HR: 15:10h
AN: U23B-06    [Abstracts]
TI: Extracting in situ cosmogenic 14C from olivine: significance for the CRONUS-Earth project
AU: Pigati, J S
EM: pigati@geo.arizona.edu
AF: University of Arizona, Geosciences Department and NSF-Arizona AMS Facility, Tucson, AZ 85721-0077 United States
AU: * Lifton, N A
EM: lifton@geo.arizona.edu
AF: University of Arizona, Geosciences Department and NSF-Arizona AMS Facility, Tucson, AZ 85721-0077 United States
AU: Quade, J
EM: jquade@geo.arizona.edu
AF: University of Arizona, Geosciences Department and NSF-Arizona AMS Facility, Tucson, AZ 85721-0077 United States
AU: Jull, A T
EM: lifton@geo.arizona.edu
AF: University of Arizona, Geosciences Department and NSF-Arizona AMS Facility, Tucson, AZ 85721-0077 United States
AB: One of the main goals of the Cosmic-Ray-prOduced NUclide Systematics on Earth (CRONUS-Earth) project is to compare production rates of in situ cosmogenic nuclides (CNs) at several well-dated locations in various rock types. Quartz is the most commonly used target mineral for several CNs (e.g., ¹⁰Be, ²⁶Al, ²¹Ne, ¹⁴C), but is generally absent in mafic volcanic terrains, where flows of different ages can constrain temporal variations in CN production at a given location. Because of its short half-life (5.73 ka), in situ cosmogenic ¹⁴C (in situ ¹⁴C) can be particularly useful for elucidating temporal variations in CN production over much shorter time scales than other CNs. While CNs such as ³⁶Cl and ²¹Ne can be measured in both mafic and felsic rocks, clearly it would be advantageous to measure in situ ¹⁴C in mafic rocks as well. As such, we have worked to develop reliable protocols to extract in situ ¹⁴C from olivine. We conducted numerous stepped combustion experiments testing the efficacy of various chemical pretreatments. We were able to extract a stable and reproducible in situ ¹⁴C component from olivine using a LiBO₂ flux, following pretreatment with dilute HNO₃. However, measured concentrations in olivine (normalized to SiO₂ composition) from two known-age basalt flows, the Tabernacle Hill flow (17.3ñ0.4 ka in age) in central Utah and the McCarty's flow (3.0ñ0.2 ka in age) in western New Mexico, were 3 to 5 times lower than predicted in situ ¹⁴C concentrations based on measurements in quartz. This discrepancy appears to arise from (1) a synthetic spinel-like mineral formed during our extraction process by the chemical interaction of the Al₂O₃ sample boat and olivine dissolved within the LiBO₂ flux, and (2) undissolved pyroxene phenocrysts (difficult to separate in quantity from olivines). Although we do not fully understand how the formation of the synthetic mineral may affect carbon atoms liberated from olivine, the concentration of in situ ¹⁴C atoms that we measured is directly proportional to the Fe-to-total-cation (Fe:TC) ratio of each sample. After applying simple correction factors based on the Fe:TC ratio and the percentage of pyroxene in the sample, measured in situ ¹⁴C concentrations were indistinguishable from predicted values at both calibration sites. Because the mineralogical composition (~30% fayalite, 70% forsterite) of the olivines studied here is common in basalt flows elsewhere and the Fe:TC correction factor appears to be predictable, in situ ¹⁴C can now be applied to CN research in basaltic terrains, complementing other CN measurements made for CRONUS-Earth.
DE: 1094 Instruments and techniques
DE: 1105 Quaternary geochronology
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
SC: Union [U]
MN: Fall Meeting 2005