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

Abstract

ABSTRACT The 87Sr/86Sr ratios of sandstone cements reflect the isotopic composition of Sr released into the pore fluid by different rock and mineral constituents. However, little is known about the extent to which the isotopic compositions of Sr in cements reflect local or regional variations in sandstone compositions. The 87Sr/86Sr ratios of sandstone cements permit identification of the major sources of Sr in the pore fluid and indicate the dimensions of the aquifer system within which the pore fluid was isotopically homogenized. Since the abundance of radiogenic 87Sr is continually increasing by decay of 87Rb, isotopic compositions of Sr may also suggest the sequence and time of cement formation in sandstones that contain det ital 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 87Sr/86Sr ratios of 0.7093 and 0.7133. In contrast to the detrital fractions of the sandstones, the cements contain Sr that is isotopically homogeneous over distances of 70 kilometers 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 87Sr86Sr ratio of the cements suggest that the pore waters were homogeneous on a regional basis and were not locally controlled. The slight difference in the isotopic composition of Sr in the cements of the Arikaree and Ogallala Groups may have resulted either from decay of 87Rb during the time interval of about 20 million years 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 Sr indicates that Precambrian plagioclase, Paleozoic marine carbonates and Tertiary volcanic ash were the dominant sources of Sr in the pore solution and that Precambrian K-feldspar was the principal contributor of radiogenic 87Sr.