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

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