掲載種別：研究論文（学術雑誌）  

<title>Abstract</title>Terrestrial records of the last geomagnetic reversal often have few age constraints. Chronostratigraphy using suborbital-scale paleoceanic events during marine isotope stage 19 may contribute to solving this problem. We applied the method to an 8 m long, high-resolution paleomagnetic record from a loess sequence in China and revealed millennial-to-sub-centennial scale features of the Matuyama–Brunhes (MB) transition. All samples were subjected to progressive thermal demagnetization with 14–15 steps up to 650–680 °C. As a result, 96% of the samples yielded a high-quality remanent magnetization. The MB transition terminated with a 75 cm thick zone with nine polarity flips. The polarity flip zone, dated at about 779–777 ka, began between the warm events “I” and “J” and terminated at the end of the cooling event coincident with the lowest axial-dipole strength interval. Most polarity flips occurred within 70 years. The virtual geomagnetic poles (VGPs) in the upper polarity flip zone clustered in the SW Pacific region, where the MB transitional VGPs from lavas of the Hawaiian and Canary Islands and lacustrine deposits of Java also clustered. These sites were probably dominated by dipolar fields. The absence of transitional fields across polarity flips implies a short time span for averaging fields due to a thin loess-magnetization lock-in zone. The reverse-to-normal polarity reversal dated at about 778 ka in Lingtai occurred at the end of the SW Pacific VGP zone, an important key bed for MB transition stratigraphy. The reversal is a good candidate for the main MB boundary. We found an excursion at about 766 ka spanning about 1 ka.

DOI： 10.1186/s40645-020-00337-z

その他URL： https://link.springer.com/article/10.1186/s40645-020-00337-z/fulltext.html

担当区分：筆頭著者,　責任著者   掲載種別：研究論文（学術雑誌）  

<title>Abstract</title>We evaluated water distributions in deformed quartz in schists along the Asemi River, Central Shikoku, in the Sanbagawa Metamorphic Belt, Japan, using infrared spectroscopic (IR) mapping. The water trapped in quartz as molecular H₂O showed a broad IR absorption band at 2800–3750 cm⁻¹. A necessary step before assessing the quartz water content was to evaluate and compare six previously proposed IR calibrations in terms of the molar absorption coefficients of H₂O (L/mol H₂O cm²). The coefficients vary from 24,100 to 89,000 L/mol H₂O cm², and the values of the coefficients show a rough increase with increasing component of structural –OH in the IR spectra. We used Paterson’s calibration, which does not require input regarding the mineral species, but which was modified in his paper for measurements of molecular H₂O in quartz. The absorption coefficient is 38,000 L/mol H₂O cm². IR mapping was performed on Sanbagawa metamorphic rocks with increasing grades of metamorphism, where the mean grain size of quartz increases from ~ 40 to ~ 120 µm. The absorption bands that are only from the quartz can be distinguished on the basis of microstructural observations and the corresponding mapping results. The IR spectra of quartz commonly show dominant molecular H₂O bands at 2800–3750 cm⁻¹ with no additional bands associated with crystalline –OH when only quartz is measured. The water contents of quartz in all our samples were 40–310 ppm, and these values are about one-third of previously reported values measured using point analyses with the unified Paterson’s calibration. This difference seems to reflect the incorporation of phyllosilicates in previous measurements that showed a broad band around 3600 cm⁻¹. The lowest and highest water contents in our quartz samples are associated with intragranular water and grain boundary water, respectively. We estimated the grain boundary widths to be at most ~ 10 nm on the basis of the water contents at grain boundaries.

DOI： 10.1186/s40623-019-1117-4

その他URL： http://link.springer.com/article/10.1186/s40623-019-1117-4/fulltext.html

担当区分：筆頭著者,　責任著者   掲載種別：研究論文（学術雑誌）  

Abstract. Grain growth of quartz was investigated using two quartz samples
(powder and novaculite) with water under pressure and temperature
conditions of 1.0–2.5 GPa and 800–1100 ∘C. The compacted powder
preserved a substantial porosity, which caused a slower grain growth than in
the novaculite. We assumed a grain growth law of
dn-d0n=k0fH2Orexp⁡(-Q/RT)t
with grain size d (µm) at time t (seconds), initial grain size
d0 (µm), growth exponent n, a constant k0 (µmn MPa−r s−1), water fugacity
fH2O (MPa) with the exponent r,
activation energy Q (kJ mol−1),
gas constant R, and temperature T in
Kelvin. The parameters we obtained were n=2.5±0.4, k0=10-8.8±1.4, r=2.3±0.3, and Q=48±34 for the powder and n=2.9±0.4, k0=10-5.8±2.0, r=1.9±0.3, and Q=60±49 for the novaculite. The grain growth parameters obtained for
the powder may be of limited use because of the high porosity of the powder
with respect to crystalline rocks (novaculite), even if the differences
between powder and novaculite vanish when grain sizes reach ∼70 µm. Extrapolation of the grain growth laws to natural conditions
indicates that the contribution of grain growth to plastic deformation in the
middle crust may be small. However, grain growth might become important for
deformation in the lower crust when the strain rate is &lt; 10−12 s−1.

DOI： 10.5194/se-10-621-2019