Helium Diffusion Kinetics Research Articles

Aragonite (U-Th)/He geochronology: Full retentivity across geologic timescales (104–107 yr)

In this study we explore aragonite retentivity to helium by comparing (U-Th)/He ages to independent age constraints provided by U/Th and U-Pb dating. We utilize aragonite speleothems from Botovskaya cave in Siberia and Soreq cave in Israel where the current temperature is ∼1 °C and ∼20 °C, respectively and is almost fluctuations-free. We then explore the retentivity of the dated aragonite samples using data from petrographic thin sections and thermal models.The obtained (U-Th)/He ages are statistically identical to independent age constraints extending backwards 6 Ma. This demonstrates that aragonite with relatively common grain size (20–500 μm) is retentive to helium at surface temperatures of 0-20 °C for millions of years. Moreover, its retentivity is higher than expected using currently available helium diffusion kinetics data for aragonite. Our results indicate that aragonite (U-Th)/He geochronology is feasible for aragonite held at near surface temperatures even when the sample grain size is relatively small. Aragonite samples that experienced post-depositional heating may lose helium. (U-Th)/He ages of such samples can be used to constrain paleo surface temperatures and the timing of tectonic and thermal events.

Cosmogenic 3He paleothermometry on post-LGM glacial bedrock within the central European Alps

Abstract. Diffusion properties of cosmogenic 3He in quartz at Earth surface temperatures offer the potential to directly reconstruct the evolution of past in situ temperatures from formerly glaciated areas, which is important information for improving our understanding of glacier–climate interactions. In this study, we apply cosmogenic 3He paleothermometry to rock surfaces gradually exposed from the Last Glacial Maximum (LGM) to the Holocene period along two deglaciation profiles in the European Alps (Mont Blanc and Aar massifs). Laboratory experiments conducted on one representative sample per site indicate significant differences in 3He diffusion kinetics between the two sites, with quasi-linear Arrhenius behavior observed in quartz from the Mont Blanc site and complex Arrhenius behavior observed in quartz from the Aar site, which we interpret to indicate the presence of multiple diffusion domains (MDD). Assuming the same diffusion kinetics apply to all quartz samples along each profile, forward model simulations indicate that the cosmogenic 3He abundance in all the investigated samples should be at equilibrium with present-day temperature conditions. However, measured cosmogenic 3He concentrations in samples exposed since before the Holocene indicate an apparent 3He thermal signal significantly colder than today. This observed 3He thermal signal cannot be explained with a realistic post-LGM mean annual temperature evolution in the European Alps at the study sites. One hypothesis is that the diffusion kinetics and MDD model applied may not provide sufficiently accurate, quantitative paleo-temperature estimates in these samples; thus, while a pre-Holocene 3He thermal signal is indeed preserved in the quartz, the helium diffusivity would be lower at Alpine surface temperatures than our diffusion models predict. Alternatively, if the modeled helium diffusion kinetics is accurate, the observed 3He abundances may reflect a complex geomorphic and/or paleoclimatic evolution, with much more recent ground temperature changes associated with the degradation of alpine permafrost.

Effect of tungsten/graphene/tungsten interface on helium diffusion kinetics and mechanical properties and defects of tungsten as first wall material - first principle calculation

In Tokamak device, helium atoms caused by the fusion reaction will generate helium bubbles and point defects after entering the first wall material composed of tungsten metal, which will seriously affect the stability of the first wall material. Therefore, we designed the tungsten/graphene/tungsten interface material as a new first wall material. It was found from first-principles calculations that the interface can trap helium atoms to reduce the diffusion of helium atoms from the interface to the tungsten phase again. The interface also traps vacancies and accelerates the recombination of vacancies and self-interstitial atoms. Using the quasi-harmonic Debye model, we found that the presence of the graphene layer can also improve the ductility, at a cost of reducing the mechanical modulus of the tungsten metal.

A helium-based model for the effects of radiation damage annealing on helium diffusion kinetics in apatite

Widely used to study surface processes and the development of topography through geologic time, (U–Th)/He thermochronometry in apatite depends on a quantitative description of the kinetics of 4He diffusion across a range of temperatures, timescales, and geologic scenarios. Empirical observations demonstrate that He diffusivity in apatite is not solely a function of temperature, but also depends on damage to the crystal structure from radioactive decay processes. Commonly-used models accounting for the influence of thermal annealing of radiation damage on He diffusivity assume the net effects evolve in proportion to the rate of fission track annealing, although the majority of radiation damage results from α-recoil. While existing models adequately quantify the net effects of damage annealing in many geologic scenarios, experimental work suggests different annealing rates for the two damage types. Here, we introduce an alpha-damage annealing model (ADAM) that is independent of fission track annealing kinetics, and directly quantifies the influence of thermal annealing on He diffusivity in apatite. We present an empirical fit to diffusion kinetics data and incorporate this fit into a model that tracks the competing effects of radiation damage accumulation and annealing on He diffusivity in apatite through geologic time. Using time–temperature paths to illustrate differences between models, we highlight the influence of damage annealing on data interpretation. In certain, but not all, geologic scenarios, the interpretation of low-temperature thermochronometric data can be strongly influenced by which model of radiation damage annealing is assumed. In particular, geologic scenarios involving 1–2 km of sedimentary burial are especially sensitive to the assumed rate of annealing and its influence on He diffusivity. In cases such as basement rocks in Grand Canyon and the Canadian Shield, (U–Th)/He ages predicted from the ADAM can differ by hundreds of Ma from those predicted by other models for a given thermal path involving extended residence between ∼40–80 °C.

Solubility and trapping of helium in apatite

A fundamental but unquantified assumption in U-Th/He dating of apatite is that grains do not incorporate extraneous helium by solution or other processes, but large age dispersion seen in some samples suggests that this assumption might be violated. Our laboratory experiments show that helium solubility in apatite is quite low and unlikely to lead to age dispersion in most samples. However, in some samples highly variable and sometimes large helium uptake suggests that apatite grains can trap helium in microvoids that could be derived from fluid inclusions or other microstructures, a conclusion supported by crushing and step-heating experiments. The presence of such microvoids raises the possibility that closure and age systematics could be complicated either by trapping of internally generated radiogenic helium and/or alteration of helium diffusion kinetics by impeding diffusion.

Zircon (U‐Th)/He thermochronology of Neoproterozoic strata from the Mackenzie Mountains, Canada: Implications for the Phanerozoic exhumation and deformation history of the northern Canadian Cordillera

AbstractSedimentary strata of the Neoproterozoic Mackenzie Mountains Supergroup (MMSG) and Windermere Supergroup (WSG) occupy the cores of anticlines in the Mackenzie Mountains of the Canadian Cordilleran Foreland Belt. Stratigraphic and structural evidence suggest that these rocks have undergone several episodes of burial and unroofing relatively intact. We report single‐grain detrital muscovite 40Ar/39Ar and zircon (U‐Th)/He (ZHe) data from a suite of samples across the fold‐thrust belt and the Neoproterozoic stratigraphic record. The strata have not reached high enough temperatures to reset the muscovite 40Ar/39Ar system, and instead our detrital muscovite data refine Tonian‐Cryogenian depositional ages. Single‐crystal ZHe dates range from 432 ± 35 to 46 ± 4 Ma, indicating that MMSG and WSG strata have not been heated sufficiently to fully reset the ZHe system. These factors make the Neoproterozoic strata an attractive natural laboratory to test the utility of the zircon radiation damage and annealing model on the quantification of thermal histories from detrital zircon populations that have accumulated radiation damage over long geologic timescales. Thermal modeling reveals that (1) a substantial sedimentary package was deposited following the Devonian and removed during Permo‐Triassic cooling, and (2) the Cordilleran deformation front propagated through the study area from the Albian to the Paleocene, with a moderate increase in cooling rates between 75–67 Ma in the southwest and 60–55 Ma at the deformation front. Ultimately, relationships between radiation damage and helium diffusion kinetics in zircon explain substantial ZHe date dispersion and elucidate the temperature‐time history of the northern Canadian Cordillera.

Radiogenic Source Identification for the Helium Production-Diffusion Equation

AbstractKnowledge of helium diffusion kinetics is critical for materials in which helium measurements are made, particulary for thermochronology. In most cases the helium ages were younger than expected, an observation attributes to diffusive loss of helium and the ejection of high energy alpha particles. Therefore it is important to accurately calculate the distribution of the source term within a sample. In this paper, the prediction of the helium concentrations as function of a spatially variable source term are considered. Both the forward and inverse solutions are presented. Under the assumption of radially symmetric geometry, an analytical solution is deduced based on the eigenfunction expansion. Two regularization methods, the Tikhonov regularization and the spectral cutoff regularization, are considered to obtain the regularized solution. Error estimates with optimal convergence order are shown between the exact solution and the regularized solution. Numerical examples are presented to illustrate the validity and effectiveness of the proposed methods

Apatite (U–Th)/He thermochronometry using a radiation damage accumulation and annealing model

Helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal, a quantity that varies over the lifetime of the apatite. Using recently published laboratory data we develop and investigate a new kinetic model, the radiation damage accumulation and annealing model (RDAAM), that adopts the effective fission-track density as a proxy for accumulated radiation damage. This proxy incorporates creation of crystal damage proportional to α-production from U and Th decay, and the elimination of that damage governed by the kinetics of fission-track annealing. The RDAAM is a version of the helium trapping model (HeTM; Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium diffusion kinetics in apatite. Earth Planet. Sci. Lett. 249, 148–161), calibrated by helium diffusion data in natural and partially annealed apatites. The chief limitation of the HeTM, now addressed by RDAAM, is its use of He concentration as the radiation damage proxy for circumstances in which radiation damage and He are not accumulated and lost proportionately from the crystal. By incorporating the RDAAM into the HeFTy computer program, we explore its implications for apatite (U–Th)/He thermochronometry. We show how (U–Th)/He dates predicted from the model are sensitive to both effective U concentration (eU) and details of the temperature history. The RDAAM predicts an effective He closure temperature of 62 °C for a 28 ppm eU apatite of 60 μm radius that experienced a 10 °C/Ma monotonic cooling rate; this is 8 °C lower than the 70 °C effective closure temperature predicted using commonly assumed Durango diffusion kinetics. Use of the RDAAM is most important for accurate interpretation of (U–Th)/He data for apatite suites that experienced moderate to slow monotonic cooling (1–0.1 °C/Ma), prolonged residence in the helium partial retention zone, or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss. Under common circumstances the RDAAM predicts (U–Th)/He dates that are older, sometimes much older, than corresponding fission-track dates. Nonlinear positive correlations between apatite (U–Th)/He date and eU in apatites subjected to the same temperature history are a diagnostic signature of the RDAAM for many but not all thermal histories. Observed date-eU correlations in four different localities can be explained with the RDAAM using geologically reasonable thermal histories consistent with independent fission-track datasets. The existence of date-eU correlations not only supports a radiation damage based kinetic model, but can significantly limit the range of acceptable time-temperature paths that account for the data. In contrast, these datasets are inexplicable using the Durango diffusion model. The RDAAM helps reconcile enigmatic data in which apatite (U–Th)/He dates are older than expected using the Durango model when compared with thermal histories based on apatite fission-track data or other geological constraints. It also has the potential to explain at least some cases in which (U–Th)/He dates are actually older than the corresponding fission-track dates.

The influence of artificial radiation damage and thermal annealing on helium diffusion kinetics in apatite

Recent work [Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium diffusion kinetics in apatite. Earth Planet. Sci. Lett. 249(3–4), 148–161] revealing a correlation between radiogenic 4He concentration and He diffusivity in natural apatites suggests that helium migration is retarded by radiation-induced damage to the crystal structure. If so, the He diffusion kinetics of an apatite is an evolving function of time and the effective uranium concentration in a cooling sample, a fact which must be considered when interpreting apatite (U–Th)/He ages. Here we report the results of experiments designed to investigate and quantify this phenomenon by determining He diffusivities in apatites after systematically adding or removing radiation damage. Radiation damage was added to a suite of synthetic and natural apatites by exposure to between 1 and 100 h of neutron irradiation in a nuclear reactor. The samples were then irradiated with a 220 MeV proton beam and the resulting spallogenic 3He used as a diffusant in step-heating diffusion experiments. In every sample, irradiation increased the activation energy ( E a ) and the frequency factor ( D o / a 2) of diffusion and yielded a higher He closure temperature ( T c ) than the starting material. For example, 100 h in the reactor caused the He closure temperature to increase by as much as 36 °C. For a given neutron fluence the magnitude of increase in closure temperature scales negatively with the initial closure temperature. This is consistent with a logarithmic response in which the neutron damage is additive to the initial damage present. In detail, the irradiations introduce correlated increases in E a and ln( D o/a 2) that lie on the same array as found in natural apatites. This strongly suggests that neutron-induced damage mimics the damage produced by U and Th decay in natural apatites. To investigate the potential consequences of annealing of radiation damage, samples of Durango apatite were heated in vacuum to temperatures up to 550 °C for between 1 and 350 h. After this treatment the samples were step-heated using the remaining natural 4He as the diffusant. At temperatures above 290 °C a systematic change in T c was observed, with values becoming lower with increasing temperature and time. For example, reduction of T c from the starting value of 71 to ∼52 °C occurred in 1 h at 375 °C or 10 h at 330 °C. The observed variations in T c are strongly correlated with the fission track length reduction predicted from the initial holding time and temperature. Furthermore, like the neutron irradiated apatites, these samples plot on the same E a − ln( D o / a 2) array as natural samples, suggesting that damage annealing is simply undoing the consequences of damage accumulation in terms of He diffusivity. Taken together these data provide unequivocal evidence that at these levels, radiation damage acts to retard He diffusion in apatite, and that thermal annealing reverses the process. The data provide support for the previously described radiation damage trapping kinetic model of Shuster et al. (2006) and can be used to define a model which fully accommodates damage production and annealing.

Radiation damage and helium diffusion kinetics in apatite

Experimentally determined diffusion coefficients for 39 different samples of apatite demonstrate that the closure temperature (Tc) for helium retention in apatite spans a wider range than previously recognized: from 44 ± 4 C to 116 ± 18 C (for 10 C/ Myr) and correlates with the radiogenic He concentration ([He]) in a given sample. We find no correlation between helium diffusion kinetics and apatite chemistry, including the F/Cl ratio. We argue that [He] is a measurable proxy for the radiation damage which accumulated within each crystal over geologic time. As the volume density of lattice damage sites increases, apatite becomes more helium retentive. This implies that helium retentivity, and hence the effective helium diffusion kinetics, is 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. Calibrated with diffusion kinetics of 39 different samples of apatite, we present a simple, quantitative trapping model which relates diffusivity to both temperature and [He]. This previously proposed model (Farley, 2000) consists of two Arrhenius relations: one for volume diffusion through undamaged mineral lattice and one for the release of helium from the damage traps back into the undamaged lattice. The model predicts much of the observed log-linear correlation between Tc and [ He]. By inserting this function into a He production-diffusion calculation, the trapping model predicts: (i) that the effective He closure temperature of apatite will vary with cooling rate and effective U concentration (eU) and may differ from 70 C by up to ±15 C, (ii) the depth of the He partial retention zone will depend on accumulation time and on eU, and (iii) samples subjected to reheating after the accumulation of substantial radiation damage will be more retentive than previously expected. These predictions are consistent with recent observations of unexpected apatite (UTh)/He ages in some settings, most noteably (Flowers et al., 2006).

4He/3He Thermochronometry: Theory, Practice, and Potential Complications

Thermochronometry most often involves the determination of a cooling age from parent and daughter abundances within an entire crystal or population of crystals (Dodson 1973). Complementary information exists in the spatial concentration distribution of the daughter, C ( x,y,z ), within a single crystal. By combining a bulk cooling age with C ( x,y,z ) on the same sample, it is possible to place tight limits on the sample’s time-temperature ( t-T ) path. Techniques for this kind of analysis have been developed for several different parent/daughter systems including U-Th-Pb and K-Ar (Harrison et al. 2005). Here we describe how this approach is applied to the (U-Th)/He system. The particular attraction of the (U-Th)/He method is its sensitivity to uniquely low temperatures. For example, the nominal 4He closure temperatures (at 10 °C/Myr) for apatite, zircon and titanite are 70 °C, 180 °C, and 200 °C, respectively (Reiners and Farley 1999, 2001; Farley 2000; Reiners et al. 2002, 2004). In the case of apatite, we will show that significant diffusive mobility of 4He occurs at temperatures just slightly higher than those of the Earth’s surface. In this chapter, we present an overview of the 4 He / 3 He thermochronometry technique in which the natural spatial distribution of 4He is constrained by stepwise degassing 4He/3He analysis of a sample containing synthetic, proton-induced 3He. We present the fundamental theory, assumptions, practical aspects of proton irradiation and stepwise 4He/3He analyses, as well as several example applications of 4He/3He thermochronometry. In particular, we illustrate how the 4He/3He technique can be used to determine the helium diffusion kinetics and constrain the natural 4He distribution within an individual crystal or a small population of crystals, and how this information can be …