway

The determination of the thermal (temperature–time) histories of high‐P metamorphic terranes has been commonly based on the concepts of slow cooling and closure temperatures. In this paper, we find that this approach cannot reconcile a geochronological data set obtained from the amphibolite‐facies allochthonous Leknes Group of the Lofoten islands, Norway, which reveals an extremely complex thermal history. Using detailed results from several different geochronometers such as ⁴⁰Ar/³⁹Ar, Rb–Sr and U–Pb, we show that a model invoking multiple, short‐lived thermal pulses related to hot‐fluid infiltration channelized by shear zones can reconcile this complicated data set. This model suggests that hot fluids infiltrated throughout basement shear zones and affected the overlying cold allochthon, partially resetting U/Pb rutile and titanite ages, crystallizing new zircon and produced identical ⁴⁰Ar/³⁹Ar and Rb/Sr ages in muscovite, biotite and amphibole in various rocks throughout the region. This paper shows the enormous potential of coupling laser Ar‐spot data with thermal modelling to identify and constrain the duration of short‐lived events. An optimal P–T–t history has been derived by modelling the age data from a previously dated large muscovite crystal (Hames & Andresen, 1996, Geology, 24 :1005) and using Zr‐in‐rutile thermometry which is consistent with all geochronological data and geological constraints from the basement zones and allochthon cover. This tectonothermal model history suggests that there have been three episodic hot‐fluid and ⁴⁰Ar‐free infiltration events, resulting in the total resetting of Ar ages during the Scandian (425 Ma) for 1 Ma at 650°C and two reheating events at 415 Ma for 400 ka at 650°C and at 365 Ma for 50 ka at 600°C, which are modelled as thermal spikes above an ambient temperature of 300°C. Independent confirmation of these parameters was provided by Pb‐diffusion modelling in rutile and titanite. The model suggests that the amphibolite facies rocks of the Leknes Group probably remained cold before being exhumed for at least 60 Ma (425–365 Ma) and successfully explains the presence of different minerals that crystallized or were totally/partially reset in the allochthon and in the basement. The migration of hot fluids for short periods of times within conduits extending through the basement and allochthon rock units is likely associated with episodic seismic activity during the Caledonian orogeny.     

A comparison of methods for low streamflow estimation from spot measurements

Low streamflow statistic estimators at ungauged river sites generally have large errors and uncertainties. This can be due to many reasons, including lack of data, complex hydrologic processes, and the inadequate or improper characterization of watershed hydrogeology. One potential solution is to take a small number of streamflow measurements at an ungauged site to either estimate hydrogeologic indices or transfer information from a nearby site using concurrent streamflow measurements. An analysis of four low streamflow estimation techniques, regional regression, regional plus hydrogeologic indices, baseflow correlation, and scaling, was performed within the Apalachicola–Chattahoochee–Flint watershed, a U.S. Geological Survey WaterSMART region in the south‐eastern United States. The latter three methods employ a nominal number of spot measurements at the ungauged site to improve low streamflow estimation. Results indicate that baseflow correlation and scaling methods, which transfer information from a donor site, can produce improved low streamflow estimators when spot measurements are available. Estimation of hydrogeologic indices from spot measurements improves regional regression models, with the baseflow recession constant having more explanatory power than the aquifer time constant, but these models are generally outperformed by baseflow correlation and scaling.     

Future developments in modelling and monitoring of volcanic ash clouds: outcomes from the first IAVCEI-WMO workshop on Ash Dispersal Forecast and Civil Aviation

As a result of the serious consequences of the 2010 Eyjafjallajökull eruption (Iceland) on civil aviation, 52 volcanologists, meteorologists, atmospheric dispersion modellers and space and ground-based monitoring specialists from 12 different countries (including representatives from 6 Volcanic Ash Advisory Centres and related institutions) gathered to discuss the needs of the ash dispersal modelling community, investigate new data-acquisition strategies (i.e. quantitative measurements and observations) and discuss how to improve communication between the research community and institutions with an operational mandate. Based on a dedicated benchmark exercise and on 3 days of in-depth discussion, recommendations have been made for future model improvements, new strategies of ash cloud forecasting, multidisciplinary data acquisition and more efficient communication between different communities. Issues addressed in the workshop include ash dispersal modelling, uncertainty, ensemble forecasting, combining dispersal models and observations, sensitivity analysis, model variability, data acquisition, pre-eruption forecasting, first simulation and data assimilation, research priorities and new communication strategies to improve information flow and operational routines. As a main conclusion, model developers, meteorologists, volcanologists and stakeholders need to work closely together to develop new and improved strategies for ash dispersal forecasting and, in particular, to: (1) improve the definition of the source term, (2) design models and forecasting strategies that can better characterize uncertainties, (3) explore and identify the best ensemble strategies that can be adapted to ash dispersal forecasting, (4) identify optimized strategies for the combination of models and observations and (5) implement new critical operational strategies.     

Tungsten isotopes in ferroan anorthosites: Implications for the age of the Moon and lifetime of its magma ocean

New W isotope data for ferroan anorthosites 60025 and 62255 and low-Ti mare basalt 15555 show that these samples, contrary to previous reports [Lee, D.C., et al., 1997. Science 278, 1098-1103; Lee, D.C., et al., 2002. Earth Planet. Sci. Lett. 198, 267-274], have a W isotope composition that is indistinguishable from KREEP and other mare basalts. This requires crust extraction on the Moon later than ∼60 Myr after CAI formation, consistent with ¹⁴⁷Sm-¹⁴³Nd ages for ferroan anorthosites. The absence of ¹⁸²Hf-induced ¹⁸²W variations in the Moon is consistent with formation of the Moon at after CAI formation that has been inferred based on the indistinguishable ¹⁸²W/¹⁸⁴W ratios of the bulk Moon and the bulk silicate Earth. The uncertainties on the age of the Moon and the age of the oldest lunar samples currently hamper a precise determination of the duration of magma ocean solidification and are consistent with both an almost immediate crystallization and a more protracted timescale of ∼100 Myr.     

Identifying transit corridors for elephant using a long time-series

The role of corridors in mitigating the effects of landscape fragmentation on biodiversity is controversial. Recent studies have highlighted the need for new approaches in corridor design using long-term datasets. We present a method to identify transit corridors for elephant at a population scale over a large area and an extended period of time using long-term aerial surveys. We investigated environmental and anthropogenic factors directly and indirectly related to the wet versus dry season distribution of elephant and its transit corridors. Four environmental variables predicted the presence of elephant at the landscape scale in both seasons: distance from permanent water, protected areas and settlements and vegetation structure. Path analysis revealed that altitude and monthly average NDVI, and distance from temporary water had a significant indirect effect on elephant distribution at local scale in dry and wet seasons respectively. Five transit corridors connecting Tarangire National Park and the northern as well as south-eastern wet season dispersal areas were identified and matched the wildlife migration routes described in the 1960s. The corridors are stable over the decades, providing landscape connectivity for elephant. Our approach yielded insights how advanced spatial analysis can be integrated with biological data available from long-term datasets to identify actual transit corridors and predictors of species distribution.