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ItemAn Investigation of Fission Tracks in Monazite: Development of a New Ultra-Low Temperature Fission Track Thermochronometer( 2022)Monazite, a rare-earth element (REE) phosphate mineral, is found as an accessory in a variety of rock types. Suitable uranium and thorium content make it a useful mineral for isotopic and chemical dating using the (U-Th)/He and U-Th-Pb methods. However, unlike other uranium-bearing minerals, apart from a few reconnaissance studies, its potential for fission-track dating has not been systematically investigated. Earlier studies produced very young ages suggesting that fission tracks may be annealed at very low temperatures. This study explores the fission track properties of monazite and presents the findings of a new track etching protocol and thermal annealing experiments. These are accompanied by a case study in SW Japan, demonstrating how a new fission track thermochronometer can be applied in a young and small orogenic belt. The previously reported concentrated (12M) HCl etchant at 90 degrees C for 45 min was found to cause grain loss from epoxy mounts and high degrees of grain erosion. Therefore, in efforts to reduce these hindrances, an alternative etching protocol of 6M HCl at 90 degrees C for 60 – 90 min has been established for monazite after testing several alternative etchants. However, it was found in an isothermal annealing experiment that ~4 percent annealing occurs after one hour exposure to this etching temperature. Thus, a key concern is that some track annealing could occur during track etching before the etchant reaches the track ends. To investigate this, possibility the application of focused ion beam scanning electron microscopy was used to mill progressively into implanted 252Cf fission tracks after slight etching, followed by an etch-anneal-etch experiment. Results showed that the etchant penetrated to the track ends in <15 min, suggesting less than ~1 percent of fission track length reduction is likely to occur during etching. Other etching experiments performed show that crystal settling during standard epoxy mounting means that (100) faces are preferentially displayed so that subsequent experiments were weighted towards this orientation. The size and shape of well-etched spontaneous fission track openings in monazite were also constrained to be rhombic in shape. Average rhombic etch pit diameters Dpc and Dpb, parallel to the crystallographic c- and b-axes on (100) faces are 0.81 +/- 0.20 micrometer and 0.73 +/- 0.26 micrometer, respectively. An angular distribution experiment on (100) faces found that spontaneous fission tracks initially etch anisotropically, being preferentially revealed at an azimuth of 90 degrees to the crystallographic c-axis up to ~60 min of etching. As etching continues, however, the distribution becomes progressively more uniform and is essentially isotropic by 90 min. Electron microprobe analyses also showed a correlation between etching rate and elemental composition, with over-etched grains tending to have higher U and Th concentrations, also suggesting a radiation damage effect. A series of isochronal laboratory annealing experiments were then performed on collimated 252Cf fission tracks implanted into monazite crystals on both (100) and ~(001) faces over 1, 10, 100 and 1000 hour schedules at temperatures between 30 degrees C and 400 degrees C. In all cases, the mean equivalent confined track length was always less than that in unannealed control samples. Monazite fission track annealing also appears to be anisotropic, with tracks on surfaces perpendicular to crystallographic c-axis consistently annealing faster than those parallel to the (100) face. To investigate how mean track lengths decreased as a function of time and temperature, one parallel and two fanning Arrhenius models were fitted to the empirical dataset. The temperature limits of the monazite partial annealing zone (MPAZ) were defined as length reductions to 0.95 (lowest) and 0.5 (highest) for these experiments. Extrapolation of the laboratory experiments to geological timescales indicates that for a heating duration of 107 years, estimated temperature ranges of the MPAZ are -71 to 143 degrees C (both +/- 6-21 degrees C, 2 standard errors) for the best fitting linear fanning model (T0 = infinity). If a monazite fission-track closure temperature is approximated as the mid-point of the MPAZ, it is estimated that the closure temperature (Tc) for fission tracks in monazite ranges between ~45 and 25 degrees C over geological timescales of 106 – 107 yrs, making this system potentially useful as an ultra-low temperature thermochronometer. Even ambient surface temperatures remain well within the MPAZ over these time scales. The final chapter of this study presents a low-temperature thermochronology study of Cretaceous granitoid samples from the Ryoke belt, located in eastern Yamaguchi and Nara Prefectures, SW Japan. Historically, low-temperature thermochronology techniques such as apatite fission track (AFT) and apatite (U-Th-Sm)/He (AHe) have been limited in their applicability to uncover the neotectonic evolution of Japan. This is predominantly due to the young age and small amount of total denudation the Japanese island arc has experienced since initiation of uplift. However, the monazite fission track (MFT) system provides an opportunity for the first time to directly analyse the neotectonic and denudation history of this area. Zircon (U-Th)/He (ZHe), AFT and AHe data and modelled thermal histories reveal Late Cretaceous - Pliocene cooling related to paleo-Izanagi and Pacific plate subduction along the eastern Eurasian continental margin. MFT dating reveals Plio – Pleistocene central ages interpreted to reflect elastic loading caused by Philippine Sea plate subduction since the Middle - Late Miocene, along with Quaternary collision of NE and SW Japan at the Itoigawa-Shizuoka Tectonic Line (ISTL). Estimated denudation rates based on MFT dating are in the order of 0.10 – 0.47 mm/yr and 0.15 – 0.56 mm/yr in the eastern Yamaguchi and Nara Prefectures, respectively, which are in accord with estimated rates calculated using geomorphological and altitude dispersion methods. No relationship with topography or geomorphological factors has been established to explain the higher denudation rates in the Nara Prefecture. Instead, differences are likely to reflect variations in the tectonic regime, timing of uplift and uplift mechanisms of the two regions.