Application of Continuous Ramped Heating to Assess Dispersion in Apatite
(U-Th)/He Ages: A case study from Transantarctic Mountains of southern
Victoria Land
Abstract
Application of apatite (U-Th)/He thermochronology has been hindered by
incomplete understanding of single-grain age dispersion often displayed
by samples, particularly those from older, slowly cooled settings. To
assess the capability of continuous ramped heating (CRH) to explain
dispersion, we performed a study on an apatite suite from Cathedral
Rocks in the Transantarctic Mountains (TAM) that have high age
dispersion. Examining 132 apatite grains from a total of six samples, we
confirmed earlier apatite (U-Th)/He results showing that measured AHe
ages have at least three-fold intra-sample dispersion with no obvious
relationships between ages and effective uranium concentration (eU) or
grain size. CRH results on these apatites yielded two groups. Those with
younger ages, characterized by single-peak incremental 4He gas-release
curves, displayed simple volume diffusion behavior. In contrast, grains
with older ages generally show anomalous gas release in the form of
sharp spikes and / or extended gas-release at high temperatures (i.e.,
>= 800 °C). Well-behaved apatites still show considerable
age dispersion that exceeds what grain size, radiation damage, and
analytical uncertainty can explain, but this dispersion appears to be
related to variations in 4He diffusion kinetics. The screened AHe ages
from well-behaved younger apatite grains together with kinetic
information from these grains suggest that the sampled region
experienced slow cooling prior to rapid cooling (rock exhumation)
beginning ca. 35 Ma. This interpretation is consistent with other
studies indicative of an increase in exhumation rates at this time,
possibly related to the initiation of glaciation at the Eocene-Oligocene
climate transition. An attempt to correct anomalous older apatite ages
by simply removing extraneous gas-release components is proposed yielded
some ages that are too young for the samples’ geologic setting,
suggesting that the factors that lead to anomalous laboratory release
behavior can impact both the expected radiogenic component as well as
those that are extraneous. From our observations we conclude that: (1)
CRH analysis can serve as a routine screening tool for AHe dating and
offers opportunities to reveal first-order kinetic variations; (2)
model-dependent age correction may be possible but would require some
means of estimating the broad proportions of 4He components incorporated
into grains before and after closure to diffusion, and (3)
interpretation of highly dispersed AHe ages requires assessment of
individual-grain diffusion kinetics beyond that predicted by
radiation-damage models. We also infer that many apatite grains contain
imperfections of varying kinds that contribute significantly to kinetic
variability beyond that associated with radiation damage.