The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 122, Issue 3
Displaying 1-4 of 4 articles from this issue
Articles
  • Case study for the Sakurajima-Bunmei tephra in the Miyazaki Plain
    Masafumi Ikuta, Masakazu Niwa, Tohru Danhara, Tohru Yamashita, Seiji M ...
    2016 Volume 122 Issue 3 Pages 89-107
    Published: March 15, 2016
    Released on J-STAGE: June 21, 2016
    JOURNAL FREE ACCESS
    Supplementary material
    Refractive index measurements of volcanic glass shards with known hydration characteristics enable the discrimination of pumice samples from multiple historic eruption events of a single volcano. We analyzed mineral compositions, undertook a morphological classification of volcanic glass shards, and measured the refractive index of glass shards and orthopyroxene from products of the Bunmei, Anei, and Taisho eruptions of Sakurajima volcano. The refractive indices of hydrated and non-hydrated parts within each individual glass shard were measured separately. Concentrations of 58 elements in the glass shards were determined using a femtosecond laser ablation-inductively coupled plasma-mass spectrometry technique, to compare elemental concentrations of the sampled glass shards. The analyses successfully discriminated the Bunmei, An-ei, and Taisho eruption products.
    Pumice layers in core samples obtained from the southern part of the Miyazaki Plain were identified as the Sakurajima-Bunmei tephra, based on the analyses described above and radiocarbon age data for coaly materials in the pumice layers. The distribution of the pumice layer suggests that the Sakurajima-Bunmei tephra reached the southern part of the Miyazaki Plain as a pumice fall deposit.
    Download PDF (8493K)
  • Takahiro Yamamoto
    2016 Volume 122 Issue 3 Pages 109-126
    Published: March 15, 2016
    Released on J-STAGE: June 21, 2016
    JOURNAL FREE ACCESS
    Supplementary material
    Akagi Volcano, a large composite volcano located on the volcanic front of NE Japan arc, consists of older and younger ones. The latter is subdivided into the volcanic-cone-building stage (ca. 220-150 ka) erupting voluminous andesite lava flows, the pumice-eruption stage (ca. 150-44 ka) ejecting many andesite-dacite pumice falls and pumice flows, and the post-caldera stage (44-30 ka) effusing rhyolite lava domes. Geochemical correlations between pumice flow and pumice fall deposits have improved our understanding of the eruptive history of the pumice eruption stage, using a new magma-discharge time-step diagram. After the effusive volcanic-cone-building stage, the magma-discharge rate had temporally declined until the vigorous pumice-eruption sub-stage (ca. 60-44 ka). Geochemical features of trace element abundance suggest that the products of Akagi Volcano were generated by interactions between mantle-derived magma and lower-crustal materials, along with amphibole fractionation. The contributions of the crustal melt to the generation of felsic magma was grater during the pumice-eruption stage than during the volcanic-cone-building stage. Increases in the magma-discharge rate appear to have corresponded to the injection of voluminous mantle-derived magma into the lower crust.
    Download PDF (9034K)
Report
  • Sotaro Baba, Issei Uchima
    2016 Volume 122 Issue 3 Pages 127-132
    Published: March 15, 2016
    Released on J-STAGE: June 21, 2016
    JOURNAL FREE ACCESS
    Garnet-bearing Shimanto pelitic schists occur in the Nago area of north-central Okinawa Island, Japan. The schists consist mainly of phengite, chlorite, quartz, albite, and epidote. They show narrow compositional banding, with bands composed of phengite + chlorite-rich and quartz-rich layers. The banded layers are deformed into isoclinal folds. The garnet occurs as euhedral grains with adjacent pressure shadows of mica and chlorite. The peak pressure-temperature conditions of metamorphism obtained from empirical garnet + muscovite + plagioclase thermobarometry are 4.1-4.6 kbar and 516-568°C, respectively. We propose that these conditions be used as reference values, because of the low almandine content of garnet. This is the first report of garnet-bearing pelitic schist from Okinawa Island.
    Download PDF (5378K)
Pictorial
feedback
Top