Isotopic investigation of an aragonite marble Orcas Island, Washington
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Date
1979-06
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Publisher
The Ohio State University
Abstract
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.
Description
Research Award 1978-79