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Abe, Noriaki; Hoshi, Hiroyuki*; Haji, Toshiki*; Sato, Katsushi*; Niki, Sota*; Hirata, Takafumi*; Iwano, Hideki*; Danhara, Toru*
Chishitsugaku Zasshi (Internet), 131(1), p.59 - 70, 2025/04
To constrain the upper limit of the depositional age of the Tanabe Group, Early-Middle Miocene forearc basin deposit, this study obtained the detrital zircon U-Pb ages separated from eight sedimentary rock samples. We obtained 221 concordant ages from 330 zircon grains. The youngest grain age was 19.4 Ma, suggesting the upper limit of the depositional age is younger than 19.4 Ma, and no grain showed the depositional age indicated by foraminifera (16.3-15.1 Ma). Large-scale volcanic activities occurred in the Middle Miocene around Kii Peninsula, mainly after 15 Ma. The lack of grain younger than 15 Ma suggests that the Tanabe Group was deposited before 15 Ma. Sluggish volcanic activities in the provenance areas before 15 Ma may have caused the lack of zircons having ages close to the depositional period inferred from foraminifera.
Nakajima, Toru; Fukuda, Shoma; Sueoka, Shigeru; Niki, Sota*; Kawakami, Tetsuo*; Danhara, Toru*; Tagami, Takahiro*
Geochronology (Internet), 6(3), p.313 - 323, 2024/07
In this study, we explored the impacts of radiation damage and chemical composition on the etching time of fission tracks in monazite. Despite the potential of monazite fission-track (MFT) dating as an ultralow-temperature thermochronology, the comprehensive effects of radiation damage and non-formula elements, especially on the etching rate of MFTs, remain unexplored, and established analytical procedures are lacking. We quantified the degree of radiation damage (dFHWM) in 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, the Toya ignimbrite (ca. 0.1 Ma) and the Kurobegawa granodiorite (
0.8 Ma), both of which are types of monazite from Quaternary geological units, have dFHWM values of 0.27 and 0.55 cm
, respectively. In contrast, the Muro ignimbrite (ca. 15 Ma) has a dFHWM value of 4.01 cm, while the Kibe granite and the Sagawa granite, both of which are Cretaceous granitoids, yielded 7.35 and 6.31 cm, respectively. MFT etching of these samples according to the existing recipe (6M HCl at 90
C for 60-90 min) was completed at 1200, 860, 210, 120, and 90 min for the Toya ignimbrite, Kurobegawa granodiorite, Muro ignimbrite, Sagawa granite, and Kibe granite, respectively.
Sawada, Hikaru*; Niki, Sota*; Nagata, Mitsuhiro; Hirata, Takafumi*
Minerals (Internet), 12(1), p.107_1 - 107_15, 2022/01
Times Cited Count：8 Percentile：46.72(Geochemistry & Geophysics)The Oeyama ophiolite is one of the oldest components in Japanese Islands and important to reveal the initiation of plate subduction along proto-Japan. This study performed U-Pb-Hf isotopic and trace element analyses of zircon in gabbroic rocks. The weight mean of 
Pb/
U dates from zircons of the Oeyama ophiolite is 544 
4 Ma (2 sigma). Trace element analysis of the zircons exhibit that the host rock was derived from the mantle depleted of incompatible elements like mid-oceanic ridge basalt. The present igneous age and geochemical feature of the zircons is consistent with previous work for other part of the Oeyama ophiolite. Zircon Lu-Hf isotopic analysis also indicate that the gabbroic rock was derived from the depleted mantle domain. The Hf isotopic signature is more depleted than those of the zircons in jadeitite associated with the Oeyama ophiolite. This result implies that the older crustal material was involved into the initial oceanic plate subduction along the proto-Japan arc.
U
Th isotope analysis of GSJ reference rocks by solid nebulization ICP mass spectrometry utilizing the femtosecond laser ablation system with a galvanometric opticsKagami, Saya; Yokoyama, Tatsunori; Niki, Sota*; Sakata, Shuhei*
no journal, ,
The UTh disequilibrium systematics in Quaternary volcanic rocks has key information about origin of melt generation and chronology of magmatism for understanding magmatic processes. The UTh isotope analysis of whole-rock samples have conventionally required complicated procedure for chemical separation, and been conducted by TIMS or SIMS. The main nalytical issues on these ion sources are the difficulty of producing large ion currents, resulting in long analysis time and worsening the analysis throughput. Nowadays, the high-temperature ICP ion source with high ionization capacity is a viable option for high-sensitive analysis of refractory elements including U and Th. In addition, laser ablation (LA) sampling technique can directly introduce solid materials to ICP without sample decomposition and chemical separation. However, previous LA systems have the upper limitation of ablation size, which constrains the application to bulk elemental and isotopic analysis. Recently, the femtosecond LA (fsLA) system with a galvanometric optics has been developed, and the combination of the high-repetition-rate laser and high-speed scanning mirrors enable to ablate large volumes. This fsLA technique is called solid nebulization since the resulting ion signal profile is stable as well as that of solution nebulization ICP-MS. In this study, this solid nebulization was applied to the UTh isotope analysis of GSJ reference rocks for the proof-of-concept study. The solid nebulization ICP-MS system is a quadrupole-based ICP tandem mass spectrometer coupled with a fsLA system at TGC, JAEA. This system has a capability to remove matrix-derived polyatomic interference ions by kinetic energy discrimination in the ICP-MS system and achieve the abundance sensitivity required for UTh isotope analysis. The glass beads of GSJ reference rocks were prepared. In the case of the ablation area of 300 
300 
m
in single analysis, the analytical accuracy of 
Th/
Th and Th/U ratios were 5% and 10%, respectively. Quaternary volcanic rocks frequently have UTh disequilibrium with 10% or higher in Th/Th, and the technique developed here is fully applicable to such whole-rock samples.
Nakajima, Toru; Fukuda, Shoma; Niki, Sota*; Sueoka, Shigeru; Kawakami, Tetsuo*; Danhara, Toru*; Tagami, Takahiro*
no journal, ,
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.
Nakajima, Toru; Niki, Sota*; Kudo, Shumpei*; Kawakami, Tetsuo*; Higashino, Fumiko*; Hirata, Takafumi*; Sakai, Harutaka*
no journal, ,
The orthogneisses widely distributed in the Himalayas are thought to be originated from granites intruded during the early Paleozoic Bhimphedian orogeny on the northeastern margin of Gondwana, although details are not clear due to the poor of metamorphic minerals such as garnet. In this study, we focus on the internal microstructure and inclusions of zircons in orthogneisses in the Central Himalaya and attempt to decipher the early Paleozoic magma-fluid activity, which was overprinted by high-temperature metamorphism accompanied with Cenozoic Himalayan orogeny. In this study, the internal microstructure of the zircon was divided from the inside into 1) the inner-core, 2) the outer-core, 3) the dark annulus, and 4) the metamorphic rim based on optical microscope and cathodoluminescence (CL) observation. Each of these domains is cut by the more outer domain. 1) the inner-core shows oscillatory or sector zoning in the CL image, with U-Pb date of 
1000 Ma. 2) the outer-core shows an oscillatory zoning in the CL image, with U-Pb date between 510 and 460 Ma. In addition to mineral inclusions such as quartz and biotite, the outer-core encloses primary fluid inclusions of medium-salinity and polyphase mineral inclusions. Euhedral quartz, K-feldspar, chalcopyrite, fluorite, sphalerite, and native bismuth occur in some of the polyphase inclusions, indicating that the outer-core crystallized from a highly differentiated S-type granitic melt. 3) The dark annulus is recognized in the CL image as a dark annular domain, rich in P, Y, REE, and U, with U-Pb date of 490-440 Ma. The dark annulus is rich in mineral inclusions such as xenotime, monazite, and thorite, as well as high-salinity primary fluid inclusions. These internal microstructures and low Gd/Yb ratio of the dark annulus suggest the garnet breakdown and zircon dissolution-reprecipitation accompanied with high-salinity fluid influx during late stage of magmatism.
Abe, Noriaki; Hoshi, Hiroyuki*; Haji, Toshiki*; Sato, Katsushi*; Niki, Sota*; Hirata, Takafumi*; Iwano, Hideki*; Danhara, Toru*
no journal, ,
no abstracts in English
Ogita, Yasuhiro; Niki, Sota*; Nagata, Mitsuhiro; Hirata, Takafumi*; Yuguchi, Takashi*
no journal, ,
Alteration process in granitic rocks is important for mass transfer through rock-fluids interactions and the formation of microcracks as a pathway of ground water. Timescale of alteration in granitic rocks is enough unrevealed because limited methodologies are available. Focusing on biotite chroritization and following precipitation of titanite (sphene) during hydrothermal alteration in granitic rocks, hydrothermal titanite leads to geochronological constraint on timescale of alteration. This study reports U-Pb age of hydrothermal titanite from Tono Plutonic Complex (TPC), Kitakami Mountains, northeast Japan, and interprets its age comparing other geochronological data. Thinsection observation suggests that titanite from the TPC sample was secondary precipitated related to biotite chloritization during subsolidus stage. In thinsections, titanite is not enough to analyze using laser ablation-inductively coupled plasma-mass spectrometry. This study conducted U-Pb analysis for separated titanite and obtained age of 118.71.5 Ma. The occurrence of titanite shows that titanite was precipitated accompanied biotite chloritization during hydrothermal alteration in the TPC. Thus, the age of titanite is potentially able to constrain on hydrothermal activity during subsolidus stage of TPC. Moreover, the obtained age falls within zircon U-Pb age (1172 Ma) and biotite K-Ar age (122.62.7 Ma) reported from central facies of TPC. This indicates hydrothermal alteration process related to biotite chloritization has experienced short period during cooling on TPC.
Kagami, Saya; Yokoyama, Tatsunori; Niki, Sota*; Sakata, Shuhei*; Umeda, Koji*; Okada, Rina*; Kondo, Misaki*
no journal, ,
Tephras from submarine eruptions are easily affected by secondary deposition and alteration and the constraints on their geochronology may be difficult. In this study, we conducted zircon U-Th disequilibrium dating by LA-ICP-MS for Zenikame-Menagawa tephra (Z-M), which was erupted from Zenakame volcano on the seafloor of the Tsugaru Strait and is a valid age indicator for MIS3. The zircons in the Toya pumice were measured to evaluate the accuracy of the method and their isochron age of 128 ka was obtained. This result is consistent with the previous reported data by the same method used in this study. We measured the zircons from Z-M and found that many of the zircons did not record eruption ages, and that many grains had U-Th disequilibrium model ages of about 100 ka. In the future, we will attempt U-Th disequilibrium dating of each zircon along with understanding its characteristics (shape, chemical composition and so on).
Nagata, Mitsuhiro; Fukuda, Shoma; Sueoka, Shigeru; Yokoyama, Tatsunori; Kagami, Saya; Niki, Sota*; Iwano, Hideki*; Danhara, Toru*; Ogita, Yasuhiro; Kajita, Yuya*; et al.
no journal, ,
no abstracts in English
Fukuda, Shoma; Niki, Sota*; Ogita, Yasuhiro; Kondo, Hidee*; Teshima, Katsuya*
no journal, ,
Nakajima, Toru; Niki, Sota*; Kudo, Shumpei*; Kawakami, Tetsuo*; Higashino, Fumiko*; Hirata, Takafumi*; Sakai, Harutaka*
no journal, ,
no abstracts in English
Yamada, Raiki*; Nagata, Mitsuhiro*; Sawada, Hikaru*; Niki, Sota*; Ogita, Yasuhiro; Ouchi, Wataru*; Aoyama, Shinnosuke*; Hirata, Takafumi*
no journal, ,
no abstracts in English
