Amidon, William H. and Farley, Kenneth A. (2010) Mass spectrometric ^3He measurement in ^4He-rich phases: Techniques and limitations for cosmogenic ^3He dating of zircon, apatite, and titanite. Geochemistry Geophysics Geosystems, 11 (10). Art. No. Q10004 . ISSN 1525‐2027 http://resolver.caltech.edu/CaltechAUTHORS:20110103-094612966
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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.
|Additional Information:||© 2010 American Geophysical Union. Received 14 April 2010; accepted 29 July 2010; published 8 October 2010. This manuscript benefited greatly from reviews by Rainer Wieler and two anonymous reviewers. This work was supported by NSF‐EAR 0921295.|
|Subject Keywords:||helium; MAP 215-50; abundance sensitivity; noble gas; sensitivity|
|Official Citation:||Amidon, W. H., and K. A. Farley (2010), Mass spectrometric 3He measurement in 4He-rich phases: Techniques and limitations for cosmogenic 3He dating of zircon, apatite, and titanite, Geochem. Geophys. Geosyst., 11, Q10004, doi:10.1029/2010GC003178|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||12 Jan 2011 19:55|
|Last Modified:||26 Dec 2012 12:47|
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