※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
|
||||
| 11.
論文 |
論文
|
荒井, 章司 ; 阿部, なつ江 ; Arai, Shoji ; Abe, Natsue
概要:
As a follow-up of a symposium on mantle xenoliths held at Kanazawa University in 2004, we here publish a special issue c
…
omposed of seven review articles dealing with mantle xenoliths. Mantle-derived xenoliths of peridotite and related rocks play an important role as a direct insight into Earth’s interior, and have been examined in various ways. We would like to widely introduce the current situation of mantle xenolith studies to non-specialists as well as to enlighten young scientists as to the mantle materials and their importance.
続きを見る
|
||||
| 12.
論文 |
論文
|
Morishita, Tomoaki ; Arai, Shoji ; Tamura, Akihiro ; Ishida, Yoshito
概要:
金沢大学フロンティアサイエンス機構<br />A peculiar amphibolite rich in deep vivid green-colored amphibole porphyroblast was found as a bo
…
ulder in the Chiroro River of Hidaka Town, Hokkaido, Japan. The amphibolite mainly consists of deep vivid green-colored amphibole (porphyroblast), colorless amphibole (matrix) and plagioclase with a small amount of chlorite, epidote and chromian spinel. Deep vivid green-colored amphibole porphyroblast is more than 20 modal % in the rock. Chromian spinel rarely occurs as inclusions in deep vivid green-colored amphibole porphyroblast. The Cr2O3 content of the deep vivid green-colored domain is usually ∼1 wt% (up to 2.5 wt% nearby chromian spinel) whereas that of colorless amphibole in the matrix is low, < 0.1 wt%. Deep vivid green-colored amphibole porphyroblast is heterogeneous in chemical compositions within each grain and is divided into three domains as follows: (1) high-Cr2O3 (∼1 wt%, up to 2.5 wt%) and Al2O3 (> 10 wt%) green-colored domain, (2) relatively high-Cr2O3 (∼1 wt%) and low- Al2O3 (< 10 w%) green-colored domain and (3) low-Cr2O3 (< 0.1 wt%) and Al2O3 (< 10 wt%) colorless domain. Chondrite-normalized rare earth element (REE) patterns of both deep vivid green-colored amphibole porphyroblast and colorless amphibole in the matrix are characterized by low light REE with positive Eu anomaly. The Cr#[= Cr/(Cr + Al) atomic ratio] and TiO2 content of chromian spinel in the amphibolite are similar to those in a serpentinite complex of the Kamuikotan Zone. The amphibolite was formed by either amphibolitization of a rock containing chromian spinel derived from the surrounding serpentinite complex or extensively metasomatized serpentinite due to interaction with surrounding metamorphic rocks.
続きを見る
|
||||
| 13.
論文 |
論文
|
荒井, 章司 ; Arai, Shoji
|
||||
| 14.
論文 |
論文
|
荒井, 章司 ; 阿部, なつ江 ; Arai, Shoji ; Abe, Natsue
概要:
This article reviews interpretations of the geological and petrological nature of the Moho, which is defined as a discon
…
tinuity in terms of Vp, with a view to preparing for the Mohole on the ocean floor in IODP. We strongly propose discarding non-seismic terms for the Moho, such as “petrologic Moho”. The nature of the Moho has been controversial for a long time; an isochemical phase transition boundary between gabbro (crust) and eclogite (mantle) was favored for the Moho by some researchers, while a chemical boundary between mafic rocks (crust) and peridotite rocks (upper mantle) is now favored by a majority of researchers. Boundaries between completely or partially serpentinized peridotite and fresh peridotite may be applicable as the Moho at some parts of the ocean floors of a slow-spreading ridge origin. Antigorite serpentinite can be expected to be observed at the lowermost crust if the Moho is the serpentinization front at the stability limit of serpentine. The Moho beneath the Japan arcs can be estimated using mafic-ultramafic xenoliths in Cenozoic volcanics. Peridotitic rocks scarcely mix with feldspathic rocks, indicating that the Moho at that location is the boundary between feldspathic rocks (mostly mafic granulites; crust) and spinel pyroxenites (mantle). Possible fossil Mohos are observed in well-preserved ophiolites, such as the Oman ophiolite. Two types of Moho are distinct in the Oman ophiolite; gabbro-in-dunite Moho, where a gabbro band network in dunite changes upward to the layered gabbro within a few to several tens of meters, and dunite-in-gabbro Moho, where late-intrusive dunites intruded into gabbros. The former is of a primary origin at a fast-spreading ridge, and the latter is of a secondary origin at a subduction-zone setting in the obduction of the oceanic lithosphere as an ophiolite. The gabbro/peridotite (dunite) boundary as the primary Moho forms in embryo as a wall of melt conduit at fast-spreading ridges as well as at the segment center of slow-spreading ridges. The oceanic primary Moho is modified to various degrees by magmatism, metamorphism and tectonism in subsequent arc and continental environments. The gabbro-in-dunite Moho formation in the Oman ophiolite is an embryo of this modification. We expect in-situ sampling across the primary oceanic Moho formed at a fast-spreading ridge through the Mohole of IODP. Ultra-deep drilling at gabbro/peridotite complexes exposed on the ocean floor is indispensable for our understanding of the suboceanic upper mantle. Studies on appropriate ophiolites and deep-seated xenoliths from oceanic areas should complement the Mohole and other ultra-deep drillings to grasp the whole picture of the oceanic upper mantle.
続きを見る
|
||||
| 15.
その他 |
その他
|
二ノ宮, 小満 ; 荒井, 章司 ; Ninomiya, Chima ; Arai, Shoji
概要:
[研究概要]
|
||||
| 16.
その他 |
その他
|
石丸, 聡子 ; 荒井, 章司 ; Ishimaru, Satoko ; Arai, Shoji
概要:
[研究概要]
|
||||
| 17.
その他 |
その他
|
村山, 豪 ; 荒井, 章司 ; Murayama, Takeshi ; Arai, Shoji
概要:
[研究概要]
|
||||
| 18.
論文 |
論文
|
荒井, 章司 ; Arai, Shoji
概要:
今年度は特にアダカイトの関与によって形成されたと思われる捕獲岩の研究が進展した.スペイン,タジャンテおよび唐津高島で得られた,かんらん石と反応して生成された斜方輝石に富む捕獲岩(あるいは脈)を詳細に検討した.単斜輝石の微量元素組成やREEパ
…
ターンはアダク島(アリューシャン)のアダカイト中の斑晶のそれちとほぼ一致することが判明した.従って,これらの単斜輝石はアダカイト的なメルトと平衡であり,関与したメルトの性質が判明した.これらの斜方輝石に富むパイロクシナイトはアダカイトやマントル・ウェッジ中の通路を表していると思われる.アダカイトの化学組成変化はかんらん石の消費+斜方輝石の晶出により基本的にコントロールされていることが明かとなった.また,タジャンテのかんらん岩中のフェルシック脈の周囲に形成されているオルソパイロクシナイが明かとなった.また,タジャンテのかんらん岩中のフェルシック脈の周囲に形成されているオルソパイロクシナイト中にはNbに富むルチルが存在しており,全体としてLILEに比べてHFSEに富むことが判明した.これにより,上記の反応はスラブメルトにHFSEの負の異常を与える可能性があることが判明した.このような斜方輝石に富む岩石は,産状や輝石の組成からフルイドとかんらん岩の反応によって生成されたと考えられる斜方輝石(カムチャツカ弧,アバチャ火山のものなど)と区別される.アセトル・ウェッジはスラブメルトやスラブ起源のフルイドによりシリカに富むことが明確となった.また,スラブとマントル・ウェッジの反応のアナログとして注目される三郡変成岩中の落合-北房かんらん岩の全岩の微量元素組成を検討した結果,RbやCsなどが変成岩からかんらん岩に付加していることがわかった.またこれらの元素はNaトレモライトに含まれることがわかった.<br />Orthopyroxene-rich xenoliths from Tallante, Spain, and Takashima, SW Japan, have been investigated. They were farmed by a reaction between the slab melt(adakite melt) and the mantle peridotite. Their dinopyroxene is similar in trace-element characteristics and REE pattern to clinopyroxene phenocrystin adakite from Adak Island of the Aeutian arc. This may indicate that an adakitic melt was involved information of the opx-rich pyroxenite xenoliths from Tallante and Takashima. They possibly derived from the conduit of adakitic melt within the mantle wedge. The chemical compositional trend of adakite is, therefore, basically controlled by precipitation of orthopyroxene at the expense of olivine. The orthopyroxenite from Tallante contains some HFSE-rich minerals such as Nb-rich rutile, and is enriched with HFSE relative to LILE. This means that the reaction above can yield negative spikes of HFSE to the fractionating slab-derived melt. The orthopyroxene in such pyroxenites, interpereted here to be a slab-melt/peridotite reaction product, is distinctively different from the oryhopyroxene that is interpreted to be fluid/peridotite reaction product, described from Avahc and Iraya volcanoes. We have stated a geochemical analysis of metamorphosed/metasomatized peridotite of the Ochiai-Hokubo comprex, emplaced into the Sangun highP/low T schist, as an analogue to mantle-wedge peridotite interacting with slab. The peridotite is clearly enriched with Rb and Cs in addition to H2O, and these dements are concentrated in metasomatically formed Na-trermolite.<br />研究課題/領域番号:13440161, 研究期間(年度):2001-2003
続きを見る
|
||||
| 19.
論文 |
論文
|
荒井, 章司 ; Arai, Shoji
概要:
1.上部マントルのかんらん岩の岩石学的・構造学的調査により,レールゾライトからハルツバーガイトに渡る広い岩相変化,組織の分布からみた変形履歴,鉱物の結晶構造の変化,マイロナイトの成因などが判明した.また白金族元素の濃集したクロミタイトが発見
…
された.2.後期貫入岩体を3つのタイプに分類した.急冷相のピクライト玄武岩に伴うコマチアイト様超マフィック噴出岩が発見された.現在,その詳細な性格付けを急いでいる.3.海嶺のセグメント構造が斑れい岩層の変化から確定された.セグメント中心部の派生的なマグマ溜りの周辺部にノーライトが貫入し,岩脈群の一部が輝石グラニュライト相までの変成作用を受けている.またマグマ溜りルーフゾーンの構造と進化過程が判明した.4.上部溶岩の単斜輝石斑晶の化学組成を精密に測定し,3種類に分類し,各々の成因の概要を明らかにした.またボニナイトを発見した.さらに,オブダクション期に活動したサラヒ火山岩類について予察的なデータを得た.5.バチナ・コースト地域のグゼイン鉱床の生成環境の鉱床学的・地球化学的検討を行い,その詳細を明らかにした.すなわち,変質玄武岩中の緑泥石に熱水から亜鉛,ニッケルが選択的に固定し,鉱体ではこれらに成分に乏しくなった.また,オマーン・オフィオライトの熱水形成では水/岩石比が20程度で2〜4という理論値をはるかに上回っていることが明らかになった.6.玄武岩〜ドレライトの変成作用,特にプレーナイト-アクチノ閃石相内の温度構造を考察し,変成温度が層序的に下位に向かって上昇していることを確認した.7.メタモルフィックソールの剪断がNE-SW方向に起こり,上盤がSW方向に移動したことが分かった.マントルウェッジ直下から数十m以内で,塩基性角閃岩は,多様な流動形態を示すことがわかった.<br />1. We recognized a wide petrographical variation of peridotite, its deformational history, cystallographical change of minerals and origin of mylonites within the upper mantle section of the northern Oman ophiolite. High-PGE chromitite was also discovered.2. We classified the late-intrusive rocks into three types in terms of petrography. Komatititic extrusives were found associated with picrite basalt, a quench phase of the late intrusives.3. The segmetation structure of a mid-ocean ridge was estimated by the strutural change of gabbroic layers. Noritic gabbro intruding around the derivative magma chmabers around the segment center impose contact metamorphism of up to granulite facies on surrounding sheeted dike complex. The structure and evolutional history of the roof zone of magma chambers were clarified.4. The upper lavas can be classified into three groups by high-accuracy chemical analysis of their Cpx phenocrysts, and the genesis was estimated for individual groups. Boninite was newly found. Preliminary data on the Salahi volcanics that erupted at the obuduction stage were presented.5. Hysrothermal metal mineralization was geologically and geochemically examined to understand its genesis. Reaction between aqueous fluid and chlorite in meta-basalts is of vital importance for their characteristics of Zn and Ni contents. The water/rock ratio in the hydrothermal fluid formation is around 20, that is far higher than the calculated values, 2 to 4.6. The thermal structure of the ocean-floor metamorphism was examined in basalt to dolerite to confirm a temprature increase downward.7. The shear sense on the metamorphic sole is NE-SW and the hanging side was confirmed to have moved toward SW.The basic amphibolite of the sole demonstrates variable flow textures within several tens of meters from the contact with overlying ultramafic rocks.<br />研究課題/領域番号:11691121, 研究期間(年度):1999-2000<br />出典：「海洋地殻-マントル系の生成と改編: オマーン・オフィオライトの例」研究成果報告書 課題番号11691121（KAKEN：科学研究費助成事業データベース（国立情報学研究所）） 本文データは著者版報告書より作成
続きを見る
|
||||
| 20.
論文 |
論文
|
荒井, 章司 ; Arai, Shoji
概要:
1.MORBの生成と深部岩石の関係を探るために,太平洋ヘス・ディープと,カリブ海中央ケイマン・トラフから得られた超マフィック岩の再検討し,比較した.両者は,それぞれ高速拡大軸,低速拡大軸下の深部物質を代表している.両者のマントルかんらん岩は
…
それぞれハルツバーガイト(H),レールゾライト(L)であり大きく異なる.一方,ダナイト〜トロクトライト(D-T)はスピネルのCr#が0.5〜0.6,0.4〜0.5とわずかな違いしかない.この違いは,比較的未分化なMORBの組成が拡大速度にわずかに依存する事と調和的であり,これらは未分化MORBと化学的平衡にある.D-Tと未分化MORBは,既存のかんらん岩とメルトの相互反応の産物であろう.2.それぞれ高速,低速拡大軸の海洋マントル起原であるオマーン・オフィオライトのマントル部分,およびリザードかんらん岩体を調べ,実際の海洋底の岩石との比較を試みた.オマーン・オフィオライトではヘス・ディープのものと類似するHが卓越する.ただし,大平洋では未発見のLがかなり普遍的に存在しており,レールゾライト〜ハルツバーガイトへの変化の詳細を知ることができる.リザードかんらん岩では,スピネルのCr#が0.1〜0.5のL〜Hが卓越し,Cr#0.3程度のスピネルを含む調和性ダナイトを伴う.3.超低速拡大軸である南西インド洋海嶺(アトランティスII断裂帯)のかんらん岩を予察的に調べた.ほぼ同一斜面で深部から浅部へ向かってL〜Hの変化が認められた.Hにはしばしばダナイト的な部分があり,またガブロ質やパイロクシナイト質の脈〜層がしばしば認められる.この深さ方向の岩相変化はオマーンなどのオフィオライトでは認められており,海嶺軸下での溶融過程を制約するものとして重要である.今まで海洋底では未確認であり,岩相変化の幅と空間スケールの解析を引き続き行う予定である.<br />1. Ultramafic rocks obtained from Hess Deep, EPR, and from Mid-Cayman Trough, the Caribbean, were petrologically examined to their difference, if any, depending on the spreading rate. The mantle peridotite is harzburgite (Hess Deep) or lherzolite (MCT), but the dunite-troctolite are only slightly different between the two. This difference is concordant with the primitive MORB compositional variation that is only slightly dependent on spreading rate. The dunite-troctolite are, thus, possibly in equilibrium with the primary MORB.Both the dunite-troctolite and MORB are the product of reaction between high-pressure MORB and peridotite at the uppermost mantle.2. The mantle peridoitites in the Oman ophiolite, possibly formed at a fast-spreading ridge, and the Lizard peridotite, possibly formed at a slow-spreading ridge were compared with the ocean-floor peridotites. The mantle section of the Oman ophiolite is dominated by harzburgite similar to that from Hess Deep. Cpx-bearing harzburgite and lherzolite were found in deeper parts of the mantle section, and the melting process beneath a fast-spreading ridge can be discussed in detail. The Lizard peridotite is dominantly lherzolite to harzburgite with concordant dunite which has spinel with Cr# of 0.3.3. The mantle peridotite from the Atlantis Bank, SWIR, was examined to know the melting process beneath a ultra-slow spreading ridge. An upward lithological variation from lherzolite to harzburgite on a slope of the Atlantis II FZ was confirmed, which can give us constraints on the space and temperature legime of mantle melting beneath the ridge.<br />研究課題/領域番号:10440152, 研究期間(年度):1998-2000<br />出典：「海洋底深部岩石からみたMORBの成因」研究成果報告書 課題番号10440152（KAKEN：科学研究費助成事業データベース（国立情報学研究所）） 本文データは著者版報告書より作成
続きを見る
|
||||
| 21.
論文 |
論文
|
荒井, 章司 ; Arai, Shoji
概要:
上部マントルかんらん岩とメルトの相互反応をかんらん岩および噴出岩より読み取る研究を行い,以下の事実が明らかになった.1.高枯渇度かんらん岩には,水の存在下で生成されたもの(神居古潭帯)や,無水条件下で形成されたもの(パプア)がある.2.オマ
…
ーン・オフィオライトのモホ遷移帯は基本的にはハルツバ-ジャイトと玄武岩質マグマとの反応の産物であり,太平洋ヘス・ディープのダナイト・トロクトライト・ガブロと類似している.3.海洋底玄武岩の形成モデルを提唱した.海洋底玄武岩は拡大速度によらずほぼ一定の化学組成を有することが知られている.一方,マントルかんらん岩は,拡大速度に依存したかなり大きな不均質性を有する.太平洋とケイマン・トラフのモホ遷移帯のダナイトの組成がほぼ類似していることから,初生マグマ(組成は拡大速度に依存して異なる)がかんらん岩とより低圧で反応することにより,はぼ同様のダナイトと二次的なマグマを生ずるらしい.4.かんらん岩とメルトの相亙反応による二次的メルトには,クロムなどのコンパティブル元素とナトリウム,水などのインコンパティブル成分が同時に濃集する.二次メルトからの晶出物の代表的なものはポディフォーム・クロミタイトである.5.クロミタイトは中程度に枯渇したハルツバ-ジャイトに特徴的に含まれる.また,そのスピネルのCr#は0.8前後のことが多く,海洋的というより島弧的環境を示唆する.6.ピクライト玄武岩はかんらん岩-メルト相互反応物の噴出相である.かんらん岩捕獲岩-アルカリ玄武岩の反応はピクライト的メルト(クリスタル・マッシュ)生成過程のよい類似物である.<br />The interaction between upper mantle peridotite and melt has been studied both for deep-seated rocks and for effusive rocks. Our results are summarized as follows.1. The highly depleted peridotite is possibly polygenetic. The Cr# of spinel in high-Mg harzburgite is higher in the Papuan ophiolite than in the Kamuikotan zone. the melting condition was hydrous in the Kamuikotan dunite-harzburgite and was anhydrous in the Papuan ophiolite.2. The Moho transition zone of the Oman ophiolite is basically composed of the interaction product between harzburgite and MORB.The rocks are similar to the equivalents from Hess Deep, the Pacific Ocean, a fast-spreading ridge system.3. The primitive MORB is possibly in equilibrium with dunite (and troctolite) of the Moho transition zone, an interaction product between more primitive MORB and mantle peridotite. The oceanic dunite is slightly variable in petrological characteristics whereas the mantle peridotite is highly variable depending on the spreading rate.4. The secondary melt produced by the peridotite/melt interaction can concentrate both some compatible elements simultaneously and incompatible components, leading to precipitate podiform chromities.5. The podiform chromitite is characteristically hosted by moderately depleted harzburgite with small smount of cpx and chromian spinel of moderate Cr# (around 0.5). Their chromian spinel often has high Cr#s (around 0.8), indicating supra-subduction zone environment for their formation.6. The picrite basalt is an effusive facies of the peridotite/melt interaction product. The reaction product between peridotite xenolith and its host alkali basalt is a good analogue of the picritic mixture.<br />研究課題/領域番号:07454131, 研究期間(年度):1995-1996<br />出典：「マグマ-マントル相互反応と未分化マグマの形成」研究成果報告書 課題番号07454131 (KAKEN：科学研究費助成事業データベース（国立情報学研究所）) 本文データは著者版報告書より作成
続きを見る
|
||||
| 22.
論文 |
論文
|
荒井, 章司 ; Arai, Shoji
概要:
日本列島に産する上部マントル起源の捕獲岩を地質学的・岩石学的に検討した結果、以下の事実が判明した。(1)日本列島の下部地殻〜上部マントルは極めて不均質である。不均質性をもたらす要因として、ダナイト〜パイロクシナイト(グル-プI)からなるキュ
…
ムラス・マントルの発達の程度の違い、マントルかんらん岩の岩石学的性質(枯渇度)の違い、新しい、Fe、Alに富むマントル〜下部地殻物質(グル-プII)のネットワ-クの付加の程度の違い、および加水化の程度の違いである。(2)アルカリ玄武岩マグマなどによる深部岩石の選択的捕獲が認められる。前述の極めて若井(多くは15Ma以後の)岩石(グル-プII;一部は未固結?)がネットワ-ク状に存在する場合、アルカリ玄武岩マグマはそこを選択的に通過してしまい、その部分にもともと存在していたグル-プIの岩石を捕獲しない。かわりにグル-プIIの岩石やメガクリストが捕獲されることになる。(3)日本列島下の上部マントルかんらん岩における枯渇度の多様性は、相伴う集積岩(特に、ダナイト)の多少によるように思える。すなわち、ダナイトを初めとする集積岩捕獲岩を伴う場合、マントルかんらん岩は、高Cr#スピネルを有するレ-ルゾライトまたはハルツバ-ジャイトを必ず含む(黒瀬、野山岳、荒戸山、新宮)。これは、最近のアルプス型かんらん岩体における観察事実と調和的である(例えば、Nicolas,1989;Takahashi,1991)。(4)日本列島の深部の基本的構造は、深部よりマントルかんらん岩、ダナイト〜ウェブステライト、かんらん石ガブロ(またはそれの変成したスピネルウェブステライト)、輝石ガブロ(マフィック・グラニュライト)である。一部は後に加水され、ホルンブレンド・ウェブステライト、ホルンブレンド・ガブロ(または角閃岩)などが生じている。<br />Petrological characteristics of the upper mantle-lower crust beneath the Japan arcs can be deduced from xenoliths as follows. (1) Rocks constituting the upper mantle-lower crust beneath the Japan arcs are very heterogeneous in terms of thickness of cumulus mantle (Group I ultramafic rocks), degree of refractoriness of mantle peridotites, degree of accretion of young Fe, Al-rich rocks (Group II), and degree of hydration. (2) Deep-seated rocks can be selectively incorporated into host magmas. Uprising magmas may tend to selectively pass through young dikelets and may not catch wall rocks of the dikelets. (3) The heterogeneity of degree of refractoriness of mantle peridotites may be controlled by abundance of associated cumulative ultramafic rocks, especially of dunites. Mantle peridotite xenoliths which are associated with large amounts of dunite xenoliths are usually harzburgite or refractory lherzolite with high-Cr spinel. Formation of dunites within the mantle peridotites makes depletion aureoles around the dunites, enhancing the heteorgeneity of the mantle peridotites. (4) The deep part of the Japan arcs is composed of, in an an ascending order, mantle peridotites (harzburgite and/or Iherzolite), dunite-websterites, olivine gabbros (oe thier metamorphosed equivalents, spinel websterites), and pyroxene-gabbros (mafic granulites). Part of them have been later hydrated to be hornblendebearing peridotites, harnblende websterites and hornblende gabbros (or amphibolites).<br />研究課題/領域番号:02640626, 研究期間(年度):1990-1991<br />出典：「日本列島産上部マントル捕獲岩カタログ作成のための基礎的研究」研究成果報告書 課題番号02640626(KAKEN：科学研究費助成事業データベース（国立情報学研究所）) 本文データは著者版報告書より作成
続きを見る
|
||||
| 23.
論文 |
論文
|
荒井, 章司 ; Arai, Shoji
概要:
地球内部のグローバルな物質的境界のうち,最浅部のモホ(モホロビチッチ不連続面の岩石学的実体とその形成過程を研究した。これらの成果は将来の海洋域でのマントル掘削(モホール)実施時の基本的指針となる。ある種の海洋リソスフェア(白亜紀)の断片であ
…
るオマーン・オフィオライトでは, 10m～1kmのモホ遷移帯(MTZ)が存在する。MTZはダナイト,ウェールライトを主とするが,厚いものほどMgに富み,基本的にマントル・ハルツバーガイトとMORBの反応帯の性格を有する。島弧では,集積岩の付加および加水作用等の二次的改変が激しく, MTZの多様性と不均質性が著しい。<br />Petrologic nature of Moho(Mohorovicic discontinuity), the shallowest of global physical discontinuities of the Earth's interior) has been investigated. Results of this study will guide the oceanic mantle drilling(Mohole) in the future. In the Oman ophiolite, a slice of Cretaceous oceanic lithosphere, the Mono transition zone(=MTZ), from 10 m to 1 km in thickness, is found between layered gabbro and the mantle peridotite(mainly harzburgite). The MTZ, mainly composed of dunites and wehrlites, becomes Mg-rich with an increase of its thickness, showing characteristics of a MORB/harzburgite reaction product. Beneath the island arc, the MTZ varies its petrologic characteristics through accretion of cumulates and hydration, and shows intense heterogeneities and varieties.
続きを見る
|
||||
| 24.
論文 |
論文
|
荒井, 章司 ; Arai, Shoji
概要:
ボウ・アゼールオフィオライト(モロッコ)は原生代のオフィオライトとしては例外的に保存がよいということであるが,それでも断片化および岩石の変成/変質の程度は高い。かんらん岩,ガブロ(主として等方ガブロ),わずかな岩脈群および火山岩が露出してい
…
る。マントル部は比較的広く露出しているが蛇紋岩化は完全であり,ハルツバーガイトはフォリエーションが発達する。ダナイトはすべて調和性であり,パイロクシナイトのみ非調和性である。小規模な調和性クロミタイトが認められる。ガブロとのコンタクト近くではダナイトが卓越し,ガブロのバンド(厚さ数m以下)に富み,モホ遷移帯の岩相を示す。かんらん岩は残留組織およびクロムスピネル組成から岩相や初生的性質を推定した。ハルツバーガイトのスピネルのCr#は6.6を越え,枯渇度が高い。クロミタイトのスピネルのCr#も0.7を越える。この枯渇したマントルは,原生代のオフィオライトの特徴であると思われる。また,オマーンなどの顕生代オフィオライトに顕著なウェールライト質の後期貫入岩体も見いだされた。総体的に,マントル部分の枯渇度が高いことを除けば,顕生代オフィオライトと類似な性質を示すことがわかった。ボウ・アゼールオフィオライトで特筆すべきは,蛇紋岩中の二次的磁鉄鉱(しばしばNiを含む)のクロット〜脈状の集積と,Mn,Co,Niに富む(含む)二次的クロムスピネルの生成であろう。これらは,かんらん石の蛇紋石化に伴う鉄族元素の移動と集積を示唆する。ボウ・アゼールオフィオライトはその境界部で大規模なCo鉱床が形成されていることで有名であるが,蛇紋岩化との関係が注目される。<br />The Bou-Azzer ophiolite, Morroco, of Proterozic age has been known to have a well-preserved ophiolite stratigraphy. It is significantly dismembered, being composed of peridotites, isotropic gabbro, dolerite dikes and volcanic rocks. The rocks are highly altered or metamorphosed. Completely serpentinized harzburgite with strong foliation comprises the mantle section of the ophiolite. Small amount of dunite and pyroxenite are concordant and discordant, respectively. Small concordant chromitite pods are not rare. Dunite is predominate near the contact with the gabbroic section, having frequent gabbroic bands. This lithology is equivalent to the Moho transition zone. Relic textures and spinel chemistry constrained initial lithology and chemical characteristics of the peridotites. The Cr# of spinet is >0.6 in harzburgite and >0.7 in chromitite, indicating a refractory character of the mantle section of the Bou-Azzer ophiolite. Late-intrusive wehrlite was found in the gabbroic rocks. Constituent rocks are basically similar to those in the Phanerozoic ophiolites, such as the Oman ophiolite, except for the refractory character of the mantle section. The highly refractory character of the mantle section may be one of characteristics of the Proterozoic ophiolite.To be interesting, secondary clots or veins of magnetite (sometimes Ni-bearing) are common to the serpentinized harzburgite. Secondary chromian spinels with appreciable amounts of Mn, Co and Ni are occasionally found in harzburgite. These elements were expelled from olivine upon serpentinization because serpentine minerals are low in them relative to olivine. They are mobile and could precipitate to form magnetite clots/veins of were incorporated in chromian spinels. The mobility of these elements are possibly related to precipitation as ores (arsenides) (= Bou-Azzer Co ores) around the boundary of the ophiolitic serpentinite.<br />研究課題/領域番号:16403010, 研究期間(年度):2004-2006<br />出典：「原生代ボウ・アゼールオフィオライト(モロッコ)調査」研究成果報告書 課題番号16403010 (KAKEN：科学研究費助成事業データベース（国立情報学研究所）) 本文データは著者版報告書より作成
続きを見る
|
||||
| 25.
論文 |
論文
|
荒井, 章司 ; Arai, Shoji
概要:
クロマイトの成因に関しては,オマーンオフィオライトの地殻部のポディフォームクロミタイトの研究により,飛躍的な発展をとげたと考える.このクロミタイトは後期貫入岩とそれに取り込まれたガブロの部分溶融メルト(シリカに富む)の境界付近にレンズ状に生
…
じており,産状的にはポディフォームである.一方,スピネルは細粒で完全な自形であり,マトリックスには斜長石が卓越する.スピネルはクロム,鉄に富んでいる.これらの特徴は層状貫入岩体の層状クロミタイトのものと一致する.すなわち,このクロミタイトは今まで記載されたことのない,オフィオライトで層序的に最上位のポディフォームクロミタイトであり,層状クロミタイトの特徴も合わせ持つ,両者の中間的なものである.この層状クロミタイトに組成の類似したクロミタイトは,後期貫入岩を作った玄武岩質マクマとガブロの部分溶融メルトの混合により形成されたと解釈できる.従って,層状クロミタイトはIrvine(1975)が最初に主張したように,玄武岩質マグマとマグマ溜りの周囲の地殻岩が部分溶融したシリシックメルトの混合でできたものであろう.Arai and Yurimoto(1994)のかんらん岩中のポディフォームクロミタイトの成因論を合わせ考えると,クロミタイトの生成にはそのタイプを問わず,2種類のマグマの混合が不可欠であることがわかる.一方はかんらん石-スピネルに飽和した玄武岩質のものであり,他方はシリカに富むメルト(通常のポディフォームクロミタイトではマントル斜方輝石の分解溶融によるもの,層状クロミタイトでは地殻の岩石の部分溶融メルト)である.また,マントルのポディフォームクロミタイトの生成は,斜方輝石か非調和融解するような最上部マントルの方がより効果的に行われると思われる.白金族元素鉱物の生成にも2種類のマグマの混合は有利てある.玄武岩マグマが白金族元素を,シリカに富むメルトは硫黄を供給する.<br />An integrated model for chromitite genesis has been established through the petrological research of upper crustal podifarm chromitite that we found from the Oman ophiolite. Ihis chromitite is lenficular and was formed at the boundary between the late-intrusive peridotite and felsic partial meltfrom gabbroic blocks captured by the late intrusion. Despite the podiform character, its spinel is perfectly euhedral and fine and has a matrix of plagioclase. Spinel is rich in Cr and total Fe. These features are similar to those of the stratiform chromitite. In short, this chromitite has an intermediate character between the podiform and stratiform chromitites. It was formed by mixing of two kinds of magma, a basaltic one from the late intrusive body and a felsic one from the partial melt of gabbro. The chemical similarity between the upper crustal chromitite from Oman and the stratifrom chromitite may mean that the latter is also formed by mixing of magmas, a basaltic magma and a silicic one formed by partial melting of wall of the magma reservoir as first suggested by Irvine(1975). Combined with an interpretation of Arai and Yurimoto(1994) on the origin of podiform chromitite in peridotite, the mixing of two kinds of magma, basaltic and silicic ones, is indispensable to genesis of chromitite irrespective of type. This mechanism favors the precipitation of primary PGM by supplies of PGE from basalt and of S from a silicic melt.<br />研究課題/領域番号:13554017, 研究期間(年度):2001-2003<br />出典：「クロムおよび白金族元素鉱床の統一的成因論および探査/評価システムの確立」研究成果報告書 課題番号13554017 (KAKEN：科学研究費助成事業データベース（国立情報学研究所）) 本文データは著者版報告書より作成
続きを見る
|
