INFRARED STIMULATED LUMINESCENCE EMPLACEMENT AGES FOR OBSIDIAN BUTTE RHYOLITE DOME (IMPERIAL COUNTY, CALIFORNIA)

Obsidian Dome is a small (~10^-3 km³) rhyolite dome at the southernmost limit of the Salton Buttes, a chain of five compositionally similar domes extruded at the southern terminus of the San Andreas fault. One of these domes (Red Island) has yielded a (U-Th)/He zircon eruption age of 2.48±0.47 ka (2σ) which is significantly younger than previous age determinations based on K-Ar geochronology. Here, we present preliminary infrared stimulated luminescence (IRSL) ages for sediment underlying Obsidian Butte in order to assess the timing of dome emplacement, and if this eruption was contemporaneous with that of Red Island. Three core samples 6.5 cm in diameter and ~10 cm in height from the Union Oil geothermal well 86-2 were selected from depths of 43.8, 49.6, and 72.8 m. The two shallow samples are from massively bedded volcaniclastic sediment with ~50% of pumiceous ash below an ~30 m thick lava flow. They are separated from the base of the flow by ~10 m of unconsolidated deposits including volcaniclastic sand, mud, and pumice, and lack any evidence for baking. The lowest sample is a fine-grained consolidated sand devoid of volcanic glass shards. The outermost 1 cm of the cores was sawed to remove light-exposed material. The center portions of the cores were then disintegrated and sieved. K-feldspar grains (175-200 µm) were separated by wet sieving, floatation in lithium metatungstate (LMT) heavy liquid (<2.58 g/cm³), and treatment with dilute HF. Single grain IRSL analysis was conducted at UCLA using a Risø TL-DA-20 instrument. The dominant K-feldspar population yielded ages (in descending stratigraphic order) of 3.7±0.6 ka, 3.0±0.5 ka, and 4.4±1.0 ka. Subordinate older and younger OSL age populations exist. These probably represent incomplete resetting and anomalous fading, respectively, and are still under investigation. Stratigraphic relations suggest a single depositional event for the volcaniclastic strata that immediately predated the emplacement of Obsidian Butte. The combined OSL age for the volcaniclastic samples of 3.3±0.5 ka overlaps within 2σ error with the (U-Th)/He age of Red Island dome. This implies a near-coeval emplacement of both domes. Very recent and concordant ages from two different chronometers underscore the potential for future volcanic activity for the Salton Buttes.