COSMOGENIC 3HE AGES OF BASALTIC ANDESITE FLOWS FROM MILLER KNOLL, PANGUITCH LAKE, UT: USING THE ISOCHRON TECHNIQUE TO OVERCOME LOW-GAS CRUSHES

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 ⁴⁰Ar/³⁹Ar or K-Ar techniques can provide reliable ages. However, when rocks are younger, they often lack enough daughter product to resolve an age. Cosmogenic techniques, and particularly ³He, provide an alternative for determining the ages of relatively recent mafic/intermediate lava flows. Requirements for using cosmogenic ³He techniques are: 1) rocks have phenocrysts of olivine or pyroxene, 2) flows are un-weathered and have original flow surfaces, and 3) trapped magmatic gasses can be corrected for. Morphologically young looking basaltic andesite flows south of Miller Knoll, near Panguitch Lake on the Markagunt Plateau in southern Utah meet these requirements. We took two samples in close proximity from two different areas of the flows and separated both olivine and pyroxene. The typical protocol is to take splits of the mineral phase and crush them on-line to determine an inclusion hosted magmatic ³He/⁴He component. Then the powders are heated in a furnace to release the total ³He and ⁴He component. The crushed component is subtracted to determine the cosmogenic component. Unfortunately in the case of the Miller Lake flows, we measured no gas on the on-line crushes. 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 ³He/⁴He of the total gas released from furnace heating is plotted vs. 1/⁴He. If the samples plot on a line, then the resulting y-intercept is the magmatic ³He/⁴He and the slope of the line is the cosmogenic ³He component, which determines the exposure age. Our data create good isochrons (MSWD = 0.58 and 0.11) with magmatic ³He/⁴He of 4.7 to 4.9 Ra. Concentrations of cosmogenic ³He are 2.13 and 2.61 x10⁷ atoms g^-1after correction for radiometric ⁴He 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 the CRONUS-Earth online exposure age calculator, we determine exposure ages of 31±3 ka for the upper part of the flow and 35±4 ka for the lower part of the flow.