PRELIMINARY CHARACTERIZATION OF A POTENTIAL MESOZOIC SANIDINE MONITOR STANDARD FOR 40AR/39AR GEOCHRONOLOGY

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.