ERUPTION AGE OF THE SANTA CLARA BASALT FLOW, SOUTHWEST UTAH FROM COSMOGENIC 3He SURFACE EXPOSURE AGES AND PREVIOUS CONSTRAINTS

The Santa Clara basalt flow descends from the upper reaches of Snow Canyon into the town of Santa Clara, just to the west of Saint George, Utah. It is imperative that we accurately date enigmatic eruptions of the Basin and Range, like this one, so we can prepare for future eruptions and understand how they fit into the broader tectonic models of the region. The Santa Clara lava flow is too young to be dated by potassium-argon. The timing of eruption was previously constrained by ¹⁴C dating of charcoal just below the flow and optically stimulated luminescence (OSL) dating of sand deposited before the eruption. These published estimates bracket the timing of eruption between 32,600 ± 400 cal yr (¹⁴C) and ~40,350 ± 2600 cal yr (OSL) (Willis and Hayden, 2015). To refine the timing, we applied cosmogenic ³He exposure age dating to three widely spaced patches of the flow surface south of Snow Canyon. Measurements were made in the University of Utah Noble Gas Lab. We used the ‘standard’ crush technique to determine the magmatic ³He/⁴He ratio of olivine separates which we subtracted from measured ³He totals. To account for radiogenic ⁴He affecting the ³He/⁴He mantle ratios we used the R-correction factor. We used the University of Washington online exposure age calculator, the LSDn scaling routine and the global ³He production rate dataset to determine exposure ages. No corrections for shielding or erosion were applied. Crush He ratio values ranged from 6.7 to 7.4 R/R_air, reasonable values for these continental basalts. They will be further refined this Spring with additional analyses. Ten exposure ages range from 27 ± 1 ka to 32 ± 2 ka with a weighted mean age of 29.5 ± 0.8/1.3 ka (MSWD = 2.1) and a sharp peak in the kernel density plot at 28-30 ka. One outlier exposure age of 18 ka is from an area likely affected by intermittent sand cover. These ages agree very well with previous dates. Our ages are younger than OSL ages of sand under the flows of 40-41 ka that pre-date flow emplacement (Willis and Hayden, 2015). Our ages are in great agreement with a ¹⁴C age from charcoal likely burned during flow emplacement of 27,270 ± 250 ¹⁴C years (Willis and Hayden, 2015) which calibrates to a 2-sigma age range of 29.1—29.8 cal. ka using Calib8.2 with the IntCal 20 curve. Note this ¹⁴C age is re-calibrated compared to the previously published age from Willis and Hayden (2015).