IN SITU X-RAY DIFFRACTION STUDIES OF DENSE HYDROUS MAGNESIUM SILICATES AT MANTLE CONDITIONS

Abstract

Lizardite was pressurized up to about 28 and 20 GPa and heated at 1310 (+/-50) and 1100 (+/-100)°C; antigorite was pressurized at about 25 and 22 GPa and heated at ~1000°C, with a Nd:YAG laser using the double-side laser heating technique. Samples were then examined in situ by energy dispersive synchrotron X-ray diffraction. Our results show that phase E, superhydrous phase B, and phase D are indeed dense silicate phases that are stable at high P-T conditions and quenchable to ambient conditions. Some unidentified peaks observed in our runs indicate the existence of unknown hydrous phases. The lattice parameter and bulk modulus of phase E are in good agreement with previous measurements. The linear moduli of the three axes of superhydrous phase B are kao=482 (+/-36) GPa, kbo=544 (+/-62) GPa, and kco=472 (+/-66) GPa and the bulk modulus of superhydrous phase B is 156 (+/-2) GPa. The linear moduli of the three axes of Fe-bearing phase D (phase Dant) are kao=422 (+/-8) GPa and kco=404 (+/-4) GPa and the bulk modulus is 134 (+/-5) GPa, which is ~19% smaller than the Fe-free phase D reported previously. Our results indicate that phase E, superhydrous phase B, and phase D are indeed thermodynamically stable phases at mantle conditions.