Abstract: Isotopic Composition of Strontium in Evaporites from Ochoan Series of New Mexico and Cayugan Series of Ohio

E. L. Tremba, G. Faure

Sr⁸⁷/Sr⁸⁶ ratios of the water-soluble phases of evaporites from the Ochoan Series of New Mexico and the Cayugan Series of Ohio were measured in an attempt to determine their initial Sr⁸⁷/Sr⁸⁶ ratios. Twenty-seven samples of primarily halite with negligible amounts of rubidium from a 100-ft section of the Salado Formation (Ochoan) of New Mexico had a mean Sr⁸⁷/Sr⁸⁶ ratio of 0.7077 ± 0.0001 (one standard deviation). This value is similar to the Sr⁸⁷/Sr⁸⁶ ratio of 0.7073 reported for aragonitic marine fossils of similar ages. Samples of primarily sylvinite from the first and third potassium ore zones within the section had radiogenically enriched Sr⁸⁷/Sr⁸⁶ ratio as high as 0.7723.

On the other hand, four samples of primarily halite from the F unit (Cayugan) of Ohio had a mean Sr⁸⁷/Sr⁸⁶ ratio of 0.7093 ± 0.0001. This value is statistically greater than the Sr⁸⁷/Sr⁸⁶ ratio of 0.70785 for Late Silurian sea water determined by interpolation of Sr⁸⁷/Sr⁸⁶ ratios reported for aragonitic marine fossils of Late Ordovician (0.7079) and Middle Devonian (0.7078) ages. Possible explanations for this lack of agreement include the following. (1) Strontium in the F unit may have been isotopically reequilibrated following deposition. (2) The Sr⁸⁷/Sr⁸⁶ ratio of Late Silurian sea water was about 0.7093. (3) Strontium in the waters of the Ohio basin may not have been of the same isotopic c mposition as strontium in sea water of the same age. Analyses of samples from above and below the potassium ore zones in the section of the Salado Formation studied suggest that the isotopic reequilibration of strontium is less likely than one of the other two possibilities.

AAPG Search and Discovery Article #90978©1975 GCAGS-GC Section SEPM Annual Meeting, Jackson, Mississippi