The effects of secular disequilibrium on (U-Th)/He systematics and dating of Quaternary volcanic zircon and apatite

摘要: The (U–Th)/He dating method applied to U-rich phases such as zircon and apatite has sufficient sensitivity precision be of potential use for relatively recent geologic events volcanic eruptions. However, in with crystallization ages less than ∼1 Ma, chemical fractionation within the ^(238)U decay series may modify He ingrowth rate, causing computed from secular equilibrium age equation incorrect. resulting systematic error depends on [^(230)Th/^(238)U] activity ratio dated phase when it is erupted, eruption age. Zircons, which exclude Th relative U, will likely have ‘ages’ that underestimate by up a few tens %, decreasing increasing Apatites tend accommodate U little fractionation, so nearly concordant If minerals are erupted immediately after crystallization, disequilibrium effect can reasonably accounted based Th/U systematics. crystals reside unknown but potentially long periods magma chamber, degree reduced prior onset accumulation. analyses co-genetic fractionate U/Th differently, like zircon, used better constrain age, well provide insights into chamber residence time. We illustrate this approach zircons apatites Pleistocene-age Rangitawa Tephra, New Zealand.