Kenji Notsu

Strontium isotope composition in volcanic rocks from the northeast Japan arc

Abstract In the Northeast Japan arc, a number of Quaternary volcanoes form a long, narrow belt, parallel to the Japan Trench. 87 Sr/ 86 Sr ratios were determined in 52 specimens of volcanic rocks from 27 volcanoes in the Northeast Japan arc area. The results reveal that the ratios change systematically in space. Decreasing 87 Sr/ 86 Sr ratios across the arc were confirmed over a wide area of Northeast Japan. In the same direction, increases in both Rb and Sr contents were also found. The regular trends are considered to be a strong constraint for elucidation of subduction-originated magma genesis at the Eurasia plate vs. Pacific plate boundary. In the northern region of the Northeast Japan arc, 87 Sr/ 86 Sr ratios in volcanic rocks along the volcanic front were almost constant (0.7038–0.7045) and slightly higher than those from the Izu-Ogasawara arc (0.7032–0.7038). This suggests that “interactions” between the Eurasia plate and the Pacific plate, and those between the Philippine Sea plate and the Pacific plate are slightly different. The southern region of the Northeast Japan arc, where the direction of the volcanic front bends from southward to westward, showed anomalously high 87 Sr/ 86 Sr ratios, reaching to 0.7077. This region coincides with the triple junction of the Eurasia, Pacific and Philippine Sea plates, suggesting “anomalous interaction” at the triple junction.

Hydrogen Release: New Indicator of Fault Activity

The hydrogen concentration in soil gas has been measured in the area around the Yamasaki Fault, one of the active faults in southwestern Japan. Degassing of a significant amount of hydrogen (up to more than 3 percent by volume) has been observed for sites along the fault zone. The hydrogen concentration in soil gas at sites away from the fault zone was about 0.5 part per million, almost the same as that found in the atmosphere. The spatial distribution of sites with high hydrogen concentrations is quite systematic. A hypothesis on the production of hydrogen by fault movements is postulated.

High temperature heating of the Allende meteorite

Abstract Allende bulk samples were heated up to 2000°C by DC arc and resistance heating. The chemical composition of evaporation residues was measured by instrumental neutron activation analysis (INAA) and electron probe microanalyser (EPMA). The evaporation sequence of elements observed was: (Na)-(Fe, Ni, Co, Au, Ir)—(Fe, Mn, Cr) (Mg, Si)-(Al, Ca, Ti, Sc, REE). Chemical compositions of 65% and 4% residual fractions were in agreement with those of a chondrule and a Ca, Al-rich inclusion of the Allende meteorite, respectively. Iridium was lost from residual solid in the early stage of heating. The early loss of the refractory siderophile elements may provide a clue to distinguish between two possibilities for the origin of Allende Ca, Al-rich inclusions: early condensates vs. evaporation residue. Ca, Al-rich inclusions (CAI) in the Allende meteorite may provide information on the physical and chemical conditions prevailing in the early solar system. So far we have two possibilities for the origin of CAI: 1. (1) High temperature condensates from a “hot” solar nebula (GROSSMAN, 1972, 1973; WANKE et al., 1974); 2. (2) Evaporation residues of the pre-existing primordial dust in the early solar system (KURAT, 1970; TANAKA and MASUDA, 1973; CHOU et al., 1976; CLAYTON, 1977). A condensation origin has been supported by theoretical considerations of equilibrium condensation from a cooling nebula, while an evaporation origin has received no endorsement from either theoretical or experimental approaches. We have heated bulk Allende samples to determine whether or not the chemical composition of CAI can be derived in the course of fractional vaporization of pre-solar matter which, in this experiment, is approximated by the bulk Allende composition.

Geochemical implications from Sr isotopes and K-Ar age determinations for the Cook-Austral islands chain

Abstract Sr isotopes and K-Ar ages were determined for volcanic rocks from three islands, Samoa, Rarotonga and Rurutu in the Austral-Gilbert-Marshall chain. We have established that Rurutu originated from the same “hot spot” as Tubuai and Raivavae in the Cook-Austral chain as indicated by its 11 m.y. K-Ar age and ( 87 Sr 86 Sr ) ratios. The ( 87 Sr 86 Sr ) ratios for Rurutu, Tubuai and Raivavae lie in the restricted range 0.7026–0.7035. K-Ar ages of all volcanic rocks from Samoa and Rarotonga are all less than 4 m.y. and their ( 87 Sr 86 Sr ) ratios are much higher than those in Rurutu, Tubuai and Raivavae. These findings suggest that Samoa and Rarotonga did not originate in a single hot spot for the Cook-Austral islands.

Strontium isotopic relations of bimodal volcanic rocks at Kikai volcano in the Ryūkyū arc, Japan

Bimodal associations of mafic and silicic rocks are observed at Kikai volcano in the Ryūkyū arc, Japan. We determined 87Sr/86Sr ratios of 16 representative volcanic rocks of this volcano. The ratios scatter in the narrow range between 0.70477 and 0.70508, despite the varying SiO2 content of the rocks, except for one sample with a ratio of 0.70539. This suggests that both basaltic and rhyolitic rocks at Kikai volcano are generated from the same source material, in contrast to the separate origin required for the bimodal volcanism typically observed in the western United States. The origin of the bimodal volcanism in the island-arc setting is distinctly different from that in the continental setting, in view of source materials of basaltic and rhyolitic magmas.

Two overlapping plates subducting beneath central Japan as revealed by strontium isotope data

Abstract In central Japan, the Pacific plate subducts westward beneath the Eurasian plate and the Philippine Sea plate subducts northwestward into the mantle wedge between the Eurasian plate and the subducted Pacific slab. There, the Northeast Japan arc is joined to the Izu-Ogasawara arc. We determined 87 Sr/ 86 Sr ratios and Rb and Sr contents for 47 volcanic rock samples from 15 Quaternary volcanoes in central Japan and summarized the geographical distribution of the ratios. The general trend of slowly increasing 87 Sr/ 86 Sr ratio from the back-arc side toward the volcanic front in the Northeast Japan arc is broken by a marked high ratio (above 0.7060) centered around Akagi volcano located at the southernmost region of the arc. Elsewhere, the ratio along the volcanic front in this arc varies within the range 0.7038 to 0.7045. The marked high 87 Sr/ 86 Sr ratio is considered to be due to the addition of slab-derived components transported by the Philippine Sea plate to the magma-generating region in the mantle wedge beneath central Japan. Therefore, the geographical distribution of the high ratio may correspond to that of the Philippine Sea slab-derived components in the mantle wedge and we may draw the underground outline of the Philippine Sea plate. This outline implies that an aseismic portion of the Philippine Sea plate continues a few tens km ahead of the seismic one. A belt of low 87 Sr/ 86 Sr ratios from the Izu Peninsula northwestward along the northern end of the Izu-Ogasawara arc coincides with the zone where the subducting Philippine Sea plate is not observed seismologically, while it is detected seismologically on both sides of the belt.