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.