apatite, and titanite
نویسندگان
چکیده
[1] Recent calibration studies have expanded the range of target minerals suitable for cosmogenic He dating to include U and Th‐rich phases such as zircon, apatite, and titanite. These minerals often contain large amounts of radiogenic He that present several analytical challenges for precise and accurate He determinations. In this paper we document the abundance sensitivity and changes in the absolute sensitivity and time evolution of the He signal over a wide range of He pressures in a MAP 215‐50 noble gas mass spectrometer. Large (>50%) decreases in sensitivity with He amount arising from space charge effects were observed but can be corrected for using an isotope dilution–like technique in which He spike is added to a sample midway through the mass spectrometric analysis. Large amounts of He also cause the time evolution of the 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 He > 1 mmol and that He/He ratios of greater than ∼5 × 10 are required to routinely measure He to better than 20% precision. We present some general considerations by which to assess the probability of success of measuring cosmogenic He in these phases as a function of elevation, exposure age, and helium cooling age.
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تاریخ انتشار 2010