Photon Factory Activity Report 2005 #23 Part B (2006)
High Pressure Science

NE5C/2004G047

Structural study on hydrous ultrabasic magma in MgO-SiO2-H2O with pressures
Akihiro YAMADA*1, Toru INOUE1, Takanori KAWAMURA1, Isamu YOSHIMI1, Takumi
KIKEGAWA2
1
Geodynamics Research Center, Matsuyama, Ehime 790-8577, Japan
2
High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan
Introduction
We have found that the liquidus phases drasticallychange to more silica-rich composition with increasingpressure under hydrous condition. As the result, thecomposition of liquid generated in the deep mantle isenriched in MgO component (e.g. [1]). This implies thestructural changes of the hydrous melts and the changesof mechanism on water solution in magma, because
liquidus phases and melting relations of silicates arestrongly affected by melt structures. However, direct
observational studies on hydrous silicate melts under highpressure have not been conducted so far because ofexperimental difficulties. Hydrous silicate glasses
quenched from hydrous melts at high pressure have beenused for experimental constrains (e.g. [2]). In order toperform direct measurement of hydrous magma underhigh pressure, we had developed the new capsule system
for the in-situ X-ray experiment using synchrotron X-ray
radiation for structural study on hydrous magma underhigh-pressure and temperature conditions [3].
In the present report, we introduce the structuralchange on the hydrous melt in Mg2SiO4-MgSiO3-H2O
determined by using the new capsule system, whichcorresponds to eutectic composition of major mineral in
the Earth’s mantle.
Experimental
Starting materials were prepared by mixing Mg(OH)2and SiO2, in order to be the compositions of Mg2SiO4MgSiO3-H2O (Fo-En-W), which is 18.3 wt% H2O baringsystem. The experiments were conducted using MAX80
cubic type press installed at AR-NE5C beamline. Pressuretransmitting medium were made of Boron + epoxy resinor pyrophyllite with the shape of 9- and 7-mm cubes.
Sample chamber was put in the center of pressuretransmitting medium and thermocouple were not used inorder to get high quality diffraction data. Instead of nouse of thermocouple in diffraction experiments forhydrous melts, we carried out temperature calibration
experiment using the same cell assemblage.
Diffraction angles were fixed at several arbitrary angles
within the range form 4° to 25° to obtain wide scatteringvector Q range (Q= 4πEsinθ/12.398, where E is energy).
Effective X-ray source intensity in the AR-NE5C bending
magnet beamline was determined by Monte Carlo method
developed by [4] from each diffraction patterns.
The hydrous magmas were enclosed in single
crystalline diamond sleeve whose ends were sealed by Pt

lids under high P-T condition. The details were describedin [3].
Results
The striking features in the structure factors, S(Q)s,were found in the FSDP (First Sharp Diffraction Peak)
which may be related to SiO4 tetrahedral network insilicate melts(e.g. [5]). New peak at around 3 Å-1
appeared and became intense with increasing pressure.
The position of FSDP shifted to higher Q position from
2.16 to 2.27 Å-1 at pressures between 1.92 and 6.5 GPa.
Those positions of FSDP were very high Q but thesignificant pressure dependence on the shift of theposition was not observed comparing with anhydrous
Mg-silicate melt reported by [6]. These observations
indicate that nearly all network of the melt seems to be
depolymerized by the H2O and MgO component.
We can derive real space information of local structurein the melt from the Radial Distribution Function (RDF)
by Fourier transform of S(Q)s. Almost no change in firstpeaks in the RDFs was observed (i.e. 1.63 Å). This result
indicates that SiO4 tetrahedron exists mainly as the local
structural units of present melt at least to 6.5 GPa. On theother hand, the distance of Si-Si pair shifted significantly
from 3.14 Å to 2.91 Å at pressures between 1.92 and 3.06
GPa. However, at higher pressures between 3.06 and 6.5
GPa (across 5 GPa), the peak did not change so much,which was approximately 2.91 Å. That may be anevidence for the change of mode that H2O dissolve tosilicate melt. Indeed the liquidus phase in hydrous magma
was changed at around 5 GPa reported by [1].
Our analyses suggest that, in the low-pressure region
where silica-rich magma is ganerated, H2O may dissolvewith dividing Si-O-Si network in magma (i.e. Si-OH).
Oppositely, in the region where MgO-rich magma isgenerated, water may prefer to dissolve with divalentcation to form Mg-OH, which was suggested by [2].
References[1] T. Inoue, Phys. Earth Planet. Inter. 85, 237-263 (1994)[2] X. Xue et al., Geochim. Cosmochim. Acta., 68, 5027-5057(2004)[3] T. Inoue et al., PF Activity Report 2004, 22, 206, (2005)[4] K. Funakoshi, PhD Thesis, Tokyo Inst. Technol., (1997)[5] S. Susman et al., Phys. Chem. Glasses., 31, 144-150 (1990)[6] N. Funamori et al., Geophys. Res. Lett. 109, B03203 (2004)*a-yamada@sci.ehime-u.ac.jp

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