Cosmogenic ^(3)He and ^(21)Ne production rates calibrated against ^(10)Be in minerals from the Coso volcanic field

Abstract

This study calibrates the production rate of cosmogenic ^(3)He in pyroxene, olivine, garnet, zircon and apatite as well as ^(21)Ne in quartz and pyroxene against the known production rate of ^(10)Be in quartz. The Devil's Kitchen rhyolite from the Coso volcanic field in southeastern California (elev. ~ 1300 m) was chosen for this study due to its young age (~ 610 ka) and diverse mineral assemblage. Based on ^(10)Be, our two rhyolite samples have apparent exposure ages of ~ 49 and 93 ka, indicating substantial erosion after eruption. Combining data from the two samples, we estimate sea level high latitude ^(3)He spallation production rates of 145 ± 11, 141 ± 16, and 144 ± 30 at g^−1 a−^1 (2σ) for pyroxene, olivine and spessartine garnet respectively. For zircon and apatite, we estimate apparent 3He spallation production rates of 114 ± 8 and 149 ± 28 at g^−1 a^−1 (2σ) respectively. The rates for zircon and apatite are reported as apparent production rates because we do not explicitly address the redistribution of spallation produced ^(3)He from adjacent minerals. These estimates quantitatively account for production of ^(3)He from both cosmogenic and radiogenic neutron reactions on ^(6)Li within the analyzed phases and also implanted from nuclear reactions in neighboring minerals; the high U, Th and Li content of this rhyolite provides a particularly rigorous test of this correction. We estimate ^(21)Ne production rates of 17.7 ± 1.6 and 34.1 ± 3.2 at g^−1 a^−1 (2σ) in quartz and pyroxene (Fe/Mg = 0.7 by mass) respectively. Although high U and Th contents create the potential for significant production of nucleogenic ^(21)Ne, this component is small due to the young eruption age of the rhyolite.