Cosmogenic 3He exposure ages of basaltic flows from Miller Knoll, Panguitch Lake, Utah: Using the alternative isochron approach to overcome low-gas crushes

Abstract

Determining the ages of young volcanic rocks is important for understanding the tectono-magmatic development of geologic terranes. Usually, if rocks are old enough the 40 Ar/ 39 Ar or K–Ar techniques can provide reliable ages. However, when rocks are younger, they often lack enough daughter product to resolve an age. Cosmogenic 3 He methods provide an alternative for determining the eruption age of relatively recent mafic/intermediate lava flows. We sampled morphologically young basaltic andesite flows south of Miller Knoll, near Panguitch Lake on the Markagunt Plateau in southern Utah. We took two samples in close proximity from two different areas on the flows and separated both olivine and pyroxene. The typical protocol is to crush mineral separates on-line to determine an inclusion-hosted magmatic 3 He/ 4 He component. Then the powders are heated in a furnace to release the total 3 He and 4 He component and the crushed component is subtracted to determine the cosmogenic 3 He component. Unfortunately, in the case of the Miller Lake flows, 3 He yield from on-line crushes was below detection. An alternative isochron approach, which obviates the need for crush data, was first described by Cerling and Craig (1994) and more fully by Blard and Pik (2008). In this approach the 3 He/ 4 He of the total gas released from furnace heating is plotted vs. 1/ 4 He. If the samples plot on a line, then the resulting y-intercept is the magmatic 3 He/ 4 He and the slope of the line is the cosmogenic 3 He component, which determines the exposure age. Our data create good isochrons (MSWD = 0.76 and 0.14) with magmatic 3 He/ 4 He of 4.7–4.9 Ra. Concentrations of cosmogenic 3 He are 2.13 and 2.61 × 10 7 atoms g −1 after correction for radiometric 4 He using the R correction factor and measured and estimated U and Th concentrations in whole rock and minerals, respectively. Using the LSDn scaling routine and an online exposure age calculator, we determine zero erosion exposure ages of 32 ± 3 ka for the upper part of the flow and 34 ± 4 ka for the lower part of the flow. • We determined 3 He exposure ages of a basaltic flow on the Markagunt Plateau, Utah. • Olivine separates had low gas crushes and unresolvable crush/magmatic 3 He/ 4 He ratios. • We utilized the alternative isochron approach for determining cosmogenic 3 He. • Zero erosion exposure age (LSDn) of the Black Rock Valley flow is 32–34 ± 4 ka.