Yoji Arakawa

Anorthite megacrysts from island arc basalts

Abstract Anorthite megacrysts are common in basalts from the Japanese Island Arc, and signally rare in other global fields. These anorthites are 1 to 3 cm in size and often contain several corroded Mg-olivine inclusions. The megacrysts generally range from An94Ab4Ot2 to An89Ab6Ot5 (Ot: other minor end-members, including CaFeSi3O8, CaMgSi3O8, AlAl3SiO8, □Si4O8) and show no chemical zoning. They often show parting. Redclouded megacrysts contain microcrystals of native copper with a distribution reminiscent of the shape of a planetary nebula. Hydrocarbons are also present, both in the anorthite megacrysts and in the olivines included within them. Implications of lateral variations in the Fe/Mg ratio of the included olivines, in Sr-content and in Sr-isotope ratio of the anorthite megacrysts with respect to the Japanese island arc, relate to mixing of crustal components and subducted slab-sediments into the basaltic magmas.

NdSr isotopic and geochemical characteristics of two contrasting types of calc-alkaline plutons in the Hida belt, Japan

Abstract Sr and Nd isotopic and chemical compositions have been determined for Mesozoic plutons in the Hida belt (Japan), which occupied a marginal part of the Sino-Korean Craton before the Miocene opening of the Japan Sea. Associated magmatic activity produced calc-alkaline plutons with a wide spectrum of lithology, chemistry and internal structure. Two types of plutons (Types 1 and 2 ) are classified on the basis of their chemical and structural characteristics. Type- 1 plutons are characterized by a limited variation in initial Sr and Nd isotope values ( 87 Sr 86 Sr = 0.7044−0.7054 ; ϵNd = −0.8 to + 5.5), irrespective of the rock compositions, whereas Type-2 plutons show a wide range of isotope values ( 87 Sr 86 Sr = 0.7055−0.7105 ; ϵNd = −10.3 to + 0.7) which form a linear data array connecting the most primitive gabbro and basement rocks in an ϵ Nd − 87 Sr 86 Sr diagram. The distinction in isotope systematics between the two types of plutons is due to the degreeof interaction between the middle Proterozoic crust and parental mafic to intermediate magmas. Isotopic and elemental signatures of Type- 1 plutons point to a magmatic differentiation by fractional crystallization as the main evolutional process. In contrast, Type- 2 plutons require a mixing process between parental mafic magma and felsic magma derived from middle Proterozoic basement crust. The lowest Sr and highest Nd isotope compositions of the mafic rock units in each pluton are believed to represent the isotopic compositions of the parental magmas, and possibly those of their source regions (subcontinental mantle and/or mafic lower crust). The two contrasting types of plutons reflect the across-arc difference in crust-mantle structures and the change of tectonic framework around the Hida belt during Mesozoic time.

Sr–Nd isotopic and chemical characteristics of the silicic magma reservoir of the Aira pyroclastic eruption, southern Kyushu, Japan

Abstract Sr and Nd isotope and geochemical investigations were performed on a remarkably homogeneous, high-silica rhyolite magma reservoir of the Aira pyroclastic eruption (22,000 years ago), southern Kyushu, Japan. The Aira caldera was formed by this eruption with four flow units (Osumi pumice fall, Tsumaya pryoclastic flow, Kamewarizaka breccia and Ito pyroclastic flow). Quite narrow chemical compositions (e.g., 74.0–76.5 wt% of SiO2) and Sr and Nd isotopic values ( 87 Sr / 86 Sr =0.70584–0.70599 and eNd=−5.62 to −4.10) were detected for silicic pumices from the four units, with the exception of minor amounts of dark pumices in the units. The high Sr isotope ratios (0.7065–0.7076) for the dark pumices clearly suggest a different origin from the silicic pumices. Andesite to basalt lavas in pre-caldera (0.37–0.93 Ma) and post-caldera (historical) eruptions show lower 87 Sr / 86 Sr (0.70465–0.70540) and higher eNd (−1.03 to +0.96) values than those of the Aira silicic and dark pumices. Both andesites of pre- and post-caldera stages are very similar in major- and trace-element characteristics and isotope ratios, suggesting that the both andesites had a same source and experienced the same process of magma generation (magma mixing between basaltic and dacitic magmas). Elemental and isotopic signatures deny direct genetic relationships between the Aira pumices and pre- and post-caldera lavas. Relatively upper levels of crust (middle–upper crust) are assumed to have been involved for magma generation for the Aira silicic and dark pumices. The Aira silicic magma was derived by partial melting of a separate crust which had homogeneous chemistry and limited isotope compositions, while the magma for the Aira dark pumice was generated by AFC mixing process between the basement sedimentary rocks and basaltic parental magma, or by partial melting of crustal materials which underlay the basement sediments. The silicic magma did not occupy an upper part of a large magma body with strong compositional zonation, but formed an independent magma body within the crust. The input and mixing of the magma for dark pumices to the base of the Aira silicic magma reservoir might trigger the eruptions in the upper part of the magma body and could produce a slight Sr isotope gradient in the reservoir. An extremely high thermal structure within the crust, which was caused by the uprise and accumulation of the basaltic magma, is presumed to have formed the large volume of silicic magma of the Aira stage.

Strontium isotopic characteristics of arc volcanic rocks at the initial stage of subduction in western Japan

Abstract In western Japan, the Philippine Sea plate subducts northwestward from the Ryukyu Trench and the Nankai Trough, to form two volcanic arcs, the Ryukyu and Southwest Japan arcs, respectively. These arcs are thought to be in the initial stage of subduction, because microearthquake data show that the subducting slab extends only a short distance from the trenches. In the Southwest Japan arc, volcanoes are aligned parallel to the Nankai Trough, although the leading edge of the subducted slab does not reach under the volcanic arc. To the south, the subducted slab reaches just under the Ryukyu volcanic arc. We determined 87Sr/86Sr ratios and Rb and Sr contents for 39 volcanic rock samples from 14 Quaternary volcanoes of both arcs and summarized the spatial distribution of the ratios. The ranges of 87Sr/86Sr ratios of volcanic rocks from the Ryukyu and Southwest Japan arcs are 0.7040–0.7069 and 0.7035–0.7066, respectively. In the Ryukyu arc, the regional variation in 87Sr/86Sr ratios of volcanic rocks is hidden under the wide scatter of ratios of different volcanic rocks from each one volcano. In the Southwest Japan arc, the ratios of volcanic rocks from volcanoes on the volcanic front vary in the along-arc direction; 0.7040–0.7048 for the western part, 0.7035–0.7039 for the central part and 0.7048–0.7066 for the eastern part. The across-arc variations in the ratios are different from place to place. These isotopic features suggest the absence of slab-derived components in the source mantle and reflect the heterogeneous nature of 87Sr/86Sr ratios in the upper mantle, modified by crustal processes. This coincides with the seismological observation that the subducting Philippine Sea slab does not reach under the volcanic arc at all.

Two types of granitic intrusions in the Hida belt, Japan: Sr isotopic and chemical characteristics of the Mesozoic Funatsu granitic rocks

Abstract Initial 87 Sr 86 Sr ratios, major- and trace-element compositions have been determined for the Late Triassic to Early Jurassic granitic rocks (Funatsu granitic rocks) in the Hida belt, Japan. Six intrusions, which are exposed in a transverse section of the belt, were selected for analyses. Based on the characteristics of initial 87 Sr 86 Sr ratios (SrI-values) and element concentrations, the above intrusions are classified into two types. One is the intrusion with nearly constant SrI-values over the variation of rock compositions. The Utsubo (SrI=0.7053) and Hirose (0.7044) intrusions in this study, and the already reported Funatsu and Shimonomoto intrusions (both 0.7048) belong to this group, and rocks in this group are interpreted to have been formed mainly by fractional crystallization from mafic magma that originated in the lower crust or upper mantle. In contrast, the rocks in the other four intrusions (Hodatsusan, Shokawa, Yatsuo and Nagarehayama) show relatively higher and variable SrI-values (0.7047–0.7105). Two-stage models, crustal mixing by the parental mafic magma at mid-crustal levels and subsequent differentiation by fractional crystallization, are interpreted to have been the main mechanisms for producing granitic rocks in these intrusions. Two contrasting types of Sr isotopic and chemical compositions recognized in the intrusions of the Funatsu granitic rocks, which are attributed to a difference of the predominant mechanism (crustal mixing or differentiation), correlate well with two geological features observed in the Hida belt. One is the existence or absence of shear zones which contemporaneously formed with granitic emplacement, and the other is the pattern of chemical zonation of the intrusion. Intrusions with a wide range of high SrI-values are accompanied by a dextral shear zone in one margin and show a reverse- (less normal-) type chemical zonation. On the other hand, intrusions with constant SrI-values do not relate with shear deformation and have normal chemical zonation. Considering these correlations, it is concluded that the stress state in middle-to upper-crustal levels is one of the important factors controlling the mechanisms of magma formation and way of emplacement.

Strontium isotopic and chemical variations of the granitic rocks in the Tsukuba district, Japan

Initial 87Sr/86Sr ratios, major and trace element compositions have been determined for the Paleogene granitic rocks in the Tsukuba district, Japan. Isotopic ages strongly suggest that the granitic rocks (seven units) were continuously emplaced and solidified during a short time interval. Initial 87Sr/86Sr ratios for seven granitic units vary from 0.7082 to 0.7132, while sedimentary and metasedimentary country rocks have ratios at the time of granitic magma emplacement ranging from 0.7149 to 0.7298. Continuous linear arrays for the granitic rocks in the diagrams of initial 87Sr/86Sr ratios versus some chemical parameters can be explained by either of following two processes. One is the assimilation — fractional crystallization (AFC) process between the parental magma (SiO2 of 68% and initial ratio of 0.7078) and sedimentary country rocks, and the other is magma mixing process between above parental magma and sediment derived acidic magma (melt) (SiO2 of 75%). The high initial ratios (0.7078–0.7098) for basic rocks such as gabbro or diorite in the Tsukuba district and the similar characteristics observed in the rocks of Ryoke belt (SW Japan) suggest that the parental magma had the same source region as the basic rocks, probably the lower crustal source.

Strontium isotopic compositions of Mesozoic granitic rocks in the Hida belt, central Japan: diversities of magma sources and of processes of magma evolution in a continental margin area

Abstract The Sr isotopic compositions of Late Triassic to Early Jurassic Funatsu granitic rocks in the Hida belt, Japan, were determined and variations of the compositions within single intrusions and on a regional scale were compared with previously reported data. Relatively low and constant (or narrow range of) initial 87 Sr 86 Sr ratios of granitic rocks within an intrusion (0.7044-0.7055) are found mostly in the outer part of the belt, while intrusions with high and wide ranges of initial ratios (0.7056-0.7105) are situated in the inner part. This difference in initial ratios within an intrusion is due to the different degrees of mixing between the parental mafic magma from the lower crust or upper mantle and the middle to upper crustal (or crust-derived) materials. On a regional scale, a smooth and regular increase of the ratios from 0.7044 to 0.7057, from outer to inner part, is outlined by the lowest ratio in each intrusion and this almost coincides with a trend given by the ratios of mafic rocks (or mafic enclaves) in the intrusion. This suggests a gradual change of source materials in the lower crust or upper mantle. The degree of crustal contributions to the parental magma, lesser in the outer part and larger in the inner part of the Hida belt, shows close relationships to some geophysical factors, such as the emplacement depth and uplift rate of mafic magma from deeper levels and stress state (extensional or compressional) in the middle to upper crustal levels. These factors are probably due to the plate tectonic configuration in the continental margin area where the Hida belt was included.

Absence of Elovl6 attenuates steatohepatitis but promotes gallstone formation in a lithogenic diet-fed Ldlr(-/-) mouse model.

Nonalcoholic steatohepatitis (NASH) is a progressive form of nonalcoholic fatty liver disease (NAFLD) that can develop into liver cirrhosis and cancer. Elongation of very long chain fatty acids (ELOVL) family member 6 (Elovl6) is a microsomal enzyme that regulates the elongation of C12–16 saturated and monounsaturated fatty acids (FAs). We have previously shown that Elovl6 plays an important role in the development of hepatic insulin resistance and NASH by modifying FA composition. Recent studies have linked altered hepatic cholesterol homeostasis and cholesterol accumulation to the pathogenesis of NASH. In the present study, we further investigated the role of Elovl6 in the progression of lithogenic diet (LD)-induced steatohepatitis. We showed that the absence of Elovl6 suppresses hepatic lipid accumulation, plasma total cholesterol and total bile acid (BA) levels in LDL receptor-deficient (Ldlr−/−) mice challenged with a LD. The absence of Elovl6 also decreases hepatic inflammation, oxidative stress and liver injury, but increases the formation of cholesterol crystals in the less dilated gallbladder. These findings suggest that Elovl6-mediated changes in hepatic FA composition, especially oleic acid (C18:1n-9), control handling of hepatic cholesterol and BA, which protects against hepatotoxicity and steatohepatitis, but promotes gallstone formation in LD-fed Ldlr−/− mice.

Native Zinc, Copper, and Brass in the Red-Clouded Anorthite Megacryst as Probes of the Arc-Magmatic Process

This review of red-clouded feldspars [1] addresses the fundamental factors that underlie our understanding of a red schiller caused by reflection from submicroscopic inclusions of hematite, Fe203. Phenocrysts of aventurine labradorites [2, 3] contain microscopic metallic coppers causing a pink schiller. A recent finding from native coppers included by red-clouded anorthites [4] has stressed the importance of broadening this traditional interpretation. Furthermore, detection of hydrocarbons within the red-anorthite megacrysts provides mineralogists a clue as to their formation mechanism [5]. Microinclusions scattered throughout red-clouded anorthite megacrysts in the olivinebasaltic lava from the island of Hachijojima, Japan, were identified as native zinc, copper, and brass by comparison with the peak shift and band shape of Xray ZnLa and CuLc~ spectra of the standard samples measured by EPMA. This contribution reports the characterization of micron-sized native copper, zinc, and brass responsible for red aventurization within anorthite megacrysts, their petrogenetic and metallogenetic significance to the arc-magmatic process, and availability of chemical shift analyses of ZnLc~ and CuLa spectra by EPMA for micro-mineral identification.

Two types of gabbroic xenoliths from rhyolite dominated Niijima volcano, northern part of Izu-Bonin arc: petrological and geochemical constraints

Abstract We examined the petrography, petrology, and geochemistry of two types of gabbroic xenoliths (A- and B-type xenoliths) in olivine basalt and biotite rhyolite units among the dominantly rhyolitic rocks in Niijima volcano, northern Izu-Bonin volcanic arc, central Japan. A-type gabbroic xenoliths consisting of plagioclase, clinopyroxene, and orthopyroxene with an adcumulate texture were found in both olivine basalt and biotite rhyolite units, and B-type gabbroic xenoliths consisting of plagioclase and amphibole with an orthocumulate texture were found only in biotite rhyolite units. Geothermal- and barometricmodelling based on mineral chemistry indicated that the A-type gabbro formed at higher temperatures (899–955°C) and pressures (3.6–5.9 kbar) than the B-type gabbro (687–824°C and 0.8–3.6 kbar). These findings and whole-rock chemistry suggest different parental magmas for the two types of gabbro. The A-type gabbro was likely formed from basaltic magma, whereas the B-type gabbro was likely formed from an intermediate (andesitic) magma. The gabbroic xenoliths in erupted products at Niijima volcano indicate the presence of mafic to intermediate cumulate bodies of different origins at relatively shallower levels beneath the dominantly rhyolitic volcano.