IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1D-P2

Date/Time: July 24 Poster

The 1707 AD eruption of Fuji Volcano, Japan: Magmatic Processes and Timescales
Elisabeth Widom, David C Kuentz
Dept. of Geology and Environmental Earth Science, Miami University (Ohio), USA
E-mail: widome@MiamiOH.edu

Fuji Volcano, one of the most hazardous volcanoes in Japan, has erupted primarily basaltic magmas throughout
its history. However, the most recent eruption in 1707 AD produced a chemically zoned deposit with compositions
ranging from basalt to dacite. This eruption was one of the largest (0.7 km3 d.r.e.) and most explosive eruptions
in Fuji’s history, and related tephra deposits a few centimeters in thickness reached as far as Tokyo, approximately
100 km from the vent [1-3]. Prior studies have proposed a variety of processes to explain the chemical variations inthis deposit, including liquid immiscibility [4], magma mixing involving magmas from discrete magma chambers[2], and fractional crystallization with minor crustal assimilation and basaltic magma injection [5]. In order tofurther constrain the timescales of magma evolution leading to the 1707 eruption, we have evaluated a suiteof samples from throughout the 1707 AD deposit for 238U-230Th-226Ra and Ba systematics. Major and traceelement variations, as well as constant Sr, Nd and Pb isotopes throughout the Fuji 1707 AD deposit are consistent
with fractional crystallization of plagioclase, pyroxene, olivine and magnetite, and minor apatite as the dominant
process leading to chemical variations. U-series disequilibria data show that all samples are U-enriched, consistent
with derivation from a mantle wedge that has been fluxed by hydrous slab fluids [6; this study]. Essentially constant
230Th/232Th, but decreasing 238U/232Th with increasing SiO2 and 230Th/238U close to radioactive equilibrium inthe most evolved samples could be explained by assimilation of old, low U/Th crust with a 230Th/232Th ratio similar
to the Fuji basalts. However, Os isotope variations indicate that the most evolved samples have been affectedby less than 0.2 percent crustal assimilation, which should have an insignificant effect on the U-Th disequilibria.
Fractionation of apatite during magma differentiation can alternatively explain the variations in 238U/232Th. The
constant 230Th/232Th requires this fractionation to take place over a timescale of less than 10 ka. However, activity
ratios of 226Ra/230Th greater than 1 in all samples [6; this study] indicate that magma fractionation timescales
may be substantially shorter. Decreasing 226Ra/230Th with increasing SiO2 can be explained by instantaneous orcontinuous fractionation models, and indicate timescales of magma fractionation from parental basalts to dacite on
the order of 103 years. References: [1] Miyaji (1984), Bull. Volcanol. Soc. Japan 29, 17-30. [2] Yoshimoto et al.
(2004), Proc. Japan Acad., Ser. B 80, 103-106. [3] Miyaji et al. (2011), JVGR 207, 113-129. [4] Kawamoto (1992),
Int. Geological Congress Abs. 29, 558. [5] Watanabe et al. (2006), JVGR 206, 1-19. [6] Kurihara et al. (2008),
Radioisotopes 57, 471-483.

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