(U-Th)/He dating of volcanic phenocrysts with high-U-Th inclusions, Jeniez Volcanic Field, New Mexico

We measured (U-Th)/He ages of fayalite and hedenbergite phenocrysts with britholite and clevkinite inclusion from the similar to 1.6 Ma Otowi (Lower) Member of the Bandelier Tuff, Jernez Volcanic Field. U and Th, the major parent nuclides of radiogen ic 4 He, are concentrated in britholite and chevkinite inclusions and are essentially absent from the host grains. We analyzed 36 fayalite (21 non-abraded + 15 abraded) and 13 hederibergite (7 non-abraded + 6 abraded) phenocrysts, and applied alplia-ejection corrections assuming uniform intracrystalline distributions of inclusions and parent nuclides in the phenocrysts. A subset of phenocrysts was subjected to abrasion to remove the alpha-ejection affected regions. The alplia-ejection correction factors (F-T) of the non-abraded samples range between 0.90 and 0.96, whereas those of the abraded samples are virtually unity. The fayalite and hedenbergite crystals yield mean ages of 1.60 +/- 0.10 Ma (S.D.) and 1.69 +/- 0.13 Ma, respectively. The overall mean age of 1.62 +/- 0.11 Ma (S.D.) is indistinguishable from previously published Ar-40/(39) Ar ages of 1.61 +/- 0.01 Ma (1 sigma). Our results Suggest that other inclusion-bearing volcanic plienocrysts and their host units may be dateable by the (U-Th)/He method. (c) 2005 Elsevier B.V. All rights reserved.