AGE OF THE OCMULGEE LIMESTONE FROM THE K-AR AGE OF GLAUCONY GRAINS, COASTAL PLAIN, GEORGIA

The late Eocene Ocmulgee Limestone contains small amounts of dark grey-green to green-black botryoidal grains at 20-22 feet above the basal contact at its type locality at Taylor's Bluff (Pulaski County) Georgia. These grains contain glauconite and were separated for K-Ar study. A technique in which potassium is measured on the solid residue remaining after argon extraction was used to obtain the K-Ar age of a small amount (18 mg) of glaucony grains remaining after SEM and mineral study. Since only one sample, weighed into a copper-foil capsule, is used for both potassium and argon measurement in this technique, sample inhomogeneity does not contribute to error in the age value, nor does any error in weighing. Argon is extracted by heating the capsule and holding it between 1000°C and 1050°C for 10 minutes within a fused-quartz glass tube connected to the argon extraction manifold. The argon is diluted with an Ar-38 spike of known amount and argon isotopic composition, cleaned by reaction of other gases with heated titanium, and isotopically analysed. The capsule and the residue remaining within it is then digested with a mixture of hydrofluoric, nitric, and perchloric acids. After evaporation of SiF₄ and excess acid, the remaining salts are dissolved in a Cs-bearing solution for potassium measurement by flame atomic absorption spectrophotometry. An age of 33.7 ± 1.0 Ma (5.58% K) from the 18-mg glaucony sample places this interval of the Ocmulgee Limestone in a numerical time range that is consistent with the late Eocene/early Oligocene biostratigraphic age for the Ocmulgee Limestone. Subsequent work with other glaucony samples, including the interlaboratory reference sample GL-O, has confirmed that essentially all of the argon is extracted from glaucony heated as described above and that all of the potassium remains within the copper-foil capsule. This verifies the technique described herein as an accurate way to determine K-Ar age values for small amounts of glaucony.