Intracrystalline microstructures in alkali feldspars from fluid-deficient felsic granulites: a mineral chemical and TEM study

Samples of essentially “dry” high-pressure felsic granulites from the Bohemian Massif (Variscan belt of Central Europe) contain up to 2-mm-large perthitic alkali feldspars with several generations of plagioclase precipitates in an orthoclase-rich host. The first generation takes the form of lenses homogeneous in size, whereas the size of a second generation of very thin albite-rich precipitates is more variable with comparatively high aspect ratios. In the vicinity of large kyanite, garnet or quartz inclusions, the first generation of plagioclase precipitates is significantly less abundant, the microstructure is coarser than in the remainder of the perthitic grain and the host is a tweed orthoclase. The first generation of precipitates formed at around 850 °C during the high-pressure stage (16–18 kbar) of metamorphism. Primary exsolution was followed by primary coarsening of the plagioclase precipitates, which still took place at high temperatures (850–700 °C). The coarsening was pronounced due to the access of fluids in the outer portions of the perthitic alkali feldspar and in more internal regions around large inclusions. The second generation of albite-rich precipitates was formed at around 570 °C. TEM investigations revealed that the interfaces between the second-generation plagioclase lamellae and the orthoclase-rich host are coherent or semi-coherent. During late evolutionary stages of the perthite, albite linings were formed at phase boundaries, and the perthitic microstructure was partially replaced by irregularly shaped precipitates of pure albite with incoherent interfaces. The albitization occurred below 400 °C and was linked to fluid infiltration in the course of deuteric alteration. Based on size-distribution analysis, it is inferred that the precipitates of the first generation were most probably formed by spinodal decomposition, whereas the precipitates of the second generation rather were formed by nucleation and growth.     

A hit-and-run giant impact scenario

The formation of the Moon from the debris of a slow and grazing giant impact of a Mars-sized impactor on the proto-Earth (Cameron and Ward [1976]. Lunar Planet. Sci. Conf.; Canup and Asphaug [2001]. Nature 412, 708) is widely accepted today. We present an alternative scenario with a hit-and-run collision (Asphaug [2010]. Chem. Erde 70, 199) with a fractionally increased impact velocity and a steeper impact angle.     

Biosiliceous particle flux in the Southern Ocean

The flux of diatom valves and radiolarian shells obtained during short-term and annual sediment trap experiments at seven localities in the Atlantic sector of the Antarctic Ocean (in the Drake Passage, Bransfield Strait, Powell Basin, NW and SE Weddell Sea and the Polar Front north of Bouvet Island) is summarized and discussed. The deployment of time-series sediment traps provided annual flux records between 1983 and 1990. The biosiliceous particle flux is characterized by significant seasonal and interannual variations. Flux pulses, accounting for 70–95% of the total annual flux, occur during austral summer, with a duration ranging between about 2 and 9 weeks. The annual values of vertical diatom and radiolarian flux range between 0.26 × 10⁹ and more than 26 × 10⁹ valves m⁻² and between 0.21 × 10⁴ and 70 × 10⁴ shells m⁻², respectively. Interannual differences in the particle flux range over a factor of 10. Grazers play an important role in controlling the quantity, timing and pattern of the vertical biosiliceous particle flux.The flux pattern of diatoms and radiolarians is similar at most of the sites investigated and shows a close relationship between the production of siliceous phytoplankton and proto-zooplankton. At some sites, however, the radiolarian flux pattern indicates probably phytoplankton production which is not documented by direct signals in the trap record.During their transfer through the water column to the ocean floor, the composition of the biosiliceous particles is altered mechanically (breakdown by grazing Zooplankton) and by dissolution, which significantly affects especially diatoms and phaeodarians in the upper portion of the water column and at the sediment-water interface.Significant lateral transport of suspended biosiliceous particles was observed in the bottom water layer in regions adjacent to shelf areas (Bransfield Strait), and in the vicinity of topographic elevations (Maud Rise), indicating considerable redistribution of biogenic silica in these regions.