Low-temperature behaviour of ammonium ion in buddingtonite [N(D/H) 4 AlSi 3 O 8 ] from neutron powder diffraction

The structural response of buddingtonite [N(D/H) 4 AlSi 3 O 8 ] on cooling has been studied by neutron diffraction. Data have been collected from 280 K down to 11 K, and the crystal structure refined using the Rietveld method. Rigid-body constraints were applied to the ammonium ion to explore the structural properties of ammonium in the M-site cavities at low-temperature. Low-temperature saturation is observed for almost all the lattice parameters. From the present in situ low-temperature neutron diffraction studies, there is no strong evidence of orientational order–disorder of the ammonium ions in buddingtonite.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46909/1/269_2004_Article_425.pd

Cosmic ray oriented performance studies for the JEM-EUSO first level trigger

JEM-EUSO is a space mission designed to investigate Ultra-High Energy Cosmic Rays and Neutrinos (E > 5 ⋅ 1019 eV) from the International Space Station (ISS). Looking down from above its wide angle telescope is able to observe their air showers and collect such data from a very wide area. Highly specific trigger algorithms are needed to drastically reduce the data load in the presence of both atmospheric and human activity related background light, yet retain the rare cosmic ray events recorded in the telescope. We report the performance in offline testing of the first level trigger algorithm on data from JEM-EUSO prototypes and laboratory measurements observing different light sources: data taken during a high altitude balloon flight over Canada, laser pulses observed from the ground traversing the real atmosphere, and model landscapes reproducing realistic aspect ratios and light conditions as would be seen from the ISS itself. The first level trigger logic successfully kept the trigger rate within the permissible bounds when challenged with artificially produced as well as naturally encountered night sky background fluctuations and while retaining events with general air-shower characteristics

High-grade K-feldspar veining in granulites from the Ivrea-Verbano Zone, northern Italy: Fluid flow in the lower crust and implications for granulite facies genesis

We present observations of extensive networks of K-feldspar microveins associated with quartz and plagioclase in granulite facies rocks from the Ivrea-Verbano Zone, northern Italy. Back-scattered electron images of the granulite facies samples show a continuous system of K-feldspar veins along quartz/plagioclase and plagioclase/plagioclase grain boundaries that can be traced across the entire length of a thin section. Quartz grain rims in contact with these veins show evidence of extensive corrosion. In contrast, contact between the veins and the orthopyroxene and garnet grains is very clean, with no signs of secondary alteration. K-feldspar + quartz + plagioclase, of the same compositions as the veins, are commonly found as near-rim inclusions within garnet. Microprobe traverses from the interiors of the K-feldspar veins to their rims show increasing Ba, indicating a metasomatic origin. Some veins display a boundinaged texture, possibly related to the D-2 deformation event in the Ivrea-Verbano Zone and therefore related to the emplacement of the mafic underplating responsible for initiating granulite facies metamorphism. The K-feldspar also exists as patches of variable size in plagioclase (5-50% of the grain), superficially resembling exsolution antiperthite. The composition of the K-feldspar ``lamellae`` in the antiperthite is close to that of the veins, especially with regard to the Ba content. Approximately half of the plagioclase grains contain little K-feldspar. Remarkably, the K-feldspar veins and patches diminish in both scope and size with decreasing metamorphic grade and eventually disappear once amphibolite facies is reached. We postulate that the K-feldspar veins in the Ivrea-Verbano Zone granulites are evidence of pervasive, high-temperature, low-H2O activity brines and that these brines were an important dehydration mechanism from amphibolite grade to granulite grade. Alkali- and volatile-rich mafic underplating, represented today by the basal Mafic Formation in the Ivrea-Verbano Zone, is the most likely source of the heat and fluids responsible for the genesis of these granulites. This mechanism may have application to other granulite facies terranes where similar K-feldspar microveining has been observed

High-grade localized metasomatic alteration of the granitic gneiss surrounding a clinopyroxene-rich pegmatoid dyke: Sondrum stenhuggeriet, Halmstad, SW Sweden

The Sondrum stone quarry (Halmstad, SW Sweden) exposes a transition from migmatized granitic gneissic country rock into a foliated clinopyroxene-free granitic gneiss, and then a central pegmatoid dyke dominated by clinopyroxene megacrysts. This transition zone represents a fracture-controlled, fluid-alteration zone that developed under conditions of 650-700 degrees C and 790MPa. Mineral chemical trends in F, Cl, Fe, Ti, Mn and Y are interpreted as documenting a fluid infiltration event associated with the formation of the pegmatoid dyke. Fluid inclusions from the pegmatoid dyke are CO2 dominant, whereas in the surrounding country rock they are dominated by H2O-NaCl-CaCl2. Fluid inclusions from the intermediate foliated clinopyroxene-free granitic gneiss are a mixture of the two types. The pegmatoid dyke appears to have originated from a high Ca activity, Fe-Mg enriched, fluid-rich granitic melt with a CO2 component, which was emplaced along a tectonic fracture in a regionally migmatized granitic gneiss in the lower crust. This was accompanied by limited partial melting of the surrounding granitic gneiss. The Ca activity of the melt was high enough to allow for the formation of clinopyroxene megacrysts as opposed to orthopyroxene. H2O-enriched fluids expelled from the crystallizing pegmatoid dyke, which retained the majority of the CO2 helping to stabilize the clinopyroxene. The expelled fluids coarsened and chemically affected the surrounding country rock resulting in trends in the mineral and fluid inclusion chemistry seen today in the foliated and regional granitic gneiss