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Fe–Mn crusts were recovered from the western escarpment of the Bonin Ridge in the Izu–Bonin fore-arc region (dive site #824: 28.612°N, 141.803°E) at water depths of c. 2900 m using the Shinkai 6500 submersible during cruise YK 04–05. Major and trace element data and XRD mineralogy indicate that the crusts are hydrogenetic in origin. We present profiles of variations in Pb isotope composition measured in-situ by laser ablation MC-ICP-MS across two of the crusts. The isotopic variations are systematic and can be matched up between the two crusts, indicating similar growth rates. The crust Pb isotope composition rules out any local source for Pb from within the Izu–Bonin–Mariana arc system, either from hydrothermal activity or through leaching of volcanic detritus. Input of a globally well-mixed volcanic Pb component, either from aerosols or as an absorbed component on aeolian dust, has been proposed as a mechanism to explain the Pb isotope composition of Central Pacific deep water. However, the Izu–Bonin crusts are displaced to lower 206Pb/204Pb and higher 208Pb/204Pb, which requires an additional Pb source. One possibility is that as water is advected from the south, outboard of the Luzon–Ryukyu–Honshu arc system, it is progressively polluted by Pb derived by weathering and erosion of these young island arc volcanic systems. Using a constant Co-flux model, growth rates are estimated at ~ 7–13 mm/Ma, which would suggest that these crusts provide a record of changes in the composition of deep water in the Izu–Bonin fore-arc region of the western Pacific Ocean over the last 4–8 Ma. Over this interval, the main feature has been a progressive decrease in 207Pb/206Pb (0.843 to 0.839) and 208Pb/206Pb (2.088 to 2.080) with time. The interior parts have compositions similar to those of crusts from the Izu–Bonin fore-arc, while the rims have compositions similar to crusts from the central Western Pacific. 相似文献
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Elyakom Vadislavsky Yoav Yair Carynelisa Erlick Colin Price Eran Greenberg Roy Yaniv Baruch Ziv Nàama Reicher Adam Devir 《Journal of Atmospheric and Solar》2009,71(17-18):1835-1839
We report synchronized optical observations of sprites in Israel during the winters of 2006/7–2007/8. Based on several events, we suggest that the elements of columniform sprites are organized in spaced intervals on the circumference of a circle centered directly above, or a little offset, to the vertical direction from the parent lightning. In 2D images most of the cases show columns to be arranged in highly eccentric elliptical forms or in straight rows. The analysis of the optical images provided the geometrical dimensions of the columns and their spatial organization. We used an electrostatic model of the QE field with reasonable assumptions on the location and magnitude of the cloud charge center, constrained by ELF evaluation of the Charge Moment Change in the parent flash, to show that the observed diameter of the columnar arrangement closely matches the conventional breakdown field line contour at the same altitude. 相似文献
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Jakob K. Jakobsen Christian Tegner C. Kent Brooks Adam J. R. Kent Charles E. Lesher Troels F. D. Nielsen Michael Wiedenbeck 《Contributions to Mineralogy and Petrology》2010,159(1):61-79
Troctolite blocks with compositions akin to the Hidden Zone are exposed in a tholeiitic dyke cutting across the Skaergaard
intrusion, East Greenland. Plagioclase in these blocks contains finely crystallised melt inclusions that we have homogenised
to constrain the parental magma to 47.4–49.0 wt.% SiO2, 13.4–14.9 wt.% Al2O3 and 10.7–14.1 wt.% FeOT. These compositions are lower in FeOT and higher in SiO2 than previous estimates and have distinct La/SmN and Dy/YbN ratios that link them to the lowermost volcanic succession (Milne Land Formation) of the regional East Greenland flood basalt
province. New major- and trace element compositions for the FG-1 dyke swarm, previously taken to represent Skaergaard magmas,
overlap with the entire range of the regional flood basalt succession and do not form a coherent suite of Skaergaard like
melts. These dykes are therefore re-interpreted as feeder dykes throughout the main phase of flood basalt volcanism. 相似文献
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