206 Geochemistry I

Tuesday, 27 September 2016
1:30 PM - 5:30 PM
Room 403

NEUTRON ACTIVATED 80KR++ INTERFERING 40AR/39AR GEOCHRONOLOGY

Paper number: 206-2
Program: Technical Sessions
Time: 1:45 PM – 2:00 PM

Authors

Abstract

Various interfering reactions producing Ar isotopes during neutron irradiation from Cl, Ar, K, and Ca have been previously detailed and are routinely corrected for. Interference from doubly charged 80Kr (80Kr++) on 40Ar (mass/charge ratio difference of 0.0042 Da) is generally not considered. We show that significant amounts of 80Kr are produced through the activation reaction 79Br(n,β)80Kr. While the computed fission spectrum averaged cross section of the reactions is ~48 mb—compared to ~113 mb for 39K(n,p)39Ar—we observe ~10 times higher production rates of 80Kr compared to 39Ar in the Oregon State University fission reactor. For the herein used MAP-215 we determined an 80Kr+/80Kr++ ionization ratio of ~3.3. We show possible influence of 80Kr++ interference on 40Ar/39Ar dates for samples with elevated—but realistic—Br concentrations in the percent range. Most Br in samples occurs in fluid inclusions. Traditionally, strong 80Kr++ interference from decrepitating fluid inclusions would be erroneously interpreted as 40ArE. The herein newly described interference can be corrected by using the parallel 81Br(n,β)82Kr reaction.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-280326

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