New technology of mass spectrometry for isotope analyses has been greatly advanced for recent years. The advances are especially for improvements of precision and for microanalysis. Precisions of isotope ratios have been improved by using an inductively coupled plasma mass spectrometer with multiple ion collectors. Capabilities of microanalysis have been improved by secondary ion mass spectrometry. These advances introduce new insights for origin of the Solar System and evolution of the Earth.
Diamond - formed during the collision of continents - has been found in continent-continent collision zones (orogens) of Phanerozoic age only, which may indicate a change in plate tectonic conditions within the last 600 Ma.
We use mineral Al isopleths, isotope systems and (micro-)structural relations in mantle fragments enclosed in the Scandinavian Caledonides to demonstrate that the tectonic transport of these `orogenic peridotites' towards 200 km depth, the deep diamond stability field, occurred at the Early Silurian and destroyed old (Proterozoic) pyroxene exsolution microstructures in garnet. Post-peak metamorphic conditions in peridotite overlap with those in surrounding Early Devonian crustal eclogite and imply a long-lived continental plate exhumation through the entire Silurian without major crustal melting. Cold collision regimes are capable to preserve ultra-high pressure metamorphosed continental margins in a mature planet.
The volume of magma of the petit spot volcanoes (Hirano et al., 2006) has not been estimated but certainly must be several orders of magnitude less than surrounding Cretaceous volcanoes on the NW Pacific Plate. Note that we found 0-1, 2.5, 4.2, 6.0 and 8.6 Ma volcanoes, implying episodic eruption of magma over a distance of 600 km of plate motion on the flexural Pacific Plate before its subduction but with low volumes of magma production.
地殻内に定置したマグマがどのくらいの速さで冷却するのかを理解することは，マグマの物質的進化や貫入マグマを熱源として生成する二次的なマグマ（地殻溶融メルト）の量を定量的に議論する上で重要である。この問題の本質は，マグマの熱対流に伴う熱流束を定量的に理解することであるため，これまで主に理論的手法で研究が行われてきた。しかし，提示されたモデルを天然の観察に基づいて検証することが困難であるため，この問題は十分に解明されてこなかった。そのような状況において，Kuritani et al. (2007) は，利尻火山の噴出物についての物質科学的・理論的検討に基づき，利尻火山下マグマ溜まりの温度の時間変化を推定することに成功した。そこで本研究では，この計算結果を解析することにより，マグマ溜まりの熱進化の一般的な理解に繋げる試みを行った。
We applied our newly developed intergranular enrichment factor, which can examine the intergranular components, to reported geochemical data obtained from numerous mantle xenoliths collected from elsewhere. The analysis based on the enrichment factor, interface thickness, and grain size in rocks reveals that the enrichments of incompatible elements at intergranular regions are typically ~10^3 larger than that expected from equilibrium chemical segregation at interfaces (i.e., grain and interphase boundaries). The best explanation for the observed intergranular enrichment is the presence of ~0.1 vol% of melt in all rocks. We propose that such specific amount of melt corresponds to the critical melt fraction, where melt starts to separate from the rock, eventually appearing at the Earth's surface.
Peridotite xenoliths from Avacha volcano, southern Kamchatka, are mostly clinopyroxene-poor (< 2 vol.%) harzburgite, and have been metasomatized by SiO2-rich melt/fluid. The metasomatism is characterized by the formation of secondary orthopyroxene replacing olivine. We collected extremely fine-grained peridotite, composed of fine (< 100 µm) minerals (sometimes extremely fine, less than 10 µm), as well as coarse-grained ones. The degree of metasomatism, i.e., the amount of secondary orthopyroxene replacing olivine, of the fine-grained peridotites is usually higher than the coarse-grained ones. Among the fine-grained peridotites, we found one peridotite xenolith (sample #159) containing domains composed of extremely high-Ni (up to 5.3 wt% of NiO) olivine, associated with secondary orthopyroxene, high-Ni MSS (monosulfide solid solutions), and Fe-Ni sepiolite. MSS do not show any exsolution and are chemically homogeneous. The Ni enrichment shows outward diffuse within individual circular domains (< 1 mm across), which are disseminated in the whole thin section. The orthopyroxene (Mg# [= Mg/(Mg + total Fe) atomic ratio]; 0.93) interstitial to the high-NiO olivine and chromian spinel (Cr# [= Cr/(Cr+Al) atomic ratio]; 0.53) also have high NiO contents, up to 1.1 wt% and 1.8 wt%, respectively. This result denies that the formation of high-Ni olivine due to metasomatic formation of low-Ni orthopyroxene, and strongly suggests that Ni addition had occurred after formation of the secondary orthopyroxene. Nickel-rich sepiolite (NiO = 21.6 wt%) is found the center of the Ni-rich domain. We interpreted that the sepiolite is an alteration mineral from some Ni-rich agent, probably Ni-rich sulfide melt, that caused the Ni enrichment of olivine and other minerals. We propose that the sulfide melt play a role as an important carrier of Ni to metasomatize mantle peridotite beneath Avacha volcano. Sulfur is supplied to the sub-arc mantle from subducting slab.
Calcium transport was estimated in the Hijiori Hot Dry Rock geothermal system, Yamagata, Japan. During 3 month circulation test from Jun 2002 to Aug 2002, we collected and calculated mass of scale samples: calcite and aragonite at the pipeline of 2 wells and anhydrite at the deep production well. Also we estimated supplied calcium to production well from the fluid chemistry at the injection and production wells. Then 2.6 ton calcium was supplied from anhydrite of reservoir granite rock and 3.3 ton calcium precipitated as calcite or aragonite in production wells with fluid temperature and pressure decreasing.
Combined thermobarometric and Os isotopic data for peridotite xenoliths carried in alnöite intruding crust on the Ontong Java Plateau reveal that the basal section of subplateau lithospheric mantle (95-120 km) includes pieces of mantle with unradiogenic 187Os/188Os ratios as low as 0.1156, which give Proterozoic model ages down to ∼1.8 Ga. In contrast, shallower portions (<95 km) yield a restricted range of 187Os/188Os ratios from 0.1223 to 0.1272, illustrating an affinity with modern abyssal peridotites. This contrasting Os isotope data between shallow and deep portions suggest their derivations from different reservoirs. We infer that the subplateau lithosphere forms a genetically unrelated two-layered structure, comprising shallower oceanic lithosphere and deeper impinged material, which involved a recycled Proterozoic lithosphere as heterogeneous components.
H2O and CO2 contents in cordierite from four different high and ultrahigh temperature metamorphic terranes have been qualitatively analyzed using Laser Raman spectroscopy. Raman spectrum of cordierite suggests that H2O is dominant for Ryoke, Namaqua and Mather metamorphic rocks, and that both H2O and CO2 are detected but are not abundant for Napier metamorphic rocks.
チェコ共和国Gföhl Unit Nove Dvoryでは1000℃，＞４Gpa （地下約150ｋｍ）の最高形成条件を示す超高圧エクロジャイトとザクロ石カンラン岩（Medaris et al., 1990: Nakamura et al,. 2004）がミグマタイト的な片麻岩に取り囲まれて産している。このミグマタイト的な片麻岩には超高圧変成作用を示す証拠はこれまで見つかっておらず、超高圧変成岩が見かけ低圧のミグマタイトに取り込まれた産状を示す中国Dabie-Suluと非常に良く似ている。本研究では、チェコ共和国Nove Dvoryの超高圧変成岩を取り囲む酸性質岩石の変成履歴を復元するため、顕微ラマン分光分析器、カソードルミネッセンスを用いてジルコン中の微細包有物を詳細に分析した。ジルコン中の微細包有物を分析した結果、一部の試料のSiO2包有物から、464 cm-1と393 cm-1に加え、超高圧変成作用の指標となるコース石と解釈されている521 cm-1と179 cm-1の弱いラマンピークの信号を確認した。カソードルミネッセンスを用いた観察により、ほとんどのジルコンはオシレタリーゾーニングを示すコア部とその周囲を不連続に取り囲むリム部に区分できる。コース石と解釈されるラマンピークを示すSiO2包有物は全てジルコンのコア部にのみ存在している。ジルコンのリム部にあるSiO2包有物は全て石英であった。
Non-metamorphosed conglomerate bed overlies the Choloktor suite which consists of pelitic schists, garnet-amphibolites and eclogites in the Atbashy area, Northern Tien-Shan, Kyrgyzstan, and it consists mainly of subrounded cobbles and boulders. The clasts include eclogites and garnet-amphibolites, and two events of HP metamorphism are distinguished in the eclogites.
The first event consists of a prograde stage of the greenschist facies, the peak eclogitic stage of 460 - 590 °C at 15 kbar and a retrograde stage of the greenschist facies. The second event of metamorphism consists of the blueschist facies and the epidote-amphibolite facies of prograde stage and the greenschist facies of retrograde stage.
The eclogite clasts from the conglomerate bed have a different P-T history from the eclogites of the Choloktor suite.
中国蘇魯帯の仰口では、Ye et al. (2000)によって、7GPa以上の超高圧変成条件が報告されている。この値は、ザクロ石に含まれる細粒単斜輝石やルチルの包有物を母相からの析出物と解釈し、majorite的な組成を復元したことによる。我々は、彼らがmajoriteの復元に用いたザクロ石と類似した組成の粗粒ザクロ石をペリドタイト中のザクロ石単斜輝石層から見出した。このザクロ石は比較的均質なコア(prp32-42alm41-45)と、最外縁部に向かってMgOが増加するリム(prp58-62alm25)で構成されている。Yoshida et al. (2004)は、ペリドタイトの最高変成条件を700‐800°C, 3.1-4.1 GPaと推定した。この推定値はYe et al. (2000)の値と大きく異なる。我々が研究した粗粒ザクロ中には、定向性のないmmサイズの単斜輝石が包有されている。これら粗粒単斜輝石がザクロ石成長時の初生包有物である場合、“majorite”の組成を復元する根拠は無くなる。
The Sam Nua Depression Zone is a NW-SE trending tectonic unit exposed over 400 km from Northern Vietnam through Laos to Southwestern China. It represents the Northern periphery of Indochina block contiguous to South China block by Songma fault. It consists of Paleozoic, Mesozoic and Cenozoic sedimentary and igneous rocks with various chemical compositions. The Paleozoic passive continental margin sediments and the associated mafic-ultramafic rocks represent the remnants of Paleo-oceanic lithosphere that accreted into the northern edge of the Indochina block while the Triassic terrigenous, terrigeno-effusive and igneous rocks are related with continental arc setting. The Jurrasic and Cenozoic igneous rock occurrences preserve chemical characteristics of within plate tectonic setting.
The Shyok suture zone in Ladakh Himalaya, NW India separates the Asian plate in the north from the Kohistan-Ladakh island arc in the south. The albite porphyroblast bearing basic schists occurring in the zone have a mineral assemblage of amphibole, albite, epidote, zoisite, clinozoisite, chlorite, calcite, quartz and opaques that are fine-grained except for albite. The albite porphyroblasts appear normally in a zone of 30cm to 1m wide and most of them occur as lens-shaped grains with pressure solution edges, of which the grain size is 1-3mm. They occur generally as aggregates of smaller albite grains. The schists record the low pressure and temperature condition or the lower greenschist facies. There is no record suggesting any high-pressure conditions so far.
The southern India terrene is composed of several Proterozoic granulite blocks dissected by major deep-crustal shear zones welded onto the Archean Dharwar Craton in the north. The existing geochronological studies indicate that the crustal growth occurred at 3000 - 2100 Ma while the peak ultrahigh-temperature metamorphism at 550 - 580 Ma. We estimate the cooling history of various metapelites and orthogenesis collected systematically from north to south of the southern granulite terrene. The Kerala Khondalite Belt gives 451 - 465Ma while the Kodaikanal Metapelite Belt yields 507 - 512 Ma, suggesting the exhumation rate in the south must be slower as compared to that in the northern part of southern granulite terrene.
This study first reports Ca-rich garnet-bearing metapelitic rocks with high pressure granulite facies mineral assemblages in the low P/high T Higo metamorphic rocks. Peak pressure and temperature condition of the metapelitic rocks is estimated to be 12 kbar and 800oC. The peak P-T condition almost corresponds to extrapolated P-T conditions of metamorphic rocks of GCII zone. Whole-rock major elements analyses and mineral equilibria modeling show that the Ca-rich garnet-bearing metapelitic rocks were formed by effective melt extraction from the lowest crust. The modeling also shows that influx of H2O in the mid- to lower crust was required for high proportion of melt needed to form the diatexite, and that the metatexite did not involve little added H2O.
EPMA analysis on a thin section of a pelitic schist sample from the Sambagawa metamorphic belt in central Shikoku revealed that two groups of garnet grains with different chemical composition are layered within a hand sample. The difference in size and chemical composition observed in garnet is consistent with that caused by the variation of bulk rock (i.e., chlorite) chemistry, based on the prediction by the forward model. It suggests that rocks with slightly different bulk rock chemistry were sandwiched after the peak of the metamorphism, simultaneously forming the mineral foliation.