Cosmogenic and nucleogenic ^3He in apatite, titanite, and zircon

Abstract

Cosmogenic ^3He was measured in apatite, titanite, and zircon and cosmogenic ^(21)Ne in quartz at 13 depth intervals in a 2.7-m long drill core in a Miocene ignimbrite from the Altiplano of Bolivia. All three ^3He depth profiles as well as the ^(21)Ne profile attenuate exponentially with depth, indicating that both of these isotopes are cosmogenic in origin with no significant contribution from other sources. The attenuation lengthscale for ^3He production of Λ = 180 ± 11 g/cm^2 is consistent with expectations for neutron spallation, and is identical to that found for the cosmogenic ^(21)Ne in quartz. By normalizing the measured ^3He concentrations to ^(21)Ne and using the independently known cosmogenic ^(21)Ne production rate, the apparent cosmogenic ^3He production rates in apatite, titanite, and zircon were respectively found to be 112, 97, and 87 atoms/g/yr at sea-level and high latitude. The formal uncertainty on these estimates is ~20% (2σ), and arises in equal parts from uncertainties in the measured ^3He/^(21)Ne ratios and the uncertainty in the ^(21)Ne production rate. However an additional factor affecting the apparent ^3He production rate in these phases arises from the long stopping range of spalled ^3He and tritium (which decays to ^3He). Because all three accessory phases have higher mean atomic number than major rock-forming minerals, they will have lower 3He production rates than their surroundings. As a consequence the long stopping ranges will cause a net implantation of ^3He and therefore higher apparent production rates than would apply for purely in-situ production. Thus these apparent production rates apply only to the specific grain sizes analyzed. Analysis of sieved zircon aliquots suggests that a factor of 2 increase in grain size (from ~50 to ~100 μm cross-section) yields a 10% decrease in apparent production rate. While this effect warrants further study, the grain sizes analyzed here are typical of the accessory phases commonly encountered, so the apparent rates provide an appropriate starting place for surface exposure dating using ^3He in these minerals.