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

Petrology and mineralogy of granulite-facies mafic xenoliths (Sardinia, Italy): evidence for KCl metasomatism in the lower crust

Here new mineralogical data is presented on the occurrence of K-feldspar in granulite-facies metagabbronorite xenoliths found in recent alkaline lavas from Western Sardinia, Italy. The xenoliths originated from the underplating of variably evolved subduction-related basaltic liquids, which underwent cooling and recrystallisation in the deep crust (T=850–900 °C, P=0.8–1.0 GPa). They consist of orthopyroxene+clinopyroxene+plagioclase porphyroclasts (An=50–66 mol%) in a granoblastic, recrystallised, quartz-free matrix composed of pyroxene+plagioclase (An=56–72 mol%)+Fe–Ti oxides±K-feldspar±biotite±fluorapatite±Ti-biotite. Texturally, the K-feldspar occurs in a variety of different modes. These include: (1) rods, blebs, and irregular patches in a random scattering of plagioclase grains in the form of antiperthite; (2) micro-veins along plagioclase–plagioclase and plagioclase–pyroxene grain rims; (3) myrmekite-like intergrowths with Ca-rich plagioclase along plagioclase–plagioclase grain boundaries; and (4) discrete anhedral grains (sometimes microperthitic). The composition of each type of K-feldspar is characterized by relatively high albite contents (16–33 mol%). An increasing anorthite content in the plagioclase towards the contact with the K-feldspar micro-vein and myrmekite-like intergrowths into the K-feldspar along the plagioclase–K-feldspar grain boundary are also observed. Small amounts of biotite (TiO2=4.7–6.5 wt.%; F=0.24–1.19 wt.%; Cl=0.04–0.20 wt.%) in textural equilibrium with the granulite-facies assemblage is present in both K-feldspar-bearing and K-feldspar-free xenoliths. These K-feldspar textures suggest a likely metasomatic origin due to solid-state infiltration of KCl-rich fluids/melts. The presence of such fluids is supported by the fluorapatite in these xenoliths, which is enriched in Cl (Cl=6–50% of the total F+Cl+OH). These lines of evidence suggest that formation of Kfeldspar in the mafic xenoliths reflects metasomatic processes, requiring an external K-rich fluid source, which operated in the lower crust during and after in-situ high-T recrystallisation of relatively dry rocks

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