Henry P. Scott1, Zhenxian Liu2, Russell J. Hemley2, and Quentin Williams3
1Indiana University South Bend
2The Geophysical Lab
3University of California at Santa Cruz
h.scott@gl.ciw.edu
NSLS-U2 (DAC)
We have collected far infrared (FIR) absorption spectra for talc and lawsonite to 30 and 25 GPa respectively using the synchrotron source at beamline U2A of the NSLS . The talc spectra gradually evolve across the pressure range of this study, however, no abrupt discontinuities in mode shifts are observed. By 30 GPa the spectrum simply consists of two broad absorption features. We also present high pressure mid infrared spectra for talc, and note continuous mode shifts across the entire pressure range. The hydroxyl stretching vibration is resolved to the highest pressure reached in this study. FIR spectra for lawsonite also display gradual modifications, but all modes are resolved in our highest pressure spectrum. However, a discontinuity is observed in mode shifts at approximately 8 GPa; this result is consistent with previous MIR studies of this phase which document a phase transition at 8.6 GPa. Notably, the gradual changes in FIR spectra observed for both phases are fully reversable upon decompression, and when returned to ambient pressure the spectra are indistinguishable from those taken prior to compression. We conclude that in cool subduction environments both phases may exist metastably to depths well beyond their expected high pressure and high temperature stability limit (~5 GPa at 700°C for talc and ~11 GPa at 800°C for lawsonite).