T104 What’s the Cosmognosis? Recent Advances in Understanding Earth and Planetary Processes with Cosmogenic Nuclides (Posters)

Monday, 23 September 2024
8:00 AM - 5:30 PM
Hall D

COSMOGENIC 3He DATING OF OLIVINE WITH TIGHTLY RETAINED MANTLE 3He, VOLCANO MOUNTAIN, YUKON

Paper number: T104-1
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

We present a step-heat method for isolating cosmogenic 3He (3Hec) 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 RA. Based on the 3He/4He ratio of 8.1 ± 0.2 RA 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 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. Regardless, the high proportion of mantle 3He in the powder fusion yields highly uncertain 3Hec 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 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. 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.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-401457

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