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

NE5C/2004G225

Kinetics of albite breakdown at high pressure
Tomoaki KUBO*1, Takumi KATO1, Makoto KIMURA2, Takumi KIKEGAWA3
Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, 812-8581, Japan
2
Institute of Astrophysics and Planetary Science, Ibaraki University, Mito 310-8512, Japan
3
KEK-PF, Tsukuba, Ibaraki 305-0801, Japan

Introduction
High-pressure transformations in plagioclase occur insubducting oceanic crust associated with the gabbroeclogite transformation and in parent bodies of heavily
shocked chondritic meteorites. Although equilibriumphase relations for these transformations have been wellstudied, the processes of plagioclase breakdown arepoorly known, which makes it difficult to constrain thedepth of eclogitisation in subducting plates and P-T-t
conditions during shock events in parent bodies. Here we
report experimental results on mechanisms and kinetics ofsodic plagioclase (albite) breakdown examined by in-situ
X-ray observations.
Experimental
Starting material used in this study is natural albite,(Ab98.0An0.4Or1.6) which decomposes into jadeite +quartz (coecite) at around 1-2 GPa. In-situ X-ray
diffraction experiments were carried out using multi-anvil
high-pressure apparatuses MAX-80 installed at KEK,
Japan. Kinetics of albite breakdown was observed at 2.26.0 GPa and 273-923K by taking time-resolved X-ray
diffraction profiles every 10-300 seconds. Transformation
microstructures and water contents of recovered samples
were investigated using SEM and FTIR, respectively.
Results and discussion
Time-resolved X-ray diffraction patterns during thetransformation are shown in Fig. 1. In case of the grainsize of about 20 micron in the present study, the reaction
proceeds rapidly at temperatures of 673-773K andoverpressure (dP) of more than 2 GPa from theequilibrium boundary. We observed that the albite
breakdown becomes very slow near the equilibriumboundary (dP of about 0.5 GPa) even at higher
temperature of 923K probably due to difficulties ofnucleation processes. SEM observations show that thebreakdown of albite occurs by grain-boundary nucleationand growth processes. Water contents of samples
recovered were estimated to be 0.2-0.5 wt% H2O on thebasis of FTIR measurements.
Kinetic data obtained indicates that the growth distanceis mostly proportional to t^0.5 (t: time), which implies thediffusion-controlled growth mechanisms (Fig. 2).
Extrapolations of obtained growth kinetics suggest thatthe breakdown of albite with the grain size of 5 mm can4complete in 10 years at 573K. Growth kinetics in thisreaction is enough fast compared to geological time

6scales (i.e., 10 years) The depth of the albite breakdown
in subducting plates is possibly controlled by nucleationqz
kinetics.
qzjjjj jjj

484 secevery
40 sec177 secevery
10 sec

590°C 2 sec
100°C
3035

a a aa a a gaaaa aa

404550

Energy (keV, 2theta=5.0°)
Fig. 1 Time-resolved XRD patterns showing albite (a) breakdown into
jadeite (j) and quartz (q) at 2.7 GPa and 863K.

2.7GPa, 590°C, 0.5wt%H2O
k = 2.2x10-7, n = 0.59

1Transformed volume fraction

1

2.7GPa, 590°C, 0.2wt%H2O
k = 1.6x10-7, n = 0.54

0.8

3.1GPa, 400°C, 0.5wt%H2O-8k = 2.0x10 , n = 0.56

0.60.4

3.1GPa, 300°C, 0.5wt%H2O
k = 1.6x10-8, n = 0.44
0.2

3.1GPa, 400°C, 0.2wt%H2O
k = 2.7x10-8, n = 0.29
001000 2000 3000 4000 5000 6000 7000 8000

Duration (sec)Fig. 2 Time dependence of transformed volume fraction. Kinetic data
were fitted using the rate equation for grain-boundary reaction. Growthdistance at time t can be described by ktn. Rate constants k and n areshown.

* kubotomo@geo.kyushu-u.ac.jp

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