Radiation Damage In Zircon Research Articles

Effect of chemical composition on zircon radiation damage dating: Implications for low-temperature thermochronology

Zircon radiation damage dating is a low-temperature thermochronological method that can reveal the cooling histories of magmatic intrusions and discriminate sedimentary provenance in combination with other dating methods. This method has broad application prospects because of its advantages of non-destructive, high efficiency, and capable of double (or multiple) dating, involving only multiple measurements by Raman spectrometer and laser ablation-inductively coupled plasma-mass spectrometry. However, several factors, such as zircon chemical composition and the non-uniformity of radiation damage annealing kinetics, can cause poor precision when using this method and thus restricts its wide application. This study examined the effect of chemical composition (P, Ti, Dy, Th, U, and Hf) on Raman spectra using synthetic zircon crystals grown in a lithium-molybdate flux. The results show that the full width at half-maximum (FWHM) of the ν3(SiO4) band has positive linear correlations with the concentrations of P, Ti, Dy, Th, and U in decreasing order of influence, while the FWHM is unaffected by Hf at concentrations <1 wt.% but increases at concentrations >10 wt.%. Furthermore, the Raman shift is negatively correlated with Th, U, and Dy concentrations, positively correlated with Hf, and shows no obvious correlation with Ti and P. Thus, our study shows that chemical composition is a non-ignorable factor for calculating zircon radiation damage age using Raman spectroscopy, especially for zircon with relatively high concentrations of P, rare earth elements (REEs), Th, U, and Hf. The obtained multiple linear regression equation provides a potential means for estimating the FWHM at zero dose and implication for improving the dating precision of this method. In addition, the observed effects of REEs, Th, U, and Hf on the Raman shift of the ν3(SiO4) band indicate that chemical composition in zircon might affect the estimation of the P-T conditions of geological processes when using entrapped zircon inclusions in host minerals or the field of zircon as an in situ pressure sensor in hydrothermal experiments. Our study suggests that zircon radiation damage dating, excluding geochemical effects, will be more accurate for addressing lower-temperature geological processes.

Multi‐Band Raman Analysis of Radiation Damage in Zircon for Thermochronology: Partial Annealing and Mixed Signals

AbstractFour zircon Raman bands were previously calibrated to give consistent estimates of the accumulated self‐irradiation α‐dose in unannealed volcanic samples. Partial annealing of radiation damage produces inconsistent values because of differences in the relative annealing sensitivities. The damage estimates based on the external rotation band (DER) at ∼356 cm−1 and that based on the ν2(SiO4) band (D2) at ∼438 cm−1 are the most and least sensitive to damage annealing. The D2/DER‐ratio thus provides a numerical estimate of the extent of geologic annealing that a zircon sample has experienced. This ratio characterizes the thermal history of a zircon sample but also its state of radiation damage during the course of its geologic history, and thus the manner in which this state influences other thermochronologic methods. Meaningful interpretation of the zircon Raman age requires that the spectra are free of measurement artifacts. The major artifacts result from micrometer‐scale gradients of the damage densities within a zircon grain due to uranium and thorium zoning. The micrometer‐sized sampled volume may span different densities, producing overlapping spectra, causing apparent peak broadening, overestimated damage densities, and zircon Raman ages. The D3/D2‐ratio of the damage densities calculated from the ν3(SiO4) and ν2(SiO4) bands, most and least affected by overlap, is an efficient indicator of a meaningless signal. It reveals overlap in annealed and unannealed samples, because the used bands have similar responses to annealing. Multi‐band Raman maps can be converted to damage‐ratio maps for screening zircon mounts, and selecting spots for thermochronologic investigations.

Late Oligocene–Early Miocene Detachment Faulting in Crystalline Basement, Mecca Hills, CA, Documented With Zircon (U–Th)/He Date‐eU‐Visual Radiation Damage Patterns

AbstractZircon (U–Th)/He (zircon He) thermochronometry and visual radiation damage relationships reveal the timing and patterns of Orocopia Schist and North American basement exhumation in the Mecca Hills, CA. Zircon crystals from these units were selected to maximize the spectrum in visual metamictization and effective U (eU) concentrations, and thus He retentivity, in each sample. Zircon He dates (n = 31 analyses) from six of seven samples are ∼24 Ma regardless of eU, despite a large spread in eU (&gt;2,800 ppm) across all grains. One sample (n = 7 analyses) yields a mean ∼65 Ma date. Uniform zircon He dates over a range of eU concentrations and their preserved visual metamictization indicate these grains experienced a thermal history that induced complete He loss, but did not anneal the damage. This is consistent with recent work that indicates zircon radiation damage anneals at higher temperatures than parameterized in current damage annealing models. The zircon He data patterns, together with new zircon U‐Pb results and thermal history modeling, reveal the two basement units in the Mecca Hills exhumed simultaneously at ∼24 Ma, likely by the Orocopia Mountains detachment fault. This requires both basement units lie in the footwall of the detachment, and implies the breakaway zone of the Orocopia Mountains detachment fault was west of the present‐day Mecca Hills basement exposures and has been translated by the San Andreas fault. Combined with prior work, our data suggest large‐magnitude late Oligocene–early Miocene extension in the southwestern Cordillera was more widespread than previously recognized.

Implementation of an Alpha Damage Annealing Model for Zircon (U‐Th)/He Thermochronology With Comparison to a Zircon Fission Track Annealing Model

AbstractRadiation damage exerts a fundamental control on He diffusion in zircon, which manifests as correlations between (U‐Th)/He date and effective uranium concentration. These correlations can be exploited with modeling to explore long‐term thermal histories. This manuscript focuses on one such model, the zircon radiation damage accumulation and annealing model (ZRDAAM) of Guenthner et al. (2013), https://doi.org/10.2475/03.2013.01, by integrating newly defined alpha damage annealing kinetics measured by Ginster et al. (2019), https://doi.org/10.1016/j.gca.2019.01.033, into ZRDAAM. I explore several consequences of this alpha damage annealing model as it relates to (U‐Th)/He date‐effective uranium (eU) correlations, using representative time‐temperature paths and previously published results. Comparison between the current version of ZRDAAM, which uses fission track annealing, and the new annealing model demonstrates that, for thermal histories with prolonged periods at low temperatures (&lt;50°C), alpha dose annealing kinetics yield slightly younger model dates at low to moderate eU concentrations, older dates at moderate to high eU, and substantially younger dates at the highest eU concentrations. The absolute eU concentrations over which the differences are observed varies for a given thermal history, so these ranges should be interpreted as relative or proportional. Younger model dates at high eU in most thermal histories result from lower amounts of annealing that occur with the Ginster et al. (2019) alpha dose annealing kinetics. This annealing model comparison illustrates that the choice of annealing kinetics has the greatest influence over model output for thermal histories involving either prolonged time periods in the 200–300°C temperature window, or a late‐stage reheating event.

Mapping radiation damage zoning in zircon using Raman spectroscopy: Implications for zircon chronology

Recent research has demonstrated the importance of understanding alpha radiation damage in zircon in order to effectively interpret the thermochronologic significance of (U-Th)/He dates and to appropriately select laser ablation U/Pb reference standards. Direct measurements of alpha radiation damage in zircon are most easily done by using Raman spectral response as a proxy. Current Raman microscopes provide the opportunity to quantify not just a crystal's bulk radiation damage, but to map intracrystalline variations in radiation damage related to radionuclide zoning. Here we illustrate the procedure through a detailed study of zoned Proterozoic zircon crystals from the Adirondack Mountains of New York state. Although previous Raman studies have focused on the response of the v3(SiO4) stretching band (1008 cm−1) to radiation damage, we demonstrate that the internal v2(SiO4) bending band (439 cm−1) and the external Eg band (357 cm−1) are in some cases more useful – and in the case of the Eg band more responsive – proxies. We present curves that permit α dose estimates to be made from Eg and v2 band widths over a range of ~2 × 1017 to 1.8 × 1018 α/g. We then illustrate how these radiation damage maps can be used to visualize and interrogate intracrystalline variations in any damage-dependent material property in zircon, using helium diffusivity as an example, and discuss the implications for (U-Th)/He and U/Pb chronology of ancient, zoned zircon.

Thermochronological insights into the morphotectonic evolution of the Eastern Highlands, Zimbabwe: Implications for thermal history modelling of multi-thermochronometer data

The Eastern Highlands form a north-south trending high relief region along the eastern border of Zimbabwe, which is also spatially coincident with the eastern Zimbabwe cratonic margin. Uncertainty surrounds when this mountain range formed and how the region has evolved, particularly as there are no Phanerozoic cover rocks preserved. Here, we use low-temperature thermochronology (apatite fission track, apatite (U–Th-Sm)/He and zircon (U–Th)/He dating) and inverse thermal history modelling to reveal the morphotectonic evolution of this region since the mid-Paleozoic. The data suggest that structural reactivation occurred in the Late Devonian and Jurassic-Cretaceous that caused tectonic denudation and crustal cooling in the northern Eastern Highlands. The first period is thought to be associated with post-orogenic extension following Pan-African activity. The ensuing denudation continued into the Triassic and substantiates previous suggestions that these mountains were a source region of Lower Karoo sediments. The Jurassic-Cretaceous episodes are most likely associated with stress transmission from the progressive breakup of Gondwana. These events include the mid-to Late Cretaceous reactivation of the Mutare Shear Zone, which caused upward displacement (∼800 m) of the southern Eastern Highlands. The Eastern Highlands then entered a relatively quiescent phase with all samples residing close to the surface (<60 °C) since at least the beginning of the Cenozoic, which limits subsequent denudation of these mountains to less than ∼3 km. An investigation of the different multi-thermochronometer modelling combinations reinforces concerns with the current zircon radiation damage accumulation and annealing model (ZRDAAM). However, the findings also suggest that presenting the observed versus predicted apatite fission track length distribution should be an essential reporting requirement for thermal history modelling.

A borehole investigation of zircon radiation damage annealing

AbstractThis contribution reports Raman radiation damage measurements of zircons from the Kontinentale Tiefbohrung, on the western border of the Bohemian Massif. The mean wavenumbers (ω3) and widths (Γ3) of the ν3(SiO4) Raman band are constant down to 3 km, decrease (ω3) resp. increase (Γ3) between 3 and 5 km, and are again constant between 5 and 7 km. Uniform high Γ3 values associated with ω3 values close to those of undamaged zircon between 5 and 7 km are interpreted as due to residual damage predating the exhumation of the Bohemian Massif. A superimposed post‐exhumation signal indicates full damage retention down to 3 km depth, partial annealing between 3 and 5 km, and zero retention at greater depth. An attempt to calculate radiation damage ages gives results of a meaningful order of magnitude but also exposes difficulties associated with dating basement samples with complex damage accumulation and annealing histories.

Annealing kinetics of radiation damage in zircon

Abstract Models of noble-gas diffusion in minerals are essential for thermochronologic interpretations used for understanding the timing and rates of a range of geologic processes including exhumation and burial. For some minerals, diffusive daughter loss depends not only on temperature but also radiation damage. This is particularly true for the zircon (U-Th)/He system. Consequently, realistic interpretation of zircon (U-Th)/He thermochronology needs to account for both accumulation as well as annealing of radiation damage as a function of time and temperature. To date, models use etchable fission track annealing as a proxy for bulk radiation damage annealing. Here we present experimental annealing data and models of bulk radiation damage annealing kinetics in zircon. We show that bulk radiation damage annealing requires significantly higher temperatures and longer durations than that of etchable fission tracks. When fission tracks are completely annealed, bulk radiation damage has been annealed by only 30–50%. Consequently, a zircon (U-Th)/He thermochronology model that uses a fission track annealing algorithm will overestimate annealing resulting in erroneous estimates of He diffusivities and He loss. A time-temperature (t-T) history derived using the fission track annealing algorithm can, therefore, differ significantly from a t-T path derived using bulk radiation damage annealing kinetics. We also show that fractional-annealing-progress depends on the extent of accumulated radiation damage. Accordingly, we present distinct annealing models for low-, moderate-, and high-damage zircon. Each model comprises three annealing regimes with distinct kinetic parameters. The low-fractional annealing (low-φ) regime applies at low temperatures and short heating durations, whereas the high-φ regime applies at high temperatures and long durations. Between the two is a transition regime. We attribute the change in annealing kinetics to the presence of multiple damage-induced defect types that anneal with different activation energies and, therefore, at distinct time-temperature conditions. Comparison with other studies suggest that the low-φ regime is dominated by annealing of point defects that anneal at low temperatures, whereas the high-φ regime is dominated by epitaxial growth and annealing of isolated, stable point defects that anneal with high activation energies.

(U‐Th)/He Thermochronology and Zircon Radiation Damage in the South American Passive Margin: Thermal Overprint of the Paraná LIP?

AbstractNew zircon and apatite (U‐Th)/He data (AHe and ZHe) from the crystalline basement of the South American passive margin in southern Brazil present a wide distribution of Phanerozoic apparent ages, recording its prerift evolution. Its geological significance can be investigated by modeling the influence of the accumulated radiation damages on the measured crystals in its He ages during a long upper crust residence. Despite the presence of a significant fault in the studied area, the samples essentially constrain a uniform thermal history for the entire study area. Zircon helium ages spread from 472 to 26 Ma and correlate strongly with the radioactive element content of the crystals. Although the region was probably covered by Paleozoic sediments of the Paraná Basin, the ZHe system experienced a more intense thermal overprint, most likely the onset of the Paraná Large Igneous Province (LIP), causing its dispersion. The nearby intrusion of a feeding system of the LIP, the Florianópolis Dyke Swarm, may have contributed for keeping geothermal gradients elevated for a period long enough to promote the observed reset. After the extrusion of the volcanism, the study area was probably covered by up to 2 km of basalt floods, imprinting temperatures above the AHe partial retention zone on the crystalline basement. These were eroded during rapid cooling between 75 and 55 Ma, as indicated by thermal modeling. Additional analyses with Raman spectroscopy were used for calculating radiation damage ages in the zircon crystals measured for (U‐Th)/He, resulting in a comparable set of ages.

Annealing history of zircons from Apollo 14083 and 14303 impact breccias

AbstractBreccia boulders scattered around the 25 Ma Cone Crater near the Apollo 14 landing site provide a potential source of material ejected from the underlying Fra Mauro formation, which is interpreted to form from the ejecta blanket of the ~3.9 Ga Imbrium impact. However, questions remain as to whether all or some of the rocks collected are from the Fra Mauro Formation. In this contribution, we present new Raman measurements on zircons from impact breccias 14083 and 14303 that indicate a zircon radiation damage age of the breccias of 3900 ± 120 (2σ) Ma. These ages are compatible with a history of thermal shock during their emplacement as part of the Imbrium impact ejecta blanket. In contrast, previously published Raman analyses of zircons from a sample of breccia 14311 have a younger radiation damage age of 3410 ± 80 (2σ) Ma, confirming that this breccia had a different thermal history and was possibly unrelated to the Fra Mauro Formation. The radiation damage ages of breccias 14083 and 14303, in combination with previously published apatite U‐Pb ages, place constraints on the thermal conditions within the Imbrium ejecta blanket.

Zircon grain selection reveals (de)coupled metamictization, radiation damage, and He diffusivity

Broad variability in intrasample effective U concentration (eU) in zircon crystals is required to characterize zircon (U-Th)/He (zircon He) date-eU relationships that resolve thermal histories in ancient rocks. We describe a new protocol for selecting grains for zircon He thermochronometry that leverages visual crystal metamictization, a proxy for radiation damage accumulation, to generate a range in intrasample eU and zircon He date-eU/metamictization trends. Plane polarized light images, eU concentration, and zircon He data from 59 individual grains from six Proterozoic and three Archean North American granitoids reveal a progressive increase in crystal opacity and discoloration that correspond to increasing eU in each sample. Zircon eU ranges from 89 to 1885 ppm across the dataset and individual samples have eU variations of 538 ppm to 1579 ppm. Three Proterozoic samples yield distinct negative zircon He date-eU patterns with individual dates of ~908 to 20 Ma. Archean grains collectively define a negative date-eU trend from ~661 to 4 Ma. In contrast, three other Proterozoic samples yield uniform ~25 Ma zircon He dates over an 1800 ppm eU range, despite increasing eU with increasing metamictization. Simple thermal history models that account for coevolution of zircon radiation damage and He diffusion demonstrate visible metamictization and diffusivity are either coupled or decoupled depending on a sample's thermal history. Zircon He date-eU/metamictization pediments develop when variable but high eU grains with variable visual damage and closure temperature close with respect to He at the same time. Uniform dates with progressive increases in eU and visual damage, reflect decoupled He diffusion and visual damage, are associated with rapid cooling, and/or imply some forms of damage in zircon may not fully anneal.

Phanerozoic Morphotectonic Evolution of the Zimbabwe Craton: Unexpected Outcomes From a Multiple Low‐Temperature Thermochronology Study

AbstractThe fragmentary Phanerozoic geological record of the anomalously elevated Zimbabwe Craton makes reconstructing its history difficult using conventional field methods. Here we constrain the cryptic Phanerozoic evolution of the Zimbabwe Craton using a spatially extensive apatite (U‐Th‐Sm)/He (AHe), apatite fission track (AFT), and zircon (U‐Th)/He (ZHe) data set. Joint thermal history modeling reveals that the region experienced two cooling episodes inferred to be the denudational response to surface uplift. The first and most significant protracted denudation period was triggered by stress transmission from the adjacent ~750–500 Ma Pan‐African orogenesis during the amalgamation of Gondwana. The spatial extent of this rejuvenation signature, encompassing the current broad topographic high, could indicate the possible longevity of an ancient topographic feature. The ZHe data reveal a second, minor denudation phase which began in the Paleogene and removed a kilometer‐scale Karoo cover from the craton. Within our data set, the majority of ZHe ages are younger than their corresponding AHe and AFT ages, even at relatively low eU. This unexpectedly recurrent age “inversion” suggests that in certain environments, moderately, as well as extremely, damaged zircons have the potential to act as ultra‐low‐temperature thermochronometers. Thermal history modeling results reveal that the zircon radiation damage accumulation and annealing model (ZRDAAM) frequently overpredicts the ZHe age. However, the opposite is true for extremely damaged zircons where the ZHe and AHe data are also seemingly incompatible. This suggests that modification of the ZRDAAM may be required for moderate to extreme damage levels.

Empirical constraints on the effects of radiation damage on helium diffusion in zircon

In this study, we empirically evaluate the impact of radiation damage on zircon (U-Th)/He closure temperatures for a suite of zircon crystals from the slowly cooled McClure Mountain syenite of south-central Colorado, USA. We present new zircon, titanite, and apatite conventional (U-Th)/He dates, zircon laser ablation (U-Th)/He and U-Pb dates, and zircon Raman spectra for crystals from the syenite. Titanite and apatite (U-Th)/He dates range from 447 to 523Ma and 88.0 to 138.9Ma, respectively, and display no clear correlation between (U-Th)/He date and effective uranium concentration. Conventional zircon (U-Th)/He dates range from 230.3 to 474Ma, while laser ablation zircon (U-Th)/He dates show even greater dispersion, ranging from 5.31 to 520Ma. Dates from both zircon (U-Th)/He datasets decrease with increasing alpha dose, indicating that most of the dispersion can be attributed to radiation damage. Alpha dose values for the dated zircon crystals range from effectively zero to 2.15×1019α/g, spanning the complete damage spectrum.We use an independently constrained thermal model to empirically assign a closure temperature to each dated zircon grain. If we assume that this thermal model is robust, the zircon radiation damage accumulation and annealing model of Guenthner et al. (2013) does not accurately predict closure temperatures for many of the analyzed zircon crystals. Raman maps of the zircons dated by laser ablation document complex radiation damage zoning, sometimes revealing crystalline zones in grains with alpha dose values suggestive of amorphous material. Such zoning likely resulted in heterogeneous intra-crystalline helium diffusion and may help explain some of the discrepancies between our empirical findings and the Guenthner et al. (2013) model predictions. Because U-Th zoning is a common feature in zircon, radiation damage zoning is likely to be a concern for most ancient, slowly cooled zircon (U-Th)/He datasets. Whenever possible, multiple mineral-isotopic systems should be employed to add additional, independent constraints to a sample’s thermal history.

Electron-beam-induced annealing of natural zircon: a Raman spectroscopic study

The annealing of radiation damage in zircon by low-energy electron irradiation was explored systematically. Natural zircon samples spanning a wide range of self-irradiation damage were irradiated with the focused electron beam of an electron probe microanalyser. The effects of beam current and irradiation time were tested systematically, and the changes in zircon were measured using Raman spectroscopy. Our results confirm the damage-annealing effect of an accelerated electron beam. We demonstrate that non-thermal annealing occurs through electron-enhanced defect reactions and that the extent of the annealing is a function of both the irradiation time and the beam current. The complete annealing of radiation damage in zircon by an accelerated electron beam was not possible under the conditions of our experiments. Our results indicate that Raman band broadening in ion-irradiated zircon can possibly be explained through phonon confinement, as the estimated domain sizes of the crystalline volume amid recoil clusters decrease with increasing α dose. The results underlay the importance of doing Raman spectroscopy before electron-beam and ion-beam analysis. To avoid unwanted beam-induced annealing of damage in zircon during EPMA analysis, the electron energy transferred per volume unit of sample should be minimised, for instance by keeping the integrated charge low and/or by defocusing the electron beam.

Long-term tectonothermal history of Laramide basement from zircon–He age-eU correlations

The long-term (>1 Ga) thermal histories of cratons are enigmatic, with geologic data providing only limited snapshots of their evolution. We use zircon (U–Th)/He (zircon He) thermochronology and age-composition correlations to understand the Proterozoic–Phanerozoic thermal history of Archean Wyoming province rocks exposed in the northern Laramide ranges of western North America. Zircon He ages from the Wind River Range (54 dates) and Bighorn Mountains (32 dates) show negative correlations with effective uranium (eU), a proxy for radiation damage. Zircon dates from the Bighorns are between 960 Ma (low-eU) and 20 Ma (high-eU) whereas samples from the Wind Rivers are between 582 Ma (low-eU) and 33 Ma (high-eU). We applied forward modeling using the zircon radiation damage and annealing model ZrDAAM to understand this highly variable dataset. A long-term t–T path that is consistent with the available geologic constraints successfully reproduced age-eU correlations. The best fit to the Wind Rivers data involves two phases of rapid cooling at 1800–1600 Ma and 900–700 Ma followed by slower cooling until 525 Ma. During the Phanerozoic, these samples were heated to maximum temperatures between 160 and 125 °C prior to Laramide cooling to 50 °C between 60 and 40 Ma. Data from the Bighorn Mountains were successfully reproduced with a similar thermal history involving cooler Phanerozoic temperatures of ∼115 °C and earlier Laramide cooling between 85 and 60 Ma. Our results indicate that age-eU correlations in zircon He datasets can be applied to extract long-term thermal histories that extend beyond the most recent cooling event. In addition, our results constrain the timing, magnitude and rates of cooling experienced by Archean Wyoming Province rocks between recognized deformation events, including the >1 Ga period represented by the regionally-extensive Great Unconformity.

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.

Annealing of radiation damage in zircons from Apollo 14 impact breccia 14311: Implications for the thermal history of the breccia

AbstractImpact breccia 14311, was collected from the Apollo 14 landing site as a potential sample of the underlying Fra Mauro Formation. Published zircon U‐Pb ages of &gt;4000 Ma date the source material of the breccia and the apatite U‐Pb age of ~3940 Ma is interpreted as dating thermal resetting of the apatite U‐Pb systems. In this contribution we present new age information on the late stage thermal history of the breccia based on the annealing of radiation damage in the zircons. From Raman spectroscopic determination of the radiation damage within SIMS analytical spots on the zircons and the U and Th concentrations determined on these spots, we demonstrate that the radiation damage in the zircons has been annealed and we estimate the age of annealing at 3410 ± 80 Ma. This age is interpreted as a cooling age following heating of the breccia to above the annealing temperature of ~230 °C for stage 1 radiation damage in zircon, but below the temperature needed to reset the U‐Pb system of apatite (~500 °C). It is proposed that this thermal event was associated with the prolonged period of Mare volcanism, from 3150 to 3750 Ma, that generated massive basalt flows in the vicinity of the sample location.

MD study of radiation damage in zircon in connection with the problem of utilization of high-level waste

MD study of radiation damage in zircon in connection with the problem of utilization of high-level waste

Quantification of -particle radiation damage in zircon

Analysis of radiation damage in natural mineral analogs such as zircon is important for the evaluation of the long-term behavior of nuclear waste forms and for geochronology. Here we present results of experiments to determine the partitioning of radiation damage due to the heavy nuclear recoil of uranium and thorium daughters and the a-particles ejected in an α-decay event in zircon. Synthetic polycrystalline zircon ceramics were doped with 10 B and irradiated in a slow neutron flux for 1, 10, and 28 days to achieve the reaction 10 B + n → 7 Li + α (+2.79 MeV), creating an α event without a heavy nuclear recoil. The 7 Li atoms produced in the nuclear reaction were directly detected by NMR “spin-counting”, providing a precise measurement of the α-dose applied to each sample. The amount of damage (number fraction and volume fraction) created by each α-event (one α-event being a 7 Li + α-particle) has been quantified using radiological nuclear magnetic resonance and X-ray diffraction data. The number of permanently displaced atoms in the amorphous fraction was determined by 29 Si NMR to be 252 ± 24 atoms for the 10 B(n,α) event when the heavy recoil is absent, which is broadly in agreement with ballistic Monte Carlo calculations. The unit-cell swelling of the crystalline fraction, determined by X-ray diffraction, is small and anisotropic. The anisotropy is similar to that observed in ancient natural samples and implies an initial anisotropic swelling mechanism rather than an anisotropic recovery mechanism occurring over geological timescales. The small unit-cell volume swelling is only ∼6% of the expansion frequently attributed to α-particles associated with an actinide α-decay event. The lattice parameters indicate a volume increase as a function of a dose of 0.21 A 3 /10 18 α-events/g, which is significantly less than the increase of 3.55 A 3 /10 18 α-events/g seen in Pu-doped zircon and 2.18 A 3 /10 18 α-events/g seen in natural zircon. It is concluded that the heavy recoil plays a more important role in unit-cell swelling than previously predicted. The likely mechanism for such an effect is the rapid, and thus defect-rich, recrystallization of material initially displaced by the heavy recoil.

Zircon radiation damage ages

The suggestion that the accumulation of radiation damage in zircons could be used as a dating method was first proposed in the 1950s. In the original technique radiation damage was determined by X-ray diffraction however, this is not suitable for the small sample analysis needed in modern geochronology. It is now possible to measure radiation damage and U and Th contents on micron-sized areas on the polished surface of zircon grains using Raman spectroscopy and SIMS mass spectrometry. This opens the way for a reassessment of the potential of radiation damage ages and the purpose of this contribution is to re-examine the technique through its application to zircons from two granitoids from the Archean Yilgarn Craton and ancient detrital zircons from the Jack Hills in Western Australia. The three examples show internally consistent radiation damage ages that are also in accord with independent geochronological evidence. The 420±110Ma radiation damage age for the Darling Range granite is coincident with 500–400Ma biotite Rb–Sr ages in the same region of the Yilgarn Craton. Likewise a tonalite from the Youanmi Terrane in the Craton with a 1650±150Ma zircon radiation damage age, lies within the domain of a 1600Ma event recorded by biotite Rb–Sr ages. The Jack Hills zircons have a 1120±130Ma radiation damage age that is explained by a complex damage accumulation and annealing history culminating in a mild heating event indicated by biotite Ar–Ar ages of about 1140Ma. The positive results for the three case histories suggest that radiation damage ages could play a useful role in dating low temperature thermal events.