Cosmogenic ³He dating of olivine with tightly retained mantle ³He, Volcano Mountain, Yukon

We present a step-heat method for isolating cosmogenic ³He (³He_c) from mantle He in olivine xenocrysts to date the eruption of four of the morphologically youngest nephelinite lava flows from Volcano Mountain (VM), the youngest cone in the Fort Selkirk volcanic field in Yukon, Canada. In these olivines, the standard procedure of powdering grains to <30 µm failed to effectively remove mantle helium prior to fusion: samples from four different flows yielded unusually high powder fusion ⁴He concentrations of 1.8 to 6.3 pmol g⁻¹, with ³He/⁴He ratios of 7.9 to 9.6 R_A. When combined with the ³He/⁴He ratios obtained by crushing (average 8.1±0.2 R_A), these measurements yield Holocene cosmogenic exposure ages but with very large uncertainties arising from the large mantle ³He correction. The inability to effectively isolate ³He_c from these samples likely arises from the survival of small (≪30 µm) fluid inclusions hosting mantle He through the powdering step. The presence of such unusually small fluid inclusions may relate to the origin of the olivines as disaggregated peridotite xenoliths rather than the more commonly analyzed olivine phenocrysts. We circumvented this problem by step-heating powdered olivine in steps of 800, 1000, and 1400 °C. Helium isotopic systematics indicate that 70 %–90 % of ³He_c was released in the low temperature step, and the rest was released in the middle temperature step. By the highest temperature step, the released He had a mantle-like ³He/⁴He ratio. Combining results from the step-heating and crush–fusion methods, we determined that the four Volcano Mountain lava flows erupted approximately coevally, at 10.5±1.7 ka.