Etching of fission-track in Quaternary monazite samples

Nakajima, Toru   ; Fukuda, Shoma   ; Niki, Sota*; Sueoka, Shigeru   ; Kawakami, Tetsuo*; Danhara, Toru*; Tagami, Takahiro*

We conducted monazite fission-track (MFT) etching experiments for Quaternary monazite dating. Quite low-temperature annealing of the MFT system (45-25 degree: Jones et al., 2021) has been proposed, and application as an ultra-low-temperature thermochronometer is expected. The practical application of MFT dating gives us access to a geological event at ultra-low temperatures, such as shallow crustal denudation and faulting. Jones et al. (2019) examined the etching conditions and suggested that the etching rate can vary between grains depending mainly on the accumulated radiation damage. Since the monazites used in previous studies were pre-Mesozoic (Weise et al., 2009; Jones et al., 2021), younger monazites with less radiation damage would be expected to have higher etching resistance. In this study, we attempted to etch FT using the Quaternary monazites, which are expected to have less radiation damage, to investigate appropriate etching conditions and discuss a relation between etching rate and radiation damage. Monazites from the Toya Ignimbrite (ca. 0.1 Ma: Niki et al., 2022) and the Kurobegawa Granite (ca. 0.8 Ma: Ito et al., 2013) were used in this study. Euhedral monazite-(Ce) crystals have weak magnetism and can be separated by the conventional magnetic separation method. The Cretaceous monazite from the Kibe Granite (ca. 98 Ma: Skrzypek et al., 2016) was also etched for comparison.