Magma Mixing in the 874 AD Hrafntinnuhraun Rhyolite Eruption

The exposed Icelandic crust contains ~10% rhyolite and ~90% basalt, and there is ample evidence in both recent and ancient rocks of interactions between rhyolitic and basaltic magmas. A spectacular and little-studied example is the c. 874 AD rhyolite eruption of Hrafntinnuhraun at the Torfajökull volcano, where after an initial explosive (Plinian) phase a large rhyolite lava field formed. In this lava field, one vent produced a hybrid lava which is a mechanically mixed blend of approximately 83% rhyolite and 17% basalt. Polarized light microscopy was conducted on a suite of Hrafntinnuhraun samples that include flow-banded rhyolites, vesicular basaltic enclaves, and hybrid lavas. The flow-banded rhyolites are nearly aphyric, containing plagioclase and augite phenocrysts, with minor hornblende and biotite. The enclaves contain abundant plagioclase, augite, and olivine. The hybrid lavas contain plagioclase, augite, and olivine, but also contain minor amounts of biotite and hornblende. To better evaluate the magma mixing process, one hybrid lava sample (83:17 rhyolite:basalt) was analyzed by EPMA. This sample contains mineral cargos belonging to both the rhyolite and basalt end-member magmas. Plagioclase compositions range from An22 to An87, with anorthite-rich grains displaying resorption textures. Of particular interest are olivine crystals derived from the basalt, which contain Fo-rich (~Fo80) interiors and narrow rims (~5- 10µm) with lower Fo content (~Fo70). Follow-up work on diffusion modelling will hopefully yield a timescale, thus providing valuable and new information on mixing processes in the conduit prior to the eruption and cooling of this hybrid lava

Geology of Tindfjallajökull volcano, Iceland

The geology of Tindfjallajökull volcano, southern Iceland, is presented as a 1:50,000 scale map. Field mapping was carried out with a focus on indicators of past environments. A broad stratocone of interbedded fragmental rocks and lavas was constructed during Tindfjallajökull’s early development. This stratocone has been dissected by glacial erosion and overlain by a variety of mafic to silicic volcanic landforms. Eruption of silicic magma, which probably occurred subglacially, constructed a thick pile of breccia and lava lobes in the summit area. Mafic to intermediate flank eruptions continued through to the end of the last glacial period, producing lavas, hyaloclastite-dominated units and tuyas that preserve evidence of volcano-ice interactions. The Thórsmörk Ignimbrite, a regionally important chronostratigraphic marker, is present on the SE flank of the volcano. The geological mapping of Tindfjallajökull gives insights into the evolution of stratovolcanoes in glaciated regions and the influence of ice in their development

Widespread tephra dispersal and ignimbrite emplacement from a subglacial volcano (Torfajökull, Iceland)

The tephra dispersal mechanisms of rhyolitic glaciovolcanic eruptions are little known, but can be investigated through the correlation of eruptive products across multiple depositional settings. Using geochemistry and geochronology, we correlate a regionally important Pleistocene tephra horizon—the rhyolitic component of North Atlantic Ash Zone II (II-RHY-1)—and the Thórsmörk Ignimbrite with rhyolitic tuyas at Torfajökull volcano, Iceland. The eruption breached an ice mass >400 m thick, leading to the widespread dispersal of II-RHY-1 across the North Atlantic and the Greenland ice sheet. Locally, pyroclastic density currents traveled across the ice surface, depositing the variably welded Thórsmörk Ignimbrite beyond the ice margin and ~30 km from source. The widely dispersed products of this eruption represent a valuable isochronous tie line between terrestrial, marine, and ice-core paleoenvironmental records. Using the tephra horizon, estimates of ice thickness and extent derived from the eruption deposits can be directly linked to the regional climate archive, which records the eruption at the onset of Greenland Stadial 15.2

Complex circular subsidence structures in tephra deposited on large blocks of ice: Varða tuff cone, Öræfajökull, Iceland

Several broadly circular structures up to 16 m in diameter, into which higher strata have sagged and locally collapsed, are present in a tephra outcrop on southwest Öræfajökull, southern Iceland. The tephra was sourced in a nearby basaltic tuff cone at Varða. The structures have not previously been described in tuff cones, and they probably formed by the melting out of large buried blocks of ice emplaced during a preceding jökulhlaup that may have been triggered by a subglacial eruption within the Öræfajökull ice cap. They are named ice-melt subsidence structures, and they are analogous to kettle holes that are commonly found in proglacial sandurs and some lahars sourced in ice-clad volcanoes. The internal structure is better exposed in the Varða examples because of an absence of fluvial infilling and reworking, and erosion of the outcrop to reveal the deeper geometry. The ice-melt subsidence structures at Varða are a proxy for buried ice. They are the only known evidence for a subglacial eruption and associated jökulhlaup that created the ice blocks. The recognition of such structures elsewhere will be useful in reconstructing more complete regional volcanic histories as well as for identifying ice-proximal settings during palaeoenvironmental investigations

Measurements of Higgs boson production and couplings in diboson final states with the ATLAS detector at the LHC

Measurements are presented of production properties and couplings of the recently discovered Higgs boson using the decays into boson pairs, H →γ γ, H → Z Z∗ →4l and H →W W∗ →lνlν. The results are based on the complete pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at centre-of-mass energies of √s = 7 TeV and √s = 8 TeV, corresponding to an integrated luminosity of about 25 fb−1. Evidence for Higgs boson production through vector-boson fusion is reported. Results of combined fits probing Higgs boson couplings to fermions and bosons, as well as anomalous contributions to loop-induced production and decay modes, are presented. All measurements are consistent with expectations for the Standard Model Higgs boson

Standalone vertex finding in the ATLAS muon spectrometer

A dedicated reconstruction algorithm to find decay vertices in the ATLAS muon spectrometer is presented. The algorithm searches the region just upstream of or inside the muon spectrometer volume for multi-particle vertices that originate from the decay of particles with long decay paths. The performance of the algorithm is evaluated using both a sample of simulated Higgs boson events, in which the Higgs boson decays to long-lived neutral particles that in turn decay to bbar b final states, and pp collision data at √s = 7 TeV collected with the ATLAS detector at the LHC during 2011

Measurement of the top quark pair cross section with ATLAS in pp collisions at √s=7 TeV using final states with an electron or a muon and a hadronically decaying τ lepton

A measurement of the cross section of top quark pair production in proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 7 TeV is reported. The data sample used corresponds to an integrated luminosity of 2.05 fb -1. Events with an isolated electron or muon and a τ lepton decaying hadronically are used. In addition, a large missing transverse momentum and two or more energetic jets are required. At least one of the jets must be identified as originating from a b quark. The measured cross section, σtt-=186±13(stat.)±20(syst.)±7(lumi.) pb, is in good agreement with the Standard Model prediction

Measurement of the top quark-pair production cross section with ATLAS in pp collisions at \sqrt{s}=7\TeV

A measurement of the production cross-section for top quark pairs(\ttbar) in $pp$ collisions at \sqrt{s}=7 \TeV is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in two different topologies: single lepton (electron $e$ or muon $\mu$) with large missing transverse energy and at least four jets, and dilepton ($ee$, $\mu\mu$ or $e\mu$) with large missing transverse energy and at least two jets. In a data sample of 2.9 pb-1, 37 candidate events are observed in the single-lepton topology and 9 events in the dilepton topology. The corresponding expected backgrounds from non-\ttbar Standard Model processes are estimated using data-driven methods and determined to be $12.2 \pm 3.9$ events and $2.5 \pm 0.6$ events, respectively. The kinematic properties of the selected events are consistent with SM \ttbar production. The inclusive top quark pair production cross-section is measured to be \sigmattbar=145 \pm 31 ^{+42}_{-27} pb where the first uncertainty is statistical and the second systematic. The measurement agrees with perturbative QCD calculations.Comment: 30 pages plus author list (50 pages total), 9 figures, 11 tables, CERN-PH number and final journal adde

Expected Performance of the ATLAS Experiment - Detector, Trigger and Physics

A detailed study is presented of the expected performance of the ATLAS detector. The reconstruction of tracks, leptons, photons, missing energy and jets is investigated, together with the performance of b-tagging and the trigger. The physics potential for a variety of interesting physics processes, within the Standard Model and beyond, is examined. The study comprises a series of notes based on simulations of the detector and physics processes, with particular emphasis given to the data expected from the first years of operation of the LHC at CERN