The genesis of podiform chromitites in the Samail ophiolite, the sultanate of Oman : constraints from the inclusions

• 資料種別: 博士論文
• タイトル: The genesis of podiform chromitites in the Samail ophiolite, the sultanate of Oman : constraints from the inclusions
• 著者・編者: Yao, Yuan
• 出版年月日等: 2020-03-23
• 出版年（W3CDTF）: 2020-03-23
• 並列タイトル等: オマーン国サマイル・オフィオライトのポディフォーム型クロミタイトの成因 : 包有物からの制約
• 授与機関名: 新潟大学
• 授与年月日: 2020-03-23
• 授与年月日（W3CDTF）: 2020-03-23
• 報告番号: 甲第4763号
• 学位: 博士(理学)
• 博論授与番号: 甲第4763号
• 本文の言語コード: eng
• 一般注記: Podiform chromitites have provided us valuable information on various mantle processes, including melt-mantle reaction, deep-seated magmatic evolution, and mantle dynamics. Recently, many kinds of inclusions have been found in podiform chromitites from Samail ophiolite, such as platinum group element minerals, silicate minerals, even ultra-high-pressure mineral: moissanite. Melt inclusions have the potential to provide important information on the primary melt that crystallized host minerals, such as the chemical composition of the melt and the pressure and temperature conditions when inclusions are trapped. After the melt inclusions are captured, they can be regarded as a closed system or an isolated system, together with the host mineral. After cooling, the melt inclusions would turn into multiphase solid inclusions (MSI). Consequently, melt inclusions can be regarded as a time capsule that stores the information about the physical and chemical conditions of the system from the time of inclusion formation. In addition, by examining the composition of multiple melt inclusions, it is possible in some cases to deduce the liquid line of descent of magmatic systems. Most studies of multiphase solid inclusions in chromites have focused on the composition of the daughter minerals, and the genesis and evolution of the inclusions are poorly understood. In this study, we used high-resolution X-ray computed tomography (HRXCT) and scanning electron microscopy (SEM) to investigate the evolution of multiphase solid inclusions in chromites from the Samail ophiolite in Oman. The studied samples were collected from three localities: (1) a banded chromitite mine in the massive dunite of the crust-mantle transition zone (MTZ) in the Samail massif, (2) chromitite veins, from ICDP Oman Drilling Project (OmanDP) Hole CM2B in the MTZ, in Wadi Zeeb of the Wadi Tayin massif and (3) a podiform chromitite mine in the western part of the Fizh massif. High–resolution X-ray computed tomography (HRXCT) was used and combined with scanning electron microscopy (SEM) to obtain 3D and 2D images of multiphase inclusions within chromite from the Samail ophiolite. Inclusions are rare in the podiform chromitite samples. However, they are common in the banded chromitite samples and in chromitite veins samples in the OmanDP cores. The multiphase inclusions range in diameter from 5μm to 200μm, contain pargasite, aspidolite, high-Cr# (= Cr / [Cr + Al] atomic ratio) (Cr#>60) chromite lining, diopside, enstatite, pentlandite and so on. In banded samples and OmanDP samples, the necking–down of the earlier large melt inclusions produced various assemblages of daughter minerals in the small inclusions were observed. The necking-down may explain the heterogeneity of melt inclusions. In banded samples, the host chromite with a skeletal morphology was observed. Moreover, the 3D HRXCT images for the spatial distributions of inclusions indicate the rapid growth of the host chromite provided cages/hopper to trap melt inclusions. These two critical observations were consistent with the rapid cooling of chromitite forming melt. After trapping melt inclusions, the chromite continued to grow on the inner wall. Sometimes this overgrowth is recognized as a high-Cr# chromite lining due to the rapid cooling. The high-temperature homogenized experiments for the banded sample were conducted to study the compositions of melt inclusions. Because the temperature of 1200°C was not high enough, the high-Cr# chromite lining and possible residual phases were remained after the melting. Thus, the homogenized glass could not represent the parental melt trapped in the host chromite. The compositions of melt inclusions were calculated by using areas of daughter minerals, including high Cr# chromite lining. The results show the Cr2O3 content of the trapped melt was up to 9.6%. Such parental melt with high-Cr2O3 content may have been responsible for the formation of podiform chromitites in the Samail ophiolite. In-situ U-Pb dating of apatite inclusions was carried out by Nano-SIMS. The young model age (130.1 ± 55.1 Ma) of apatite indicates the MTZ chromitites were related to the formation of Samail ophiolite at fast-spreading ridge (94 – 95 Ma). The parental melt of chromitites was super-saturated in chromite. Meanwhile, the cooling rate of chromitites parental melt may control the host chromite grow mechanism and inclusions trapping mechanism. For the genesis of MTZ chromitites, the constraints from the melt inclusions were rapid cooling, super-saturated chromite parental melt, and young age. The reason for the rapid cooling of MTZ chromitites or their parental melt requires further discussion. In contrast, the mantle chromitite had a slow cooling rate.; 新大院博(理)甲第453号; 開始ページ : 1; 終了ページ : 131
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• コレクション（共通）: 障害者向け資料
• コレクション（障害者向け資料：レベル1）: テキストデータ
• コレクション（個別）: 国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文; https://dl.ndl.go.jp/collections/A00014
• 収集根拠: 博士論文（自動収集）
• 受理日（W3CDTF）: 2020-09-07T06:04:08+09:00
• 作成日（W3CDTF）: 2020-08-31
• 記録形式（IMT）: application/pdf
• オンライン閲覧公開範囲: 国立国会図書館内限定公開
• デジタル化資料送信: 図書館・個人送信対象外
• 遠隔複写可否（NDL）: 可
• 掲載誌（URI）: http://hdl.handle.net/10191/00051774
• 連携機関・データベース: 国立国会図書館 : 国立国会図書館デジタルコレクション; https://dl.ndl.go.jp