熊谷 英憲

The vent fluids of the Kairei Hydrothermal Field (KHF) on the Central Indian Ridge near the Rodriguez Triple Junction are atypical. They have a very high H2concentration, a relatively high Si concentration, and a remarkably low CH4/H2ratio. Recently, particular attention has been paid to the KHF, because the hydrothermal fluids are suggested to support a hydrogen-based hyperthermophilic subsurface lithoautotrophic microbial ecosystem (HyperSLiME), which is considered to be a likely modern analogue for the early Earth ecosystems prior to photosynthesis. Despite the increasing interest in the fluid chemistry and associated biota, the origin of the unusual chemistry of the hydrothermal fluids is still uncertain. Here we suggest interaction of seawater with troctolite recently discovered from small hills near the KHF, provide a possible explanation for the composition of the KHF fluids. Dives with the manned submersible Shinkai 6500 recovered plagioclase dunite, troctolites, and olivine gabbros, which generally constitute the lower oceanic crust. Microscopic observations revealed that olivine crystals in the samples were partly to completely replaced by serpentine and magnetite, indicating the generation of H2by serpentinization reactions between olivine and hydrothermal fluids. Theoretical model calculations predict that the high H2and high Si concentrations of the hydrothermal fluids can be attributed to serpentinization of the troctolites and subsequent hydrothermal reactions with basaltic wall rocks under the KHF. The unique geological setting of the Kairei hydrothermal system, where deep crustal rocks are emplaced in the shallower part of oceanic crust, is considered to be responsible for the unusual chemistry of the Kairei hydrothermal fluids driving the occurrence of the HyperSLiME. © 2009 Elsevier B.V. All rights reserved.

四重極型質量分析計を装備した超高真空試料真空破砕装置を実験室外に移動して分析を行うことで, 希ガスおよび共存する炭酸ガス, 水, 窒素のガス分析を機動的に行う可能性を検証した. 移動に際し装置超高真空部の封止に注意を払うことで, 移動後も実験室内での運用と同程度の10<sup>-5</sup> Pa台の圧力が維持された. 火山ガスを大量に含む玄武岩質ガラス6K834R4の分析において, ベースラインからの見かけ変動値は最少で, 装置内の分圧として7×10<sup>-7</sup> Paであった. 今回の結果により, 移送および組み立てにおいて十分な配慮があれば, 所要の電力が供給された観測船や車両において, 流体中の微量ガスのその場分析が可能であるという見通しが得られた.

 Mid-ocean ridge basalt (hereafter, MORB) is a final product of melt generated from the partial melting of mantle peridotite, following reaction with mantle and/or lower crustral rocks, fractionation at a shallower crust and other processes en route to seafloor. Therefore, it is difficult to estimate melting processes at the upper mantle solely from any investigations of MORB. In contrast to the restricted occurrence of peridotite of mantle origin in particular tectonic settings (<i>e.g.</i>, ophiolites, fracture zones, or oceanic core complexes), the ubiquitous presence of MORB provides us with a key to understanding global geochemical variations of the Earth's interior in relation to plate tectonics.<br> In fact, MORB has been considered to show a homogeneous chemical composition. In terms of volcanic rocks from other tectonic settings (<i>e.g.</i>, island arc, continental crust, ocean island), this simple concept seems to be true. However, recent investigations reveal that even MORB has significant chemical variations that seem to correspond to location (Pacific, Atlantic, and Indian Oceans). These observations suggest that the mantle beneath each ocean has a distinct chemical composition and an internally heterogeneous composition.<br> In this paper, global geochemical variations of MORB in terms of major and trace element compositions and isotope ratios are examined using a recently compiled database. The compilation suggests that MORB has heterogeneous compositions, which seem to originate from a mixture of depleted mantle and some enriched materials. Coupled with trace element compositions and Pb-isotope ratios, there seems to be at least two geochemical and isotopic domain of the upper most mantle: equatorial Atlantic-Pacific Oceans and southern Atlantic-Indian Ocean. Material (melt and/or solid) derived from plume, subducted slab, subcontinental crust, or fluid added beneath an ancient subduction zone is a candidate to explain the enrichment end-member to produce heterogeneous MORB.<br> Because MORB is heterogeneous, using a tectonic discrimination diagram that implicitly subsumes homogeneous MORB or its mantle sources should be reconsidered. Further investigations, particularly of off-axis MORB, are needed to understand the relationship between heterogeneous compositions of MORB and geophysical parameters (<i>e.g.</i>, degree of melting, temperature, spreading rate, crustal thickness, etc). In addition, the role of the MOHO transitional zone should be investigated to interpret the chemical characteristics of MORB.

 A large number of intraplate volcanoes erupted two to several hundred kilometers off the fast-spreading East Pacific Rise (EPR). These volcanoes consist of large lava fields, monogenetic volcanoes, and linear chains of monogenetic volcanoes and volcanic ridges. Large lava fields of 7-26 km<sup>3</sup> in volume are known at 8°N, 14°S, and 16°S within 2-19 km from the rise axis and from the top 75-100 m of ODP Site 1256 on the 15 Ma Cocos plate. Monogenetic volcanoes form within ∼20 km from the rise axis or on the basement < 200 kyr, and are evenly distributed over the rise axis. Linearly aligned volcanoes and volcanic ridges occur farther from the rise axis than large lava fields and monogenetic volcanoes, and run subparallel to the direction of the Pacific plate motion. The Sojourn Ridge, the largest volcanic ridge, extends up to 440 km in length and is several hundred cubic kilometers in volume. Eruptive ages along a volcanic ridge and a volcano chain contradict the hot-spot origin of these volcanic features. Negative free-air and residual mantle Bouguer anomalies correlate well with the linearly aligned volcanoes and volcanic ridges, suggesting excess magma supply beneath the volcanic edifices. Seismic experiments show volcanic ridges have no keel below the Moho, indicating compensation of surface loading by plate flexure and underplating. <br> Whole rock compositions of off-ridge volcanoes have a much wider spectrum than the adjacent axial lavas, spanning from depleted NMORB through TMORB to isotopically fertile EMORB. Some off-ridge lavas could be produced by the fractional crystallization of the same parent magma as the adjacent axial lavas. However, most off-ridge lavas originate from different parent magmas than the neighboring axial lavas. Some TMORB magmas including the 14°S large lava field are the mixing product of the NMORB and EMORB magmas. Copious differentiated lavas of the large lava fields require a large magma chamber as a the site for crystallization differentiation and magma mixing. The lava geochemistry of off-ridge volcanoes strongly suggests the presence of a magma source that is independent of the axial magma plumbing system.<br> Seismic tomography and seafloor compliance measurements beneath the northern EPR indicate that the presence of melt across the rise axis is restricted in a narrow zone ∼4 km in width through the crust, but has a 10-14 km wide distribution in the uppermost mantle. Broad distribution, volume, and geochemistry of off-ridge monogenetic volcanoes and large lava fields strongly suggest that the off-ridge volcanoes originated from the Moho transition zone (MTZ). The MTZ is formed by a reaction between the uprising magma and the host mantle peridotite, leaving replacive dunite that experienced variable depletion and enrichment processes. Passive asthenospheric upwelling beneath the fast-spreading ridges produces a broad partial melt zone, through which magma ascends and accumulates beneath the off-ridge lithosphere. More depleted off-ridge magmas than axial magmas differentiate and mix with residual magmas in the MTZ, and react with variably enriched, impregnated dunite, resulting in variety of off-ridge lava compositions.<br> Small clusters of volcanoes and linear volcano chains are created by partial melting in asthenospheric return flows or local instability of the thermal boundary layer beneath the cooling lithosphere. Linear volcano chains will develop into long and robust volcanic ridges extending several hundred kilometers in length.

 The hydrothermal circulation of seawater in the oceanic lithosphere is an important factor controlling seawater chemistry, compositions of subducted materials returned to the mantle and microbial activity. We summarize the results of hydrothermally altered rocks taken directly from the ocean floor in terms of major and trace elements combined with petrographic descriptions. Hydrothermal circulation starts at the spreading axis where magmatic heat from a basaltic crustal formation is available (high temperature of > 350°C). Low-temperature alteration (< 150°C) may persist for > a million of years through the ridge flanks. Due to ridge flanks occupying large regions of the seafloor, changes in chemistry, mineralogy and physical properties of the oceanic lithosphere are accompanied by geochemical fluxes that may be even larger than those at the ridge axis. Two deep drill holes, DSDP/ODP 504B and 1256D, allow an examination of downhole variations of hydrothermal alteration in basaltic rocks, and dolerite in the extrusive and sheeted dike sequence. Recent direct sampling from the ocean floor reveals that gabbros and peridotites crop out in significant amounts on the ocean floor, particularly in the slow-spreading ridges. The chemical behavior of these originally deep-seated rocks during hydrothermal circulation thus has a large impact on global mass budgets for many elements.<br> Previous studies on the ocean floor have been mainly conducted in the Atlantic Ocean and the Pacific Ocean. We present our results on hydrothermally altered basaltic rocks, gabbros and peridotites recovered from the Indian Ocean. Basaltic samples dredged from the first segment of the Southwest Indian Ridge near the Rodriguez Triple Junction are classified into three types—a fresh lavas, low-temperature altered rocks and high-temperature altered rocks. Petrological and geochemical features of these rocks are basically comparable to those of the basaltic rocks in DSDP/ODP Hole 504B, which suggests generalities in alteration processes and chemical exchange fluxes during hydrothermal activity across all world oceans. Gabbros and peridotites were sampled from an oceanic core complex, which was composed of tectonically exposed footwalls of detachment faults, from the Central Indian Ridge. Less deformed serpentinized and gabbros were recovered from the ridge-facing slope, whereas highly deformed schist-mylonites of a mixture of these rocks were recovered from the top of the surface (<i>i.e.</i>, detachment fault). Efficient localization of strain was probably due to the formation of secondary minerals (<i>e.g.</i>, talc, chlorite, serpentine) onto large, discrete shear zones where fluid was introduced locally. In-situ microanalysis of trace elements of the primary minerals and their secondary minerals revealed that selective elements, such as Rb, Sr, Ba, Pb and U, are enriched in the secondary minerals. Although oceanic core complexes are places that allow cross-sectional samplings of deep-seated rocks (<i>i.e.</i>, gabbros and peridotites) in the oceanic lithosphere, we should keep in mind the implications of the results for the normal oceanic lithosphere. To understand the nature of the oceanic lithosphere, a close linkage between the ophiolite study and a number of deep holes in the oceanic lithosphere, including a deep hole through the crust-mantle boundary, is required.

Laser fusion measurement for a single grain of phenocryst or of in-situ measurement of less-abundant minerals found on thin sections is established for K-Ar dating method. For such kind of samples, Ar-Ar dating is applied widely to obtain radiometric ages because the Ar-Ar method is independent of the site difference between K and Ar in the specimen. However, Ar-Ar dating raises at least two difficulties: 1) the method requires a control area to treat radioactive samples that were irradiated with neutrons in a nuclear reactor before the analysis to produce <sup>39</sup>Ar from <sup>39</sup>K; 2) quantities of nuclides produced by irradiation mask information about the original isotope ratios in rock and mineral samples. Consequently, detailed correction using the initial noble gas isotope ratio is inapplicable, which poses a serious problem, especially for recent samples or samples with low K concentrations, which are expected to include minute amounts of radiogenic Ar. In these cases, large uncertainty is brought to ages useless by the masking of the original isotope ratio. Herein, we report an un-irradiated and un-spiked laser fusion K-Ar dating method, with which we can analyze both Ar and K for the identical grains or positions on a thin section. This is mainly attributable to the following protocols: 1) K measurement following/after laser fusion Ar measurement applied to the retrieved single mineral grain itself; and 2) in-situ laser fusion Ar measurement applying to the epoxy resin mounted grain, where its K-content measured using EPMA. This method is expected to enable acquisition of precise radiometric ages of young lavas or of low K samples having a low <sup>40</sup>Ar/<sup>36</sup>Ar ratio.

Because of chemical inertness of noble gases, their isotopic compositions trapped in mantle-derived xenoliths provide valuable information about mantle processes. Here we present a review of noble gas studies of mantle xenoliths from several tectonic settings with specific attention to mantle metasomatism. Numerous metasomatic traces have been identified as noble gas isotopic anomalies found in fluid or melt inclusions or in minerals of metasomatic origin in the suboceanic and subcontinental lithosphere. The noble gas isotopic ratio of MORB source, which is generally regarded as representing the suboceanic upper mantle, is characterized by a quite uniform <sup>3</sup>He/<sup>4</sup>He ratio and a high <sup>40</sup>Ar/<sup>36</sup>Ar ratio of up to 40000. On the other hand, low <sup>3</sup>He/<sup>4</sup>He and <sup>40</sup>Ar/<sup>36</sup>Ar ratios compared to those of MORBs have been reported from some subcontinental ultramafic xenoliths. This phenomenon is explainable in terms of metasomatism by a slab-derived component at the continental/convergent plate margin causing enrichment of U and Th, parent nuclides of <sup>4</sup>He, and of atmospheric Ar in the mantle wedge. Metasomatic signatures attributable to deep mantle plume are observable as a higher <sup>3</sup>He/<sup>4</sup>He ratio than the MORB value and a distinct trend in Ne three-isotope plot from that of MORBs, both in oceanic and continental areas. In addition, noble gas isotope exchange between the mantle xenolith and its host magma are often observed. By applying several methods for extraction of noble gases and careful selection of samples, noble gases can serve as a powerful tool to distinguish these metasomatic agents. Furthermore, noble gas analysis of small pieces of mantle xenoliths or individual fluid/melt inclusion using a laser microprobe in combination with other analytical techniques for detection of major volatile components, such as micro-Raman spectroscopy, will clarify the origin of volatiles in mantle xenoliths.

Kumagai, H, 2004, 'Correction to “Noble gas signatures of abyssal gabbros and peridotites at an Indian Ocean core complex”', <i>Geochemistry Geophysics Geosystems</i>, vol. 5, no. 2.

金沢大学フロンティアサイエンス機構

Kumagai, H & Kaneoka, I, 1998, 'Variations of noble gas abundances and isotope ratios in a single MORB pillow', <i>Geophysical Research Letters</i>, vol. 96, no. B7, p. 119.

Since noble gases are chemically inert and include both radiogenic (+ nucleogenic) and stable isotopes, they are quite useful to identify the characteristics of magma sources, which were established on global scale and affected mainly by time-integrated effects through the history of the Earth. Based on noble gas isotope signatures, at least four different components have been classified: M (MORB) source, P (plume) source, A (atmospheric) component and C (crustal) component. Other types such as island arc type source (Ac) can be explained by the mixture of them. However, there remain problems concerning their definite values which are significant to evaluate their distribution quantitatively in the Earth's interior. To infer those values, it is essential to evaluate the secondary effects properly which occur during magma transportation and extrusion and affect their primary signatures. In this paper, examples of secondary contamination by groundwater and sea water on the noble gas signatures are shown for volcanic lavas of the Unzen Volcano and a MORB glass, respectively. Further, coupled or decoupled characteristics are discussed among each isotope systematics.

Kumagai, H, Kaneoka, I & Ishii, T, 1996, 'The active period of the Ayu Trough estimated from K-Ar ages:the southeastern spreading center of Philippine Sea Plate.', <i>GEOCHEMICAL JOURNAL</i>, vol. 30, no. 2, pp. 81-87.