Photon Factory Activity Report 2014 #32 (2015) B

AR-NE5C / 2013G140

PVT equation of state of lawsonite
Nao CAI1, Toru INOUE *1 and Takumi KIKEGAWA2
1Geodynamics Research Center (GRC), Matsuyama, Ehime 790-8577, Japan2
Photon Factory, Tsukuba, Ibaraki 305-0801, Japan
1 Introduction
Lawsonite [CaAl2Si2O7(OH)2·H2O] is a commonhydrous phase in metabasalts and metagreywackes, and isbelieved to be the last hydrous phase in the mid ocean
ridge basalts (MORB) system in subduction zones. The
previous experimental results in CaO-Al2O3-SiO2-H2Osystem (CASH) have revealed that lawsonite can bestable up to 10 GPa and 1230 K, which allows it to retain
the water to the depth of ~300 km in the descending slab.
Thus lawsonite can be a very important water carrier into
the deep mantle. Several experiments have already been
conducted under high pressure and high temperature toclarify the compressibility of lawsonite [1-3], but thereported values of the bulk modulus show a large
discrepancy ([1] 96 GPa; [2] 191 GPa, etc). Here we
conducted in situ X-ray diffraction experiments oflawsonite under high pressures and high temperatures todetermine the equation of state under high pressure andhigh temperature.
2 Experiment
The starting “lawsonite” sample was synthesized
at 10 GPa and 1273K for 10 hours by a Kawai-type multianvil apparatus installed in GRC, Ehime University.
High-pressure and high temperature in situ X-ray
experiments were conducted at the AR-NE5C beam lineof PF-AR in KEK, Tsukuba, Japan. The 6-6 typecompression method was used [4]. The TEL of thesecond stage anvils made by WC was 4 mm. Afterobtaining the diffraction patterns at ambient condition,
the cell was first compressed to a desired load, and thenheated to a temperature within the stability region torelease the deviatoric stress. After collecting thediffraction patterns for both sample and pressure standard,
the temperature decreased at 100 K interval to collect thediffraction patterns. The generated pressure values werecalculated from the equation of state (EoS) of NaCl [5].
3 Results and Discussion
Figure 1 shows the pressure and temperature conditions
where we collected the data. The obtained diffraction
patterns were fitted by the program XRayAna, using atleast 14 sharp diffraction peaks to calculate the lattice
parameters.
Figure 2 shows the pressure-volume-temperature (P-VT) dataset, and the data were fitted by using 3rd order
Birch-Murnaghan equation of state. The bulk modulus of
lawsonite has second order dependence on temperature.
Thus fitting the high temperature data yielding:=674.3(4) Å3,= 128.7(15) GPa,
= -0.047(8)GPa K-1,= 0.028(6) 10-3 GPa K-2, and α= 3.1(3) 10-5 K-1.
was fixed to 4. The present dataset
was less scattering than the previous studies. Using thoseparameters one can calculate the density and thedecomposition boundaries of lawsonite under highpressure and high temperature. This work is now
submitted to an international journal [6].

Figure 1 The pressure-temperature pathways for the insitu experiments.

Figure 2 Compressibility of Lawsonite with 3rd order
Birch-Murnanham fitting at temperatures of 300, 373,473, 573, 673, 773, 873 and 973 K, respectively.
References[1] Comodi&Zanazzi, Am. Mineral., 81, 833 (1996)[2] Holland et al., Am. Mineral., 81, 341 (1996)[3] Daniel et al. Phys. Chem. Minerals, 26, 406 (1999)[4] Nishiyama et al., High Press. Res., 28, 307 (2008)[5] Decker, J. Appl. Phys., 42, 3239 (1971)[6] Nao et al., J. Mineral. Petrol. Sci. (submitted)*inoue@sci.ehime-u.ac.jp