--> Abstract: Fundamental Limitations in the Use of <SUP>87</SUP>Sr/<SUP>86</SUP>Sr in Temporal Correlation of Paleozoic Rocks, by S. C. Ruppel; #90937 (1998).

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Abstract: Fundamental Limitations in the Use of 87Sr/86Sr in Temporal Correlation of Paleozoic Rocks

RUPPEL, STEPHEN C., Bureau of Economic Geology, The University of Texas at Austin, Austin. TX, 78713-8924

In recent years, the record of change in seawater 87Sr/86Sr during the Phanerozoic has been repeatedly updated. This revision is largely the result of better measurements made possible by improvements in technique and instrumentation. Advancements in technique have come mostly in sample selection and preparation. In large part, the new data have served to refine the secular curve of 87Sr/86Sr change and increase resolution. For the Paleozoic, however, these new, higher quality analyses have begun to raise fundamental questions about the limitations of 87Sr/86Sr chemostratigraphy as a relative dating technique for these rocks. Comparisons of 87Sr/86Sr data obtained from brachiopods and conodonts, the phases thought to be the best recorders of unaltered seawater 87Sr/86Sr reveal major discrepancies and indicate that a reevaluation of these data and their sources is necessary.

Empirical observations of 87Sr/86Sr variations in brachiopods and conodonts show that both can undergo strontium isotope exchange during diagenesis. For brachiopods, cathodoluminescence has been widely used to avoid diagenetically altered phases; for conodonts, preanalysis treatment has been employed. With all samples it is generally assumed that the lowest ratios are nearest true seawater chemistry.

Despite these precautions, recent conodont and brachiopod data from equivalent Devonian strata display significant variations in 87Sr/86Sr This fact implies that either (1) substantial diagenetic overprint exists in these samples that is not definable using currently employed techniques or (2) the seawater chemistry recorded by these samples is characterized by short-term temporal or geographical variations. At present, the uncertainties surrounding these possibilities severely undermine the potential for high-resolution 87Sr/86Sr chemostratigraphy in the Paleozoic.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah