Cosmogenic and Nucleogenic 3He in Apatite, Titanite, and Zircon

Abstract

Cosmogenic He-3 was measured in apatite, titanite, and zircon and cosmogenic Ne-21 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 He-3 depth profiles as well as the Ne-21 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 He-3 production of Lambda=180 +/- 11 g/cm(2) is consistent with expectations for neutron spallation, and is identical to that found for the cosmogenic Ne-21 in quartz. By normalizing the measured He-3 concentrations to Ne-21 and using the independently known cosmogenic Ne-21 production rate, the apparent cosmogenic He-3 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 similar to 20% (2 sigma), and arises in equal parts from uncertainties in the measured He-3/Ne-21 ratios and the uncertainty in the Ne-21 production rate. However an additional factor affecting the apparent He-3 production rate in these phases arises from the long stopping range of spalled He-3 and tritium (which decays to He-3). Because all three accessory phases have higher mean atomic number than major rock-forming minerals, they will have lower He-3 production rates than their surroundings. As a consequence the long stopping ranges will cause a net implantation of He-3 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 similar to 50 to similar to 100 mu m 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 He-3 in these minerals.