Hidenori Kumagai

<jats:p>The peak height comparison K-Ar dating method was applied to very young (younger than ca. 35 ka) vesiculated bombs from three pyroclastic units (Komakusadaira Pyroclastics, Kattadake Pyroclastics, and Umanose Agglutinate) from Zao Volcano in northeast Japan. Because the 38Ar/36Ar ratios differed from 0.187, the mass fractionation was corrected for all measurement data. The obtained K-Ar ages are 30.0 ± 32.2 ka (no. 1-1, weakly vesiculated), 37.4 ± 64.8 ka, and 33.0 ± 58.0 ka (nos. 2-1, 2-2, weakly vesiculated) for the samples from Komakusadaira Pyroclastics; 12.6 ± 46.7 ka (no. 3-1, highly vesiculated) for a sample from Kattadake Pyroclastics; and 5.3 ± 44.9 ka, 5.7 ± 14.3 ka (no. 4-1, 4-2, weakly vesiculated), and 17.3 ± 79.3 ka (no. 5-1, moderately vesiculated) for the samples from Umanose Agglutinate on a 1σ error basis. For all cases, the obtained ages’ errors exceed 100%, with none involved within the error range. Therefore, the following age value upper limits are considered statistically significant: no. 1-1 is ≤94.4 ka, 2-1 is ≤167.0 ka, 2-2 is ≤149.0 ka, 3-1 is ≤106.0 ka, 4-1 is ≤95.1 ka, 4-2 is ≤34.3 ka, and 5-1 is ≤175.9 ka, considering 2σ errors. The obtained ages are generally consistent with those previously estimated using 14C dating.</jats:p>

Abstract The Martian atmospheric Ne may reflect recent gas supply from its mantle via volcanic degassing, due to its short (∼100 Myr) escape timescale. The isotopic ratio of the Martian atmospheric Ne would therefore provide insights into that of the Martian mantle, further suggesting the origin of Mars volatiles during planetary formation. Mass spectrometric analysis of the Martian atmospheric Ne, however, has faced challenges from interference between ²⁰Ne⁺ and ⁴⁰Ar⁺⁺. Previous studies using a polyimide membrane for ²⁰Ne/⁴⁰Ar separation were limited by the drawbacks of elastomeric O-rings to support the membrane, such as low-temperature intolerance, outgassing, and the need to endure environmental conditions during the launch and before/after landing on Mars. This study proposes a new method employing a metal C-ring to secure a 100 μm polyimide sheet within vacuum flanges. Environmental tests, including vibration, shock, extreme temperatures, and radiation exposure, were conducted on the gas separation flanges. Pre- and post-test analyses for He, Ne, and Ar demonstrated the membrane-flange system’s resilience. Gas permeation measurements using terrestrial air effectively permeated ⁴He and ²⁰Ne, while reducing ⁴⁰Ar by more than six orders of magnitude. This study achieved a &lt;3% accuracy in determining the ²⁰Ne/²²Ne ratio, sufficient for assessing the origins of Ne in the Martian mantle. Furthermore, experiments with a 590 Pa gas mixture simulating the Martian atmosphere achieved a 10% accuracy for the ²⁰Ne/²²Ne isotope ratio, with gas abundances consistent with numerical predictions based on individual partial pressures. These results validate the suitability of the developed polyimide membrane assembly for in situ Martian Ne analyses.

We determined the K-Ar ages of phengite occurring as a gangue silicate mineral in the massive sulfide ores from the Kotsu and Iyo Besshi-type volcanogenic massive sulfide (VMS) deposits in the Sanbagawa high-P/T metamorphic belt, southwest Japan. The K-Ar ages of phengite are dated to be ca. 45 or 85 Ma and these ages correspond to the timing of initial or terminal stages of regional retrograde metamorphism. Contrastingly, the Re-Os isochron ages of sulfide mineral aggregates from the Kotsu and Iyo Besshi-type VMS deposits are ca. 145 Ma, which is much older than those of phengite K-Ar ages. Since exactly the same ore samples were used for both K-Ar and Re-Os geochronology, it was confirmed that the K-Ar isotope system was reset due to the regional metamorphism up to lower epidote-amphibolite facies, although the Re-Os isotope system retained the original geochemical and isotopic composition even though the regional (Sanbagawa high-P/T) metamorphism.

地質試料に対する希ガス同位体分析は、大気混染との戦いである。一方、今世紀に入っての二次イオン質量分析計(SIMS)やレーザーアブレーション誘導結合プラズマ質量分析計(LA-ICP-MS)などの局所同位体分析の発展は地球史初期の様相をはじめ地質試料から多様な情報を引き出しつつある。熱年代学にも活用されている希ガス同位体分析がこの流れにどの程度追随できるかは予断を許さない。この小論では、一世代前の希ガス質量分析装置を用いた筆者らの局所微量分析の試みを報告する。

<title>Abstract</title>Seafloor massive sulphide (SMS) deposits, modern analogues of volcanogenic massive sulphide (VMS) deposits on land, represent future resources of base and precious metals. Studies of VMS deposits have proposed two emplacement mechanisms for SMS deposits: exhalative deposition on the seafloor and mineral and void space replacement beneath the seafloor. The details of the latter mechanism are poorly characterised in detail, despite its potentially significant role in global metal cycling throughout Earth’s history, because in-situ studies require costly drilling campaigns to sample SMS deposits. Here, we interpret petrographic, geochemical and geophysical data from drill holes in a modern SMS deposit and demonstrate that it formed via subseafloor replacement of pumice. Samples from the sulphide body and overlying sediment at the Hakurei Site, Izena Hole, middle Okinawa Trough indicate that sulphides initially formed as aggregates of framboidal pyrite and matured into colloform and euhedral pyrite, which were replaced by chalcopyrite, sphalerite and galena. The initial framboidal pyrite is closely associated with altered material derived from pumice, and alternating layers of pumiceous and hemipelagic sediments functioned as a factory of sulphide mineralisation. We infer that anhydrite-rich layers within the hemipelagic sediment forced hydrothermal fluids to flow laterally, controlling precipitation of a sulphide body extending hundreds of meters.

During the cruise CK16-05 aboard the D/V Chikyu, we conducted natural gamma-ray logging with a through-the-bit logging system by using a memory-type geothermal exploration tool that has so-far been used only in terrestrial geothermal wells. This logging system was firstly tried during the cruise CK16-01 to reduce the cost and increase the efficiency of drilling surveys, but issues from vibration and collision impact within the drill hole were found. Based on these results, we re-examined and modified the logging tools and operation methods. As a result, we successfully obtained highly accurate and continuous data of natural gamma-ray intensity, temperature, and pressure within the drill holes at five sites in the Izena Hole, mid-Okinawa Trough, and accomplished significantly lower-cost logging at one order of magnitude lower than that of the Logging While Drilling (LWD) system. At three sites within the Izena Hole hydrothermal field, the gamma-ray intensity exhibited significant variations along with lithological changes composed of pumice, hemi-pelagic sediment, massive sulfide, silicified rock, and a K-rich altered clay layer. On the other hand, at the other two sites located ca. 330 and 670 m N-NNW from the hydrothermal sulfide mound, subseafloor strata were dominated by pumice with minor amounts of hemi-pelagic sediment and the gamma-ray intensity correlated well with the density and porosity of the core samples determined by the multi-sensor core logger (MSCL) and discrete sample measurements. Our results demonstrate that gamma-ray logging with the through-the-bit logging system is highly effective in identifying and characterizing the subseafloor lithology as well as the thickness of the massive sulfide layer, and can compensate for the often inevitable poor core recovery rate during coring operations at seafloor hydrothermal deposits. The versatility of this system could be further improved with additional sensor measurements (e.g., resistivity) in future drilling operations at seafloor hydrothermal fields.

Kumagai, H, Adrovic, F, Iwase, R, Kinoshita, M, Machiyama, H, Hattori, M &amp; Okano, M, 2012, , InTech.