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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 63 (1979)

Issue: 3. (March)

First Page: 533

Last Page: 533

Title: Isotopic Composition and Sources of Strontium in Sandstone Cements in High Plains Sequence of Wyoming and Nebraska: ABSTRACT

Author(s): Kenneth O. Stanley, Gunter Faure

Article Type: Meeting abstract

Abstract:

The 87Sr/86Sr ratios of sandstone cements reflect the isotopic composition of strontium released into the pore fluid by different rock and mineral constituents. However, little is known about the extent to which the isotopic compositions of strontium in cements reflect local or regional variations in sandstone compositions. The 87Sr/86Sr ratios of sandstone cements permit identification of the major sources of strontium in the pore fluid and indicate the dimensions of the aquifer system within which the pore fluid was isotopically homogenized. Since the abundances of radiogenic 87Sr is continually increasing by decay of 87Rb, isotopic compositions of strontium may also suggest the sequence and time of cemen formation in sandstones that contain detrital mineral and rock grains having high Rb/Sr ratios.

After removing the calcite, montmorillonite, or zeolite cements, sandstones from the Arikaree and Ogallala Groups of the High Plains sequence (Oligocene to Pliocene) have 87Sr/86Sr ratios ranging from 0.7065 for plagioclase arenite to 0.7491 for arkosic arenite; rhyolitic vitric ash samples have intermediate ratios of 0.7093 and 0.7133. In contrast to the detrital fractions of the sandstones, the cements contain strontium that is isotopically homogeneous over distances of 70 km or more. Calcite and montmorillonite cements from the Arikaree Group (Oligocene-Miocene) have an 87Sr/86Sr ratio of 0.7103, whereas calcite and clinoptilolite from the Ogallala Group (Miocene-Pliocene) yield 0.7112. The ratio of the cements suggests that the pore wate s were homogeneous on a regional basis and were not locally controlled. The slight difference in the isotopic composition of strontium in the cements of the Arikaree and Ogallala Groups may have resulted either from decay of 87Rb during the time interval of about 20 m.y. between lithogenesis of the Arikaree and Ogallala Groups or from differences in their mineral compositions. A quantitative model for mixing of different isotopic varieties of strontium indicates that Precambrian plagioclase, Paleozoic marine carbonate rocks, and Tertiary volcanic ash were the dominant sources of strontium in the pore solution and that Precambrian K-feldspar was the principal contributor of radiogenic 87Sr.

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