※一部利用できない機能があります
| 1.
論文 |
論文
|
荒井, 章司 ; 石丸, 聡子 ; Arai, Shoji ; Ishimaru, Satoko
概要:
金沢大学理工研究域自然システム学系<br />本論文では島弧マグマ中および島弧に噴出した他のマグマ中の捕獲岩，さらに前弧域の海底に露出するかんらん岩を島弧リソスフェアマントルを代表するものとして，その岩石学的性質（モード組成, 鉱物化学組成
…
, 平衡条件, 酸化還元状態）を総括した．特に火山前線～前弧域の枯渇したハルツバーガイトの存在は上部マントル・ウェッジを特徴づける．それらは高度部分溶融の融け残りである．また，それらの多くは交代作用（シリカの付加および加水作用）を被っている．トレモラ閃石の存在は，低温および/または枯渇した（Alに乏しい）化学組成を示唆し，特徴的である．特に太平洋西部の火山前線～前弧域のものは比較的高い酸素フガシティーを示し，島弧マントルを強く特徴づけている．ただし，いくつかの島弧かんらん岩は低い酸素フガシティーを示すことが注目される．特に，マントル・ウェッジ深部には強い還元的な流体が存在している。<br />The mantle wedge is an important locus for material recycling, magma generation, and fluid transportation from the slab, and should therefore be thoroughly examined to better understand these processes. Peridotite xenoliths transported to the surface by arc magmas, or by other magmas, may be representative of the upper part (lithosphere) of the mantle wedge. Fore-arc peridotites exposed on the seafloor also represent the uppermost part of the mantle wedge. We summarize their modal composition, mineral chemistry, equilibrium temperature, and redox state, and discuss the implications for mantle-wedge processes. The arc peridotites are thought to derive mainly from the spinel to plagioclase-peridotite stability fields. They are varied in character, depending on their history as well as the tectonic setting (e.g., fore-arc, volcanic front, and back-arc) of their source regions. Some arc peridotites, especially those from the fore-arc to the volcanic front, are harzburgites and contain high-Mg olivine and high-Cr spinel, with high degrees of partial melting. They also show metasomatism, silica enrichment (i.e., formation of secondary orthopyroxene at the expense of olivine), and hydration (i.e., precipitation of Ca-amphiboles and/or phlogopites). The presence of tremolite, which is indicative of low temperatures and/or depleted (Al-poor) chemistry, is characteristic of sub-arc mantle peridotites. The equilibrium temperature is relatively low (＜1100℃) with the exception of the Noyamadake peridotites, SW Japan arc, which are characterized by high temperatures (～1200℃). Some peridotites from the Western Pacific show high oxygen fugacities relative to abyssal peridotite (although a few peridotites show relatively low oxygen fugacities or contain secondary veins composed of highly reduced minerals such as metals and alloys). This indicates the importance of local reducing agents in the mantle wedge.
続きを見る
|
||||
| 2.
論文 |
論文
|
荒井, 章司 ; Arai, Shoji
概要:
金沢大学理工研究域自然システム学系<br />筆者の研究を中心として，マントル岩由来の砕屑物（特に砕屑性クロムスピネル）の解析法およびその意義を総括した．マントル岩由来の砕屑物は機械的・化学的性質が特異であり，独特の意義を有する．クロムスピ
…
ネルは岩石学的に重要ないくつかの元素を主要成分として含み，かんらん岩の成因を解釈する上で重要な鉱物である．その組成は，関与したマグマの組成の違いおよびサブソリダスでの冷却・変成により変化する．侵食・運搬・堆積・続成過程では化学組成の変化はなく，後背地の解析のよいツールとなる．砕屑性クロムスピネルはオマーン・オフィオライトマントル部のようなかんらん岩体の性質を概観するのに有効である．また，蛇紋岩メランジュのマトリックスのように強く破砕されたかんらん岩体の性質を理解するのに威力を発揮する．かんらん岩体を，相伴う砕屑性粒子と合わせて解析できればさらに有効である．環伊豆地塊蛇紋岩帯はよい適用例を示す．<br />Detrital materials supplied from mantle-derived rocks have peculiar mechanical and chemical characteristics that make them excellent sedimentological and tectonic markers. Chromian spinels are well known and favored petrogenetic indicators because they contain several important cations, including Mg2+, Fe2+, Cr3+, Al3+, and Fe3+, as their main components. The Mg/(Mg + Fe2+) ratio (= Mg#), Cr/(Cr + Al) ratio (= Cr#), and Ti concentration are important parameters that are commonly analyzed to petrologically characterize chromian spinels. The main hosts in the chromian spinels of mantle peridotites (harzburgite and lherzolite) and their serpentinized equivalents are Mg# and Cr#, which are controlled by the equilibrium temperature (degree of subsolidus cooling) and degree of melt extraction, respectively. The chromian spinels in detritus materials are chemically stable during sedimentation processes, and can thus serve as powerful indicators of the tectonic and geologic history of the hinterland. The derivation of detrital chromian spinels, peridotites (serpentinites), chromitites, or volcanics can be partially identified by analyzing their Mg#, Cr#, and Ti concentrations in combination with their textures. The petrologic character of highly sheared serpentinite, such as the matrix of a serpentinite mélange complex, sometimes yields an inconclusive geodynamic history, but this can potentially be inferred from detrital chromian spinels in nearby sediments. Detrital chromian spinels from modern sediments are also useful for obtaining a general view of large peridotite bodies, such as the mantle section of the Oman ophiolite. When we compare the detrital spinels with those from in situ rocks, we should note that the host rocks containing the detritus material have already been eroded. If we analyze the serpentinite sandstones together with their closely associated peridotite bodies, we can possibly obtain information on the petrologic heterogeneity of the upper mantle. Our petrographic investigation of the Circum-Izu Massif Serpentinite Belt, central Japan, provides a good example of such a combined analysis of these peridotite-serpentinite sandstone pairs.
続きを見る
|
||||
| 3.
その他 |
その他
|
金沢大学フロンティアサイエンス機構 ; 金子, 周一 ; 荒井, 章司 ; Kaneko, Shuichi ; Arai, Shoji
|
||||
| 4.
論文 |
論文
|
阿部, なつ江 ; 荒井, 章司 ; Abe , Natsue ; Arai, Shoji
概要:
金沢大学大学院自然科学研究科自然計測
|
||||
| 5.
その他 |
その他
|
荒井, 章司 ; 前田, 仁一郎 ; 小山内, 康人 ; 新井田, 清信 ; Arai, Shoji ; Maeda, Jin-ichiro ; Osanai, Yasuhito ; Niida, Kiyoaki
概要:
金沢大学大学院自然科学研究科自然計測
|
||||
| 6.
論文 |
論文
|
石丸, 聡子 ; 荒井, 章司 ; Ishimaru, Satoko ; Arai, Shoji
概要:
金沢大学大学院自然科学研究科自然計測
|
||||
| 7.
論文 |
論文
|
阿部, なつ江 ; 荒井, 章司 ; Abe, Natsue ; Arai, Shoji
概要:
金沢大学大学院自然科学研究科自然計測
|
||||
| 8.
論文 |
論文
|
荒井, 章司 ; 平井, 寿敏 ; 阿部, なつ江 ; Arai, Shoji ; Hirai, Hisatoshi ; Abe, Natsue
概要:
金沢大学大学院自然科学研究科自然計測
|
||||
| 9.
論文 |
論文
|
荒井, 章司 ; 阿部, なつ江 ; Arai, Shoji ; Abe, Natsue
概要:
Petological constitution of the upper mantle beneath the ocean floor has been poorly known except for oceanic fracture z
…
ones of slow-spreading ridges. Information from ophiolites may supplement the paucity of data to some extent ; however, the ophiolites should be treated carefully because of their polygenetic nature. The abyssal peridotite varies from lherzolite with Cr# of spinel of 0.1 to harzburgite with Cr# of spinel of 0.6. Dunite is relatively rare from the ocean floor. An exotic lherzolite with continental mantle signatures appears in midoceanic areas. The refractoriness of the abyssal peridotie has been proposed to correlate with the spreading rate of the ridge system, but this is false. The upper mantle beneath the ocean floor changes downwards from dunite to lherzolite via harzburgite, being independent of spreading rate. The lithological change is more abrupt in a slowspreading system than in a fast-spreading one, so it is around ridge segment boundaries rather than around the segment center on the same spreading ridge. The thin harzburite layer in slow-spreading ridges has resulted in its rarity there, and the deep seat of lherzolite in fast-spreading ridges has caused its apparent absence. The primitive MORB can be in equilibrium with dunite, which is formed along the melt conduit beneath the ridge via peridotie/melt reaction, and the dunite part is laid down by the corner flow of the mantle just below the lowermost gabbro layer as it leaves the ridge axis. We proposed the following deep ocean-floor drilling to explore scientific proglems concerning the abyssal upper mantle : (1) non-riser drilling on the “continental peridotite” to know the relationship with abyssal peridotite, and (2) non-riser or riser drilling on the ocean floor where deep-seated rocks have already been exposed to examine the deep constitution of the upper mantle. The “21st Century Mohole” drilling through the oceanic Moho should primarily be directed to the segment center of a fast-spreading ridge system. The back-arc basin such as the Sea of Japan will be the alternate for Mohole drilling because we have had relatively little information on the petrological nature of the back-arc basin lithosphere despite its importance. We can solve the “ophiolite problem” simultaneously if we are careful in choosing the drilling sites. We also propose a close linkage between the ophiolite study and ocean drilling in the coming IODP.
続きを見る
|
||||
| 10.
論文 |
論文
|
荒井, 章司 ; Arai, Shoji
概要:
Various characteristics of podiform chromitites, an enigmatic mantle rock member, are reviewed in this article. Chromiti
…
tes are composed of chromian spinel, with the general formula (Mg, Fe2+)(Cr, Al, Fe3+)2O4, and silicates (mainly olivine). The Fe3+ content is generally very low, being less than 0.1 to all trivalent cations, in mantle chromian spinels. The Mg/(Mg + Fe2+) ratio (= Mg#) changes inversely with the Cr/(Cr + Al) ratio (= Cr#), which increases with an increase of degree of partial melting of mantle peridotites. The Cr# of chromian spinel is generally higher than 0.4 (generally 0.6 to 0.8) in podiform chromitites, varying widely from 0.1 to 0.9 in the mantle peridotite. The podiform chromitite forms pod-like bodies (dimensions of up to 1.5 km × 150 m for an individual pod) with a dunite envelope, totally set within mantle harzburgite. In well-preserved ophiolites, they occur in the uppermost mantle, especially in and beneath the Moho transition zone, which is dominated by dunite. The Cr# of chromian spinel is relatively low (0.4 to 0.6) around the Moho transition zone, and high (>0.6) at deeper levels in the mantle section. Chromitites are denser and less anisotropic in Vp than peridotites, and the Vp is 8.5 to 9 km/sec depending on the proportion of chromian spinel, and higher in the former than in the latter. The podiform chromitite has been interpreted to be one of melt/rock interaction products within the uppermost mantle harzburgite; hybridization of relatively Si-rich melt formed by the breakdown of orthopyroxenes of the wall harzburgite and subsequently supplied primitive melt cause oversaturation in chromian spinel, giving rise to formation of chromitite with a dunite envelope. The fractionated melt leaving high-Cr# podiform chromitite is possibly of arc-magma affinity. Chromitites with low-Cr# (0.4 to 0.6) chromian spinel can be in equilibrium with MORB. Recently found ultra-high pressure minerals, such as diamond, moissanite, Fe-silicides and Ni-Fe-Cr-C alloys, within chromian spinel of podiform chromitites make the genetical history of chromitites highly enigmatic. A new story, which incorporates the genesis and involvement of these highly reducing, ultra-high pressure minerals, is required.
続きを見る
|
