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Apatite (U–Th)/He and fission-track dates, as well as 4 He/ 3 He and fission-track length data, provide rich thermal history information. However, numerous choices and assumptions are required on the long road from raw data and observations to potentially complex geologic interpretations. This paper outlines a conceptual framework for this path, with the aim of promoting a broader understanding of how thermochronologic conclusions are derived. The tiered structure consists of thermal history model inputs at Level 1, thermal history model outputs at Level 2, and geologic interpretations at Level 3. Because inverse thermal history modeling is at the heart of converting thermochronologic data to interpretation, for others to evaluate and reproduce conclusions derived from thermochronologic results it is necessary to publish all data required for modeling, report all model inputs, and clearly and completely depict model outputs. Here we suggest a generalized template for a model input table with which to arrange, report and explain the choice of inputs to thermal history models. Model inputs include the thermochronologic data, additional geologic information, and system- and model-specific parameters. As an example we show how the origin of discrepant thermochronologic interpretations in the Grand Canyon can be better understood by using this disciplined approach. [ABSTRACT FROM AUTHOR]

Abstract: We present new data for the age of the Mono Lake Excursion at its type locality. Using the (U-Th)/He system on allanite, we dated Wilson Creek Ash 15 (Lajoie, 1968) to 38.7±1.2ka (2 SE). The new age for this ash supports the hypothesis (Kent et al., 2002; Zimmerman et al., 2006) that the Mono Lake Excursion is coincident with, and probably the same event as, the Laschamp Geomagnetic Excursion (40.4±2ka), an event that shares similar magnetic characteristics with the excursion identified at Mono Lake. We also estimate an allanite magma residence time of slightly less than 30ka based on 230Th/238U disequilibrium and the (U-Th)/He-based eruption age. [Copyright &y& Elsevier]

We analyzed (U–Th)/He ages of titanite from both the initial pilot hole (4 km depth) and the main drill-hole (9 km depth) of the German Continental Deep Drilling Project (KTB), to empirically evaluate the titanite (U–Th)/He partial retention zone (HePRZ) and laboratory helium diffusion data. Laser (U–Th)/He ages on single titanite fragments from the KTB drill hole, uncorrected for α emission, range from ∼94 to ∼2 Ma with reproducibility better than ±9%. Nearly invariant titanite (U–Th)/He ages, concordant with reported apatite fission track data, suggest that rocks from the upper ∼3 km of the KTB drill hole rapidly cooled from temperatures >200 to <110 °C between ∼75 and 85 Ma, followed by protracted cooling. Below ∼3 km, mean titanite (U–Th)/He ages decrease systematically with increasing borehole depth and down-hole temperature in a pattern similar to the titanite HePRZ predicted on the basis of laboratory-determined He diffusivities. Our results demonstrate that experimental titanite He diffusion data are applicable in the natural setting and that titanite (U–Th)/He thermochronometry provides a means to reliably reconstruct and quantify thermal histories between ∼100 and 180 °C, bridging the temperature ranges constrained by other techniques such as 40Ar/39Ar K-feldspar multi-diffusion domain modeling (350–150 °C) and apatite fission track dating (<110 °C). [Copyright &y& Elsevier]

Using classical diffusion theory, we present a mathematical technique for the determination of 4He concentration profiles in minerals. This approach should prove useful for constraining the low-temperature cooling histories of individual samples and for correcting (U–Th)/He ages for partial diffusive loss. The calculation assumes that the mineral of interest contains an artificially produced and uniform distribution of 3He obtained by proton irradiation [Shuster et al., Earth Planet. Sci. Lett. 217 (2004) 19–32]. In minerals devoid of natural helium, this isotope allows measurement of He diffusion coefficients; in minerals with measurable radiogenic He, it permits determination of 4He profiles arising during ingrowth and diffusion in nature. The 4He profile can be extracted from stepwise degassing experiments in which the 4He/3He ratio is measured. The evolution of the 4He/3He ratio as a function of cumulative 3He released can be compared with forward models to constrain the shape of the profile. Alternatively, we present a linear inversion that can be used to directly solve for the unknown 4He distribution. The inversion incorporates a standard regularization technique to filter the influence of random measurement errors on the solution. Using either approach we show that stepwise degassing data can yield robust and high-resolution information on the 4He profile. Profiles of radiogenic He are a sensitive function of the time–Temperate (t–T) path that a cooling sample experienced. Thus, by step heating a proton-irradiated sample it is possible to restrict the sample’s acceptable t–T paths. The sensitivity of this approach was explored by forward-modeling 4He profiles resulting from a range of realistic t–T paths, using apatite as an example. Results indicate that 4He profiles provide rich information on t–T paths, especially when the profiles are coupled with (U–Th)/He cooling ages on the same sample. Samples that experienced only moderate diffusive loss have 4He concentration profiles that are rounded at the edge but uniform in the core of the diffusion domain. Such profiles can be identified by nearly invariant 4He/3He ratios after the first few to few tens of percent of 3He have been extracted by step heating. We show how such data can be used to correct (U–Th)/He ages for partial diffusive loss. [Copyright &y& Elsevier]

Apatite, titanite and olivine samples were bombarded with a ∼150 MeV proton beam to produce ∼108 atoms/mg of spallation 3He. High-precision stepped-heating experiments were then performed in which the artificial 3He and, for apatite and titanite, the natural radiogenic 4He were measured to characterize the diffusive behavior of each isotope. Helium-3 diffusion coefficients are in excellent agreement with concurrently and/or previously determined He diffusion coefficients for each mineral. Our results indicate that proton-induced 3He is uniformly distributed and that radiation damage associated with a proton fluence of ∼5×1014 protons/cm2 does not cause noticeable changes in 4He diffusion behavior in at least apatite and titanite. Proton-induced 3He can therefore be used to establish He diffusion coefficients in minerals with insufficient natural helium for analysis or those in which the natural 4He distribution is inhomogeneous. In addition, step-heating 4He/3He analysis of a mineral with a uniform synthetic 3He concentration provides a means by which a natural 4He distribution can be determined. [Copyright &y& Elsevier]

In the last decade apatite (U–Th)/He thermochronometry has emerged as an important tool for quantifying the cooling history of rocks as they pass through the upper 1–3 km of the crust. The low closure temperature of this technique (∼70°C) has gained the interest of geomorphologists and tectonocists because it is applicable to interdisciplinary studies in landform evolution, structural geology, and geodynamics. We discuss current analytical techniques, the temperature calibration of the method, and sample quality considerations. Results from 1D, 2D and 3D thermo-kinematic numerical models are used to illustrate applications of He thermochronometry to problems in tectonics and landform evolution. [Copyright &y& Elsevier]

To evaluate the potential of (U–Th)/He geochronometry and thermochronometry of zircon, we measured He diffusion characteristics in zircons from a range of quickly and slowly cooled samples, (U–Th)/He ages of zircons from the quickly cooled Fish Canyon Tuff, and age-paleodepth relationships for samples from 15 to 18 km thick crustal section of the Gold Butte block, Nevada. (U–Th)/He ages of zircons from the Fish Canyon Tuff are consistent with accepted ages for this tuff, indicating that the method can provide accurate ages for quickly cooled samples. Temperature-dependent He release from zircon is not consistent with thermally activated volume diffusion from a single domain. Instead, in most samples apparent He diffusivity decreases and activation energy (Ea) increases as cycled step-heating experiments proceed. This pattern may indicate a range of diffusion domains with distinct sizes and possibly other characteristics. Alternatively, it may be the result of ongoing annealing of radiation damage during the experiment. From these data, we tentatively suggest that the minimum Ea for He diffusion in zircon is about 44 kcal/mol, and the minimum closure temperature (Tc, for a cooling rate of 10 °C/myr) is about 190 °C. Age–paleodepth relationships from the Gold Butte block suggest that the base of the zircon He partial retention zone is at pre-exhumation depths of about 9.5–11 km. Together with constraints from other thermochronometers and a geothermal gradient derived from them in this location, the age–depth profile suggests a He Tc of about 200 °C for zircon, in reasonable agreement with our interpretation of the laboratory measurements. A major unresolved question is how and when radiation damage effects become significant for He loss from this mineral. [Copyright &y& Elsevier]

Abstract: Stepwise degassing diffusion experiments on 39 different apatite samples using radiogenic 4He and proton-induced 3He reveal a range in closure temperature (T c) from ∼ 50 to 115 °C, for a cooling rate of 10 °C/Myr. There is no correlation between helium diffusion and apatite chemistry including F/Cl ratio, but the closure temperature is positively correlated with the radiogenic 4He concentration ([4He]) in each sample. We argue that [4He] is a proxy for a sample''s natural exposure to actinide radioactivity below the closure temperature, and that helium diffusion in apatite is impeded by radiation-induced damage to the apatite structure. The kinetics must therefore be an evolving function of time; measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample. The effect of radiation damage on helium diffusion appears to far exceed other known controls on helium diffusivity, including grain size. Our diffusion data are well described by a previously proposed, quantitative model that consists of two Arrhenius relations, one for volume diffusion through undamaged mineral structure and one for release of helium from radiation damage “traps.” The unknown parameters in this “trapping model” were determined from the diffusion experiments, and allow us to develop a tentative mathematical function that relates diffusivity to temperature and [4He]. By inserting this function into a 4He production-diffusion model we have explored how these results affect the interpretation of apatite (U–Th)/He thermochronometry. The model predicts that the effective 4He closure temperature of apatite will vary with cooling rate and effective U concentration (eU) and may differ from the commonly assumed T c of 70 °C by up to ±15 °C. The 4He partial retention zone will look similar to previous expectations, but its depth will depend on accumulation time and on eU. Most notably, samples subjected to reheating after accumulation of substantial radiation damage will be more retentive than previously expected. These predictions are consistent with recent observations of unexpected apatite (U–Th)/He ages in some settings. [Copyright &y& Elsevier]

Abstract: The distinctive Durango apatite is widely distributed as a reference material for mineralogy, geochemistry and thermochronology. The apatite occurs within the Cerro de Mercado open-pit iron mine, located in the northern portion of Durango City, Mexico. It is largely confined to vugs and open fractures within the lower portion of the ore deposit. Apatite and iron ore formed in situ and concurrently along with emplacement of small felsic intrusions along the southern margin of the Chupaderos caldera complex. This activity occurred between eruptions of two major ignimbrites from the caldera complex. Four single-crystal sanidine-anorthoclase 40Ar–39Ar ages from these ignimbrites and from a conformable older unit have been used to compute a reference age of 31.44±0.18 Ma (2σ) for the apatite itself. Four additional multi-crystal measurements are in general agreement with this age. Also in agreement are a set of 24 (U–Th–Sm)/He ages recently determined directly on the apatite, which have a mean of 31.02±1.01 Ma 1σ, with a standard error (S.E.) of ±0.22 Ma. [Copyright &y& Elsevier]

(U–Th)/He geochronology of 33 goethite grains from in situ ferruginous duricrusts overlying the Paraná flood basalt in the Guarapuava region, Paraná, Brazil, reveals ages ranging from 3.6 ± 0.4 to 0.4 ± 0.1 Ma. Thirty-one grains from detrital fragments of ferruginous duricrust yield ages in the 6.2 ± 0.6 to 0.7 ± 0.1 Ma range. The results show that goethites from detrital blocks are generally older than those from the in situ ferruginous layers and that all the goethites from the Guarapuava sites are relatively young (≤ 6.2 ± 0.6 Ma). Goethites show variable U and Th contents depending on the mode of precipitation. Goethite that forms by progressive ferrugization and pseudomorphic replacement of basalts are rich in Th, U-poor, contain various mineral inclusions ( e.g. , kaolinite, rutile, ilmenite) and display a distinct “checkered” appearance. Colloform goethite precipitated within cavities and desiccation cracks contain higher U, show low Th/U values, and are free of mineral contaminants, suggesting direct precipitation from solution. The geochronological results reveal a dynamic evolution for the soils overlying the Paraná basalt, where recurrent duricrust formation and dismantling lead to the evolution of regional colluvial soil profiles while simultaneously preventing the evolution of chemically stratified lateritic weathering profiles. The modes of formation and U, Th, and He contents of goethite at Guarapuava provide useful constraints on the expected compositions and modes of formation of goethite formed on basaltic soils on Mars. [ABSTRACT FROM AUTHOR]

Regional correlation of dated weathered land surfaces provides the necessary constraints to test long-term continental landscape evolution models, but major challenges remain in properly dating these surfaces. The geomorphological province of Second Paraná Plateau, Paraná State, Brazil, is a high elevation (ca. 800 m) land surface characterized by widely distributed deep saprolites and scattered lateritic profiles (e.g., Vila Velha and Serra das Almas). Prolonged exposure to weathering and erosion has promoted the pseudo-karstic and ruiniform features that are characteristic of this landscape. In this study, 40 Ar/ 39 Ar laser incremental heating geochronology on 22 grains of supergene Mn oxyhydroxides from lateritic profiles at Vila Velha yielded results ranging from 17.2 ± 0.7 to 9.1 ± 0.7 Ma. (U–Th)/He geochronology on 28 goethite grains from the same profile yielded results ranging from 36.4 ± 3.6 to 1.0 ± 0.1 Ma, with an age cluster lying within the 17.2 ± 0.7 to 7.9 ± 0.8 Ma interval. (U–Th)/He geochronology on 17 goethite grains from the Serra das Almas lateritic profile, located 20 km from Vila Velha, yield results ranging from 35.1 ± 3.5 to 14.1 ± 1.4 Ma. The combined results for the two sites reveal a common weathering history that started ca. 35 Ma, suggesting that the Second Paraná Plateau results from regional fluvial incision and denudation before ~ 35 Ma, followed by a decline in denudation rates and proportionally more intense weathering. Consistent with the laterite profile central ages, weathering was particularly intense during the Miocene (17–8 Ma). Denudation intensified after the Pliocene. [ABSTRACT FROM AUTHOR]