RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 9, ES1002, doi:10.2205/2007ES000221, 2007

Methods

[22]  The Rb-Sr classification of carbonates has been studied using the bulk carbonate component after solving a weighed portion of the crushed sample in 10% acetic acid. Rb and Sr have been conventionally separated by the ion-exchange technique using Dowex AG50Wx8 cation exchanger (200 to 400 bags) and 2.5N HCl as an eluent [Kuznetsov et al., 2003]. Rb and Sr contents have been determined by the mass-spectrometric method of isotope dilution using a mixed 87Rb-84Sr indicator. The Sr isotopic composition has been determined by the Finnigan MAT-261 multiple collector mass-spectrometer simultaneously recording ion currents of all isotopes. Isotope studies were conducted at the Institute of Precambrian Geochronology of the Russian Academy of Sciences under the leadership of A. B. Kuznetsov. The 87Sr/86Sr average ratio normalised to 87Sr/86Sr = 0.1194 in the standard SRM 987 sample has been estimated at 0.71025 pm 0.00001.

[23]  Carbonates generally have low Rb contents and high Sr contents but can accumulate, over geological spans of time, substantial amounts of radiogenic strontium distorting the determination of the primary isotopic composition.

[24]  The primary strontium isotopic composition of carbonates, corrected for radiogenic strontium, can be calculated using the following formula [Faure, 1989]:

eq001.gif(1)

where ( 87Sr/86Sr)meas is the measured Sr ratio in the carbonate specimen, ( 87Sr/86Sr)0 is the primary Sr isotope ratio, 87Rb/86Sr is the measured ratio of the 87Rb isotope content to the 86Sr isotope content, t is time (years), l is the 87 Rb decay constant of 1.42 cdot 10-11 yr-1 [Neumann and Huster, 1974].

[25]  With t taken as 300 million years, lt of the exponential factor is ca. 10-3 and elt can be expressed as the first two members of the series 1+lt +(-lt )2/2!+ldots. Equation (1) can in this case be expressed as

eq002.gif(2)

[26]  The primary Sr isotopic composition can also be distorted due to secondary alterations of carbonates. The degree of preservation/alteration of carbonates has been estimated using published criteria [Banner and Hanson, 1990; Brand and Veizer, 1981; Khabarov et al., 2000; Veizer, 1989].

[27]  The separation of samples with disturbed and undisturbed isotopic systems has been performed using the following Mn/Sr and Fe/Sr values: < 5 and < 20 for limestones and < 10 and < 60 for dolomites, correspondingly. A differentiated approach to determining the effect of post-sedimentation alterations on primary 87Sr/86Sr ratios and carbon and oxygen isotopic compositions is used due to the fact that strontium more easily enters the crystalline lattice of calcite than that of dolomite and, for this reason, strontium concentrations in limestones can be almost ten times higher than in dolomites.


RJES

Citation: Nurgalieva, N. G., V. A. Ponomarchuk, and D. K. Nurgaliev (2007), Strontium isotope stratigraphy: Possible applications for age estimation and global correlation of Late Permian carbonates of the Pechishchi type section, Volga River, Russ. J. Earth Sci., 9, ES1002, doi:10.2205/2007ES000221.

Copyright 2007 by the Russian Journal of Earth Sciences

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