DEVELOPMENT OF A ~3.3 MA SANIDINE STANDARD FOR 40AR/39AR DATING

Deino et al. (2015AGUFM.V51A3029D) discussed the merits of developing a set of intercalibrated mineral standards for ⁴⁰Ar/³⁹Ar dating, noting that the ⁴⁰Ar*/³⁹Ar_Kratio of geological unknowns should be as close as practical to those of co-irradiated mineral standards in order to achieve best analytical practices. A scheme involving an approximately factor-of-three age progression of geological materials was proposed (0.4, 1.2, 3.3, 9.4, 28.2, 95, 320, and 1080 Ma). Here we report on initial testing of a potentially suitable sanidine separated from a ~3.3 Ma tuff (sample EV-1) from Eureka Valley on the northern edge of Death Valley National Park, California. Single-grain incremental heating ⁴⁰Ar/³⁹Ar dating of this material using the BGC Noblessemulti-collector mass spectrometer yielded an age of 3.3032 ± 0.0025 Ma (± 1stotal error = 0.07%, n= 32), using sanidine from the Alder Creek Rhyolite of California as the neutron fluence monitor with a reference age of 1.1848 ± 0.0006 Ma (Niespolo et al., 2016). All 32 sanidine grains yield apparent-age plateaus, typically across the entire gas release. The Ca/K of these analyses range from ~ 0.010 – 0.015. Thus this material passes initial examination as an age standard, to be followed by further evaluation.