The Lilloise Intrusion, East Greenland: Fractionation of a Hydrous Alkali Picritic Magma

The Lilloise is an 8 km4 km layered mafic intrusion which cutsthe plateau basalts of the East Greenland Tertiary province.Lilloise was intruded at 50 Ma, 4–5 Ma after cessationof the voluminous tholeiitic magmatism which accompanied riftingof the East Greenland continental margin. Lilloise is unusualamong layered intrusions in the province because it had a hydrousalkali picrite parent magma and generated a late-stage effluxof magmatic water from the intrusion into the aureole rocks.The three major subdivisions of the layered rocks are: olivine-clinopyroxene,olivine-clinopyroxene-plagioclase and plagioclase-amphibolecumulates. Massive subsidence of the intrusion before completesolidification resulted in deformation of the internal layeringand downturn of the bedding in the surrounding basalts. A strikingfeature of the intrusion is the injection of the layered rocksby a plexus of magmatic sheets which formed at the time of subsidence.The composition of these sheets is representative of the fractionationtrend of the intrusion and ranges from hawaiite to mildly saturatedquartz trachyte. The fractionation trend is successfully explainedby extraction of cumulus minerals of the layered rocks froma parent magma represented by alkali picrite dykes of a contemporaneousregional dyke swarm. Saturated to mildly over-saturated syenitesare a major component of the East Greenland province and theLilloise intrusion is illustrative of an important magmatictrend towards such compositions at this stage in the openingof the North Atlantic. KEY WORDS: Lilloise intrusion; East Greenland; alkali picrite magma; layered intrusion; magmatic differentiation ^*Corraponding author     

Late- and post-glacial depositional environments in the Norwegian Trench, northern North Sea

The late Weichselian sequence in the northern part of the Norwegian Trench is composed of eight units. The two lowermost units are massive, firm to stiff diamictons, interpreted to have been deposited beneath ice-streams that in all likelihood reached the shelf edge. They are overlain by glaciomarine and normal-marine sediments deposited after 15000BP. The first phase of glacial retreat from the Norwegian Trench (15000–14800 BP) was very rapid and left a thin layer of proximal sediments on top of the tills. This was followed by a period with lower accumulation rates (14800–13600 BP), probably as a result of rapid source retreat and cold meltwater inhibiting dropstone fall-out. The end of this interval marks the change from ice-stream calving in cold water to melting on land. According to lithologic and isotopic data, the maximum rate of Fennoscan-dian ice-sheet disintegration took place around 12500 BP. The water temperatures declined significantly and rates of sedimentation and ice-rafting fell in association with the Younger Dryas period. The final retreat of the ice began as early as 10 500 BP, and the transition to normal-marine sedimentation is reflected by precipitation of iron oxide followed by pyrite, reduced sedimentation rates, and a change from terrigenous to biogenic sedimentation.     

Carbonates in fractures of Martian meteorite Allan Hills 84001: Petrologic evidence for impact origin

Abstract— Carbonates in Martian meteorite Allan Hills 84001 occur as grains on pyroxene grain boundaries, in crushed zones, and as disks, veins, and irregularly shaped grains in healed pyroxene fractures. Some carbonate disks have tapered Mg-rich edges and are accompanied by smaller, thinner and relatively homogeneous, magnesite microdisks. Except for the microdisks, all types of carbonate grains show the same unique chemical zoning pattern on MgCO₃-FeCO₃-CaCO₃ plots. This chemical characteristic and the close spatial association of diverse carbonate types show that all carbonates formed by a similar process. The heterogeneous distribution of carbonates in fractures, tapered shapes of some disks, and the localized occurrence of Mg-rich microdisks appear to be incompatible with growth from an externally derived CO₂-rich fluid that changed in composition over time. These features suggest instead that the fractures were closed as carbonates grew from an internally derived fluid and that the microdisks formed from a residual Mg-rich fluid that was squeezed along fractures. Carbonate in pyroxene fractures is most abundant near grains of plagioclase glass that are located on pyroxene grain boundaries and commonly contain major or minor amounts of carbonate. We infer that carbonates in fractures formed from grain boundary carbonates associated with plagioclase that were melted by impact and dispersed into the surrounding fractured pyroxene. Carbonates in fractures, which include those studied by McKay et al. (1996), could not have formed at low temperatures and preserved mineralogical evidence for Martian organisms.     

Estimating the Asian radon flux density and its latitudinal gradient in winter using ground-based radon observations at Sado Island

Terrestrial radon-222 flux density for the Asian continent, integrated over distances of 4500 km, is estimated in two 20° latitudinal bands centred on 48.8°N and 63.2°N. The evaluation is based on three years of wintertime radon measurements at Sado Island, Japan, together with meteorological and trajectory information. A selection of 18% of observations are suitable for evaluation of an analytical expression for the continental surface flux. Various meteorological assumptions are discussed; it is found that there is a substantial effect of increased complexity of the formulation on the flux estimates obtained. The distribution of spatially integrated radon flux over the Asian landmass is reported for the first time. Expressed as geometric means and 1σ-ranges, estimated fluxes are 14.1 mBq m⁻² s⁻¹ (1σ-range: 18 mBq m⁻² s⁻¹) and 8.4 mBq m⁻² s⁻¹ (1σ-range: 10 mBq m⁻² s⁻¹) for the lower and higher latitude bands. These results constitute an annual minimum in flux densities for these regions, and are higher than previously reported. The existence of a latitudinal gradient in the continental radon source function is confirmed; the present estimate for Asia (−0.39 mBq m⁻² s⁻¹ per degree of latitude) is in agreement with the northern hemisphere terrestrial radon flux gradient proposed previously.     

Thermal and impact histories of reheated group IVA,IVB, and ungrouped iron meteorites and their parent asteroids

Abstract– The microstructures of six reheated iron meteorites—two IVA irons, Maria Elena (1935), Fuzzy Creek; one IVB iron, Ternera; and three ungrouped irons, Hammond, Babb’s Mill (Blake’s Iron), and Babb’s Mill (Troost’s Iron)—were characterized using scanning and transmission electron microscopy, electron‐probe microanalysis, and electron backscatter diffraction techniques to determine their thermal and shock history and that of their parent asteroids. Maria Elena and Hammond were heated below approximately 700–750 °C, so that kamacite was recrystallized and taenite was exsolved in kamacite and was spheroidized in plessite. Both meteorites retained a record of the original Widmanstätten pattern. The other four, which show no trace of their original microstructure, were heated above 600–700 °C and recrystallized to form 10–20 μm wide homogeneous taenite grains. On cooling, kamacite formed on taenite grain boundaries with their close‐packed planes aligned. Formation of homogeneous 20 μm wide taenite grains with diverse orientations would have required as long as approximately 800 yr at 600 °C or approximately 1 h at 1300 °C. All six irons contain approximately 5–10 μm wide taenite grains with internal microprecipitates of kamacite and nanometer‐scale M‐shaped Ni profiles that reach approximately 40% Ni indicating cooling over 100–10,000 yr. Un‐decomposed high‐Ni martensite (α₂) in taenite—the first occurrence in irons—appears to be a characteristic of strongly reheated irons. From our studies and published work, we identified four progressive stages of shock and reheating in IVA irons using these criteria: cloudy taenite, M‐shaped Ni profiles in taenite, Neumann twin lamellae, martensite, shock‐hatched kamacite, recrystallization, microprecipitates of taenite, and shock‐melted troilite. Maria Elena and Fuzzy Creek represent stages 3 and 4, respectively. Although not all reheated irons contain evidence for shock, it was probably the main cause of reheating. Cooling over years rather than hours precludes shock during the impacts that exposed the irons to cosmic rays. If the reheated irons that we studied are representative, the IVA irons may have been shocked soon after they cooled below 200 °C at 4.5 Gyr in an impact that created a rubblepile asteroid with fragments from diverse depths. The primary cooling rates of the IVA irons and the proposed early history are remarkably consistent with the Pb‐Pb ages of troilite inclusions in two IVA irons including the oldest known differentiated meteorite ( Blichert‐Toft et al. 2010 ).