21世紀 岡山の100人 岡山大学地球物質科学研究センター センター長 中村栄三

岡山大学地球物質科学研究センターは鳥取県三朝町に整然と佇んでいるが、中身は驚くほど熱い。研究成果を惜しみなく発信し、地球科学に関する先導的役割を担う。研究センターの分析能力は「世界中の他の研究室の追随を許さない」と国際評価され、その鋭い眼力は自らの研究のみならず日本の教育問題をも見据える。月刊プラザ岡山 10月号(2007) ; copyright (c) 2007（株）オークシー

Lithium- and oxygen-isotope compositions of chondrule constituents in the Allende meteorite

We report in situ ion-microprobe analyses of Li- and O-isotope compositions for olivine, low-Ca pyroxene, high-Ca pyroxene, and chondrule mesostasis/plagioclase in nine chondrules from the Allende CV3 chondrite. Based on their mineralogy and O-isotope compositions, we infer that the chondrule mesostasis/plagioclase and ferroan olivine rims were extensively modified or formed during metasomatic alteration and metamorphism on the Allende parent asteroid. We excluded these minerals in order to determine the correlations between Li and both O and the chemical compositions of olivines and low-Ca pyroxenes in the chondrules and their igneous rims. Based on the O-isotope composition of the olivines, nine chondrules were divided into three groups. Average Δ17O of olivines (Fo>65) in group 1 and 2 chondrules are −5.3 ± 0.4 and −6.2 ± 0.4‰, respectively. Group 3 chondrules are characterized by the presence of 16O-rich relict grains and the Δ17O of their olivines range from −23.7 to −6.2‰. In group 1 olivines, as Fa content increases, variation of δ7Li becomes smaller and δ7Li approaches the whole-rock value (2.4‰; Seitz et al., 2012), suggesting nearly complete Li-isotope equilibration. In group 2 and 3 olivines, variation of δ7Li is limited even with a significant range of Fa content. We conclude that Li-isotope compositions of olivine in group 1 chondrules were modified not by an asteroidal process but by an igneous-rim formation process, thus chondrule olivines retained Li-isotope compositions acquired in the protosolar nebula. In olivines of the group 3 chondrule PO-8, we observed a correlation between O and Li isotopes: In relict 16O-rich olivine grains with Δ17O of ∼−25 to −20‰, δ7Li ranges from −23 to −3‰; in olivine grains with Δ17O > −20‰, δ7Li is nearly constant (−8 ± 4‰). Based on the Li-isotope composition of low-Ca pyroxenes, which formed from melt during the crystallization of host chondrules and igneous rims, the existence of a gaseous reservoir with a δ7Li ∼ −11‰ is inferred

Silicate–SiO reaction in a protoplanetary disk recorded by oxygen isotopes in chondrules

The formation of planetesimals and planetary embryos during the earliest stages of the solar protoplanetary disk largely determined the composition and structure of the terrestrial planets. Within a few million years of the birth of the Solar System, chondrule formation and the accretion of the parent bodies of differentiated achondrites and the terrestrial planets took place in the inner protoplanetary disk 1,2 . Here we show that, for chondrules in unequilibrated enstatite chondrites, high-precision Δ17O values (where Δ17O is the deviation of the δ17O value from a terrestrial silicate fractionation line) vary significantly (ranging from −0.49 to +0.84‰) and fall on an array with a steep slope of 1.27 on a three-oxygen-isotope plot. This array can be explained by the reaction between an olivine-rich chondrule melt and an SiO-rich gas derived from vaporized dust and nebular gas. Our study suggests that a large proportion of the building blocks of planetary embryos formed by successive silicate–gas interaction processes: silicate–H2O followed by silicate–SiO interactions under more oxidized and reduced conditions, respectively, within a few million years of the formation of the Solar System

The Albedo of Ryugu: Evidence for a High Organic Abundance, as Inferred from the Hayabusa2 Touchdown Maneuver

The Hayabusa2 mission successfully collected samples from the asteroid Ryugu last year and will return these to Earth in December 2020. It is anticipated that the samples will enable the analysis of terrestrially uncontaminated organic matter and minerals. Such analyses are in turn expected to elucidate the evolution of organic matter through Solar System history, including the origination and processing of biogenically important molecules, which could have been utilized by the first organisms on Earth. In anticipation, studies have made predictions concerning the properties of Ryugu, including its composition. The spectral characteristics of Ryugu, such as albedo, have been employed to relate the asteroid to members of the carbonaceous chondrite group that have been identified on Earth. However, the recent Hayabusa2 touchdown highlights a disparity between the color of surfaces of displaced platy fragments, indicating a brightening trend for the surface exposed to space compared to that facing into the body. Here we present a mass balance calculation with reference to data from the literature, which indicates that Ryugu may contain a significantly higher abundance of organic matter (likely >50%) than the currently most accepted meteorite analogues. A high organic content may result in high levels of extractable organic matter for the second touchdown site, where the spacecraft sampled freshly exposed material. However, high abundances of insoluble aromatic/graphitic rich organic matter may be present in the first touchdown site, which sampled the surface of Ryugu that had been exposed to space. Moreover, we suggest that the potentially high organic abundance and the rubble-pile nature of Ryugu may originate from the capture of rocky debris by a comet nucleus and subsequent water-organic-mineral interactions and sublimation of water ice

Feedback of Slab Distortion on Volcanic Arc Evolution: Geochemical Perspective From Late Cenozoic Volcanism in SW Japan

Southwest Japan is an island arc formed by subduction of the Philippine Sea (PHS) plate. The Quaternary magmatism in this region is characterized by eruptions of high-Sr andesites and dacites, considered to have been derived by melting of the PHS plate. The loci of these volcanoes spatially coincide with seismic discontinuities of the subducted PHS plate. Thus, the magmatism is interpreted as the result of slab melting at the plate tears. However, the processes that promote slab tearing remain unclear. In this study, we applied geochronological and geochemical analyses to late Cenozoic volcanic rocks in southwest Japan as tracers of slab morphology. Two different magma types, ocean-island basalt (OIB) and island-arc basalt (IAB), have occurred over 12 million years (Myr). These two magmas are attributed to different integrations of melts extracted from an originally fertile mantle; the OIBs from high temperature melt (1,300–1,400°C) were extracted at a depth of 40–80 km, whereas the IABs were extracted from a shallower, lower temperature region (30–60 km, 1,200–1,350°C). Secular change in Sr enrichment of IAB likely arose due to a transition of slab-derived fluids, incorporated into magmas as they formed, from water- to melt-dominant one. Progressive shallowing of the subducted PHS plate is responsible for secular change in the properties of slab-derived fluids as well as rollback of OIB volcanoes. Production of chemically variable magmas in the Chugoku district is the surface expression of distorting slab morphology by interaction between mantle and the subducting plate

Tourmaline in a Mesoarchean pelagic hydrothermal system: Implications for the habitat of early life

The RNA World hypothesis requires the synthesis of RNA to allow the emergence of life on Earth. Hydrothermal systems have been proposed as potential candidates for constructing complex biomolecules. However, in order to successfully form RNA, it is necessary to stabilize ribose, a RNA carbohydrate component. Borate has been found to stabilize ribose. Therefore, boron rich hydrothermal systems are important environments concerning the origin of life on Earth. The 3.2-Ga Dixon Island Formation of the West Pilbara Superterrane, Western Australia, is a volcano-sedimentary sequence. The Formation represents a Mesoarchean pelagic hydrothermal system, which formed adjacent to an immature island arc. Fine-grained tourmaline, in addition to biogenic carbonaceous matter and spherulitic and tubular bacteriomorphs, are found in black chert. A boron-rich environment was responsible for the formation of these deposits. To explore the implications of such a boron enriched environment on microbial activity, modes of occurrence and chemical compositions of the tourmaline were examined. The tourmaline is schorl or dravite of the alkali tourmaline group and the boron isotope compositions range in δ11B from -7.3 to +2.6‰. The tourmaline occurs in microcrystalline quartz matrix of black chert veins that cross cut a volcanic unit and also in a bedded black chert, which overlays the volcanic unit. The volcanic unit contains highly altered zones with hydrothermal veins. The associated lithologic and stratigraphic features suggest that the black chert veins were the conduits for upward moving hydrothermal fluids, which reached the sea floor. Subsequently, the volcanic unit was covered by organic matter-rich cherty sediments that in part were fed, and/or altered, by the hydrothermal fluids. These results suggest that the origin of boron enrichment to form Dixon Island tourmaline is not the associated sedimentary mineral assemblage, which includes diagenetic clay, low-grade metamorphic mica, and organic matter. Instead, the tourmaline was directly precipitated from hydrothermal fluid, enriched in boron. Furthermore, the hydrothermal fluids had already concentrated the boron, in the Mesoarchean pelagic system, prior to the apex of organic matter production and microbial activity. Our findings support a hypothesis that the boron-enriched hydrothermal environment aided the survival and evolution of early life

Platinum-Group Element Geochemistry of the Escondida Igneous Suites, Northern Chile: Implications for Ore Formation

Platinum-group element (PGE) geochemistry may be used to constrain the timing of sulfide saturation in magmas, which influences the Cu and Au fertility of evolving magmatic systems. We report new geochronological and geochemical data, with emphasis on PGE geochemistry, for a suite of regional porphyritic hornblende–diorite intrusions and ore-bearing porphyries from the super-giant Escondida and smaller Zaldivar Cu deposits of Northern Chile. The regional dioritic intrusions have zircon U–Pb ages between 39·6 to 37·1 Ma, which overlap with the ages of the ore-bearing Escondida and Zaldivar porphyries (38·1 to 35·0 Ma). Whole-rock major and trace element, and Sr–Nd–Pb and zircon O–Hf isotope geochemistry indicate that the regional diorites and ore-bearing porphyries are co-magmatic and originated from the same mantle-derived magma by fractional crystallization, with minor contamination by Paleozoic crust (∼10%). The low concentrations of PGE in the regional diorites show that they reached sulfide saturation before the MgO content of the melt fell to 4·7 wt %, the MgO content of the most primitive sample analysed. The fraction of sulfide melt which separated from the melts that formed the regional diorites is estimated to be ∼0·12 wt %; this resulted in the partitioning of highly chalcophile elements (Au and PGE) into a sulfide phase that was retained in cumulus rocks at depth. However, the fraction of sulfide melt was too low to have a significant effect on the Cu content of the fractionating melt. As a consequence, when the evolving melt eventually reached volatile saturation, it contained enough Cu (40 ± 10 ppm) to form a super-giant Cu deposit. In contrast, Au was largely stripped from the melt by sulfide precipitation, with the result that the mineralization at Escondida is Cu dominant, with only minor Au. The Zaldivar deposit, on the other hand, contains even less Au, which is attributed to a longer fractionation interval between sulfide and volatile saturation. This study provides evidence to support previously proposed models which suggest that the timing of sulfide saturation, the amount of sulfide melt produced, the water content and oxidation state of the melt, and the magma volume are critical factors in determining the potential to form a porphyry Cu deposit. Plots of Pd/MgO against Y can be used as empirical indicators of magma fertility for porphyry mineralization, and to discriminate between Cu–Au and Cu-dominated systems, but cannot predict the size of the deposit. The super-giant status of the Escondida deposit is attributed to it being underlain by a large batholith with a calculated minimum mass of 1012 tonnes (∼400 km3).This work was supported by the Australian Research Council [DP170103140]

Lithium in garnet as a tracer of subduction zone metamorphic reactions: The record in ultrahigh-pressure metapelites at Lago di Cignana, Italy

Lithium is of great interest as a tracer of metamorphic reactions and related fluid-mineral interactions because of its potential to isotopically fractionate during inter- and intracrystalline diffusional processes. Study of its transfer through subduction zones, based on study of arc volcanic and metamorphic rocks, can yield insight regarding ocean-to-mantle chemical cycling. We investigated major- and trace-element concentrations and delta Li-7 in garnet in ultrahigh-pressure (UHP) Lago di Cignana metasedimentary rocks, relating these observations to reconstructed prograde devolatilization history. In all garnet crystals we studied, heavy rare earth elements (HREEs), Y, and Li showed strong zoning, with elevated concentrations in cores (15-50 ppm Li) and marked high-concentration anomalies (up to 117 ppm Li, 5500 ppm Y; little or no major-element shift) as growth annuli, in which some crystals showed subtle elevation in delta Li-7 greater than analytical error of similar to 3 parts per thousand (2 sigma). Rutile inclusions appeared abruptly at annuli and outward toward rims, accompanied by inclusions of a highly zoned, Ca- and rare earth element-rich phase and decreased Nb concentrations in garnet. These relationships are interpreted to reflect prograde garnet-forming reaction(s), in part involving titanite breakdown to stabilize rutile, which resulted in delivery of more abundant Y and HREEs at surfaces of growing garnet crystals to produce annuli. Co-enrichments in Li and Y + REEs are attributed to mutual incorporation via charge-coupled substitutions; thus, increased Li uptake was a passive consequence of elevated concentrations of Y + REEs. The small-scale fluctuations in delta Li-7 (overall range of similar to 9 parts per thousand) observed in some crystals may correlate with abrupt shifts in major-and trace-element concentrations, suggesting that changes in reactant phases exerted some control on the evolution of delta Li-7. For one garnet crystal, late-stage growth following partial resorption produced deviation in major- and trace-element compositions, including Li concentration, accompanied by a 10 parts per thousand-15 parts per thousand negative shift in delta Li-7, perhaps reflecting a change in the mechanism of incorporation or source of Li. These results highlight the value of measuring the major- and trace-element and isotope compositions of garnets in high-pressure and UHP metamorphic rocks in which matrix mineral assemblages are extensively overprinted by recrystallization during exhumation histories. Lithium concentrations and isotope compositions of the garnets can add valuable information regarding prograde (and retrograde) reaction history, kinetics of porphyroblast growth, intracrystalline diffusion, and fluid-rock interactions. This work, integrated with previous study of devolatilization in the Schistes Lustres/Cignana metasedimentary suite, indicates retention of a large fraction of the initially subducted sedimentary Li budget to depths approaching those beneath volcanic fronts, despite the redistribution of this Li among mineral phases during complex mineral reaction histories

Determination of Abundances of Fifty-Two Elements in Natural Waters by ICP-MS with Freeze-Drying Pre-concentration

To precisely determine the abundances of fifty-two elements found within natural water samples, with mass fractions down to fg g(-1) level, we have developed a method which combines freeze-drying pre-concentration (FDC) and isotope dilution internal standardisation (ID-IS). By sublimation of H2O, the sample solution was reduced to 0.3 mol l(-1) to avoid hydrolysis. Matrix-affected signal suppression was not significant for the solutions with NaCl concentrations lower than 0.2 and 0.1 cg g(-1) for quadrupole ICP-MS and sector field ICP-MS, respectively. The recovery yields of elements after FDC were 97-105%. The detection limits for the sample solutions prepared by FDC were <= 10 pg g(-1), except for Na, K and Ca. Blanks prepared using FDC were at pg-levels, except for eleven elements (Na, Mg, Al, P, Ca, Mn, Fe, Co, Ni, Cu and Zn). The abundances of fifty-two elements in bottled drinking water were determined from five different geological sources with mass fractions ranging from the fg g(-1) to mu g g(-1) level with high accuracy