RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 9, ES1002, doi:10.2205/2007ES000221, 2007
[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
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]:
![]() | (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
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!+. Equation (1) can in this
case be expressed as
![]() | (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.
Citation: 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.
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