the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Mar 2026
Short communication: Crystalline features of fission tracks in monazite: Evidence from swift heavy ion irradiation and transmission electron microscopy
Abstract. To further understand monazite fission-track thermochronometry, three experiments were conducted. 1) Cretaceous monazite-(Ce) was irradiated with 80 and 200 MeV Xe ions using a tandem accelerator. Transmission electron microscopy (TEM) was then used to define crystalline structures of spontaneous fission tracks in monazite. 2) Zircon powder was irradiated with 80 MeV Xe ions. TEM was subsequently used and the results were compared with those obtained from monazite. 3) Quaternary monazite-(Ce) in a resin mount was irradiated with 80 MeV Xe ions. Next, chemical treatment (etching) was performed to ascertain etchability of the ion tracks in monazite. These irradiation conditions correspond to the energies of heavily charged particles of spontaneous fission and the entire spontaneous fission event. This experiment simulated the damage process associated with spontaneous fission. First, from the TEM images of ion-irradiated Cretaceous monazite, ion track damage could be visualized as a low-density columnar region where the crystal lattice was maintained, and no sign of amorphization for both energies was observed. These results suggest that point defects accumulate around the ion path in monazite, in contrast to the amorphous features in zircon under the same irradiation conditions. Secondly, these etching experiments on ion-irradiated Quaternary monazite indicate that even non-amorphous domains can be selectively etched, which is consistent with previous studies on etching ion tracks in non-amorphous materials, as well as natural fission tracks and alternative ion tracks on monazite. These observations suggest that etchability of fission tracks in monazite is likely attributable to accumulated point defects rather than to amorphous regions. Hence, estimation of ultra-low closure temperature in the monazite fission track system should be derived from the formation of point defects rather than from amorphous regions, since the former can be easily annealed compared to the latter.
- Preprint
(4284 KB) - Metadata XML
- BibTeX
- EndNote
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-6112', Anonymous Referee #1, 06 May 2026
-
RC2: 'Comment on egusphere-2025-6112', Weixing Li, 20 May 2026
This study addresses a critical gap in fission-track thermochronology by targeting the long-standing puzzle of monazite's reported ultra-low closure temperature (~25–50°C). To investigate this phenomenon, the authors employ a methodologically sound approach: they utilize 80 MeV Xe ions to simulate the initial energy of single fission fragments, compare the resulting track structures to those in zircon, and perform chemical etching experiments to bridge laboratory simulations with natural samples. While these preliminary results provide valuable data, major revisions are required to strengthen the claims regarding track structure and to better contextualize the findings within the broader thermochronological framework. I recommend acceptance contingent upon the authors rigorously validating their structural interpretations, integrating their findings into the existing literature, and thoroughly addressing the major and minor concerns detailed below.
Major Comments:
The claim that tracks in monazite retain a crystalline structure and lack amorphous features warrants significant caution. Conventional TEM alone is fundamentally incapable of resolving individual point defects or identifying latent track structures. To definitively distinguish true track-core features from the surrounding matrix, high-resolution TEM (HRTEM) combined with fast Fourier transform (FFT) or selected area electron diffraction (SAED) is strictly required. Because the authors did not perform HRTEM, their "crystalline structure" claim cannot be verified; without high-resolution lattice imaging, any observed lattice fringes may simply be an artifact of the electron beam projecting through the overlapping, intact crystalline matrix. This methodological limitation is further compounded by contradictions with prior work: Overstreet et al. (NIMB 2022) demonstrated the amorphization of ion tracks in Sm/Tb‑monazite using conventional TEM, Raman spectroscopy, and XRD under GeV heavy‑ion irradiation. Absolute assertions such as “no sign of amorphization” should be revised to reflect the resolution boundaries of the data (e.g., “no detectable complete amorphization”), and claims regarding “accumulated point defects” must be softened to acknowledge that conventional TEM cannot resolve such sub-nanometer features. Detailed technical limitations and alternative interpretations are provided in the minor comments below.
The manuscript would be significantly strengthened by explicitly comparing monazite with other key thermochronometer minerals, particularly apatite, which was omitted in this work, across track structure, formation and annealing kinetics, etching behavior, and closure temperatures. As the most widely used fission-track mineral (Gleadow et al., Rev. Mineral. Geochem. 2002), apatite exhibits continuous, highly porous tracks that anneal via distinct mechanisms (Li et al., EPSL 2011, EPSL 2012). This contrasts sharply with zircon’s fully amorphous, continuous tracks, and helps isolate the behavior of monazite’s own low-density, potentially discontinuous domains. Establishing this comparative framework is essential to explain why monazite exhibits an even lower closure temperature than apatite, and to properly contextualize monazite tracks within the known spectrum of latent radiation damage: fully amorphous in zircon, discontinuous, low-density tracks in TiO2 (Zhai et al., NIMB 2019), and continuous, highly porous in apatite. These fundamental structural variations govern long-term thermal stability and dictate how different minerals respond to chemical etching. Integrating these cross-mineral insights would elevate the study from an isolated, monazite-specific observation to a major, broadly applicable contribution to the field of thermochronology.
Minor Comments:
- Line 12: The authors appropriately selected 80 MeV Xe ions to simulate single fission fragments (70–100 MeV). However, the higher-energy ions (e.g., 300 MeV Kr) cited from the literature exceed typical fission energies and are unsuitable for quantifying representative track lengths or annealing behaviors.
- Conventional TEM imaging (Figs. 1–3) cannot definitively confirm crystalline structure retention within monazite tracks. Obtaining high-quality cross-sectional HRTEM images of ion tracks demands precise mutual alignment of three axes: the ion irradiation direction, a low-index zone axis of the matrix, and the electron beam. Without this strict alignment, lattice fringes observed "on top" of tracks may misleadingly originate from the overlapping crystalline matrix rather than the track core itself. Consequently, HRTEM combined with FFT or SAED is necessary to rule out such alignment artifacts. Furthermore, because amorphous features in beam-sensitive minerals like monazite and apatite are easily obscured by electron-beam-induced damage, amorphous tracks are far more difficult to detect than intact lattice fringes. The authors must demonstrate that beam damage was minimized during sample tilting/observation and contextualize these local observations with more reliable bulk characterizations of amorphization, such as Raman spectroscopy and XRD (e.g., Overstreet et al., NIMB 2022).
- Line 20: Describing tracks as “low-density regions” (rather than asserting that “crystalline structure is retained”) is scientifically safer until HRTEM evidence confirms the absence of amorphous components.
- Line 61: While a significant amount of heat is generated by gamma-ray release during neutron absorption by the high Gd content in monazite, potentially reaching temperatures high enough to melt the sample, it is scientifically inaccurate to frame this mechanism as "thermal neutron shielding" since the overall proportion of absorbed neutrons remains extremely low.
- Line 76: Define the acronym “TEM” (Transmission Electron Microscopy) upon its first use to improve readability.
- Line 82: Monazite tracks appear to align with the “intermediate type” damage (discontinuous droplets or low-density domains) observed in TiO2 (Zhai et al., NIMB 2019) and CeO2 (Takaki et al., Prog. Nucl. Energy 2016), rather than being purely crystalline. While ion tracks are widely considered amorphous in most materials (including zircon, pyrochlore, quartz, and epidote), the authors should contextualize these observations within the wider thermochronology literature. For instance, apatite exhibits highly porous tracks, as demonstrated by HRTEM, Fresnel contrast, and thermal annealing studies (Li et al., PRB 2010). Because the tracks in the authors' monazite samples exhibit discontinuous droplets and low-density areas, classifying them as this "intermediate type" would be much more accurate.
- Line 87: The repetitive use of “lead to” makes the sentence awkward. Revise for clarity.
- Line 115: The text states that irradiations were performed “perpendicular to the grid surface,” but Figure 3 displays a 45° tilt. Please clarify in the Methods section that some grids were tilted pre-irradiation.
- Line 120: Note that Weise et al. (Chem. Geol. 2009) used 300 MeV ions, which exceed both single fission fragment energy (~70–100 MeV) and total fission energy (~170 MeV). In contrast, Li et al. (EPSL 2012) correctly used a representative 80 MeV Xe ion beam for single-fragment simulation.
- Line 125: This section omits critical chemical etching details (e.g., 6M HCl at 90°C, per Jones et al., Terra Nova 2019). Explicitly state these experimental parameters.
- Line 135: The absence of visible amorphous features under conventional TEM does not prove their total absence within the tracks; stringent imaging orientation requirements and electron-beam damage can easily obscure small, localized amorphous domains.
- Line 140: In monazite, 80 MeV Xe ion tracks are discontinuous from the initial stage of penetration, contrasting with apatite where tracks remain continuous until near the terminal range (Li et al., EPSL 2012). Furthermore, during annealing, continuous apatite tracks segment into discontinuous droplets driven by Rayleigh instability. This divergent behavior demonstrates that continuous, heavily damaged zones develop more readily in apatite than in monazite under identical irradiation conditions.
- Figures 1–4: The authors reported circular tracks in zircon (Figure 4) yet rectangular ones in monazite (Figure 1). However, latent track geometry is highly sensitive to the ion’s trajectory relative to the crystal lattice, necessitating a precise determination and reporting of the crystallographic orientation of the irradiated crystals. For instance, Li et al. (Am. Mineral. 2014) demonstrated via HRTEM and conventional TEM that ion tracks in zircon can appear oval or rectangular, rather than strictly circular, depending on the irradiation direction. Consequently, the circular morphology presented in Figure 4 may simply reflect a specific crystallographic section or imaging condition rather than an intrinsic, isotropic track structure.
- Line 181: The attribution of “enhanced etching resistance” in Toya-6b to “minimal radiation damage” lacks a mechanistic explanation. The authors should expand on the physical basis for this relationship—specifically, how a low baseline of cumulative damage reduces defect density or chemical reactivity (e.g., Nakajima et al., GChron 2024). Clarifying whether this resistance arises from reduced preferential leaching along damaged zones versus changes in bulk dissolution kinetics would strengthen the interpretation.
- Line 187: Overstreet et al. (NIMB 2022) identified amorphous components in Sm/Tb-monazite via TEM/XRD/Raman, even when lattice fringes remained visible in conventional TEM, proving that misalignment can mask amorphous cores. Because GeV ions possess a comparable electronic stopping power (Se) to actual fission fragments, their energy range alone does not invalidate their findings.
- Line 220: The absolute statement “no amorphization” should be revised to “no complete amorphization” or “no metamictization,” strictly following the cited reference (Nasdala et al., Sci. Rep. 2020).
- Line 230: While helium ion irradiation experiments show radiation damage at low accumulation thresholds (Nasdala et al., Sci. Rep. 2020), it is mechanistically incorrect to attribute natural alpha-particle accumulation as the primary driver of track damage. Alpha-recoil nuclei are the dominant drivers of radiation damage in natural samples, whereas alpha particles themselves primarily promote electronic ionization and structural healing.
- Line 243: Lithium fluoride is highly sensitive to electron beam irradiation, which can rapidly induce amorphous artifacts across both the matrix and track zones. Consequently, conventional TEM makes it difficult to precisely isolate the presence of amorphous tracks, because remaining crystalline structures generate strong diffraction contrast that overpowers the amorphous signal. Even if amorphous features are not directly observed at the top of the damaged region in LiF via TEM, concluding that the tracks fundamentally lack amorphous components is scientifically unjustified.
- Line 265: Individual point defects cannot be resolved via conventional TEM. Soften claims such as “point defects are present” to “damage likely includes point defects.”
- Line 300: The closure temperature of fission tracks in minerals is dictated by a complex interplay of factors, including track geometry, the structural state of the damaged core, the thermal response of the damage zone, and chemical interactions between the damaged volume and the host crystalline matrix. The authors are encouraged to expand their discussion to explain why monazite exhibits an even lower closure temperature than apatite.
Citation: https://doi.org/10.5194/egusphere-2025-6112-RC2
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 4,401 | 119 | 23 | 4,543 | 19 | 30 |
- HTML: 4,401
- PDF: 119
- XML: 23
- Total: 4,543
- BibTeX: 19
- EndNote: 30
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
The study by Fukuda et al. provides an important microstructural contribution to the developing field of monazite fission track (MFT) thermochronology by investigating the nature of latent ion tracks using swift heavy ion irradiation and transmission electron microscopy (TEM). The primary objective of the paper is to determine whether fission tracks in monazite form amorphous domains, as is typical in minerals such as zircon, or whether they instead comprise defect-rich crystalline regions. Using Xe ion irradiation at energies of 80 and 200 MeV, designed to approximate the energy range of spontaneous fission fragments, the authors show that ion tracks in monazite-(Ce) exhibit reduced-density linear features while retaining an intact crystal lattice. In contrast, zircon irradiated under similar conditions displays clearly amorphous cylindrical tracks. The study also demonstrates that ion tracks in low-damage Quaternary monazite are etchable despite the absence of amorphous domains, supporting the interpretation that etching behaviour in monazite is controlled by the presence of point defects rather than amorphous material. The authors conclude that the non-amorphous nature of monazite fission tracks may explain the very low closure temperature of the MFT system, as recovery of point defects is expected to require lower activation energy than recrystallisation of amorphous zones.
The study addresses a key unresolved question in MFT thermochronology, which is what the structural nature of latent tracks and the implications for annealing kinetics is. The use of irradiation energies comparable to spontaneous fission fragments represents an important strength of the study, as many previous ion track investigations have relied on much higher electronic stopping powers that may not accurately reproduce natural fission track formation conditions. The TEM imaging approach, including the use of underfocus and overfocus imaging conditions and tilted irradiation geometries, provides convincing qualitative evidence for the presence of linear damage zones in monazite that do not exhibit clear amorphisation. The comparison with zircon irradiated under identical conditions provides an effective internal benchmark demonstrating that the analytical approach is capable of resolving amorphous track cores when present. The etching experiment on Toya monazite also reinforces previous observations that etchability is strongly dependent on radiation damage level, and supports the emerging view that etch selectivity in monazite arises from defect-enhanced dissolution rather than removal of amorphous material. Collectively, the study contributes useful experimental evidence supporting the conceptual model that monazite fission tracks consist primarily of linear defect arrays within an otherwise crystalline lattice.
Despite these strengths, several targeted improvements would substantially strengthen the paper. This includes firstly integrating the TEM observations with independent quantification of radiation damage (most practically through Raman spectroscopy), would allow the authors to directly link microstructural observations to a measurable damage proxy. This would also enable assessment of whether track morphology varies systematically with pre-existing damage, which is central to interpreting annealing kinetics. What is meant by this is pre-existing radiation damage in monazite might influence how new fission (or ion) tracks form, which in turn affects how those tracks anneal over time and temperature. In a more damaged crystal, the lattice is already disordered, so newly formed tracks may differ in structure, such as defect density, continuity, or geometry when compared to those formed in a relatively pristine crystal. If annealing behaviour depends on the physical nature of the track (e.g., how defects are distributed and how easily they can recover), any variation in track morphology caused by differing damage states could potentially lead to differences in annealing kinetics. This is important because many models assume a single, uniform relationship between time, temperature, and track shortening, but if pre-existing damage influences track structure, then annealing behaviour may vary between grains. Therefore, understanding how track morphology changes with pre-existing radiation damage is essential for accurately interpreting and modelling monazite fission track annealing.
The study also does not consider crystallographic orientation effects on track formation. It implicitly assumes that all tracks behave similarly, however track structure may depend on direction within the crystal due to anisotropy in monazite. Without constraining crystallographic orientation, some of the observed variability in track morphology may be misattributed to intrinsic material behaviour. This represents a limitation of the current study and is an important consideration for future experiments.
The etching experiment appears to be designed primarily as a proof of concept to demonstrate that ion tracks, used here as a proxy for fission tracks, can be etched in a similar manner in monazite. In this context, the results are appropriate and support the authors’ interpretation that etchability does not require amorphization. Etch rate in monazite is likely controlled by radiation damage, where higher defect densities associated with greater damage enhance dissolution and reduce etching time, whereas low-damage crystals with fewer defects require significantly longer etching durations to reveal tracks. However, the experiment remains limited in scope, as it only compares two etching durations and does not attempt to systematically characterise etching kinetics or variability between grains. As such, while the results are consistent with previous work and sufficient to support the qualitative claim being made, they do not provide a detailed understanding of the relationship between radiation damage, defect structure, and etch behaviour. Expanding the etching dataset or integrating independent measures of radiation damage would strengthen the conclusions, but this is not essential for the primary objective of the study.
Overall, this study represents a useful and timely contribution to the understanding of latent track structure in monazite and provides experimental support for the hypothesis that MFT annealing behaviour is controlled by recovery of defect-rich crystalline domains rather than recrystallisation of amorphous track cores. The findings are consistent with recent experimental observations of annealing kinetics and etching behaviour in monazite and help strengthen the emerging conceptual framework for interpreting ultra-low temperature thermochronological behaviour. Future work integrating TEM observations with Raman spectroscopy, controlled annealing experiments, and systematic variation in radiation damage state would significantly improve the ability to quantitatively link microstructural damage characteristics with annealing kinetics models. As such, the paper provides an important foundation for further experimental work aimed at refining kinetic models for the monazite fission track system and improving confidence in the interpretation of thermal histories derived from MFT data.