15:35 〜 16:00
[SCG42-01] Development of monazite as a new ultra-low temperature fission track thermochronometer
★Invited Papers
キーワード：Monazite, Fission Track, Thermochronology, Etching, Annealing
Monazite, a rare-earth element phosphate mineral, is found as an accessory mineral 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. Previous studies produced very young ages suggesting that fission tracks may be annealed at very low temperatures. This study presents a newly developed track etching protocol and thermal annealing results, as well as a monazite fission track (MFT) case study from SW Japan.
The original method for monazite fission track etching was (12M) HCl at 90°C for 45 min, which was found to cause considerable grain loss from epoxy mounts and high degrees of grain corrosion. Therefore, after considerable experimentation to reduce these hindrances, we present an improved etching protocol of 6M HCl at 90°C for 60 – 90 min.
A series of isochronal laboratory annealing experiments were then performed on collimated 252Cf fission tracks implanted into monazite crystals on (100) and ~(001) faces over 1, 10, 100 and 1000 hour schedules at temperatures between 50°C and 400°C. In all cases, mean equivalent confined track lengths were always less than that in unannealed control samples. Monazite fission-track annealing also appears to be anisotropic, but this decreases with further etching. To investigate how track lengths decrease 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 this study. 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°C (both ± 6-21°C, 2 standard errors) for the best fitting linear fanning model (T0 = ¥). If a closure temperature is approximated as the mid-point of the MPAZ, it is estimated that the MFT closure temperature (Tc) ranges between ~45 and 25°C over geological timescales of 106 – 107 years, making this system potentially useful as an ultra-low temperature thermochronometer.
Historically, the young age and small amount of total denudation the Japanese island arc has experienced since its initiation of uplift has not been well constrained by low-temperature thermochronology techniques such as apatite fission track (AFT) and apatite (U-Th-Sm)/He (AHe). However, advances in the understanding of the MFT system have afforded an opportunity for the first time to directly analyse its neotectonic and denudation history. Low-temperature thermochronology methods have been applied to Cretaceous granitoid samples from the Ryoke belt, located in eastern Yamaguchi and Nara Prefectures, SW Japan. Zircon (U-Th)/He, 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 related to elastic loading caused by Philippine Sea plate subduction since the Middle - Late Miocene, along with the 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 agree 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.
The original method for monazite fission track etching was (12M) HCl at 90°C for 45 min, which was found to cause considerable grain loss from epoxy mounts and high degrees of grain corrosion. Therefore, after considerable experimentation to reduce these hindrances, we present an improved etching protocol of 6M HCl at 90°C for 60 – 90 min.
A series of isochronal laboratory annealing experiments were then performed on collimated 252Cf fission tracks implanted into monazite crystals on (100) and ~(001) faces over 1, 10, 100 and 1000 hour schedules at temperatures between 50°C and 400°C. In all cases, mean equivalent confined track lengths were always less than that in unannealed control samples. Monazite fission-track annealing also appears to be anisotropic, but this decreases with further etching. To investigate how track lengths decrease 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 this study. 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°C (both ± 6-21°C, 2 standard errors) for the best fitting linear fanning model (T0 = ¥). If a closure temperature is approximated as the mid-point of the MPAZ, it is estimated that the MFT closure temperature (Tc) ranges between ~45 and 25°C over geological timescales of 106 – 107 years, making this system potentially useful as an ultra-low temperature thermochronometer.
Historically, the young age and small amount of total denudation the Japanese island arc has experienced since its initiation of uplift has not been well constrained by low-temperature thermochronology techniques such as apatite fission track (AFT) and apatite (U-Th-Sm)/He (AHe). However, advances in the understanding of the MFT system have afforded an opportunity for the first time to directly analyse its neotectonic and denudation history. Low-temperature thermochronology methods have been applied to Cretaceous granitoid samples from the Ryoke belt, located in eastern Yamaguchi and Nara Prefectures, SW Japan. Zircon (U-Th)/He, 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 related to elastic loading caused by Philippine Sea plate subduction since the Middle - Late Miocene, along with the 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 agree 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.