HR: 0800h
AN: V51C-0718    [Abstracts]
TI: A Pb and Hf Isotopic Study for the Unzen Volcano
AU: * Lee, D
EM: dclee@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, 128 Academia Road. Sec. 2, Taipei, 11529, Taiwan
AU: Chen, C
EM: china@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, 128 Academia Road. Sec. 2, Taipei, 11529, Taiwan
AU: Nakada, S
EM: nakada@eri.u-tokyo.ac.jp
AF: Volcano Research Center, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032, Japan
AB: The Unzen volcano is one of the many active volcanoes in Japan. It has been active since at least 0.5 Ma, and the most recent eruption was in December of 1990. The Unzen volcano lies on the Shimabara peninsula of western Kyushu in southwestern Japan, and is within the Unzen graben that forms the northern tip of the Okinawa Trough. Two different models, mixing between an aphyric mafic magma and a felsic magma rich in crystals in the magma chamber, and assimilations of crustal materials during the ascend of a rhyodacitic magma, have been proposed to explain the evolution of the magmas for the Unzen volcano. In this study, Pb and Hf isotopic compositions of the lavas, erupted over the past 4000 years, from the Mt. Fugen of Unzen volcano have been analyzed, together with the existing Sr, Nd, and O isotopic data, in order to study the generation and evolution of the magmas for the Unzen volcano, and also to constrain the assimilation and fractionation crystallization (AFC) model proposed by previous studies. Six samples covering the eruptions from 1993 A.D. to 4000 years ago are included in this study. In general, the Pb isotopic compositions are consistent within analytical errors for all the samples, except for the systematic lower 6/4, 7/4, and 8/4 ratios for the two 1663 A.D. lavas, which also have the lowest Si contents among these samples. Similar variations between the Si content and Sr and Nd isotopic data have also been previously reported. Although the differences are smaller, Hf data also exhibit the similar variations as the Pb data for these lavas. The Pb isotopic data for the Unzen volcano are plotted very close to the MORB field in the Pb-Pb diagrams, except for the slightly elevated 7/4 and 8/4 ratios. Furthermore, the Hf and Nd data of these samples lie within the field of the mantle array in a Hf-Nd isotopic diagram. The coupled variations between Si contents and Sr and Nd isotopic data have been explained with an AFC type model. While the Pb and Hf data obtained in this study are generally consistent with such an AFC scenario, more data, especially the Pb and Hf isotopic data of the local crust in Kyushu and the older erupted lavas from the Unzen volcano are necessary in order to better constraint the generation and evolution in the magma chambers of the Unzen volcano.
DE: 1040 Radiogenic isotope geochemistry
DE: 3618 Magma chamber processes (1036)
DE: 3619 Magma genesis and partial melting (1037)
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting