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 very young nephelinites from Volcano Mountain (VM) Yukon, Canada. In these olivines, the standard procedure of powdering grains to <30 µm failed to adequately remove mantle helium prior to fusion analyses. For example, in one powder fusion the concentration of ⁴He was 109 ncc STP/g with a ³He/⁴He ratio of 8.7 ± 0.3 R_A. Based on the ³He/⁴He ratio of 8.1 ± 0.2 R_A released by crushing of the same sample, the estimated fraction of mantle ³He in the powder fusion is between 87 % and 98 % of the total ³He. 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. Regardless, the high proportion of mantle ³He in the powder fusion yields highly uncertain ³He_c exposure ages. We circumvented this problem by heating powdered olivine in a three-step heating schedule ranging from 700 to 1400°C. 80-92 % 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. Using this technique on two samples from the youngest VM flow, we obtained precise estimates of cosmogenic ³He concentrations, from which we derive an eruption age of 10.9 ka ± 1.1 ka.