ALBERT

Description: Author(s): S. Lebernegg, M. Schmitt, A. A. Tsirlin, O. Janson, and H. Rosner Periodic and cluster density functional theory (DFT) calculations, including DFT+ U and hybrid functionals, are applied to study magnetostructural correlations in spin-1/2 frustrated chain compounds Cu X 2 : CuCl 2 , CuBr 2 , and a fictitious chain structure of CuF 2 . The nearest-neighbor and second-neighbor... [Phys. Rev. B 87, 155111] Published Wed Apr 03, 2013

Description: Author(s): A. Dijon, E. Clément, G. de France, P. Van Isacker, J. Ljungvall, A. Görgen, A. Obertelli, W. Korten, A. Dewald, A. Gadea, L. Gaudefroy, M. Hackstein, D. Mengoni, Th. Pissulla, F. Recchia, M. Rejmund, W. Rother, E. Sahin, C. Schmitt, A. Shrivastava, J. J. Valiente-Dobón, K. O. Zell, and M. Zielińska Lifetimes of the 9/2 1 - and 3/2 1 - states in 63 Co and the 9/2 1 - state in 65 Co were measured using the recoil distance Doppler shift and the differential decay curve methods. The nuclei were populated by multinucleon transfer reactions in inverse kinematics. γ rays were measured with the EXOGAM Ge arra... [Phys. Rev. C 83, 064321] Published Tue Jun 21, 2011

Description: Although physiological data on microcircuits involving a few inhibitory neurons in the mammalian cerebral cortex are available, data on the quantitative relation between inhibition and excitation in cortical circuits involving thousands of neurons are largely missing. Because the distribution of neurons is very inhomogeneous in the cerebral cortex, it is critical to map all neurons in a given volume rather than to rely on sparse sampling methods. Here, we report the comprehensive mapping of interneurons (INs) in cortical columns of rat somatosensory cortex, immunolabeled for neuron-specific nuclear protein and glutamate decarboxylase. We found that a column contains ∼2,200 INs (11.5% of ∼19,000 neurons), almost a factor of 2 less than previously estimated. The density of GABAergic neurons was inhomogeneous between layers, with peaks in the upper third of L2/3 and in L5A. IN density therefore defines a distinct layer 2 in the sensory neocortex. In addition, immunohistochemical markers of IN subtypes were layer-specific. The “hot zones” of inhibition in L2 and L5A match the reported low stimulus-evoked spiking rates of excitatory neurons in these layers, suggesting that these inhibitory hot zones substantially suppress activity in the neocortex.

Description: Few terrestrial localities preserve more than a trace lithic record prior to ca. 3.8 Ga greatly limiting our understanding of the first 700 Ma of Earth history, a period inferred to have included a spike in the bolide flux to the inner solar system at ca. 3.85–3.95 Ga (the Late Heavy Bombardment, LHB)....

Description: Generation of large-volume rhyolites in the shallow crust is an important, yet enigmatic, process in the Snake River Plain and worldwide. Here, we present data for voluminous rhyolites from the 6·6–4·5 Ma Heise volcanic field in eastern Idaho. Heise is arguably the best site to evaluate shallow rhyolite genesis in the Snake River Plain; it is the youngest complete record of caldera cluster volcanism along the Yellowstone hotspot track and it culminated with the eruption of the most voluminous low- 18 O rhyolite known on Earth: the 1800 km 3 Kilgore Tuff ( 18 O = 3·4). Such low- 18 O values fingerprint meteoric waters, and thus the shallow crust. New oxygen isotope data for phenocrysts, obtained by laser fluorination, correspond to a low- 18 O magma value of 3·4 ± 0·1 (2 standard error) for Kilgore Tuff samples erupted 〉100 km apart; however, ion microprobe data for single zircon crystals show significant diversity, with 18 O values that range from –1·3 to 6·1. U–Pb zircon ages, mineral chemistry, whole-rock major and trace element geochemistry, Sr and Nd isotope data, and magmatic (liquidus) temperatures are similar and/or overlapping for all studied samples of the Kilgore Tuff. Normal- 18 O Heise tuff units that preceded the Kilgore Tuff define a temporal compositional trend in trace element concentrations, trace element ratios, and Sr and Nd isotope ratios that is consistent with fractional crystallization from a common reservoir, whereas low- 18 O Kilgore cycle units have compositions that define a sharp reversal in the temporal trend back towards the composition of the first normal- 18 O Heise tuff (6·62 Ma Blacktail Creek Tuff). The data support derivation of the voluminous low- 18 O Kilgore Tuff from remelting of hydrothermally altered ( 18 O depleted) intracaldera and subvolcanic portions of the Blacktail Creek Tuff. Single pockets of melt with variable low- 18 O values were assembled and homogenized on a caldera-wide scale prior to the climactic Kilgore Tuff eruption, and the best record of this process is provided by the 18 O diversity in Kilgore Tuff zircons. Temporal trends of oxygen isotopic depletion and recovery in rhyolite eruptions of the Heise volcanic field are clearly linked to caldera collapse events, and remarkably consistent with trends in the Yellowstone Plateau volcanic field. At Heise and Yellowstone, magmatic 18 O values can be predicted on the basis of cumulative eruptive volumes, with a decrease in 18 O by ~1 for every ~1000 km 3 of erupted rhyolite. The Kilgore Tuff of the Heise volcanic field has the same timing, magnitude of 18 O depletion, and cumulative eruptive volume as the youngest phase of voluminous rhyolitic eruptions in the Yellowstone Plateau volcanic field, indicating that the Kilgore Tuff may serve as a useful analog for these and perhaps other large-volume low- 18 O rhyolites on Earth.

Description: The 1.85 Ga Belomorian Belt, Karelia, Russia, hosts ultralow 18 O and D (as low as –27.3 and –235 standard mean ocean water [SMOW], respectively), high-Al gneisses and amphibolites that we attribute to the Paleoproterozoic "Slushball Earth" glaciation. They now occur in at least 11 localities spanning 450 km. To constrain distribution of 18 O-depleted rocks, we performed detailed field mapping in Khitostrov, where 18 O values are the lowest. Using 430 new and previously published laser fluorination isotope analyses, we show that the elongated, concentrically zoned area of 18 O depletion is greater than 6 x 2 km in areal extent, ~10 times larger than previously thought. Relationships between 17 O versus 18 O strictly adhere to the equilibrium terrestrial mass-dependent fractionation with a slope of 0.527. We also report the results of ion microprobe U-Pb geochronology of zircons coupled with co-registered oxygen isotope spot analyses for mafic intrusions and host gneisses in six localities. The 2.9–2.7 Ga gneiss zircon cores are normal in 18 O (5–7). They show truncated oscillatory cathodoluminescence (CL) patterns and rounded shape indicative of original igneous crystallization with subsequent detrital overprinting. A younger 2.6–2.55 Ga metamorphic zircon domain with normal 18 O, low Th/U, dark cathodoluminescence, and also with rounded crystal morphology is commonly preserved. Cores are surrounded by ubiquitous rims highly depleted in 18 O (re-)crystallized with Svecofennian (1.85–1.89 Ga) ages. Rims are interpreted as metamorphic due to bright and uniform CL and Th/U 〈0.05. Mafic intrusions preserve few igneous zircon crystals between ca. 2.23 and 2.4 Ga in age, but neoblastic zircon in these intrusions originated mostly during 1.85 Ga Svecofennian metamorphism. The 18 O-age relationship for metamorphic rims in zircon and corundum grains suggests that 18 O values of fluids were subtly increasing with time during metamorphism. Large metamorphic corundum grains have ~3 intracrystalline 18 O isotope zonation from –24 to –21, which likely developed during interaction with metamorphic fluids. The Zr-in-rutile geothermometer temperatures are in the range of 760 to 720 °C, in accordance with mineral assemblages and amphibolite metamorphic grade. High and irregular rare-earth element (REE) abundance in cores and rims of many zircons correlates with high phosphorus content and is explained by nanometer-scale xenotime and monazite inclusions, likely in metamict zones during 1.85 Ga Svecofennian metamorphism. A survey of oxygen isotopes in ultrahigh-pressure diamond and coesite-bearing metamorphic terrains around the world reveals normal to high- 18 O values, suggesting that the low 18 O in metamorphic rocks of Dabie Shan, Kokchetav, and in Karelia, are genetically unrelated to metamorphism. We discuss alternative ways to achieve extreme 18 O depletion by kinetic, Rayleigh, and thermal diffusion processes, and by metamorphism. We prefer an interpretation where the low- 18 O and high-Al signature of the rocks predates metamorphism, and is caused by shallow hydrothermal alteration and partial dissolution of the protolith surrounding shallow mafic intrusions by glacial meltwaters during pan-global Paleoproterozoic "Slushball Earth" glaciations between ca. 2.4 and ca. 2.23 Ga.

Description: The geological record contains evidence of volcanic eruptions that were as much as two orders of magnitude larger than the most voluminous eruption experienced by modern civilizations, the A.D. 1815 Tambora (Indonesia) eruption. Perhaps nowhere on Earth are deposits of such supereruptions more prominent than in the Snake River Plain–Yellowstone Plateau (SRP-YP) volcanic province (northwest United States). While magmatic activity at Yellowstone is still ongoing, the Heise volcanic field in eastern Idaho represents the youngest complete caldera cycle in the SRP-YP, and thus is particularly instructive for current and future volcanic activity at Yellowstone. The Heise caldera cycle culminated 4.5 Ma ago in the eruption of the ~1800 km 3 Kilgore Tuff. Accessory zircons in the Kilgore Tuff display significant intercrystalline and intracrystalline oxygen isotopic heterogeneity, and the vast majority are 18 O depleted. This suggests that zircons crystallized from isotopically distinct magma batches that were generated by remelting of subcaldera silicic rocks previously altered by low- 18 O meteoric-hydrothermal fluids. Prior to eruption these magma batches were assembled and homogenized into a single voluminous reservoir. U-Pb geochronology of isotopically diverse zircons using chemical abrasion–isotope dilution–thermal ionization mass spectrometry yielded indistinguishable crystallization ages with a weighted mean 206 Pb/ 238 U date of 4.4876 ± 0.0023 Ma (MSWD = 1.5; n = 24). These zircon crystallization ages are also indistinguishable from the sanidine 40 Ar/ 39 Ar dates, and thus zircons crystallized close to eruption. This requires that shallow crustal melting, assembly of isolated batches into a supervolcanic magma reservoir, homogenization, and eruption occurred extremely rapidly, within the resolution of our geochronology (10 3 –10 4 yr). The crystal-scale image of the reservoir configuration, with several isolated magma batches, is very similar to the reservoir configurations inferred from seismic data at active supervolcanoes. The connection of magma batches vertically distributed over several kilometers in the upper crust would cause a substantial increase of buoyancy overpressure, providing an eruption trigger mechanism that is the direct consequence of the reservoir assembly process.

Description: Author(s): D. E. McNally, J. W. Simonson, K. W. Post, Z. P. Yin, M. Pezzoli, G. J. Smith, V. Leyva, C. Marques, L. DeBeer-Schmitt, A. I. Kolesnikov, Y. Zhao, J. W. Lynn, D. N. Basov, G. Kotliar, and M. C. Aronson We present high temperature inelastic neutron scattering and magnetic susceptibility measurements of the antiferromagnetic insulator LaMnPO that are consistent with the presence of two-dimensional magnetic correlations up to a temperature Tmax≈700K≫TN=375 K, the Néel temperature. Optical transmissio... [Phys. Rev. B 90, 180403] Published Tue Nov 18, 2014