HR: 17:45h
AN: V22G-08    [PDF]
TI: Preliminary Estimate of Production Rates for Terrestrial Cosmogenic $^{38}$Ar from Calcium
AU: * Knight, K B
EM: kimmer@eps.berkeley.edu
AF: Department of Earth \& Planetary Science, University of California, Berkeley, CA 94720-4767 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Department of Earth \& Planetary Science, University of California, Berkeley, CA 94720-4767 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Farley, K A
EM: farley@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AB: Cosmogenic $^{38}$Ar, dominantly produced from targets of Ca and K (and to a lesser extent from Fe and Ti), has been used in extra-terrestrial studies for decades. Recent measurement of terrestrial cosmogenic $^{38}$Ar (Renne {\it et al.}, 2001) primarily produced by high-energy spallation on calcium has shown potential as a useful addition to stable noble gas cosmogenic geochronology. Terrestrial cosmogenic production rates for both $^{38}$Ar$_{c}$ and $^{36}$Ar$_{c}$ have not yet been empirically constrained, however, in part because simple atmospheric corrections to measured $^{38}$Ar/$^{36}$Ar ratios are impossible. We have employed a different methodology, after Turner {\it et al.}, 1971, to quantify calcium derived $^{38}$Ar$_{c}$ production. Our method requires irradiation of mineral separates, using neutron activation to create $^{37}$Ar as a proxy for the cosmogenic target calcium via the reaction $^{40}$Ca(n,$\alpha$) $^{37}$Ar. The extent of conversion due to irradiation is monitored using co-irradiation of a standard with a known composition including [Ca] and [Cl], known age and no cosmogenic exposure, analogous to standards used in $^{40}$Ar/$^{39}$Ar dating. Approximately 15-30 mg of sample is loaded into a mass spectrometer and degassed incrementally with a CO$_{2}$ laser. Measured isotopes are corrected for backgrounds, mass discrimination, radioactive decay and additional argon isotopes produced in the irradiation. Step-wise degassing allows construction of a "cosmochron" plot of $^{38}$Ar$_{c}$/$^{36}$Ar vs. $^{37}$Ar$_{Ca}$/$^{36}$Ar, with a slope representing the $^{38}$Ar$_{c}$/ $^{37}$Ar$_{Ca}$, and an intercept ideally being that of atmospheric $^{38}$Ar/$^{36}$Ar ($\sim$0.188). Deviations from atmospheric $^{38}$Ar/$^{36}$Ar ratios imply sample disturbance or further complexity. We have used this irradiation method to successfully measure $^{38}$Ar$_{c}$ from calcium bearing minerals including diopside, clinopyroxene, garnet, sphene and apatite. A first order approximation can be made of the production of $^{38}$Ar$_{c}$ from calcium by using the better-known cosmogenic $^{3}$He production rate. In this study, we collected samples primarily from the Antarctic Dry Valleys, where long exposure histories relative to low erosion rates and high latitude, along with a diverse range of lithologies maximizes cosmogenic dosage and potential target minerals. We have measured both $^{3}$He$_{c}$ and $^{38}$Ar$_{c}$ in samples with identical exposure histories, using mineral pairs where necessary due to the limited application of $^{3}$He$_{c}$ to olivine, clinopyroxene and apatite. $^{3}$He$_{c}$ is normalized to total mineral mass, reflecting its production from a range of target atoms, while $^{38}$Ar$_{c}$ is normalized to grams of Ca. Complementary measurement of these two cosmogenic isotopes yields a strong linear correlation of $^{38}$Ar$_{c}$ atoms/g Ca and $^{3}$He$_{c}$ atoms/g sample with a slope of $\sim$0.9 $\pm$ 0.4 (MSWD of 1.5), demonstrating that spallation dominated pathways and energies for formation of cosmogenic $^{3}$He and $^{38}$Ar are consistent to a first order. These data also provide an estimate of $^{38}$Ar$_{c}$ production on calcium as $\sim$100 atoms/g Ca-year at high latitude, sea-level, significantly lower than the 200 atoms/g Ca-year production estimate calculated by Lal, 1991. Further geological calibration and modeling are underway to refine these estimates.
UR: http://eps.berkeley.edu/~kimmer/38Ar.html
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting