Yudovskaya M.,University of Witwatersrand |
Yudovskaya M.,RAS Institute of Geology and Mineralogy |
Kinnaird J.,University of Witwatersrand |
Naldrett A.J.,University of Witwatersrand |
And 5 more authors.
Mineralogy and Petrology | Year: 2013
The layered Bushveld Complex hosts a number of chromitite layers, which were found to contain significant amounts of zircon grains compared with adjacent silicate rocks. Cathodoluminescent-dark, partially metamict cores and transparent rims of composite zircon grains were analyzed for trace elements with SIMS and LA-ICPMS techniques. The cores are enriched in REE, Y, Th and U and are characterized by distinctly flatter REE patterns in contrast to those of the rims and transparent homogenous crystals. Zircon from the different stratigraphic units has specific Th/U ratios, the highest of which (1.5-4) occurs in a Merensky Reef zircon core. The Ti content of Bushveld zircon ranges from 12 to 52 ppm correlating to a crystallization temperature range of 760-930 °C. The geochemical characteristics of the first zircon generation are consistent with its high-temperature crystallization as the first major U, Th and REE acceptor from a highly-evolved residue of the high-Mg basalt magma, whereas the rims and coreless crystals have crystallized from percolating intercumulus liquid of new influx of the same magma. U-Pb SHRIMP dating of zircon cores and rims does not reveal a distinguishable difference between their ages indicating the absence of inherited zircon. Concordia ages of 2,051 ± 9 Ma (2σ, MSWD = 0.1) and 2,056 ± 5 Ma (2σ, MSWD = 0.05) for zircons from the Merensky Reef and the Upper Platreef located equally near the top of the Critical Zone are in agreement with published ages for the Merensky Reef. Zircon from the deeper-seated Lower Group, Middle Group and Lower Platreef chromitites yields younger concordia ages that may reflect prolonged late-stage volatile activity. © 2013 Springer-Verlag Wien.
Grigoriev A.,Russian Research Geological Institute VSEGEI |
Zhamoida V.,Russian Research Geological Institute VSEGEI |
Spiridonov M.,Russian Research Geological Institute VSEGEI |
Sharapova A.,Russian Research Geological Institute VSEGEI |
And 2 more authors.
Climate Research | Year: 2011
The focus of this study was the 11.55 m long sediment core 303700-7, which was retrieved from the Gdansk Basin during a cruise of RV 'Poseidon' within the frame of the Russian-German Project GISEB. The core was analysed for grain size, elemental chemical composition, organic carbon and palynological spectra. The age control was based on palynostratigraphy and 7 radiocarbon datings of bulk sedimentary organic matter. These data provide a high-resolution record of climatic and marine palaeoenvironments in the Gdansk Basin for the last ca. 13 kyr, from the Bølling to late Holocene time. Sedimentation rates were estimated to vary between 0.37 and 1.62 mm yr -1. Major variations in palaeosalinity were estimated from bromine concentrations in the sediment. This method allows the first quantitative reconstruction of palaeosalinity changes in the Baltic Sea, especially profound during the Littorina and Postlittorina periods (middle to late Holocene). In addition, grain size data indicated several Littorina transgressive-regressive stages and a few episodes of increased near-bottom current activity. Our results from the Gdansk Basin are consistent with palaeoceanographic data from other deep basins of the Baltic Sea and provide new insights into the regional Holocene history. © Inter-Research 2011.