Thirteen dike swarms in Northeast Japan were dated by the K-Ar method. The K-Ar ages in this study give some crucial constraints to the volcanostratigraphy and imply that the geological situation of some dike swarms estimated in the other studies should be reexaminned while other dike swarms showed the K-Ar ages concordant with the geological ages. Available K-Ar ages of the volcanic rocks from the present and other studies indicate that the volcanic front in Northeast Japan had a temporal position about 22–23 Ma ago to the east of that in the Quaternary and that the volcanism appears to have been inactive from 17 to 21 Ma ago.

View full abstract

Pegmatoid bodies predominantly consisting of calcite and apatite occur associated with metasediments within Precambrian high-grade metamorphic basement rocks in the Fazenda das Panelas area, central Bahia State, Brazil. Petrology, trace elements, stable and radiogenic isotope chemistry of the carbonate-phosphate intrusives have been studied in order to clarify their genesis. K-Ar data give approximately 2.0b.y. that is likely to coincide with the metamorphic recrystallization of the basement rocks which occurred during the Transamazonic tectonic event. Rb-Sr data give somewhat older ages. C and O isotope data on the intrusive carbonates are distributed within the values of marine carbonates of Precambrian ages. Trace element chemistry and REE patterns in mineral phases (calcite, apatite, micas) give intermediate figures between sedimentary marine and igneous (carbonatitic) materials. The pegmatoids are supposed to have derived from the crystallization of carbonate-phosphate liquids formed by partial melting of phosphate-rich layers contained in the sedimentary sequence. The enrichment in REE and in large charged elements is explained by interaction of the phosphate-carbonate liquid with fluids of metamorphic origin. A possible paleoenvironmental characterization of the source phosphate rocks is presented.

View full abstract

In the Izu-Ogasawara arc, many Quaternary volcanoes are located in a line parallel to the Izu-Ogasawara Trench. ⁸⁷Sr/⁸⁶Sr ratios were determined for 65 samples of Quaternary volcanic rocks, 4 cognate inclusions and one sample of basement rock from 15 volcanic islands and 3 submarine volcanoes. In this arc, the ⁸⁷Sr/⁸⁶Sr ratios in several rocks from the same volcano agree with one another within 0.0002, regardless of the difference of the rock types and the eruptive stages. The ratios of the volcanoes along the volcanic front in the Izu-Ogasawara arc were constant over 1, 200 km and were all within the range between 0.7034 and 0.7039, which is significantly lower than those from the Northeast Japan arc (0.7038–0.7044) (NOTSU, 1983). In the northern region of the Izu-Ogasawara arc, across-arc ⁸⁷Sr/⁸⁶Sr ratio variation was found. The ratios decrease with increasing depth of the Wadati-Benioff zone. Even in the volcanic rocks distant from the volcanic front, the ratios (0.7033–0.7035) were slightly higher than those in normal-type MORB, so it is suggested that a small amount of oceanic sediment, altered basalt or sea water component that accompanies the subducting slab has been contributed to the source magma which originates from a partially melted, upper mantle wedge. This contribution becomes smaller with increasing distance from the volcanic front.

View full abstract

The distribution of ¹³⁷Cs in volcanic rocks was measured for sliced samples of solid lava blocks from Izu-Oshima island, Japan. The decrease in ¹³⁷Cs concentration with the increase in depth, together with the uneven spatial distribution, suggested that the fallout was responsible for the occurrence of ¹³⁷Cs and that ¹³⁷Cs might have penetrated into lavas as deep as several centimeters.

View full abstract

Depth profiles of ¹⁰Be and ⁹Be concentrations have been measured in five manganese nodules from widely varying locations of the Pacific Ocean. Mean accumulation rate of these manganese nodules ranges from 2.7 to 8.0 mm·m.y.^-1. The variation of ¹⁰Be and ¹⁰Be/⁹Be with depth or time is found to be less than ±30% for periods from 0.4–1.0 m.y. back to 6 m.y. B.P. Surface ¹⁰Be/⁹Be atomic ratio is found to be (9–13) × 10^-8, that shows authigenic nature of manganese nodule acuumulation by comparison with (12–31) × 10^-8 for deep-sea water, (2–7) × 10^-8 for clay sediments and (7–9) × 10^-8 for fossil-rich sediments.

View full abstract