熊谷 英憲

 As our understanding of seafloor hydrothermal systems grows, we recognize they are not always stable and sometimes show dramatic changes. In this review, the authors present a compilation of geochemical and geochronological studies that are helpful when investigating the evolving processes of submarine hydrothermal systems.<br> Chapter II describes the systematics and methodology of three dating techniques with discussions on their application to minerals formed by seafloor hydrothermal activities. The K-Ar (Ar-Ar) technique is popular for dating igneous rocks, but it is not appropriate for dating hydrothermal minerals because potassium is a trace component of sulfide/sulfate minerals. Following recent progress, micro-analytical techniques applying laser fusion are applicable for dating fluid inclusions and/or hydrothermal alteration minerals, which could provide important geochronological information. Uranium and thorium series disequilibrium dating have been employed for previous geochronological studies of hydrothermal minerals obtained from submarine ore deposits. To cover a wide time range, it is necessary to use various combinations of parent and daughter nuclides. Applying ESR dating to hydrothermal minerals is a rather new challenge. Although it needs several investigations to establish the methodology, it could be a useful rapid dating technique for a time range of less than one thousand years.<br> Chapter III introduces studies focusing on the evolution of seafloor hydrothermal activities over a short time scale (one week to a few years). Detection of event plumes associated with seafloor lava eruption brought an awareness of episodic hydrothermal activity triggered by magmatic perturbation. Subsequent dive studies revealed evolving geochemical processes, such as major changes of volatiles and elemental species concentrations of venting fluid. With remote real-time monitoring of acoustic T-waves generated by seafloor seismic activities, event detection and response cruises have been conducted successfully to investigate various evolving processes in more detail.<br> Chapter IV introduces studies focusing on the evolution of seafloor hydrothermal activities over a long time scale (tens of thousands of years). Radiometric dating studies of hydrothermal minerals such as sulfide and manganese oxide collected from the TAG mound, which is one of the largest hydrothermal mound structures, reveal an age distribution over at least 15000 years separated by quiescent intervals lasting up to 2000 years. On slow spreading ridges such as the Mid-Atlantic ridge, major fracture systems focus the hydrothermal discharge at one place for more than one thousand years with repeated reactivation.<br> In Chapter V, the authors discuss the direction of future studies. Although hydrothermal systems on mid-oceanic ridges have been well studied, those related to arc-backarc magmatic activities could provide more appropriate fields for studying the evolutionary process of submarine hydrothermal systems. Combining geochronological studies with geochemical and mineralogical studies would be important for reconstructing the evolution process in more detail.

Frictional melting experiments were performed on fine grain homogeneous gabbroid with high temperatures induced by frictional heating using a high-velocity apparatus. We examined whether rapid fault movement can equilibrate fault rock gas with atmospheric components by measuring volatile gas and noble gas isotopes from a gabbroid sample using a quadrupole mass spectrometer to detect released gas from the simulated fault rock.The anticipated rapid equilibration of volatiles during the frictional melting of rocks implies that the noble gas and volatile were released and mixed with the atmosphere during this experiment. Gases released from the sample were collected in a small aluminum tube in nitrogen atmosphere before and after the frictional melting experiment. The gas comprised carbon dioxide, water vapor, hydrogen, helium, and other noble gases. The He/Ar ratio and H2 concentration are higher than the pre-analysis of N2 atmosphere. This release of volatiles is consistent with the pseudotachylyte-like post experimental texture of specimen. It is also consistent with the co-seismic geochemical anomaly observed along a natural fault system.

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.

金沢大学フロンティアサイエンス機構

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.