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

Abstract. We present a step-heat method for isolating cosmogenic 3He (3Hec) 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 RA. When combined with the 3He/4He ratios obtained by crushing (average 8.1±0.2 RA), these measurements yield Holocene cosmogenic exposure ages but with very large uncertainties arising from the large mantle 3He correction. The inability to effectively isolate 3Hec 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 3Hec 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.