Isotope geochemistry of stratified Lake "A," Ellesmere Island, N.W.T., Canada: Reply

Ionic composition, salinity, temperature, pH, tritium, and stable isotope compositions of water and ions were determined for samples collected in 1969 and 1982 from different depths of stratified Lake "A" on Ellesmere Island. Tritium contents and ionic and stable isotope compositions were diagnostic of recent fresh water overlying older, deeper trapped seawater. A temperature maximum occurs at 15 m in the freshwater–seawater transition zone. Salinity, δ18O, and δD data suggest that the lake evaporated slightly and acquired about 12% fresh water prior to stratification. Individual ion concentrations reveal a slightly modified cation composition and marked depletions in sulphate and enrichments in dissolved carbonate compared with modern ocean water. The δ34S, δ18O, and δ13C data for [Formula: see text] and [Formula: see text] attest to extensive anaerobic [Formula: see text] reduction during the lake's history.

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The first results of isotopic (U-Pb, ID-TIMS) dating of individual zircon grains from dolerite dikes of the Eastern zone of the Urals

LITOSFERA ◽

10.24930/1681-9004-2020-20-2-224-230 ◽

2020 ◽

Vol 20 (2) ◽

pp. 224-230

Author(s):

V. N. Smirnov ◽

K. S. Ivanov ◽

T. V. Bayanova

Keyword(s):

Isotope Geochemistry ◽

Early Carboniferous ◽

Dynamic Mode ◽

Middle Urals ◽

Volcanic Complex ◽

The Urals ◽

Electron Multiplier ◽

The Middle Urals ◽

Eastern Zone ◽

First Results

Research subject. The article presents the results of dating two dolerite dikes differing in geochemical features from a section along the Iset river in the area of Smolinskoe settlement (the Eastern zone of the Middle Urals). Materials and methods. The dating was performed by an U-Pb ID-TIMS technique for single zircon grains using an artificial 205Pb/235U tracer in the laboratory of geochronology and isotope geochemistry of the Geological Institute of the Kola Science Centre of the Russian Academy of Sciences. The lead isotopic composition and uranium and lead concentrations were measured using a Finnigan-MAT (RPQ) seven-channel mass spectrometer in dynamic mode using a secondary electron multiplier and RPQ quadrupole in ion counting mode. Results. The dikes were dated 330 ± 3 Ma and 240 ± 2 Ma. Conclusions. The research results indicate different ages of dolerite dikes developed within the Eastern zone of the Middle Urals. The oldest of the two established age levels corresponds to the Early Carboniferous era. This fact, along with the proximity of the dolerites to the petrochemical features of the basaltoids of the Early Carboniferous Beklenischevsky volcanic complex, allows these bodies to be considered as hypabyssal comagmates of these volcanics. The youngest obtained age level – Triassic – indicates that the introduction of some dolerite dikes was associated with the final phases of the trapp formation developed rarely within the eastern outskirts of the Urals and widely further east in the foundation (pre-Jurassic basement) of the West-Siberian Plate.

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