Photon Factory Activity Report 2004 #22 Part B (2005)
High Pressure Science

6A, 18B/2004G047

In situ X-ray experiment on hydrous Mg silicate melts under high-P-T conditions
using single crystalline diamond capsule
Toru INOUE1*, Akihiro YAMADA1, Satoru URAKAWA2, Nobumasa
FUNAMORI3, Yuji HIGO1,
1
Takehiro KUNIMOTO
1
Geodynamics ResearchCenter,Ehime Univ., Matsuyama 790-8577, Ehime, Japan
2
Okayama
Univ.,Okayama 700-8530, Japan3
Univ. Tokyo, Tokyo 113-0033, Japan
Introduction
Recent studies on melting of mantle minerals haverevealed that the liquidus phases change from olivine topyroxene, and further stishovite with increasing pressure
under hydrous condition, and the compositions of theliquid generated in the deep mantle become enriched in
MgO component (e.g. Inoue, 1994[1], Yamada et al.,2004[2]). This phenomenon implies the structural changesof the hydrous silicate melts in the vicinity of the pressure
at which liquidus phases are changed, because theliquidus phases and the melting relations should bestrongly affected by the melt structures. In spite of theimportance, however, direct observation on hydroussilicate melts under high pressure has not been conducted
yet because of the experimental difficulties. A few studies
were conducted so far by analyzing hydrous silicate
glasses quenched from hydrous melt at high pressure (e.g.
Xue et al., 2004[3]). We have conducted in-situ X-ray
diffraction experiments of hydrous Mg-silicate melts
under high pressure and high temperature to constrain thestructures. Special attentions were made to obtain thediffraction by introducing the new diamond capsulesystem.
Experimental
High pressure and high temperature experiments wereconducted at AR-NE5C bending magnet beamline, using
MAX-80 cubic type high pressure apparatus. The anvil
truncation edge length was 6 mm. X-ray diffractions ofmolten hydrous silicates were measured by white X-ray(5-140 keV) energy dispersive method using Ge-SSD.
Diffraction angles were fixed at several arbitrary angles
between 3° and 25° to obtain the data in wide scatteringvector Q range (Q (Å-1) = 4πEsinθ/12.398, where E isphoton energy).
Starting materials were prepared by mixing ofMg(OH)2, MgO and SiO2, to make the systems ofMg2SiO4-H2O (Mg/Si=2.0, H2O=20.4 and 11.3 wt%),
Mg2SiO4-MgSiO3-H2O (Mg/Si=1.5 H2O=18.3 wt%) andMgSiO3-H2O (Mg/Si=1.0, H2O=15.2 wt%). Pressuretransmitting medium were made of Boron + epoxy resin(4: 1) or pyrophyllite with the shape of 9 mm cube. Prior
to X-ray diffraction experiments, we carried out
temperature calibration experiment using the same cell
assembly with W3%Re-W25%Re thermocouple. Hydrous
molten samples were enclosed by single crystalline
diamond sleeve (cut in parallel to (100)) combining with

206

Users' Reportrare-metallic lids made of platinum. Both ends ofdiamond sleeve were sealed by platinum cap. Pressures
were calculated by Pt and MgO markers put on the top ofsample chamber.
Results
We could succeed to get good quality diffraction dataof hydrous silicate melts up to 6 GPa by using diamondcapsule, and also could 8 times reduce the exposure timecomparing with the use of Ag-Pd capsule, in which wehave done experiments previously.
Structure factor S(Q) on dense hydrous Mg-silicate
melts were derived by Monte Carlo simulation method [4]in the systems Mg/Si=1.0, 1.5 and 2.0. In those S(Q)s,first sharp diffraction peaks (FSDP), which may berelated to SiO4 tetrahedral network in silicate melts, shift
to higher Q with increasing MgO content and pressure. At~ 6 GPa, FSDP on Mg/Si=2 melt is 4% higher than that of
Mg/Si=1. This indicates that MgO bearing melt is more
depolymerized. Pressure dependence on the shifts of
FSDP was quite large on the Mg/Si=1.0 system, and thevalue at ~6 GPa is 18% larger than that of ambientcondition[5]. This large contraction can be explained by
the shrink of -Si-O-Si- chain-network structure oncompression. The results of Fourier transformation ofS(Q) show that Si-O inter-atomic distance (nearest
neighbor distance in silicate melts) is slightly longer thanthe dry melt in the Mg/Si=1.0 system (1.64 Å at 3 GPaand 1.65 Å at 6 GPa ). This extension may attribute to thebonding balance of Si-O-H (repulsion between Si4+ andH+ and/ or strong O-H bonding). In addition, clearincrease of the bonding distance with pressure may show
the gradual coordination change of 4-fold to 6-fold Si.
References[1] Inoue, Phys. Earth Planet. Inter. 85, 117-120 (1994).
[2] Yamada et al., Phys. Earth Planet. Inter. 147, 45-56(2004).
[3] X. Xue et al., Geochim. Cosmochim. Acta., 68, 50275057 (2004).
[4] Funakoshi, PhD Thesis, Tokyo Inst. Technol., (1997)[5] Funamori et al., Geophys. Res. Lett. 109, B03203(2004)* inoue@sci.ehime-u.ac.jp