The 17th International Conference on Thermochronology (Thermo2021) was held in Santa Fe, New Mexico, on September 12-17, 2021. This bi-annual conference series evolved via the coalescence of the International Workshops on Fission Track Thermochronology, held since 1978, and the European Workshops on Thermochronology. It has become the premier forum for thermochronology practitioners and users to discuss fundamental and methodological topics and opportunities related to their science and its future. Each conference is independently organized, and a Standing Committee consisting of past organizers and other community members helps to ensure their continuation into the future. Thermo2021 was greatly affected by the COVID-19 pandemic. Normally the meeting would have been expected to draw ~250 attendees, but travel restrictions limited in-person attendance to 86, plus 21 remote presenters. Nearly all in-person participants were from the US, and only four were international. Talks and posters were distributed among five themes: (U-Th)/He; fission track; other thermochronometers; frontiers in data handling, statistics, interpretation methods, and modeling; and integration and interpretation. Although COVID-19 presented many challenges, it also allowed the Organizing Committee to adapt creatively and transform adversity into opportunity. In particular, the smaller number of attendees permitted more talks by students and early-career scientists, both within the theme sessions and in the Charles & Nancy Naeser Early Career Session. Discussion time was prioritized: at a Tuesday evening “swap meet” for ideas, in 30-40-minute time slots within each theme session, and in Friday afternoon breakouts for the first four themes and another dedicated to early career and DEI issues. These were used to identify emergent ideas and concerns across a broad range of topics, from the theory and practice of the various thermochronometric techniques, to their interpretation through thermal history modeling and other methods, to anticipated trends in data dissemination and management, to the needs of the next generation of thermochronologists, particularly in the US. Each Friday breakout designated a scribe who recorded the discussion and distributed their notes. Each group then designated one or more writers to transform the notes into text for this White Paper. Notes or early write-up versions were provided to the international thermochronology community, and feedback solicited. In addition, cross-cutting themes that occurred across multiple breakout groups were identified and compiled. This White Paper is the outcome of these efforts. We hope that it will serve as a record for the meeting, and an overview of where the predominantly US-based component of the thermochronology community considers the current state of knowledge to be and where future efforts should be directed, for developing both the science and its human infrastructure.

Monazite fission-track presents itself as a novel, low-temperature thermochronometer with annealing studies placing its closure temperature between ~45 and 25 °C. Previously, monazite has been unsuitable for fission-track dating due to high abundance of gadolinium and insufficient investigation of the etching protocol. Gadolinium causes self-shielding via thermal neutron capture and substantial associated nuclear heating during irradiation which prevented robust monazite fission-track dating using the traditional external detector method. Further, early etching studies were found to be extremely corrosive to monazite grains. However, developments in LA-ICP-MS fission-track analysis allow for measurement of 238U and improvements in monazite fission-track etching protocols mean that dating monazite through the fission-track method is now viable. In this study, we present monazite fission-track data from an elevation profile (2260 m, 2000 m, 1600 m, and 1200 m) from the Catalina metamorphic core complex (Catalina MCC), in southern AZ, USA. We follow the etching protocol described in Jones et al. (2019), etching the monazites in 6 M HCl for 90 minutes at 90 °C. We measure the 238U concentration via LA-ICP-MS and compare the dates to other multi-method thermochronology from the same rocks. Traditional low-temperature thermochronology (apatite and zircon fission-track, apatite and zircon (U-Th-Sm)/He) from the Catalina MCC reveals cooling at 25-20 Ma and 18-10 Ma. Preliminary monazite fission-track analysis yields a date of 6.1 ± 0.4 Ma, far younger than all the traditional thermochronometric data, in-line its far lower closure temperature. The 6 Ma monazite fission-track date is consistent with the youngest phase of hematite (U-Th)/He dates observed in the nearby Rincon metamorphic core complex and suggest that these dates correspond to the latest phase of exhumation in response to Basin and Range extension and/or climate enhanced erosion. These preliminary results show that monazite fission-track can reveal shallow crustal processes and contribute to constraining thermal histories below ~60 oC, which are traditionally difficult to resolve.

Arc-arc collision plays an important role in the formation and evolution of continents (e.g., Yamamoto et al., 2009; Tamura et al., 2010). The Izu collision zone central Japan, an active collision zone between the Honshu Arc and the Izu-Bonin Arc since the middle Miocene (Matsuda, 1978; Amano, 1991; Kano, 2002; Hirata et al., 2010), provides an excellent setting for reconstructing the earliest stages of continent formation. Multi-system geo-thermochronometry was applied to different domains of the Izu collision zone, together with some previously published data, in order to reveal mountain formation processes, i.e., vertical crustal movements. For this study nine granitic samples yielded zircon U–Pb ages of 10.2–5.8 Ma (n = 2), apatite (U–Th)/He ages of 42.8–2.6 Ma (n = 7), and apatite fission-track (AFT) ages of 44.1–3.0 Ma (n = 9). Thermal history inversion modelling based on the AFT data using HeFTy ver. 1.9.3 (Ketcham, 2005), suggests rapid cooling events confined to the study region at ~5 Ma and ~1 Ma. The Kanto Mountains are thought to be uplifted domally in association with collision of the Tanzawa Block at ~5 Ma. But this uplift may have slowed down following migration of the plate boundary and late Pliocene termination of the Tanzawa collision. The Minobu Mountains and possibly adjacent mountains may have been uplifted by collision of the Izu Block at ~1 Ma. Mountain formation in the Izu collision zone was mainly controlled by collisions of the Tanzawa and Izu Blocks and motional change of the Philippine Sea plate at ~3 Ma (Takahashi, 2006). Earlier collisions of the Kushigatayama Block at ~13 Ma and Misaka Block at ~10 Ma appear to have had little effect on mountain formation. Together with ~90° clockwise rotation of the Kanto Mountains at 12-6 Ma (Takahashi & Saito, 1997), these observations suggest that horizontal deformation predominated during the earlier stage of arc-arc collision, whereas vertical movements due to buoyancy resulting from crustal shortening and thickening developed at a later stage. References: Amano, K., 1991, Modern Geol., 15, 315-329; Hirata, D. et al., 2010, J. Geogr., 119, 1125-1160; Kano, K., 2002, Bull. EQ Res. Inst. Univ. Tokyo, 77, 231-248; Ketcham, R.A., 2005, Rev. Min. Geochem., 58, 275-314; Matsuda, T., 1978, J. Phys. Earth, 56, S409-S421; Takahashi, M., 2006, J. Geogr., 115, 116-123; Takahashi, M. & Saito, K., 1997, Isl. Arc, 6, 168-182; Tamura et al., 2010, J. Petrol., 51, 823, doi:10.1093/petrology/egq002; Yamamoto, S. et al., 2009, Gond. Res., 15, 443-453.

Factors influencing data reproducibility of fission-track (FT) thermochronology can be summarized into three main categories associated with data acquisition steps. (1) Sample preparation involves mineral separation, mounting, polishing and etching; (2) data revelation relates to instrumentation (microscope, LAICPMS, etc.) and software settings; and (3) execution depends on feature selection by the analyst. Previous committee reports and studies (Hurford A.J. 1990; Ketcham et al. 2009; Ketcham et al. 2015; Ketcham et al. 2018) have contributed significant insights into the reproducibility of fission-track data by comparing length and age measurements produced by several laboratories using their own preparation and revelation procedures. A recent attempt to isolate analyst-specific factors in length measurement using an image-based approach (Tamer et al. 2019) found that when two analysts observe the same feature and agree it is a valid track, measurement reproducibility was very good, though impacted by etching. Dispersion of individual length measurements was 0.7-1.0 µm (2 for weaker etching and 0.5-0.8 µm for stronger etching, but mean lengths were always within 0.1 µm of each other. Where the analysts disagreed more significantly, however, was in finding tracks and evaluating whether they were valid, sufficiently clear, and sufficiently etched for measurement, which led to differences of up to ~0.8 µm in mean track length. This study builds on the image-based approach to encompass more aspects of the measurement process and increase the number of analysts being compared. We will look at confined track selection in greater detail, and also study analyst decisions behind age determination, including the selection of the region of interest for counting, and identification of grain-surface features as tracks appropriate for counting. Reflected and transmitted light image stacks for 41 grains and graticules are available on a cloud platform Participants will carry out analyses of these images using their preferred approach, e.g. suitable analytical software, manual measurements or AI-based analysis. A limited license for FastTracks (v3.2) will be available for those who would like to participate but do not have measurement software. Analysts are asked to fill out a questionnaire about their fission track experience, conduct track density estimations, confined track length and Dpar measurements, and especially provide comments on all grains being analyzed or skipped. FastTracks users are asked to send the .xml files produced by the software, while other participants are asked to submit the results using a template. The results will be entirely anonymous unless the analyst states otherwise. The deadline for the submission of the results is June 1st, 2022. The results will be shared on 18th International Conference on Thermochronology.

Apatite (U-Th)/He (AHe) dating is a widely-applied thermochronological technique used to decipher low-temperature thermal histories. Accurate dates require that the results are corrected for α-ejection because 4He atoms travel ~20 µm during α-decay and a correction is required to account for He lost by this effect. Effective uranium concentrations (eU) are important for accurate AHe data interpretation because radiation damage scales with eU, which affects He retentivity. Both α-ejection correction parameter (Ft) and eU are calculated on the basis of crystal size and assuming an idealized morphology. However, the uncertainty stemming from the calculations’ assumptions depends on how much the real crystal geometry deviates from that assumed, and this uncertainty is typically not included in the propagated uncertainties on AHe data. Our goal for this study was to develop a ‘rule of thumb’ for Ft and eU uncertainties associated with the full range of commonly analyzed apatite geometries by comparing manually measured grain size and actual grain size using nano-computed tomography (nano-CT). Apatite geometry and roughness were characterized using a Grain Evaluation Matrix (GEM). The geometry of each grain was described as: A (prismatic/hexagonal), B (subprismatic), or C (rounded/ellipsoid). Surface roughness was graded from ‘least’ to ‘most’ using values from 1 to 3. The GEM allows for a single parameter (eg. B2) to succinctly classify a grain’s morphology. High resolution nano-CT scans of ~260 grains representative of those usually analyzed for AHe dates were completed and processed using Dragonfly and Blob3D. Initial analysis shows that manual grain measurements systematically overestimate the actual grain size, leading to overestimates in Ft and eU values. One correction exists for A and B grains (hexagonal) and another for C grains (ellipsoid). The correction is controlled primarily by grain size and shape, while the uncertainty on the correction appears to be controlled primarily by surface roughness. Together, this approach provides a simple and practical tool for deriving more accurate Ft and concentration values, and for incorporating this oft neglected geometric uncertainty into AHe dates.

The “Laramide-style” uplifts of North America—characterized by blocks of Proterozoic-Archean basement that were exhumed along reverse faults within the Cordilleran foreland basin—are widely interpreted to be a result of flat-slab subduction of oceanic lithosphere beneath the continent. Despite this general consensus, the causal mechanisms of basement-cored uplifts remain unclear. Assessment of the hotly debated geodynamic models that relate flat-slab subduction and upper crustal deformation hinge on the availability of accurate estimates for the timing of exhumation of Laramide uplifts. A major problem with current models is that they do not incorporate timing constraints from the Laramide region of central Montana. This region represents the northernmost extent of Laramide deformation and timing constraints are critically needed to enhance our understanding of how stress is transmitted inboard during flat-slab subduction. We present the first low-temperature thermochronological ages from the Little Belt Mountains (LBM) of central Montana, which is the northernmost Laramide-style uplift with exposed basement gneisses. Apatite fission track ages ranging from ca. 90-70 Ma suggest that the core of the LBM was exhumed through the 120-60°C apatite partial annealing zone in the Late Cretaceous. These ages corroborate recent low-T thermochronology and sedimentological work that propose an earlier than previously recognized onset of Laramide deformation in southwestern Montana and eastern Idaho (>80 Ma), but additional data are needed to reduce the uncertainty between ages and ascertain the exhumation history of the LBM. To this end, we will integrate new low-temperature thermochronology (<150°C) and associated thermal history models in order to constrain the Cretaceous tectonic evolution of the LBM and the extent of Laramide deformation in the western USA.

The zircon (U-Th)/He (ZHe) system with a typical closure temperature of ~160-200°C*1, but lower for higher radiation damaged grains*2, offers the potential for evaluating thermal histories in the uppermost ~10 km of the crust. ZHe thermochronometry has been applied to different geological settings in order to estimate tectonics, uplift and denudation, basin evolution, etc.*3, which can also contribute to evaluating long-term tectonic stabilities for the geologic disposal project. So far, the effectivity of ZHe thermochronology has been verified, however improved age standards for the method are required. To date, the method has conventionally employed zircon fission-track age standards such as the Fish Canyon Tuff (FCT) zircon*4. ZHe grain ages are sometimes over-dispersed owing to factors such as zoning of parent nuclei, radiation damage, grain size and He-bearing inclusions*2,5. Considerable parent isotope zonation was reported in some FCT crystals*6, inviting a search for alternative potential ZHe standards*7,8,9. These works reported robust ZHe data with little age dispersion because of homogeneous U-Th distribution in zircon megacrysts, making them possible reference material candidates. However, a practical issue remains because ZHe analyses of unknown samples are carried out grain-by-grain as opposed to analyzing large pieces of a single grain. We have attempted to assess suitable zircon samples as ZHe age standards by using rapid cooling rock samples of relatively young (<100 Ma) age. This is because such rock samples are expected to empirically exhibit simple thermal histories and little radiation damage. Therefore, age dispersion caused by radiation damage can be relatively small. In order to reassess previous data obtained by Tagami et al. (2003)*10, ZHe analyses of the Pliocene Utaosa rhyolite (TRG-04 and -07) and the Miocene Buluk Tuff have been carried out. In addition, OD-3 zircon*11, a zircon U-Pb age standard, was also analyzed. In this presentation, preliminary ZHe age data from these samples will be presented and compared to evaluate their suitability as ZHe reference materials e.g., FCT. References 1: Reiners et al. (2004), Geochim. Cosmochim. Acta, 68, p. 1857–1887 2: Guenthner et al. (2013), Am. J. Sci., 313, p. 145–198 3: Ault et al. (2019), Tectonics, 38, p. 3705–3739 4: Gleadow et al. (2015), Earth Planet Sci. Lett., 424, p. 95–108 5: Danišík et al. (2017), Sci. Adv., 3, p. 1–9 6: Dobson et al. (2008), Geochim. Cosmochim. Acta, 72, p4745–4755 7: Li et al. (2017), Geostand. Geoanal. Res., 41, p. 359–365 8: Yu et al. (2020), Geostand. Geoanal. Res., 44, p. 763–783 9: Kirkland et al. (2020), Geochim. Cosmochim. Acta, 274, p. 1–19 10:Tagami et al. (2003), Earth Planet Sci. Lett., 207, p. 57–67 11:Iwano et al. (2013), Isl. Arc, 22, p. 382–394 Acknowledgements This study was supported by the Ministry of Economy, Trade, and Industry (METI) of Japan.

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.

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.

Numerical thermal history modelling has become a core approach used for interpretation of low-temperature thermochronometry data. Modelling programs can find rock time-temperature (t-T) paths that fit the input data while incorporating independent geologic information about a sample’s history and leveraging the factors that impact the kinetics of each thermochronometric system (e.g., grain size, radiation damage, and composition). HeFTy (Ketcham, 2005) and QTQt (Gallagher, 2012) are two of the commonly used tools for both forward and inverse t-T modeling. The modelling process involves making key decisions about (i) data input, (ii) initial set-up of model space and parameters, (iii) kinetic model(s) (i.e. annealing, diffusion, radiation damage), and (iv) additional t-T constraints. In addition, users need to have an understanding of the statistical methods underlying the modelling approach to be able to interpret the model outputs and the relationship between the observed and predicted data. However, these modeling tools currently lack clear and accessible entry-points for all users—experienced and new thermochronologists alike—and thus for many geoscientists, there is a substantial barrier to the modeling, interpretation, and publication of thermochronologic datasets. Here we present a suite of simple forward and inverse models that we recommend everyone perform before embarking on t-T modeling in HeFTy and/or QTQt for the first time. At the core of the exercises are the six different t-T paths used by Wolf et al. (1998) to illustrate the partial-retention behavior of the apatite He system; however, this approach can easily be applied to other systems as well. This exercise not only illustrates the fundamental behavior of thermochronologic systems but also guides users through the main functionality of the modelling programs. Despite the apparent simplicity of this exercise, users will experience most of the challenges and opportunities common to thermal history modeling, including: how to enter data; error handling; how to use geologic constraints in t-T space; the non-unique nature of cooling ages; the power of grain size and eU variability; the limitations on a model’s ability to resolve the ‘right’ rock thermal history; and how to evaluate the sensitivity of model results to all these factors. These exercises were introduced in the Thermo2020/1 Sunday workshops for both QTQt and HeFTy and are more fully fleshed out in two publications currently in preparation.

Since initially developing laser ablation (U-Th)/He procedures for high-spatial-resolution dating of monazite more than a decade ago, our research group has refined the technique to the point that laser ablation dating of apatite, titanite, and zircon is now routine in the Arizona State University (Group 18) laboratories. We are actively exploring applications to additional minerals. Compared to conventional single-crystal (U-Th)/He dating, the laser ablation alternative offers some important advantages. Following appropriate analytical protocols, laser ablation dates require no alpha ejection corrections. In principle, most factors commonly believed to cause high apparent age dispersion in conventional datasets – parent element zoning, alpha particle implantation, and the presence of high-(U+Th) inclusions – can be mitigated using the laser ablation method. Analytical throughput is greatly enhanced compared to the conventional method because sample dissolution is not required for U+Th+Sm analysis. This is especially beneficial for detrital studies; in this presentation, we review examples of Group 18 research involving (U-Th)/He and U/Pb laser ablation double dating of detrital apatite and zircon. The principal limitations to the method are that: 1) relatively large grain sizes (≥ 100 μm) are sometimes required for especially young or low-(U-Th) materials; and 2) analytical uncertainties for these materials can be as much as a factor of two larger for laser ablation dates than for conventional dates due to a combination of the much smaller masses analyzed and uncertainties in the U, Th, and Sm concentrations of available appropriate standards. Frontier applications of this technology advance our understanding of the intracrystalline distribution of radiogenic 4He in accessory minerals. Here we show examples of both two-dimensional mapping of 4He in polished crystal interiors and one-dimensional depth profiling as practiced in the Group 18 laboratories. Zoning in 4He is very common in older crystals, and 4He distribution patterns can be much more complex than what might be expected simply from alpha ejection or grain-scale diffusive loss during cooling. Much of this complexity reflects non-concentric zoning in parent elements and, for older crystals, spatially variable radiation damage that results in spatially variable 4He diffusivity. The potential impacts of such phenomena on thermal and exhumation history modeling argue for a greater reliance on microanalytical procedures in (U-Th)/He thermochronology moving forward.

Zircon Raman dating is an emergent thermochronological method. It exploits the disruption of the zircon lattice due to α-disintegration of trace amounts of 238U, 235U, 232Th, and their daughters. The radiation damage broadens the Raman bands and shifts them to lower wavenumbers. The Raman bandwidths provide a sensitive measure for the accumulated lattice damage (Nasdala et al., 1995; Nasdala et al., 2001). The measured bandwidth and the effective uranium concentration define the Raman age (Härtel et al., 2021). Radiation damage anneals upon heating and the meaning of the Raman age depends on the zircon’s thermal history. The Raman age is a formation age if no annealing has taken place, or a reset age if all pre-existing damage has been annealed by a geological heating event. In the case of partial annealing, however, the Raman age is a mixed age with no obvious geological significance. Mixed ages are difficult to interpret and cannot be distinguished from reset ages using the standard procedure for annealing detection (Nasdala et al., 2001). On the other hand, inhomogeneous damage distributions due to actinide zoning within zircon grains present a problem for zircon Raman dating. Overlapping signals from more and less damaged zones lead to asymmetric Raman bands and overestimated bandwidths (Nasdala et al., 2005). We discuss Raman spectra of zircon from partially annealed samples and spectra with asymmetric bands. We introduce discrimination plots based on the 356, 439, and 1008 cm-1 bandwidths that provide a means for detecting and distinguishing asymmetry and partial annealing. We discuss examples of zircon Raman dating and present a measurement protocol.

Inverse thermal history modeling is an effective tool to explore plausible time-temperature (t-T) histories that can be used to describe the geologic history of a sample. Although in some inverse modeling exercises the input thermochronology data is consistent with a single set of t-T histories with similar heating and cooling trends, more commonly inverse models identify a range of paths with different and distinct heating and cooling histories but similarly good fits to the data. Each set of these “path families” typically requires a different geologic interpretation to explain the observed heating and cooling trend, so it is important to identify and consider all possible path families consistent with the regional geology that fit the modelled dataset before selecting a preferred geologic interpretation. Although the inverse model results are always consistent with measured data, a model’s ability to detect all possible path families is partly controlled by the model design – for example the choice of initial conditions, monotonicity settings, and forced time-temperature windows. In this exercise using the thermal history modeling program HeFTy, we illustrate the effects of model design on the inverse model results of a set of multi-chronometer datasets from the southern Patagonian Andes. We use model design to maximize the number of path families identified through inverse modeling. Once individual paths are classified according to path families, we use independently constrained regional geology to discriminate among the diverse plausible set of path families and evaluate different available geologic scenarios. Our exercise illustrates that models restricting exploration of all path families may not identify the true cooling history of the sample. Initially, it may appear challenging to interpret inverse model results that include multiple path families, but we argue that iterating between independent geologic data and modeling provides an effective tool to test the geologic plausibility of alternative heating and cooling histories. Although this exercise is executed using HeFTy, maximizing the identification of all possible path families should be an important component of model design in inverse modeling exercises using all inverse modeling programs.

The Ultraviolet Laser Ablation Microprobe (UVLAMP) method of releasing helium from samples is an excellent, but under-utilized, tool in the diverse toolkit of gas extraction approaches available to researchers working with the (U-Th-Sm)/He thermochronology method. So far, most applications have involved some form of Laser Ablation (U-Th-Sm)/He dating (LAHe) or combined LAHe and Laser Ablation U-Th/Pb double dating (LADD) (e.g. 1, 2, 3, 4, 5, 6, 7). Other applications using UVLAMP have focused on 2D-mapping of helium distributions within zircon crystals (8) and stepwise Laser Ablation Depth Profiling (LADP) of induced helium diffusional loss profiles in apatite and zircon (9, 10). Based on the latter examples the stepwise helium LADP method would appear to be an excellent method to study the intricacies associated with a variety of aspects of the (U-Th-Sm)/He dating method and the interpretation and modeling of its results. Given that it creates high resolution helium profiles from the crystal margin to its core without the need to heat the sample to release the gas. Thus, it avoids issues of within-experiment radiation damage annealing, diffusional flattening of helium zonation, and/or the sudden release of helium from fluid and/or melt inclusions that can be associated with approaches using step heating of samples to acquire similar information about the helium distribution within a sample. In this contribution we focus on the results of high spatial resolution helium LADP experiments in a variety of accessory minerals (apatite, zircon, monazite, and titanite). The experiments are intended to a) empirically determine the alpha ejection distance and how those results compare to the distance for each mineral derived from SRIM calculations (11) and b) image natural helium distribution profiles from rim to core in zircons to produce data that are equivalent to those produced by 4He/3He thermochronology (12) experiments, but without the need to proton irradiate the sample. Initial LADP results on Durango apatite yielded an alpha ejection distance that is within error of the theoretical value, while results from several larger (>5 mm) zircon crystals did not yield profiles consistent with the presence of a straightforward alpha ejection zone. The helium depth profile results from the zircons were suggestive of either natural diffusional loss profiles, showing evidence of U-Th zoning, or a combination thereof. 1 Boyce et al. GCA 70, 2006; 2 Vermeesch et al. GCA 79, 2012; 3 Tripathy-Lang et al. JGR-ES 118, 2013; 4 Evans et al. JAAS 30, 2015; 5 Horne et al. GCA 178, 2016; 6 Horne et al. CG 506, 2019; 7 Pickering et al. CG 548, 2020; 8 Danisik et al. Sci Adv 3, 2017; 9 Van Soest et al. GCA 75, 2011; 10 Anderson et al. GCA 274, 2020; 11 Ziegler and Biersack, 1985; 12 Shuster and Farley EPSL 217, 2004.

Radiation damage in zircon directly impacts diffusion of He from the crystal lattice and is a key factor in defining the kinetics of the zircon (U-Th)/He system. Damage accumulates within a crystal as a function of time and U and Th concentration, but can be thermally annealed as well. The total level of radiation damage in a zircon crystal is governed by a thermally-activated, kinetic process, which in turn influences the interpretation of zircon (U-Th)/He dates for thermal histories. Several annealing models have been defined for the zircon system based on measurements in natural crystals; however, few studies have investigated how multiple levels of radiation damage due to zonation of actinides within a crystal may influence the annealing process. Here we use Raman spectroscopy to map the full width half maximum (FWHM) of then (SiO) band, a proxy for radiation damage, in zircon crystals from the Lucerne pluton (Maine, USA) with heterogeneous distributions of U and Th. We compare FWHM maps before and after annealing these crystals at laboratory times and temperatures. These maps show that each damage zone within a single zircon acts as an isolated domain that is dictated by an independent set of annealing kinetics. Thermally activated annealing decreases radiation damage in all radiation damage zones; however, the rate of annealing is not consistent across all zones. We identify specific modes of damage in probability density plots of all measured FWHM in a crystal that are not present in pre-annealed imagery, but persist in post-annealing Raman maps, regardless of laboratory time temperature conditions: FWHM modes at 2-5 cm, 10-15 cm, and25-30 cm. We attribute these persistent damage modes to variable annealing kinetics that are partially dependent on the level of pre-annealing damage, combined with the inability of high-damage crystals, or zones within crystals, to fully recover their crystallinity. That is, some damage is permanent. These findings therefore show that zircon crystals with non-uniform distributions of U and Th can anneal to create long-lived damage zones at specific damage levels, which has implications for treating the zircon (U-Th)/He chronometer as a multi-domain diffusion system.

Abstract A new method for automatic counting of etched fission tracks in minerals is developed and recently published in Geochronology (see Nachtergaele and De Grave, 2021). Artificial intelligence techniques such as deep neural networks and computer vision were trained to detect fission surface semi-tracks on images. The deep neural networks can be used in an open source computer program for semi-automated fission track dating called “AI-Track-tive”. Our custom-trained deep neural networks use YOLOv3 object detection algorithm, which is currently one of the most powerful and fastest object recognition algorithms. Two Deep Neural Networks were trained for both apatite and mica using our training dataset with images from the available microscope. The developed program successfully finds most of the fission tracks in the microscope images, however, the user still needs to supervise the automatic counting. The presented deep neural networks have high precision for apatite (97%) and mica (98%). Recall values are lower for apatite (86%) than for mica (91%). These high values have been obtained on images using the same microscope that provided the training images. The application can be used online on the web page https://ai-track-tive.ugent.be or after download as an offline application for Windows. The online application can be used to analyse captured images and does not require installation or download. The offline application can be used for both live track recognition on live microscopy images and captured images of apatite or mica. AI-Track-tive is written in Python and can be downloaded on https://github.com/SimonNachtergaele.

Exhumation in continental and oceanic arc settings is linked to tectonic and climatic forces that result in some of the highest topography and erosion rates in the world. Regional exhumation studies of oceanic arcs are sparse due to poor exposure, accessibility, complex inherited structures, and thermal overprinting due to magmatism, reburial, and metamorphism. In contrast, the Aleutian arc has an unusually thick crust, exceptional subaerial exposure of plutons, and limited regional thermal overprinting, providing the ideal conditions for a systematic thermochronology study that investigates the mechanisms that lead to arc exhumation. By analyzing multiple chronometers (apatite and zircon (U-Th)/He, zircon U-Pb) with different temperature sensitivities within the same plutonic sample, we can constrain uplift and erosion rates over ~40 million years of Aleutian arc history. Here, we present preliminary apatite and zircon (U-Th)/He ages from 23 plutonic samples from the islands of Unalaska, Umnak, Atka, Ilak, and Amatignak. These data are part of a larger study where we will analyze a total of 78 samples collected from 11 plutons along >1400 km of the Central Aleutian Islands. Ultimately, this dataset will provide a regional framework to quantitatively assess the various proposed mechanisms for Aleutian arc exhumation, such as; 10° Pacific plate rotation, subduction of the Kula ridge, and/or pluton emplacement, each of which has predictable and testable geographic age trends. The samples analyzed are from plutons that crystallized in the Oligocene to Miocene based on zircon U-Pb dating. Both zircon and apatite (U-Th)/He ages also show an Oligocene to Miocene spread, with a majority of the ages from both chronometers pooling at 7-13 Ma and no particular geographic trend. These preliminary results suggest that a significant exhumation event occurred in the late Miocene, as has been observed in other circum-Pacific arcs. These preliminary data may support the hypothesis that the late Miocene pulse of high plutonism in the Aleutian Islands led to arc exhumation. However, additional samples need to be analyzed to provide an arc-scale view of exhumation timing, trends, and rates of the Central Aleutian arc in order to test possible uplift and erosion scenarios.

The thermochronological method was applied to metamorphic rocks distributed in eastern Nepal to elucidate the denudation process of the upper-crust of the continental collision zone. New results of systematic fission-track (FT) age dating and FT length measurements of zircon and apatite were utilized in the thermochronological inverse analysis to reconstruct the time-temperature (t-T) paths in the temperature range of 60–350°C. Eight t-T paths obtained along the across-strike section in eastern Nepal showed that the cooling process of the metamorphic rocks are characterized by 1) gradual cooling (<30°C/Myr) followed by rapid cooling (~150°C/Myr) and subsequent gradual cooling (gradual-rapid-gradual cooling: GRG cooling), 2) northward-younging of the timing of the rapid cooling since ca. 9 Ma. The observed FT ages and t-T paths were then compared with the FT ages and t-T paths obtained by forward calculations using 3-D thermokinematic models to test the following four tectono-thermal models which have been proposed in the Central and Eastern Himalayas: 1) The denudation of the upper-crust is associated with the movement of the plate boundary fault (Main Himalayan Thrust: MHT) showing flat geometry (the Flat MHT model) and 2) flat-ramp-flat geometry (the Flat-Ramp-Flat MHT model), 3) the denudation of the upper-crust is mainly controlled by the focused uplift associated with the growth of the Lesser Himalayan Duplex (the Duplex 01-03 model) or 4) slip of the splay fault of the MHT (the Splay Fault model). As a result, only the Flat-Ramp-Flat MHT model reconstructed similar t-T paths and age distribution patterns obtained from eastern Nepal. This suggests that the observed FT ages and t-T paths reflect a denudation process driven by the movement of the MHT showing a flat-ramp-flat geometry. The GRG cooling and the northward-younging of the timing of rapid cooling indicate that the flat-ramp-flat geometry of the MHT was established by ca. 9 Ma and has been stable thereafter. The result of the thermokinematic inverse analysis also indicates that the denudation rate and its spatial distribution have been stable since ca. 9 Ma.