HR: 1340h
AN: V43E-1450    [Abstracts]
TI: (U-Th)/He Dating Of Volcanic Phenocrysts With High-(U-Th) Inclusions, Bandelier Tuff, New Mexico
AU: * Min, K
EM: kyle.min@yale.edu
AF: Yale University, Dept of Geology & Geophysics P.O. Box 208109, New Haven, CT 06520 United States
AU: Reiners, P W
EM: peter.reiners@yale.edu
AF: Yale University, Dept of Geology & Geophysics P.O. Box 208109, New Haven, CT 06520 United States
AU: Nicolescu, S
EM: stefan.nicolescu@yale.edu
AF: Yale University, Dept of Geology & Geophysics P.O. Box 208109, New Haven, CT 06520 United States
AU: Wolff, J A
EM: jawolff@mail.wsu.edu
AF: Washington State University, Department of Geology P.O. Box 642812, Pullman, WA 99164 United States
AU: Mundil, R
EM: rmundil@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Winters, L R
EM: winters24@hotmail.com
AF: Washington State University, Department of Geology P.O. Box 642812, Pullman, WA 99164 United States
AB: (U-Th)/He dating is typically applied to U-Th-rich crystals of apatite, zircon and titanite for thermochronologic and geochronologic studies. In some circumstances, mineral phases devoid of U and Th but containing U-Th-rich inclusions may also be dated. The host grains may efficiently retain He released from the inclusions, simplifying the alpha ejection or diffusive loss corrections. To test the application of (U-Th)/He dating in such circumstances, we analyzed volcanic fayalite and hedenbergite crystals containing abundant U-Th-rich inclusions found from the \sim1.6 Ma Otowi (Lower) Member of the Bandelier Tuff in New Mexico. U and Th, the major parent nuclides of radiogenoic $^{4}$He, are concentrated in the britholite ((Ca, REE$^{3+}$, Na)$_{10}$(Si, P)$_{6}$O$_{24}$(OH, F) $_{2}$) and chevkinite ((Ce, La, Ca, Th) $_{4}$(Fe$^{2+}$, Mg) $_{2}$(Ti, Fe$^{3+}$)$_{3}$Si$_{4}$O$_{22}$) inclusions and are essentially absent in the host grains of fayalite and hedenbergite. A subset of phenocrysts was subjected to controlled air-abrasion to remove the alpha-ejection affected regions. We analyzed 36 fayalite (21 non-abraded + 15 abraded) and 13 hedenbergite (7 non-abraded + 6 abraded) phenocrysts, and applied alpha recoil correction assuming uniform intracrystalline distribution of inclusions and parent nuclides in the phenocrysts. The F$_{T}$ factors (alpha retentivity) of the non-abraded samples are in the range of 0.90-0.96, whereas those of the abraded samples are essentially unity. The fayalite and hedenbergite crystals yield weighted mean ages of 1.62 \pm 0.11 Ma (1 standard deviation) and 1.68 \pm 0.11 Ma, respectively. The overall weighted mean age of 1.63 \pm 0.10 Ma is identical with the previously published $^{40}$Ar/$^{39}$Ar ages of 1.61 \pm 0.01 Ma. The calculated (U-Th)/He ages may decrease by up to 0.05 myr when the secular disequilibrium of U and Th decay series is considered. Nevertheless, the two isotopic systems yield indistinguishable ages within their uncertainties. These results suggest that other inclusion-bearing volcanic phenocrysts and their host units may be dated by the (U-Th)/He method. As shown in this study, olivine and pyroxene are suitable for this approach. However, phases such as quartz, sanidine or muscovite may not retain a significant portion of trapped $^{4}$He even at low temperatures over short time scales, undermining possible application of He dating to these minerals.
DE: 8400 VOLCANOLOGY
DE: 1035 Geochronology
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting