GSA Connects 2023 Meeting in Pittsburgh, Pennsylvania

Paper No. 128-4
Presentation Time: 2:25 PM


MCALEER, Ryan1, BLAKE, David2, CARTER, Mark W.3, JUBB, Aaron4, BECKER, Naomi5, DEASY, Ryan6, FISHEL, Emma R.2, FUTRELL, Justin2, MERSCHAT, Arthur7, ODOM III, William8, POWELL, Nicholas E.6 and STOKES, Rebecca4, (1)U.S. Geological Survey, 954 National Center, Reston, VA 20192, (2)Earth and Ocean Sciences, University of North Carolina Wilmington, 601 South College Road, Wilmington, NC 28403-5944, (3)U.S. Geological Survey, Florence Bascom Geoscience Center, Reston, VA 20192, (4)U.S. Geological Survey, Geology, Energy & Minerals Science Center, 12201 Sunrise Valley Dr, MS 954, Reston, VA 20192, (5)Department of Earth & Planetary Sciences, Johns Hopkins University, 3400 N Charles Street, Baltimore, MD 21218; Florence Bascom Geoscience Center, U.S. Geological Survey, MS 926A National Center, Reston, VA 20192, (6)U.S. Geological Survey, Florence Bascom Geoscience Center, MS 926A National Center, Reston, VA 20192, (7)Florence Bascom Geoscience Center, U.S. Geological Survey, MS 926A National Center, Reston, VA 20192, (8)Florence Bascom Geoscience Center, U.S. Geological Survey, Reston, VA 20192

Sanidine, the high temperature polymorph of K-feldspar, is metastable at Earth surface conditions, making it susceptible to recrystallization over geologic time. Thus, pristine sanidine suitable for use as an 40Ar/39Ar standard is extremely rare in pre-Oligocene rocks. Lack of a reliable pre-Oligocene single-grain sanidine standard results in a significant difference in measured 40Ar/39Ar ratios between standard and unknown when analyzing Mesozoic and older samples. This mismatch can introduce systematic measurement bias, ultimately affecting the accuracy of 40Ar/39Ar age determinations.

Here we present a preliminary characterization of K-feldspar from a Mesozoic rhyolite dike just south of Brinkleyville, NC, in the eastern Piedmont. Stoddard et al. 1986 and Stoddard 1992 defined and described a series of NNE-trending sanidine-bearing dikes, and later the dike series was dated, yielding a 40Ar/39Ar plateau age of 196.0 ± 0.7 Ma (Ganguli et al., 1995). We sampled a boulder from one of these dikes to evaluate the K-feldspar for potential use as an age standard.

The rock is composed of an aphanitic groundmass, ~15-20 vol% phenocrysts, and spheres of a clay mineral that are interpreted to reflect amygdule infills. The phenocrysts are dominantly quartz and K-feldspar and typically ~4-8 mm in size. Raman spectroscopy of the K-feldspar phenocrysts confirms that they are fully disordered sanidine despite their ~200 Ma age. Scanning electron microscopy and energy dispersive spectroscopy show that the groundmass is dominated by fine intergrowth of K-feldspar, quartz, and ilmenite. The sanidine phenocrysts, with an approximate composition of Or65Ab35, are more sodic than the groundmass K-feldspar. Notably, all phenocrysts have ~100 µm thick rims comprised of vermicular intergrowths of K-feldspar, quartz, and plagioclase indicating disequilibrium between the phenocrysts and the melt. Volumetrically these reaction rims are small but complicate single crystal 40Ar/39Ar analysis. Preliminary HF leaching experiments on these intergrowths suggest they can be preferentially dissolved, leaving the sanidine cores intact, and so the problem may be surmountable. Additional leaching and 40Ar/39Ar experiments are underway to further evaluate these Mesozoic sanidine grains as a potential geochronology standard.