[SCG57-P31] Elastic wave velocity change caused lawsonite decomposition in blueschist at 1.0GPa and up to 550℃
Keywords:lawsonite, dehydration, elastic wave velocity
Here we investigated compressional (Vp) and shear (Vs) wave velocities in lawsonite blueschist to evaluate the effect of lawsonite dehydration on Vp and Vs under the condition of high pressure and temperature. Experiments were carried out at 1.0 GPa up to 550°C with piston-cylinder apparatus having 34mm bore hole. Talc, pyrophyllite, boron nitride and NaCl were used as the pressure-transmitting media. An Al2O3 buffer rod was placed between rock sample and transducer. Temperature was monitored with the alumel-chromel thermocouple placed upon the NaCl (about 2mm above top end of rock sample). P and S-wave signals were generated by a LiNbO3 transducer ( 10°Y-cut ) simultaneously. Vp and Vs measurements were carried out using pulse reflection method.
Vp and Vs markedly dropped over 350°C during temperature ramping while Vp/Vs ratio stayed constant. After the experiment, about 11 vol.% of lawsonite broke down and the sample contained newly formed anorthite which interpreted as breakdown products of lawsonite. In the run product, many spherical pores were present in the breaking lawsonite and near anorthite. We inferred that released H2O was trapped as spherical fluid inclusion in these pores. Then we conducted another experiment at 1.0GPa up to 500°C to estimate the temperature lawsonite broke down and we could not get any clues of lawsonite breakdown. As a result of these experiments, it could be inferred that lawsonite breakdown reaction occurred between 500 and 550°C by the following reaction.
Lawsonite [ CaAl2Si2O8(2H2O) ]→Anorthite [ CaAl2Si2O8 ]+ 2H2O
Furthermore, the amount of H2O released by this reaction is estimated 0.26 wt.% against the Bulk composition of the sample.
Though H2O was released between 500 and 550°C, Vp/Vs ratio stayed constant. For the reason of constant Vp/Vs ratio, we inferred that released H2O was trapped as spherical fluid inclusion. So it is thought that dehydration reaction proceeded without Vp/Vs ratio changes.