Mass spectrometric ^3He measurement in ^4He-rich phases: Techniques and limitations for cosmogenic ^3He dating of zircon, apatite, and titanite

Abstract

Recent calibration studies have expanded the range of target minerals suitable for cosmogenic ^3He dating to include U and Th-rich phases such as zircon, apatite, and titanite. These minerals often contain large amounts of radiogenic ^4He that present several analytical challenges for precise and accurate ^3He determinations. In this paper we document the abundance sensitivity and changes in the absolute sensitivity and time evolution of the ^3He signal over a wide range of ^4He pressures in a MAP 215-50 noble gas mass spectrometer. Large (>50%) decreases in sensitivity with ^4He amount arising from space charge effects were observed but can be corrected for using an isotope dilution–like technique in which ^3He spike is added to a sample midway through the mass spectrometric analysis. Large amounts of ^4He also cause the time evolution of the ^3He signal to become steeper, degrading precision of the initial peak height determination from the intercept. Taken together we find that these effects preclude reliable analysis of samples with ^4He > 1 μmol and that ^3He/^4He ratios of greater than ~5 × 10^(−10) are required to routinely measure ^3He to better than 20% precision. We present some general considerations by which to assess the probability of success of measuring cosmogenic ^3He in these phases as a function of elevation, exposure age, and helium cooling age.