A document by Hongcheng Guo. Click on the document to view its contents.

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.

Apatite (U-Th)/He (AHe) thermochronology depends on accurate knowledge of how diffusion occurs. This involves measurement of core diffusion kinetics as well as understanding the behavior of migrating He atoms. Drawing from previous studies as well as data obtained via continuous ramped heating (CRH), we assess several processes that need to be integrated into a single model for He diffusion in apatite. CRH analyses conducted at different heating rates show a kinetic response for both the “normal” lower-temperature and the higher-temperature release peaks, with peaks shifting to lower temperatures at lower heating rates. Where we do see a rollover in Arrhenius trends it also shows a kinetic response, being deferred to higher temperatures at higher heating rates, though many samples with unimodal release peaks do not show a significant rollover; fluorapatites seem to show more prominent rollover. For samples showing multiple release peaks, we find that their Arrhenius data often transition from one lower-temperature trend to another at higher temperatures that has about the same slope and thus activation energy. This looks very much like MDD behavior in K-feldspar, and MDD domain analysis fits the observed data very well, even if mechanisms involving discrete domain sizes are implausible. This interesting and unexplained result must speak to the nature of what is happening during analysis of samples having trapped He. To explore our data, we coded a simple diffusion model in which single He atoms are free to jump within a grid, but can also arrive at grid nodes designated as reversible sinks, escape from which depends on an exponentially temperature- dependent probability. The model includes radiogenic He production over geological thermal histories followed by laboratory CRH outgassing. When conditioned using D values observed for AHe, the model accurately predicts parameters such as closure temperature and fractional loss. When traps are introduced, the model simulates the essential nature of the dual-peak CRH results we see. Three important results emerge from this model. (1) Few sinks need be present. (2) Trapping occurs twice during diffusion, first in nature and then again during laboratory outgassing, meaning that the ratio of the gas amounts beneath each CRH peak overestimates the geological trapping. (2) Trapping in nature is very dependent on the sample’s thermal history: it is smallest for ancient rapid cooling and largest for samples that reside in the PRZ (allowing more radiogenic production to find traps before diffusion ceases). This model raises the possibility that complex CRH data record extended thermal-history information. If CRH and 4He/3He analysis were combined the 3He lab outgassing would record the sample’s trapping dynamics, and the 4He outgassing would reflect that plus a segment of the sample’s thermal history, which could be extracted using the 3He observations.

New and recently published U-Pb, muscovite-biotite 40Ar/39Ar, K-feldspar MDD 40Ar/39Ar, zircon and apatite (U-Th)/He, and apatite fission-track data were compiled and inverted for a comprehensive, thermal history of southern Baffin Island, Canada. This work is a contribution to the Geo-mapping for Energy and Minerals (GEM) Baffin Island initiative and Trans-GEM synthesis of the Phanerozoic exhumation history of the Canadian Shield. Southern Baffin Island is comprised of Archean plutonic basement metamorphosed during the Trans-Hudson Orogeny. Monazite U-Pb dating on the Hall Peninsula suggest peak metamorphic conditions were at ca. 1850-1820 Ma and remained at >550ºC ca. 100 My after the thermal peak [1], while 40Ar/39Ar hydrous mineral ages and modeling suggest temperatures remained at >420-450ºC ca. 150-200 My after peak conditions [2]. New apatite U-Pb age populations are in agreement and range from 1674 ± 35 Ma to 1796 ± 75 Ma (2σ), suggesting elevated post-THO temperatures at ~450ºC. During the Meso- to Neoproterozoic the Hall Peninsula region experienced prolonged slow cooling on the order of ≤0.5ºC/My until ca. 1000 Ma when cooling accelerated to ~1ºC/My due to supercontinent Rodinia assembly. Sedimentary sequences place minimum timing constraints on basement rocks being at near-surface conditions in the early Paleozoic. Preliminary results from apatite fission-track data suggest that southwest Baffin (Meta-Incognita and Hall Peninsula) was fully exhumed by Paleozoic time during basement uplift that likely exploited preexisting, regional structures. Nearby Foxe Basin sediments suggest this region of the Canadian Shield was exhumed by the Late Ordovician (ca. 450 Ma) and either remained topographically high, or experienced minor burial during subsequent continental-wide transgression and shallow marine carbonate deposition in the Silurian-Devonian. AFT data from a >1890 Ma volcanic tuff cutting the Paleoproterozoic Hoare Bay Group sediments on the easternmost Cumberland Peninsula record rapid cooling in the Jurassic. The cooling signal recorded along Cumberland Peninsula is likely due to early crustal thinning related to rifting of Greenland from mainland Canada during Pangaea breakup and aligns with a model of rift-flank uplift. AFT models are in agreement with ages of dike swarms in West Greenland given as evidence by [3] for the onset of rift extension. The summarized cooling history of southern Baffin Island suggests post-THO cooling rates of ~1-3ºC/My from ca. 1700-1500 Ma, followed by slow cooling and Mesoproterozoic cooling pulses at ca. 1300 Ma and ca. 1000-950 Ma, likely due to Rodinia assembly. Rocks have been at temperatures <100ºC since ca. 500 Ma. [1] Skipton et al., 2016, J. of Petrology, v.57(8); [2] Skipton et al., 2017, Lithos, v.284; [3] Larsen et al., 2009, J. Geol. Soc., v.166.

We performed continuous ramped heating (CRH) on apatites from various tectonic settings and found two major types of 4He outgassing behavior. Apatites with good (U–Th)/He age reproducibility show simple and unimodal incremental gas-release curves that are similar to those predicted by volume diffusion, whereas samples exhibiting greater age dispersion have complex gas-release curves that feature He ‘spikes’ and secondary gas-release peaks deferred to higher temperatures. Age dispersion from the apatites with simple outgassing behavior can be explained by variability in their relative He retentivity observed on Arrhenius arrays—with similar activation energy but different diffusivities, which may be a result of fine-scale crystal imperfections. The observed high-temperature gas component and the resulting “too-old” AHe ages, combined with an attempt at age correction based on secondary peak gas-removal, seem to indicate the existence of sink-like crystal imperfections that can trap 4He both in nature and during laboratory heating. CRH analysis at different heating rates further suggests that the second gas-release peak occurs at varying temperatures, indicating that the sink is kinetically responsive, and if characterizable, may contain additional thermal-history information. These observations suggest that (1) CRH can be deployed as a routine screening tool for (U–Th)/He dating, (2) diffusion of 4He could be complicated by imperfections beyond radiation damage, and (3) if the proposed sinks exist and retain appreciable 4He, there are opportunities to explore additional thermal histories of natural samples.