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

14C2/2004G225

Dehydration sequence of chrysotile serpentine at high pressure
1

Takuma HAYAKAWA1, Tomoaki KUBO1, Takumi KATO1, Takumi KIKEGAWA2
Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 812-8581, Japan
2
KEK-PF, Tsukuba, Ibaraki 305-0801, Japan

Introduction
Problems related to water circulation and storage in theEarth's interior are of fundamental importance tounderstand the material distribution and the chemicalevolution, because water can activate the transport
processes. Serpentine minerals (antigorite, chrysotile, andlizarlite) are produced by hydration of mantle peridotite
in ubiquitous environments, such as hydrothermal
alternation of the oceanic plate and fluid metasomatism in
the wedge mantle of the subduction zone [1,2]. Efforts
have been paid to elucidate their stablity fields and kinetic
aspects of mutual reactions, as release and accumulationof water (dehydration and rehydration) proceed
irreversibly under the geothermal conditions [3,4]. We,
here, report the dehydration sequence of chrysotile
serpentine at 3.6 GPa and its possible geoscientificimplications.
Experimental
Natural chrysotile serpentine (collected from Horomanultramafic complex, Hokkaido) was used as a starting
material. In-situ X-ray diffraction experiment wasperformed by using MAX-III system installed at BL14C2, and complementary beamless experiments byquenching method were made with MAX90 at Kyushu
University. Time-resolved X-ray diffraction profiles areacquired by energy dispersive method using Ge-SSD at afixed 2 theta angle. Before attaining the final heating
condition, the samples are annealed at 400 C for 2 hours
to realize the uniformity of the strain state.
Results and Discussion
Fig.1 shows backscattered electron image of the run
product after 120 minutes heating at 3.6 GPa and 560 C.
Dark grains are chrysotile and bright spots are magnetite
impurities. Dehydrated assemblage of forsterite + talc
appears along the grain boundaries (grey, intermediatecontrast). The textural observation suggests that thethermal decomposition of chrysotile occur with theincoherent grain boundary nucleation and growthmechanism. Fig.2 is the time-resolved X-ray diffraction
profiles at 3.6 GPa between 550 and 700 C. Chrysotile
starts to decompose almost instantly with temperatureelevation and the final assemblage at higher temperature
is found to be forsterite + enstatite
The results indicated that chrysotile decomposes in
two steps: (1) quasi-stable assemblage talc + forsteriteand (2) anhydrous assemblage of forsterite + enstatitewith released free water. Formation of quasi-stable talc inthe dehydration sequence would decrease thetransformation kinetics to the stable anhydrous

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Users' Reportassemblage, because its peculiar slippery rheology could
relieve the grain boundary strain and suppress furthernucleation. The characteristic properties of phyllosilicates
and their dehydration behavior may affect transport ofmaterials and fractionation of elements in water-available
conditions in the Earth and Martian interiors.

Fig.1. Backscattered electron image showing grain
boudary nucleation of talc + forsterite assemblage at 560
C and 3.6 GPa after 120 min heating.

Fig.2. Time-resolved profiles at 3.6 GPa, 560-700 C. Ch,
chrysotile; Fo, forsterite; En, enstatite; Au, Gold; Gra,graphite.
References[1] R.D. Hyndman and S.M. Peacock, Earth Planet. Sci. Lett.,
212, 417 (2003)[2] S. Kamiya and Y. Kobayashi, Geophys. Res. Lett., 27, 819(2000)[3] T. Komabayashi et al., Phys. Earth Planet. Inter., 151, 276(2005)[4] J.P. Perrillat et al., Earth Planet. Sci. Lett., 236, 899 (2005)*tkato@geo.kyushu-u.ac.jp