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

Abstract

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.

Introduction

Tephra deposits resulting from highly explosive eruptions commonly form distinctive stratigraphic and chronologic markers. Where tephras can be correlated over large areas, they are particularly useful for detailed correlations in both marine and terrestrial sequences and between them (e.g. [1], [2]). Many isotopic techniques are available for dating such rocks (e.g. [3]), but those used most routinely are radiocarbon dating, generally extending from >0.3 ka to <55 ka [4], [5]; K–Ar and ⁴⁰Ar/³⁹Ar dating, capable of measuring ages <2 ka in favorable circumstances (e.g. [6], [7]) which are relatively rare, and fission track dating of glass (e.g. [8]) and zircon (e.g. [9]) down to ∼100–200 ka. The usefulness of tephra deposits as time markers is often limited by a lack of suitable material to date with these techniques as well as by relatively poor dating precision. In this study, we investigate the (U–Th)/He method [10] as a potential new tool for dating young volcanic deposits. In particular, we consider the effects of U-series disequilibrium and present new (U–Th)/He analyses on apatite and zircon from three exposures of the widespread ∼340-ka Rangitawa Tephra in New Zealand.