Optically stimulated luminescence (OSL) thermochronometry is a tool for constraining cooling histories in low-temperature domains (several tens of degree Celsius) during the past 104–105 years [1][2][3]. This method is currently applied only to rapidly denuded regions (~5mm/yr when a general geothermal gradient in is assumed to be ~0.03℃/m) because luminescence signals in slowly denuded regions saturate before the rocks are exhumated to the surface. However, cooling histories in slowly denuded regions may be constrained if unsaturated samples are obtained from deep boreholes. In addition, using deep borehole core enable to compare the results between samples at multiple depths, which is useful to isolate the denudation history from other events, such as faulting or hydrothermal activity. We applied multi-OSL-thermochronometry [2] to the deep borehole core drilled at the Rokko Mountains, Japan, where slow denudation rates (0.1-1.0 mm/yr) are expected from previous studies [4][5][6]. We used the Kabutoyama core collected by National Research Institute for Earth Science and Disaster Resilience [5][7]. The total length of Kabutoyama core is 1,313 m and we collected the samples at 408, 642, 818 and 1048 m for OSL-thermochronometry. Our results showed that useful thermal information can be extracted from the infrared stimulated luminescence signals of samples collected at depths ≥408 m. We found that the sample temperatures remained around the present ambient temperature at each depth for the last 0.1 Myr, indicating that the Rokko Mountains is topographically stable, which was consistent with previous findings. Thus, the thermal denudation history of slowly denuded regions may be constrained by multi-OSL-thermochronometry using samples from deep borehole cores. However, the denudation rates in the Rokko Mountains were too low and could not be determined by this method. Further research is required to quantify the denudation rate. This study was funded by the Ministry of Economy, Trade and Industry (METI), Japan as part of its R&D supporting program titled “Establishment of Advanced Technology for Evaluating the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (Fiscal Years 2019-2021)”. References: [1] Herman et al. (2010). Earth and Planetary Science Letters, 297, 183-189; [2] King et al. (2016). Quaternary Geochronology, 33, 76-87; [3] Herman and King (2018). Elements, 14, 33-38; [4] Huzita (1968). The Quaternary Research, 7, 248-260; [5] Sueoka et al. (2010). Journal of Geography, 119, 84-101; [6] Matsuhi et al. (2014). Transactions, Japanese Geomorphological Union, 35, 165-185; [7] Yamada et al. (2012). Technical Note of the National Research Institute for Earth Science and Disaster Prevention, 371, 27p.

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.

Reconstructing thermal histories in thrust belts using thermochronometry is a widely used method to infer the age and rates of thrusting, a precondition to understanding the driving mechanisms of orogenesis. Along a thrust sheet, the time and temperature conditions at the switch between heating and cooling retrieved from thermal modeling are commonly interpreted as the onset of thrust-induced exhumation associated with thrustbelt development. In subduction orogens such as the Andes, this interpretation can be challenged by the intrinsic changes in basal heat flow imposed by changes in subduction regimes. We document a case in the northwestern Sierras Pampeanas in the Argentinean Central Andes in which independent constraints on the onset late Cenozoic thrusting derived from structural cross-cutting relationships allow us to explore alternative causes for Cenozoic cooling signals. Located at ~31°S Lat, the Villa Unión-Ischigualasto basin hosts a composite stratigraphic record associated with Triassic rifting developed onto the Paleozoic substratum of western Gondwana and the overlying Meso-Cenozoic foreland basin record. A multi-method approach including apatite fission-track, apatite and zircon (U-Th)/He analyses, vitrinite reflectance and clay mineralogy carried out along three stratigraphic profiles, and inverse thermal modeling reveals the thermal history patterns and allows inferring its triggering mechanisms. Despite an up to 5 km-thick Cenozoic overburden and unlike previously thought, the thermal peak in the basin is not due to Cenozoic burial but occurred in the Triassic, associated with an abnormally high heat flow of up to 90 mWm-2 and less than 2 km of burial, which heated the base of the Triassic strata to ~160°C. Following exhumation, attested by the development of an unconformity between the Triassic and Late-Cretaceous-Cenozoic sequences, Cenozoic re-burial increased the temperature to ~110°C at the base of the Triassic section and only ~50°C 4 km upsection, suggesting a dramatic decrease in the thermal gradient. The onset of Cenozoic cooling from those conditions occurred between ~10 and 8 Ma, approximately 5 My before the onset of thrusting that has been independently documented by exceptionally well preserved stratigraphic-cross-cutting relationships. We argue that the onset of cooling is associated with lithospheric refrigeration following a decrease in the angle of subduction of the Nazca slab, leading to the eastward displacement of the asthenospheric wedge beneath the South American plate. Our study places time and temperature constraints on an idea that has been previously discussed in the region and calls for a careful interpretation of exhumation signals in thrustbelts inferred from thermochronology only.

Most thermochronological studies aimed at constraining exhumation rates rely on bedrock datasets. Often, they involve the analysis of samples collected along an elevation profile in terrains with high relief. However, there are several limitations to this approach, most importantly access to an appropriately steep traverse and sufficient relief to overcome uncertainties, and to have a broad enough range in closure ages as a function of elevation. Detrital thermochronology offers an alternative approach which can mitigate these challenges through coordinated dating of modern river sediments using multiple thermochronologic methods. Modern detrital sediments from active catchments provide an excellent source of material, typically rich in rock-forming and accessory minerals. Detrital thermochronologic data for material in sedimentary basins has been used widely to infer exhumation histories of sedimentary source terrains, reconstruct paleorelief, and evaluate spatial and temporal variations in erosion rates; however, there have been comparably fewer studies that apply this technique to evaluate regional exhumation patterns using detrital samples from active catchments. Based on the approach presented in Gallagher and Parra, (2020), we are exploring the capability of detrital thermochronologic data to infer regional exhumation patterns in the southeastern Sierra Nevada, CA. Here the uplift history remains debated and the potential mechanism of uplift has yet to be thoroughly constrained. Many catchments along the eastern side of the Sierra Nevada exhibit advantageous characteristics for detrital thermochronologic studies, including steep topography and high relief (that make it more difficult to sample bedrock), limited lithologic variability (which minimizes point-source biasing), relatively simple geologic structure, and relatively easy access to detrital sampling localities. Additionally, the dominant source of the southeastern Sierra Nevada catchments, the igneous units of the Sierra Nevada batholith, include abundant rock-forming minerals for 40Ar/39Ar thermochronology (hornblende, biotite, and sometimes muscovite) as well as abundant accessory minerals for (U-Th)/Pb geochronology(zircon), (U-Th)/Pb thermochronology (apatite), and (U-Th)/He thermochronology (zircon and apatite). Collectively, detrital thermochronological data from these minerals can elucidate much of the post-crystallization thermal history of the eastern flank of the Sierra Nevada. Preliminary results of this technique demonstrate the potential of this cost- and labor-efficient approach for exhumation history studies.

Since the advent of particle-track methods, it has been understood that the energy loss rate of an ion changes continuously along the particle trajectory, and that energy loss rate in turn affects etching rate. As fission particles slow down and stop, their energy loss rate also drops, which in turn reduces their along-track etching velocity. Conversely, the conceptual model that underlies the way we interpret track length data is based on a more simplified paradigm of a constant along-track etching velocity, vT, with the track tip marking the transition to bulk crystal etching, vB, at its maximum etchable extent. We present a new model for the etching and revelation of confined fission tracks that incorporates and attempts to quantify variable along-track etching velocity, vT(x). The model attempts to fully represent the track-in-track (TINT) revelation process, consisting of etchant penetration along semi-tracks intersecting the polished grain surface, expansion of etchant channels to intersect latent confined tracks, etching of confined tracks, and finally selection by the analyst of tracks suitable for measurement. We successfully use the model to fit step-etching data for spontaneous and unannealed and annealed induced confined tracks in Durango apatite. All model fits support a continuous decrease in etching velocity toward track tips, and lead to a series of insights concerning the theory and practice of fission-track thermochronology. Etching rates for annealed induced tracks in Durango apatite are much faster than those for unannealed induced and spontaneous tracks, impacting the relative efficiency of both confined track length and density measurements, and suggesting that high-temperature laboratory annealing may induce a transformation in track cores that does not occur at geological conditions of partial annealing. However, we are still investigating to what degree that pattern holds for other apatite varieties. The model also quantifies how variation in track selection criteria by analysts, which we approximate as the ratio of along-track to bulk etching velocity at the etched track tip (vT/vB), is likely to play a first-order role in the reproducibility of confined length measurements, and may explain the bulk of the variability observed in inter-laboratory calibration exercises. The concept of a “fully etched track” is subjective. Finally, the model illustrates how a substantial proportion of tracks that are intersected are not measured, which in turn indicates that length biasing is likely to be an insufficient mathematical basis for predicting the relative probability of detection of different track populations.

Apatite – Ca5(PO4)3(OH,F,Cl) – is a common accessory mineral in many terrestrial rocks and has a low (~75˚C) closure temperature for He retention[1]. As such the (U-Th-Sm)/He thermochronometer has become one of the most used dating techniques to constrain low temperature cooling histories of terrestrial samples. Apatite has been documented in many meteoritic, lunar, and martian samples, and is a particularly common accessory phase in many martian lithologies[2]. In meteoritic samples, (U-Th-Sm)/He dating of apatites provides some methodological challenges in that grains are frequently anhedral making it complicated to do a FT correction[2]. Results are commonly interpreted to represent the timing of shock metamorphism related to impacts onto the (planetary) surface which often correlates with cosmic-ray exposure ages[2]. NWA 7034, and its pairings, represents a piece of martian regolith that documents events on the martian surface that span approximately 4 Ga[3]. NWA 7034 is known to contain upwards of 4 wt.% apatite as detrital mineral fragments and as accessory phases in many lithic and impact-derived clasts[4]. These apatites have been previously analyzed geochronologically via U-Pb apatite and isotopically via δD, δ37Cl, and H2O using microbeam techniques[5,6]. At present, (U-Th)/He dating of NWA 7034 and its pairings has been restricted to bulk rock analyses[4,7,8,9]. This whole-rock approach relies on assumptions that the He, and its parent isotopes, are evenly distributed throughout the sample and that resetting of the WR-He thermochronometer is universal throughout the sample in response to the thermal pulse associated with the impact. These studies have yielded widely dispersed datasets that range from 50 Ma to 200 Ma, putting some of those assumptions into question[4,7,8,9]. In this contribution, we utilize the laser ablation double-dating (LADD) technique on individual apatites present within several polished slabs of NWA 7034. This targeted approach allows us to acquire (U-Th-Sm)/He ages, U-Pb ages, and trace element information within a petrological framework. The (U-Th-Sm)/He ages will provide additional constraints on the low temperature history of NWA7034 and help test the veracity of WR-He retention ages. References: [1] Zeitler et al. 1987 GCA, [2] Min 2005 Rev. Mineral Geochem; [3] Cassata et al. 2018, SciAdv; [4] Agee et al. 2013 Sci; [5] Hu et al. 2019 MAPS; [6] Davidson et al. 2020 EPSL; [7] Cartwright et al. 2014 EPSL; [8] Lindsay et al. 2021 MAPS; [9] Stephenson et al. 2017 MAPS

We report a new series of step-etch experiments to reveal the influence of microscopy technique on track selection bias. Two different aliquots of induced tracks in Durango apatite were etched for 10-15-20-25-30s and for 20-25-30s in 5.5M HNO3 at 21°C. Three different track selection criteria were applied after the initial etch step of the etching procedure: (1) all tracks with reasonable measurability under transmitted and reflected light switch with 100x objective and 2.5X optovar magnification; (2) ”fully etched” tracks under transmitted light with 100x objective; and (3) ”fully etched” tracks under reflected light with 100x objective. Approach 1 was applied to both aliquots and the approaches 2 and 3 to the latter aliquot. Comparing the mean track lengths, approaches 2 and 3 result in similar values over all experiments, while approach 1 provides a ~0.4 μm higher in the first aliquot and ~0.3 μm lower in the second aliquot than approach 2 and 3 due to higher and lower variations of effective etch times. Comparing the c-axis angles, in the 0-30° range approach 3 provides a severely reduced fraction of tracks due to their weak appearance under reflected light. Furthermore, approach 2 provides ~%14 lower track densities comparing with approaches 1 and 3. We recommend using both transmitted and reflected light during entire track selection and measurement procedures. We are working to develop a new 2+-step etching procedure, where tracks are located after 10s of etching but measured after a second 10s etch step, resulting in better-controlled etching times while reducing the bias associated with analyst choice. Furthermore, this two-step etching procedure can be iterated for more etch-steps, by identifying newly-appeared tracks after each etch step and etching them for 10s more, to increase the number of measured tracks while maintaining consistent selection criteria.

A compilation of legacy and new low-temperature thermochronological data from the British-Irish Isles and their surrounding offshore shelves yielded c. 700 AFT ages and c. 180 AHe ages from 29 peer-reviewed papers, 27 Geotrack industry reports and several new unpublished studies from offshore Ireland. The compilation shows for the first time a regional age pattern, with older AFT ages in Scotland and Northern Ireland than in the rest of Ireland. This pattern is tentatively attributed to the influence of the Anton-Dohrn Transfer Zone (ADTZ) during an Early Cretaceous phase of plate-wide uplift that resulted in more exhumation to the SW of the transfer zone than to the NE. Caledonian faults might also create differential exhumation of the tectonic blocks between them, as is observed in the compilation of AFT data from northern Scotland and this could explain the dispersion in the timing of exhumation seen on the North Porcupine High, offshore Ireland. Finally, the Paleogene exhumation visible in the Central Irish Sea, and attributed in recent years to igneous underplating, has not been detected in the Malin Sea-Outer Hebrides, despite the area being underlain by a high-velocity body also interpreted as igneous underplating. In conclusion, a detailed analysis of a large dataset of low-temperature thermochronological has revealed the possible influence of major crustal structure on the Mesozoic exhumation of this part of the NE Atlantic Margin, with large-scale decoupling occurring at a transfer zone and medium-scale decoupling occurring along regional-scale faults. The dataset also shed some doubts on the generic nature of exhumation caused by igneous underplating which has been much discussed in recent years.

Single-aliquot (U-Th)/He dating of iron-oxides requires less mass and has a higher spatial resolution than the two-aliquot approach, and is, therefore, a more reliable tool for quantifying the timescales of weathering processes, fault activity, and the development of soils and surfaces. Highly helium-retentive hematite samples must be heated to 1000-1100 °C to completely degas He, but we show that U is lost progressively during laser-heating starting at ~900 °C, with major U-loss at ~980 °C and complete U-loss at 1050-1100 °C. This partial or complete loss of U leads to incorrect (U-Th)/He ages that appear older than the true ages. We performed a series of heating tests on hematite and goethite aliquots with independently determined (U-Th)/He ages of 10-1761 Ma in which the helium release was determined by isotope-dilution mass spectrometry and phase change was monitored by infrared spectroscopy. As hematite is heated, reduction of Fe3+ causes a phase change to magnetite and then to elemental Fe. We correlate the onset of U-loss to the phase change from hematite to magnetite. By raising the temperature of this phase transition using a high oxygen partial pressure (pO2) in the sample chamber during laser-heating, we show that the onset temperature of U-loss is equally raised. Samples can therefore be safely degassed at higher temperatures without any detectable U-loss. In our implementation of this technique, the O2 in the sample chamber is being withdrawn from a tank using a pipette and it is being released before and captured after the degassing process on activated charcoal in a cold finger with a movable LN2 Dewar flask. We describe the automation of this process for routine degassing of hematite samples. We show that an average age calculated on a reference hematite sample from replicate aliquots (n=12), which were analyzed using this procedure, has a relative uncertainty of 2% (1σ), and is within uncertainty of the previously measured two-aliquot age. We suggest this high-pO2 degassing procedure as a way to precisely and reproducibly determine single-aliquot hematite and goethite (U-Th)/He ages.

Thermal history models are interpretive tools that incorporate data from chronometers and implement the published kinetics in the context of independent constraints on a sample’s known geologic history, in order to explore specific gaps in geologic knowledge. Despite their central role in the interpretation of thermochronologic datasets, our community has no standards for what characterizes a “robust” thermal history model result, how a model result’s rigor can and should be demonstrated, and how to communicate the key layers of interpretation produce a preferred thermal (and geologic) history. As a result, and through no fault of any one study or modeling program, published models are a patchwork of modeling philosophies, assumptions, and auxiliary hypotheses that are rarely sufficiently explored—to the frustration of authors, reviewers, and readers. This patchwork can give rise to conflicting conclusions and generate apparent controversies that distract from the geologic questions at hand. Therefore, our community needs to both embrace a diversity of modeling approaches and collectively discuss and set broad expectations for what constitutes thermal history modeling best practices. Here, we argue that the fundamental characteristic of any robust thermal history model result is that it is accompanied by a clear articulation of the “why”—e.g., the reason(s) that a model produces a distinctive history, be it the power of a geologic constraint, a grain’s age, a spatial relationship between samples, the choice of kinetic model, etc. We demonstrate this approach using (U-Th)/He data from basement rocks in the Front Range, CO, which when modeled require a distinctive Neoproterozoic thermal history: heating to 235-280°C after ca. 650 Ma and then cooling to <60°C during Paleozoic time. We demonstrate “why” through a suite of models that add, modify, and remove geologic constraints and data from the preferred model. We find that a heating event is required to produce the observed zircon He age-[eU] trend because (1) there is no more than ~600 My of radiation damage accumulated in the zircon crystals, (2) the geologic record places the samples at the surface prior to 650 Ma, and (3) published Ar ages require that these rocks were colder than ~250˚C for most of the last 1.5 Gy. By identifying these key factors, our sensitivity test facilitates comparisons to other studies and directs further discussion to how confident we are in the parts of our data and model set-up that produce this distinctive result. More broadly, this exercise demonstrates one of the challenges of deep-time thermochronology: the potential to accumulate multiple auxiliary assumptions that control the model result in ways that are not obvious, even to the experienced model user, without deliberate exploration of alternative solutions—further underscoring the need for more open discussion of this topic.