Combining [^(3)He] cosmogenic dating with U–Th/He eruption ages using olivine in basalt

Abstract

This paper presents new U–Th/He crystallization and ^(3)He exposure ages on olivine for a suite of basalt samples from the Snake River Plain, Idaho. The results provide the first demonstration that U–Th/He geochronology can be successfully applied to Late Quaternary basalts using measurements of olivine phenocrysts as opposed to U-rich minerals. The approach employs abrasion to remove the outer rinds of olivine crystals to avoid problems with ^(4)He implantation from the groundmass, and a specially designed extraction system that allows U, Th and He concentrations and isotopic compositions to be measured on the same olivine grains. We demonstrate the viability of the U–Th/He technique on basalts with ages of 373 ka and 3400 ka previously dated by Ar–Ar, and show that concurrent measurements of U–Th–4He ages and 3He cosmogenic exposure ages improve the accuracy of both methods. The combined U–Th–^(4)He–^(3)He technique is applied to a series of boulders and flows from Box Canyon near Hagerman, Idaho, to evaluate the ages of the basalts and to use exposure ages to make inferences about the age and mode of formation of the canyon. The U–Th/He ages of the Box Canyon basalts are between 86 ± 12 ka and 130 ± 12 ka (1-sigma errors) and suggest at least two periods of eruption. The ages are similar to a ca. 95 ka Ar–Ar age previously assigned, and distinguish the Box Canyon basalts from nearby ca. 395 ka basalt. Exposure ages of boulders within the canyon at the mouth, head and middle of Box Canyon are 21 ± 1, 19 ± 3 and 48 ± 3 ka; scoured bedrock at the canyon head has an exposure age of 45 ± 5 ka. These data indicate that the canyon was carved before the Bonneville Flood.

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