Mathematics – Logic
Scientific paper
Jul 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002e%26psl.201..117f&link_type=abstract
Earth and Planetary Science Letters, Volume 201, Issue 1, p. 117-125.
Mathematics
Logic
17
Scientific paper
The (U-Th)/He dating method applied to U-rich phases such as zircon and apatite has sufficient sensitivity and precision to be of potential use for dating relatively recent geologic events such as volcanic eruptions. However, in phases with crystallization ages less than ~1 Ma, chemical fractionation within the 238U decay series may modify the He ingrowth rate, causing He ages computed from the secular equilibrium age equation to be incorrect. The resulting systematic error depends on the [230Th/238U] activity ratio of the dated phase when it is erupted, and on the eruption age. Zircons, which exclude Th relative to U, will likely have secular equilibrium He `ages' that underestimate the eruption age by up to a few tens of %, decreasing with increasing eruption age. Apatites tend to accommodate U and Th with little fractionation, so apatite secular equilibrium He ages will be nearly concordant with eruption age. If minerals are erupted immediately after crystallization, the disequilibrium effect can be reasonably accounted for based on Th/U systematics. However, crystals are likely to reside for unknown but potentially long periods in a magma chamber, such that the degree of secular disequilibrium will be reduced prior to the onset of He accumulation. (U-Th)/He analyses of co-genetic phases that fractionate the U/Th ratio differently, like apatite and zircon, can be used to better constrain eruption age, as well as to provide insights into magma chamber residence time. We illustrate this approach with (U-Th)/He analyses of zircons and apatites of the Pleistocene-age Rangitawa Tephra, New Zealand.
Farley Ken A.
Kohn Barry P.
Pillans Brad
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