Isotopic investigation of an aragonite marble Orcas Island, Washington
Creators:Endres, Robert A.
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Publisher:The Ohio State University
Series/Report no.:The Ohio State University. Department of Geology and Mineralogy Honors Theses; 1979
The carbonate fraction of an aragonite marble (20% aragonite, 80% calcite) from the Orcas Formation, Washington has an 87Sr/86Sr ratio = 0.70779 ± 0.00028. This is concordant with the ratio for Early to Middle Triassic sea water and thus could imply a Triassic age, consistent with fossil evidence. The 87Sr/86Sr ratio for the non-carbonate residue of the sample is 0.7123 ± 0.0007 and indicates that Sr homogenization during mid-Cretaceous metamorphism (250°C, 2.5 kB) may not have occurred, allowing the calculation of a Devonian age for the clay fraction. The strontium concentration of the carbonate (614 ppm) is consistent with the conclusion by Vance, (1975) that the aragonite is a recrystallized metastable product of original calcite. A δ18o value of +18.9°/oo and δ13c value of +3.1°/oo for the carbonate fraction is consistent with a known marine origin for the rock. A δ13c value of -26.1% for amorphous carbon of the residue fraction indicates a photosynthetic origin.
Research Award 1978-79