Short communication: Inverse correlation between radiation damage and fission-track etching time on monazite

Abstract. In this study, we explored the impacts of radiation damage and chemical composition on the etching time of monazite fission-track (MFT). Despite the potential of MFT as an ultra-low-temperature thermochronology, the comprehensive effects of radiation damage and non-formula elements, especially on the etching rate of MFT, remain unexplored, and established analytical procedures are lacking. We quantified the degree of radiation damage (ΔFHWM) of Cretaceous to Quaternary monazites distributed in the Japan arc through Raman spectroscopy and chemical composition analyses. Subsequently, MFT etching was performed to examine the correlation between these parameters and the etching time. Estimation of the degree of radiation damage showed an increase in radiation damage corresponding to the cooling age of each geological unit. For example, Monazites from Quaternary geological units, the Toya ignimbrite (ca. 0.1 Ma) and the Kurobegawa granodiorite (ca. 0.8 Ma), have ΔFHWM of 0.48 and 0.70 cm-1, respectively. In contrast, the Muro ignimbrite (ca. 15 Ma) has a ΔFHWM of 4.11 cm⁻¹, while Cretaceous granitoids, including the Kibe granite and the Sagawa granite, yielded 7.42 and 6.40 cm⁻¹, respectively. MFT etching of these samples according to the existing recipe (6M HCl at 90 °C for 60–90 minutes) was completed at 1200, 860, 210, 120, and 90 minutes for Toya ignimbrite, Kurobegawa granodiorite, Muro ignimbrite, Sagawa granite, and Kibe granite respectively. These outcomes highlight an inverse relationship between MFT etching time and the degree of radiation damage in monazite, while the correlation between MFT etching time and chemical composition was unclear. The results affirm earlier considerations that the etching rate of MFT is strongly influenced by radiation damage. Conversely, young samples with lower levels of radiation damage exhibit higher chemical resistance, suggesting that existing etching recipes may not adequately etch MFT.