A new type of orthopyroxenite xenolith from Takashima, Southwest Japan: silica enrichment of the mantle by evolved alkali basalt

We found fine-grained Fe-rich orthopyroxene-rich xenoliths (mainly orthopyroxenite) containing partially digested dunite fragments of Group I from Takashima, Southwest Japan. Orthopyroxenite veinlets, some of which contain plagioclase at the center, also replace olivine in dunite and wehrlite xenoliths of Group I. This shows high reactivity with respect to olivine of the melt involved in orthopyroxenite formation, indicating its high SiO₂ activity. The secondary orthopyroxene of this type is characterized by low Mg# [= Mg/(Mg + total Fe) atomic ratio] (down to 0.73) and high Al₂O₃ contents (5–6 wt%). It is different in chemistry from other secondary orthopyroxenes found in peridotite xenoliths derived from the mantle wedge. Clinopyroxenes in the Fe-rich orthopyroxenite show a convex-upward REE pattern with a crest around Sm. This pattern is strikingly similar to that of clinopyroxenes of Group II pyroxenite xenoliths and of phenocrystal and xenocrystal clinopyroxenes, indicating involvement of similar alkali basaltic melts. The Fe-rich orthopyroxenite xenoliths from Takashima formed by reaction between evolved alkali basalt melt and mantle olivine; alkali basalt initially slightly undersaturated in silica might have evolved to silica-oversaturated compositions by fractional crystallization at high-pressure conditions. The Fe-rich orthopyroxenites occur as dikes within the uppermost mantle composed of dunite and wehrlite overlying pockets of Group II pyroxenites. The orthopyroxene-rich pyroxenites of this type are possibly common in the uppermost mantle beneath continental rift zones where alkali basalt magmas have been prevalent.     

Geochemistry of spinel-hosted amphibole inclusions in abyssal peridotite: insight into secondary melt formation in melt–peridotite reaction

Spinel-hosted hydrous silicate mineral inclusions are often observed in dunite and troctolite as well as chromitite. Their origin has been expected as products associated with melt–peridotite reaction, based on the host rock origin. However, the systematics in mineralogical and geochemical features are not yet investigated totally. In this study, we report geochemical variations of the spinel-hosted pargasite inclusions in reacted harzburgite and olivine-rich troctolite collected from Atlantis Massif, an oceanic core complex, in the Mid-Atlantic Ridge. The studied samples are a good example to examine geochemical variations in the inclusions because the origin and geological background of the host rocks have been well constrained, such as the reaction between MORB melt and depleted residual harzburgite beneath the mid-ocean ridge spreading center. The trace-element compositions of the pargasite inclusions are characterized by not only high abundance of incompatible elements but also the LREE and HFSE enrichments. Distinctive trace-element partitioning between the pargasite inclusion and the host-rock clinopyroxene supports that the secondary melt instantaneously formed by the reaction is trapped in spinel and produces inclusion minerals. While the pargasite geochemical features can be interpreted by modal change reaction of residual harzburgite, such as combination of orthopyroxene decomposition and olivine precipitation, degree of the LREE enrichment as well as variation of HREE abundance is controlled by melt/rock ratio in the reaction. The spinel-hosted hydrous inclusion could be embedded evidence indicating melt–peridotite reaction even if reaction signatures in the host rock were hidden by other consequent reactions.     

Closed-system geochemical recycling of crustal materials in alpine-type peridotite

We examined aluminous mafic rock (with or without corundum or sapphirine) alternating with peridotite from the Ronda peridotite massif, southern Spain. On the basis of petrographic characteristics, these mafic rocks show a decompression history from high pressure (P > 1.5 GPa), but on the basis of their geochemical characteristics, they are crystal accumulates of plagioclase, clinopyroxene, and olivine formed within the lower crust (P < 1 GPa). A complex evolution history, including higher-pressure recrystallization after initial formation as cumulate gabbros at lower-pressure conditions, is proposed. The aluminous mafic rocks and their peridotite hosts are inferred to be recycled crustal materials now observed as centimeter-scale layered components in alpine-type peridotite. The rocks retained their original cumulate compositions; that is, their compositions were not affected by melting and metasomatic modifications during subduction, intense deformation within the upper mantle, and upwelling to the surface.     

Origin of fine‐grained peridotite xenoliths from Iraya volcano of Batan Island,Philippines: deserpentinization or metasomatism at the wedge mantle beneath an incipient arc?

Abstract Peridotite xenoliths from the subarc mantle, which have been rarely documented, are described from Iraya volcano of the Luzon arc, the Philippines, and are discussed in the context of wedge-mantle processes. They are mainly harzburgite, with subordinate dunite, and show various textures from weakly porphyroclastic (C-type) to extremely fine-grained equigranular (F-type). Textural characteristics indicate a transition from the former to the latter by recrystallization. The F-type peridotite has inclusion-rich fine-grained olivine and radially aggregated orthopyroxene, being quite different in texture from ordinary mantle-derived peridotites previously documented. Despite their strong textural contrast, the two types do not show any systematic difference in modal composition. The harzburgite of C-type has ordinary mantle peridotite mineralogy; olivine is mostly Fo91–92 and chromian spinel mostly has Cr#s (= Cr/[Cr + Al] atomic ratios) from 0.3 to 0.6. Olivine is slightly more Fe-rich (Fo89–91) and spinel is more enriched in Cr (the Cr#, 0.4–0.8) and Fe³⁺ in F-type peridotites than in C-type harzburgite. Orthopyroxene in F-type peridotites is relatively low in CaO (<1 wt%), Al₂O₃ (<2 wt%) and Cr₂O₃ (<0.4 wt%). The F-type peridotite was possibly formed from the C-type one by recrystallization including local dissolution and precipitation of orthopyroxene assisted by fluid (or melt) of subduction origin. Textural characteristics, however, indicate a deserpentinization origin from abyssal serpentinite of which protolith was a C-type peridotite. In this scenario the initial abyssal serpentinite was possibly dehydrated due to an initiation of magmatic activity beneath an incipient oceanic arc like Batan Island. The F-type peridotite is characteristic of the upper mantle of island arc, especially of incipient arc.     

Isotopic composition of dissolved inorganic carbon in subsurface sediments of gas hydrate-bearing mud volcanoes, Lake Baikal: implications for methane and carbonate origin

We report on the isotopic composition of dissolved inorganic carbon (DIC) in pore-water samples recovered by gravity coring from near-bottom sediments at gas hydrate-bearing mud volcanoes/gas flares (Malenky, Peschanka, Peschanka 2, Goloustnoe, and Irkutsk) in the Southern Basin of Lake Baikal. The δ¹³C values of DIC become heavier with increasing subbottom depth, and vary between ?9.5 and +21.4‰ PDB. Enrichment of DIC in ¹³C indicates active methane generation in anaerobic environments near the lake bottom. These data confirm our previous assumption that crystallization of carbonates (siderites) in subsurface sediments is a result of methane generation. Types of methanogenesis (microbial methyl-type fermentation versus CO₂-reduction) were revealed by determining the offset of δ¹³C between dissolved CH₄ and CO₂, and also by using δ¹³C and δD values of dissolved methane present in the pore waters. Results show that both mechanisms are most likely responsible for methane generation at the investigated locations.     

Magma mixing model for the genesis of middle crust in the Izu–Bonin–Mariana arc: evidence from plutonic rocks in the Mineoka-Setogawa ophiolitic mélange,central Japan

ABSTRACT

A Paleogene accretionary complex, the Mineoka–Setogawa belt is distributed adjacent to the northern portion of the collision zone between Honshu and Izu–Bonin–Mariana (IBM) arcs in central Japan, comprising a mélange of ophiolitic fragments of various sizes. The Eocene-Oligocene plutonic rocks in this belt (gabbro, diorite, and tonalite) have been interpreted as fragments brought from the deep crust beneath the IBM arc through tectonic collisions. The geochemical characteristics of the gabbro and associated basaltic dike are similar to those of the Eocene IBM tholeiitic basalt; thus, the gabbro was likely formed via the crystallization of the Eocene tholeiitic basaltic magmas, which was produced by the partial meltings of a depleted mantle wedge. A comparison with experimental results and geochemical modeling indicates that the tonalite was generated by 10–30% dehydration melting of the gabbro. Actually, Eocene–Oligocene felsic veins, which are coeval with the plutonic rocks, occur in the Mineoka–Setogawa gabbro. Plagioclase crystals in the diorite comprise Ca-rich and -poor parts in a single crystal. Their compositional characteristics are consistent with those of plagioclase in the gabbro and tonalite, respectively. The textures and chemical composition of plagioclase indicate that the diorite was formed by the mixing between mafic and silicic magmas. The whole-rock composition of the diorite also indicates the evidence for the mixing between basaltic magmas which were fractionated to variable degrees and homogeneous silicic magma. The mixing model proposed from the first direct observations of the IBM middle crust exposed on the Mineoka–Setogawa belt is applied to the genesis of the Eocene to present intermediate rocks in the IBM arc. If the continental crust were created at intra-oceanic arc settings such as the IBM arc, the magma mixing model would be one of the most likely mechanisms for the genesis of the continental crust.     

Structural changes of synthetic opal by heat treatment

The structural changes of synthetic opal by heat treatment up to 1,400 °C were investigated using scanning electron microscopy, X-ray diffraction, and Fourier transform infrared and Raman spectroscopies. The results indicate that the dehydration and condensation of silanol in opal are very important factors in the structural evolution of heat-treated synthetic opal. Synthetic opal releases water molecules and silanols by heat treatment up to 400 °C, where the dehydration of silanol may lead to the condensation of a new Si–O–Si network comprising a four-membered ring structure of SiO₄ tetrahedra, even at 400 °C. Above 600 °C, water molecules are lost and the opal surface and internal silanol molecules are completely dehydrated by heat effect, and the medium-temperature range structure of opal may begin to thermally reconstruct to six-membered rings of SiO₄ tetrahedra. Above 1,000 °C, the opal structure almost approaches that of silica glass with an average structure of six-membered rings. Above 1,200 °C, the opal changes to low-cristobalite; however, minor evidence of low-tridymite stacking was evident after heat treatment at 1,400 °C.     

A study on the duration and amplitude characteristics of earthquake ground motions

The duration, total power, rms amplitude, and peak ground acceleration (PGA) are examined using the data from the KiK-net digital borehole accelerograph arrays in Japan during three large earthquakes. We investigate correlations between the duration, PGA on the surface and hypocentral distance, instrumental seismic intensity, and the local site conditions. Also considered in this paper, are correlation between the duration and PGA. This present study suggests that PGA is inversely proportional to the duration.Comparing the duration, total power, rms amplitude, and PGA on the surface with those at the bottom of borehole, the duration and maximum amplitude characteristics of earthquake ground motions are investigated with emphasis on site amplification due to the local site conditions. The difference of amplification factors is not only site-dependent but also event-dependent. The comparison of the site amplification factors and the average shear-wave velocity in the upper 30-m showed a good agreement.This paper indicated that the duration and amplitude characteristics suffer a reciprocal effect due to the local site conditions and the site dependency of maximum amplitude PGA is greater than that of the duration. In view of the results, the duration is not less important than the maximum amplitude and frequency content in earthquake engineering. Moreover, this study provided meaningful information of input motion available to earthquake-resistant design.     

One-armed oscillations of a non-selfgravitating polytropic disk

One-armedglobal oscillations in a non-selfgravitating polytropic disk rotating around a star are investigated. The unperturbed disk is axisymmetric, geometrically thin, and extends infinitely in the radial direction keeping its thickness constant. Perturbations considered are inviscid and adiabatic. It is found that there are one-armed retrograde wave modes which are trapped in an inner region of the disk. The eignefrequency of the lowest order mode is given by _K(r _s)(z ₀/r _s)₂, wherer _s is the radius of the central star,z ₀ is the half-thickness of the disk, and _K(r _s) is the Keplerian angular frequency at the surface of the star.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Geological structure of the marginal plateau off Tottori,Southern-West of Honshu,Japan

A geological and geophysical survey off Tottori, southern-west of Honshu, Japan, was carried out by the Hydrographic Department of Japan during May and June (26 days) of 1974.In the survey area, there is marginal plateau which lies under the slope of the continental shelf adjacent to the Sanin district. By the closely-spaced surveying the marginal plateau was divided into two different zones based on differences of genesis and structure, which are identified in this paper as marginal plateaus A (west side) and marginal plateau B (east side). The tectonic boundary between marginal plateaus A and B, previously believed to exist near Tottori City, was not confirmed. An erosional plane, beneath the horizontally bedded upper layer was found to be 75 m lower than the submerged surface of marginal plateau A. Marginal plateau B is hypothesized to have been generated by the sliding of the upper layer. This is suggested by the presence of prominent slumping directed toward the bottom of the Oki Trough. This result implies that the submergence of the Oki Trough area and formation of the marginal plateau are closely related.