Combining U-Th ion microprobe analysis with (U-Th)/He dating has strong
potential for Quaternary volcano chronostratigraphy. U-Th zircon ages constrain the time
of crystallization, whereas (U-Th)/He dates the eruption. Uranium-series disequilibrium of
long-lived intermediate daughters ^{230}Th and ^{231}Pa, however, requires significant
corrections for Quaternary (U-Th)/He ages. For zircon, deficit in ^{230}Th (~75.7 ka
half-life) causes retarded ^{4}He accumulation compared to secular equilibrium and
consequently (U-Th)/He ages that appear too young. On the other hand, excesses in ^{231}Pa
(~32.8 ka half-life) are expected for zircon, leading to excess He and overestimation of
(U-Th)/He ages. Disequilibrium at the time of crystallization can be corrected through
known partition behavior of parent and daughter isotopes (D parameters), but many volcanic
rocks show evidence for protracted crystal growth and pre-eruptive residence during which
radioactive decay changes initial disequilibrium. For this reason, knowing crystallization
ages through direct measurement of individual crystals is essential.
In order to
improve disequilibrium corrections for (U-Th)/He dating, we have developed a new
computational routine (MCHeCalc) for disequilibrium correction of (U-Th)/He data based on
a Monte Carlo simulation. MCHeCalc randomly picks ages from the normal distribution
defined by the uncorrected measured (U-Th)/He ages and its uncertainty (light blue
Gaussian), and combined it with a randomly pick crystallization age from the normal
distribution represented by (U-Th)/He measured age and its uncertainty (blue Gaussian). We
calculate the deficit in |
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