HR: 17:15h
AN: V24A-06 [Abstracts]
TI: Water content of magma generated just above the 410 km seismic discontinuity
AU: * Inoue, T
EM: inoue@sci.ehime-u.ac.jp
AF: Ehime University, Bunkyo-cho 2-5, Matsuyama, 790-8577, Japan
AU: Kojima, K
AF: Ehime University, Bunkyo-cho 2-5, Matsuyama, 790-8577, Japan
AU: Irifune, T
EM: irifune@dpc.ehime-u.ac.jp
AF: Ehime University, Bunkyo-cho 2-5, Matsuyama, 790-8577, Japan
AB:
Seismological observations have revealed the existence of low velocity and high attenuation zones just above the
410 km seismic discontinuity in the Earth's mantle. It has been suggested that the existence of a small amount of
melt could be responsible for such anomalies. The density of silicate melt under dry conditions has been
measured under high pressure and found to be denser than the surrounding mantle, thereby allowing the melt to
exist at this depth. However unless water exists, it is impossible to melt the mantle in the average temperature of
~1400°C at 410 km. Sakamaki et al. (2006) examined the density of hydrous magma, and claimed
that the hydrous magma may be gravitationally stable in the case that the water content of the magma is less than
6.7 ± 0.6 wt% H2O. Nevertheless, the actual water content generated in this condition in the mantle is
still unknown. Therefore we have determined the water content in the hydrous melt generated in the condition of
410 km depth at various temperatures.
Two starting materials, pyrolite-2.9 wt% H2O and 8.3 wt% H2O, were used. The MA-8 type high
pressure apparatus was adopted in the high pressure and high temperature experiments. The experiments were
carried out at pressure of 13.5 GPa and temperatures from 900°C to 1600°C. Micro-Raman
spectroscopy and EPMA were used for identification and chemical analysis of the recovered samples. The
amounts of melt were determined from the cross sections of the recovered samples.
We found that clear melt segregations occurred above 1400°C in the 2.9 wt% H2O system and above
1100°C in the 8.3 wt% H2O system. The amount of melt increased with increasing water content and
temperature, from ~6% at 1400°C to ~44% at 1500°C in 2.9 wt% H2O system, and
from ~30% at 1100°C via ~63% at 1400°C to ~95% at 1600°C.
One of the most important results is that the water content of magma is more than 10 wt% H2O at
1400°C. Therefore, it may be difficult that such hydrous melt is gravitationally stable just above the 410 km
seismic discontinuity.
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1212 Earth's interior: composition and state (7207, 7208, 8105, 8124)
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting