Possible secondary apatite fission track age standard from altered volcanic ash beds in the middle Jurassic Carmel Formation, Southwestern Utah

Abstract

Secondary age standards are valuable in intra- and interlaboratory calibration. At present very few such standards are available for fission track dating that is older than Tertiary. Several altered volcanic ash beds occur in the Middle Jurassic Carmel Formation in southwestern Utah. The formation was deposited in a shallow marine/sabhka environment. Near Gunlock, Utah, eight ash beds have been identified. Sanidines from one of the ash beds (GUN-F) give a single-crystal laser-probe ⁴⁰Ar/³⁹Ar age of 166.3±0.8 Ma (2σ). Apatite and zircon fission track ages range from 152–185 Ma with typically 15–20 Ma errors (2σ). Track densities in zircons are high and most grains are not countable. Apatites are fairly common in most of the ash beds and have reasonable track densities ranging between 1.2–1.5 × 10⁶ tracks/cm². Track length distributions in apatites are unimodal, have standard deviations <1μm, and mean track lengths of about 14–14.5 μm. High Cl apatites (F:Cl:OH ratio of 39:33:28) are particularly abundant and large in ash GUN-F, and are fairly easy to concentrate, but the concentrates contain some siderite, most of which can be removed by sieving. GUN-F shows evidence of some reworking and detriaal contamination based on older single grain ⁴⁰Ar/³⁹Ar analyses and some rounding of grains, but the apatite population appears to be largely uncontaminated. At present BJK has approximately 12 of apatite separate from GUN-F.