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森清 寿郎, 森田 亮, 古橋 奨汰原稿種別: 研究論文
2018 年 72 巻 4 号 p. 195-211
発行日: 2018/10/25
公開日: 2020/01/09
ジャーナル オープンアクセス新潟県長岡市栃尾地域に分布する荒谷層の黒色泥岩には，管状やレンズ状，塊状など様々な形態の石灰質団塊を産する．管状団塊のあるものは玄能石を核として包有し，マントル，リムの同心円構造をもつ．これらの団塊の炭素・酸素同位体比と方解石の化学組成を求め，管状団塊ほか各種団塊の成因を研究した．団塊は玄能石包有型と玄能石欠如型に大別され，両者は炭素・酸素同位体比においても違いがある．玄能石包有型はすべて方解石1 相であるが，玄能石欠如型には方解石1 相，方解石と菱鉄鉱の2 相の団塊がある．δ13C は玄能石包有型→玄能石欠如型方解石→同2 相型の方解石→同2 相型の菱鉄鉱の順に上昇し，Fe2+ 濃度もこの順に上昇している．一方δ18O はこの順に低下する．方解石中の石灰質団塊の同位体比および化学組成は，巣穴生物の腐敗に続く，硫酸塩還元からメタン発酵までの続成作用のほぼ連続的な過程を表している．
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服部 昇, 大井 修吾, 中野 聰志原稿種別: 短報
2018 年 72 巻 4 号 p. 213-218
発行日: 2018/10/25
公開日: 2020/01/09
ジャーナル オープンアクセスClastic sandy nodules are found on a submerged terrace south of Oki-shima Islet in Lake Biwa, southwest Japan. Vivianite and siderite-rhodochrosite crystals occupy interstitial spaces between the clastic sand grains in the nodules. There are three types of nodules recognised: (1) vivianite only, (2) siderite-rhodochrosite only and (3) coexisting vivianite and siderite-rhodochrosite. The vivianite contains little manganese, and its compositional variation is very small. Some parts of the siderite are enriched with manganese, showing compositions of rhodochrosite. The vivianite and siderite-rhodochrosite in the sediments might be formed during incipient diagenesis in a reduction environment below the oxidative environment of the lake bottom. These interstitial minerals play a role of depositing iron, manganese, phosphorus, carbon and related elements in the sediments.
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昆 慶明, 保科 裕, 関東山地研究グループ原稿種別: 短報
2018 年 72 巻 4 号 p. 219-224
発行日: 2018/10/25
公開日: 2020/01/09
ジャーナル オープンアクセスThe Yotsumatayama quartz diorite body exposed at Shimonita Town in Gunma Prefecture, consists of two varieties of rocks, namely, hornblende quartz diorite and two-mica granodiorite. Zircon U-Pb ages of these granitoids are measured.
U-Pb isotope data of zircon grains from hornblende quartz diorite yields an age of 112.7 ± 3.7 Ma, and that from the two-mica granodiorite yields an age of 113.1 ± 3.6 Ma. These values are nearly similar within their respective error limits. The geochronological data time indicates the Early Cretaceous period. The earlier reported K-Ar age of hornblende (105 ± 6 Ma and 110 ± 5 Ma) might represent a cooling age in the Yotsumatayama quartz diorite, whether the U-Pb age of zircon indicates the age of crystallization. U-Pb age of the hornblende quartz diorite indicates also the Early Cretaceous period. However, two-mica granodiorite also included older zircon grains yielding U-Pb age of the Late Jurassic period ~ Early Permian period. We consider these as the inherited zircon grains from possible sedimentary protolith. The difference in the rock facies and the U-Pb age spectrum of the Yotsumatayama quartz diorite possibly support this view.
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