Slide 6
Slide 6 text
富⼠⼭のマグマだまり
Magma reservoir of Mt. Fuji
富⼠⼭ 伊⾖諸島
富⼠⼭のマグマだまりは20km付近にあり伊⾖諸島の
⽕⼭のマグマだまりより深い。
Crustal magma reservoir of Mt. Fuji is at ~20
km, deeper than that of surrounding volcanoes.
1707年宝永噴⽕は、深部から上昇したマグ
マが浅部の残存マグマと混合して発⽣した。
The 1707 Hoei eruption was triggered
by an interaction between deep mafic
and shallow felsic magmas.
藤井 (2007)
Fujii (2007)
Kaneko et al. (2010)
between K2
O levels in between compositions of melt-inclusions and
the whole rock sample containing their host olivines shown in Fig. 3 is
interpreted to result from such a process.
Alternatively, the andesitic melt-inclusions could simply have
originated from a disequilibrium compositional boundary layer that
grew around olivines rapidly due to abrupt decomposition of hydrous
basaltic magma. However, the decrease in CaO and Al2
O3
content of
the melt-inclusions with increase of SiO2
(Fig. 3) indicates that
plagioclase and/or pyroxenes were crystallized alongside olivine,
denying this possibility.
To evaluate the plotted trends in Fig. 3, petrologic mixing equations
(Wright and Doherty, 1970) were applied to the trend of “FG1053” as a
test case. Based on the calculation, the most SiO2
-rich melt-inclusion can
be successfully derived from the basaltic melt-inclusion assuming
fractionation of olivine: 3.6%, clinopyroxene: 3.8%, plagioclase: 15.2%,
and magnetite: 3.8% (Appendix D). Scattering observed in each melt-
inclusion trend likely results from analytical and corrective errors, as
well as from possible entrapment of inclusions generated in the fol-
lowing mixing process prior to an eruption.
The magmatic process mentioned above is consistent with the
zoning pattern of phenocrystic minerals. In our interpretation, pheno-
crystic olivine is assumed to be contained in the magma in the shallow
magma chamber, which sometimes has andesitic composition. Consis-
tent with this, more than two-thirds of the olivine phenocrysts show
reverse zoning, which is considered to be a consequence of mixing with
the basaltic magma from the deep chamber (Fig. 5, upper right). On the
other hand, many of the plagioclase phenocrysts whose compositions
lie near the more An-rich peak (∼An86
) of the bi-modal pattern show
normal zoning, while those near the less-calcic peak (∼An70
) show
Fig. 6. Proposed model of the magma plumbing system of Fuji. The magma plumbing
system consists of relatively deep (∼20 km) basaltic and shallow SiO2
-rich (often
andesitic) magma chambers (∼8–9 km).