Petrological modelling of granulite‐facies mafic and semipelitic migmatites from Cairn Leuchan, northeast Scotland, has provided new constraints on the pressure (P) and temperature (T) conditions of high‐grade metamorphism in the type‐locality Barrow zones. Phase diagrams constructed in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 system have constrained the P?T conditions of peak metamorphism in the Glen Muick region of the upper sillimanite zone (Sill+Kfs) to have been at least ~840°C at ~9 kbar (high‐P granulite facies). These conditions are ~120°C and ~3 kbar higher than those recorded by lower sillimanite zone (Sill+Ms) units located only a few kilometres away to the southeast at Glen Girnock, indicating the presence of a significant thermal and barometric high exposed within the Scottish Dalradian, and supporting previous suppositions of a potential tectonic break between the two regions. U–Pb zircon geochronology performed on these mafic migmatites produced ages of c. 540–470 Ma from grains with both igneous and metamorphic morphological characteristics. Their basaltic protoliths likely formed during a period of volcanism dated at c. 570 Ma, associated with passive‐margin extension prior to the onset of Iapetus Ocean closure, and high‐grade metamorphism and partial melting is interpreted to have taken place at c. 470 Ma, synchronous with sillimanite‐grade metamorphism recorded elsewhere in the Dalradian. These high‐grade Cairn Leuchan lithologies are interpreted as representing a fragment of Grampian Terrane lower crust that was exhumed via displacement along a steeply dipping tectonic discontinuity related to the Portsoy–Duchray Hill Lineament, and are not pre‐Caledonian Mesoproterozoic basement, as suggested by some previous studies. Veins within some mafic migmatites in the Cairn Leuchan area, composed almost entirely (>80%) of garnet, with minor quartz, plagioclase, amphibole, and clinopyroxene, are interconnected with leucosomes and are interpreted to represent former garnet‐bearing melt segregations that have been locally drained of almost all melt. Thus, mafic components of the lower crust, currently underlying relatively lower grade metasediments exposed to the southeast, may represent a potential source rock for widely documented, post‐orogenic felsic plutons, sills, and dykes that occur throughout the Grampian Terrane. 相似文献
We report homogeneity tests on large natural apatite crystals to evaluate their potential as U reference materials for apatite fission‐track (AFT) thermochronology by laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS). The homogeneity tests include the measurements of major element concentrations by electron probe microanalysis (EPMA), whereas for U concentration, isotope dilution (ID) ICP‐MS and laser ablation (LA) ICP‐MS were employed. Two apatite crystals are potential reference materials for LA‐ICP‐MS analysis: a 1 cm3 fraction of a Durango crystal (7.5 μg g?1 U) and a 1 cm3 Mud Tank crystal (6.9 μg g?1 U). The relative standard deviation (1 RSD) of the U concentration determined by ID‐ICP‐MS of both apatite crystals was ≤ 1.5%, whereas 1 RSD for the LA‐ICP‐MS results was better than 4%, providing sufficient homogeneity for fission‐track dating. The results on the U homogeneity for two different apatite samples are an important step towards establishing in situ dating routines for AFT analysis by LA‐ICP‐MS. 相似文献
We examined the zooplankton abundance and composition of Laguna Grande, a floodplain wetland of the Lower Paraná Basin (Argentina), during an extraordinary drought–flood cycle that affected both the environment and the biological conditions of the lake. Low waters were characterised by remarkably high conductivities and pH values, and high phytoplankton and bacterioplankton abundances with cyanobacterial blooms, while high waters showed opposite features. In relation to zooplankton, the mean abundances of all the taxonomic groups (rotifers, cladocerans, copepods, ciliates, and heterotrophic nanoflagellates) were slightly higher at low waters. Major changes were observed in the specific composition of metazooplankton: the euryhaline species assemblage that dominated in the dry warm period was replaced by several oligohaline littoral and planktonic species characteristic of the Paraná River Basin, when the water level rose. Mean species richness values at high waters doubled those of low waters and were directly correlated to water depth. Most of the rotifers of the genus Brachionus and the cladoceran Moina micrura switched from parthenogenetic to sexual reproduction during low waters, as a response to a harsh environment and crowding. We suggest that the main changes in the environmental conditions in this eutrophic floodplain lake are driven by the hydrology, which regulates the zooplankton succession. The herein described shifts in the zooplankton structure and dynamics of Laguna Grande over an extraordinary drought–flood cycle contribute to the understanding of the processes that might occur under the scenarios predicted by climate change models. 相似文献
This paper contains the results of an extensive isotopic study of United States Geological Survey GSD‐1G and MPI‐DING reference glasses. Thirteen different laboratories were involved using high‐precision bulk (TIMS, MC‐ICP‐MS) and microanalytical (LA‐MC‐ICP‐MS, LA‐ICP‐MS) techniques. Detailed studies were performed to demonstrate the large‐scale and small‐scale homogeneity of the reference glasses. Together with previously published isotopic data from ten other laboratories, preliminary reference and information values as well as their uncertainties at the 95% confidence level were determined for H, O, Li, B, Si, Ca, Sr, Nd, Hf, Pb, Th and U isotopes using the recommendations of the International Association of Geoanalysts for certification of reference materials. Our results indicate that GSD‐1G and the MPI‐DING glasses are suitable reference materials for microanalytical and bulk analytical purposes. 相似文献
We analyzed oceanographic, hydrologic and atmospheric conditions in the northern Adriatic (section between the Po River delta and Rovinj) from 1990 to 2004 in which extreme phytoplankton blooms appeared. The largest blooms occurred in February, March, July and October, and were higher in surface layers and in the western, more eutrophic part of the region. They often appeared when the Istrian Coastal Countercurrent (ICCC; strong southward current indicating a “closure” of the northern Adriatic circulation system) was pronounced. They seem to be related to the intensities of surface fluxes and Po River discharge rates. The blooms can float over the northern Adriatic for a long period (up to 30 days; typical for March) and can appear simultaneously in separate circulation systems with a different species distribution. Finally, we present a hypothesis based on comparisons of geostrophic currents and fish stock that specific February northern Adriatic oceanographic conditions play the key role in the Adriatic anchovy stock. 相似文献
Based on a series of experiments conducted by two regional climate models (RCA4 and LMDZ) with and without soil moisture-atmosphere coupling, we investigate the role of soil moisture on the occurrence of surface air temperature extremes and its persistence in Southeastern South America. Our analysis reveals that both factors, soil moisture-atmosphere coupling and relatively drier soil conditions, enhance the temperature extremes. In addition, the existence of soil-atmosphere coupling and the associated soil moisture variability is crucial for the development of the extremes in SESA. The key role of soil-atmosphere coupling is also reflected in the intrinsic persistence of hot days, which is greater in simulations with interactive soil moisture than in those with prescribed soil conditions. In the absence of soil-atmosphere coupling, the imprint of the anomalous dry (and also wet) soil conditions on the intensity and persistence of hot days is weaker.
A paleoecological analysis of the fossil record before and after the Cretaceous/Tertiary boundary indicates that the widespread extinctions and biological stresses around the boundary are best explained in terms of a sudden, significant, but short temperature rise. L. Alvarez and co-authors, having found an enrichment in iridium at the same boundary, postulated that it was associated with the impact of an extraterrestrial body. If this body struck the ocean, the water injected into the atmosphere may have led to a transient increase in the global surface temperature. This temperature pulse may have been primarily responsible for the effects observed in the biosphere. The pattern of extinction of higher plant species suggests that splash down occurred in the northern Pacific-Bering Sea area. 相似文献