Thermochronology Methods

Multi-kinetic fission-track behaviour (e.g. Issler et al., 2005) is expressed by differential track annealing between single grains and is typical in samples that have different sedimentary provenance or variable chemical composition. Fission-track annealing within the apatite Ca10(PO4)6(F,Cl,OH) crystal lattice is controlled by physical and chemical processes. Annealing is temperature- dependent, and increasing temperature results in track-length shortening and track-density reduction (Green, Duddy, Gleadow, Tingate, & Laslett, 1986), whereas increased fission-track retention is influenced by chemical composition, primarily Cl content (Green et al., 1985) and various hydroxyl/cation substitutions, such as OH, Fe2+, Na+, and REE (Barbarand et al., 2003; Carlson et al., 1999). Collectively these phenomena produce intra-sample age dispersion that can be exploited, allowing ages to be grouped into separate kinetic populations for thermal history modelling, each with their own distinct annealing characteristics. Therefore a single AFT sample with three recognized kinetic populations can act as ‘three-samples-in-one’, each behaving as an independent thermochronometer allowing us to account for an expanded range in AFT thermal sensitivity beyond the canonical 110°C, dependent upon sample chemistry and experienced thermal history.

F, Na, Mg, P, S, Cl, Ca, Mn, Fe, Sr, Y, La, Ce are acquired for apatites using a JEOL JXA-8500F field-emission electron microprobe operated at 15 kV (20 nA current) with a beam size of 5 μm. Grains are separated into their respective populations based on radial plot mixture modelling and the apatite EPMA elemental analyses on age and length grains. Elemental data and the Carlson et al. (1999) multivariate equation were used to calculate rmr0 values and converted to 'effective Cl' (eCl) using the Ketcham et al. (1999) rmr0–Cl relationship. Negative effective Cl values (rmr0 > 0.84) indicate apatite that is characterized by very low track retentivity and associated low-annealing temperatures outside the calibrated range of the Ketcham et al. (1999) AFT annealing model.

Excerpts from McDannell et al., 2019 Terra Nova v. 31, Terra Nova - Special Issue - Thermo2018

Radiation-enhanced fission track annealing revisited

ESSOAr (preprint) Geochimica et Cosmochimica Acta (published)

Apatite fission track (AFT) analysis of granitoid and metamorphic bedrock samples from the Western Superior Province (Ontario), the Churchill-Rae Province (Melville Peninsula and Southampton Island, Nunavut), and the Slave Province (Northwest Territories) show a broad range of single grain effective uranium concentrations (eU) from <1 to ~300 ppm and are among some of the oldest reported AFT ages in North America. Typically, our samples characterized by low effective Cl (<0.1 apfu) and variable eU between single grains establish a correlation between high eU and younger AFT ages, while low eU grains are older. This eU-age relationship is resonant of the Hendriks and Redfield (Earth and Planetary Science Letters, 236, 443-458, 2005) argument for α-radiation enhanced fission track annealing (REA) and is analogous to the negative age-eU correlation observed in published zircon and titanite (U-Th)/He data from slowly-cooled cratonic rocks. Low-Cl bedrock apatites show high intra-sample age variability and exhibit strong REA control on AFT ages due to protracted histories (>200-500 m.y.) at <100°C since the Precambrian. Conversely, some AFT samples with a narrower eU range and low Cl show less age dispersion and weak apparent age-eU correlation. Instances where a counterintuitive relationship exists between eU and rmro (and effective Cl), imply a complex trade-off between radiation damage and chemical composition (e.g. low Cl and REE enrichment). In all cases, the samples fail the canonical χ2 test to evaluate if grains are from a single age population (χ2 <5%) and have characteristic “open jaw” radial plots, generally considered to indicate multiple age populations. Previous assessments of the influence of REA on AFT age were based on evaluating central age and mean track length, which potentially mask high single-grain age scatter and REA effects. Therefore, it is crucial that bedrock samples exhibiting high age scatter are evaluated in terms of intra-sample compositional heterogeneity. AFT samples with relatively low Cl concentrations are especially prone to greater REA control of cooling ages and underscores routine acquisition of compositional data for AFT datasets. Our broad range in single-grain AFT ages (with no other clear, strong compositional controls) supports the notion that radiation damage affects both the AFT and (U-Th)/He thermochronometers in slowly-cooled settings and must be accounted for during thermal modeling and interpretation.

Characterization of helium release from apatite by continuous ramped heating

Idleman et al., 2018, Characterization of helium release from apatite by continuous ramped heating

McDannell et al., 2018, Screening apatites for (U-Th)/He thermochronometry via continuous ramped heating: He age components and implications for age dispersion

Knowledge of the kinetic behavior of He in apatite and other U‑ and Th‑bearing minerals comes largely from detailed step-heating experiments, yet such experiments are time-consuming and are rarely performed during routine thermochronological studies using the U-Th/He method. We propose a new analytical method for measuring both the bulk 4He abundance and the kinetics of He release in apatite. Using this method He is extracted from samples by continuous heating following a ramped temperature schedule under static vacuum conditions, and the evolved He is measured periodically as it accumulates in the extraction system. Continuous ramped heating (CRH) experiments can be conducted using instrumentation available in most noble-gas thermochronology labs but require particular attention to temperature control, measurement linearity and dynamic range, and suppression of active gases co-evolved with He. CRH experiments require little more time than conventional single-step heating measurements but yield a detailed record of He release not provided by conventional methods. Kinetic parameters for He diffusion in Durango apatite derived from continuous heating data agree well with those obtained from published step-heating studies. The continuous record of He release obtained from CRH experiments also provides important information about the siting of He and the presence of multiple He components in apatite, some of which may be responsible for the anomalous U‑Th/He ages and high age dispersion that characterize some dated apatites. As such the CRH method shows promise as a useful sample screening tool for apatite U‑Th/He thermochronology.

photo: Granites outside the town of Uliastai, Mongolia copyright K. McDannell