Changes in the cell parameters of antigorite close to its dehydration reaction at subduction zone conditions

Abstract

The unit-cell parameter a of antigorite (usually expressed as the polysome m value) has been de- termined as a function of temperature (T) and pressure (P) in the range of 600–650 °C, 25–45 kbar in weeklong piston-cylinder experiments. A well-characterized natural antigorite (with m = 16 and less abundant m = 15) was used as a starting material that coexisted with olivine, chlorite, Ti-humite, and aqueous fluid at run conditions. Transmission electron microscope (TEM) measurements on selected focused ion beam (FIB) wafers showed that antigorite m values after the experiments varied between 14 and 22. More than 40 punctual analyses for each run condition were acquired to determine the range and the primary m value. The most frequent antigorite m-value decreased systematically from 17–19 at 600 °C to 15–16 at 650 °C. The spacing of the m-isolines is getting narrower as the antigorite break- down reaction is approached. The topology of them-isolines is similar to that previously characterized for the simple MgO-SiO2-H2O (MSH) system. However, the isolines are shifted to about 50–100 °C higher temperatures due to the incorporation of Al into antigorite. Powder samples and FIB wafers of natural antigorite from the Tianshan UHP belt (China) with peak metamorphic conditions of ~35 kbar, ~520 °C were also investigated with TEM. Low Al-antigorite formed at peak metamorphic conditions displays a peak m value of 20–21, whereas high-Al antigorite formed during isothermal decompression displays a lower m value of 19. Combination of our results with the published data of m values from metamorphic antigorite that experienced various conditions allowed construction of aP-T-m diagram that can be used in future studies to better constrain formation conditions of serpentinites. The decrease of m values and the increase of Al in antigorite with increasing temperature result in small, continuous dehydration whereby the H2O content of antigorite changes from 12.4 to 12.1 wt%. Therefore, it is expected that a pore fluid is present during the prograde deformation of serpentinites. TEM observa- tions showed that antigorite adjusted its Al content by segregation of chlorite at the nanoscale. Together with the observation that multiple m values are always present in a single sample, this result indicates that full equilibration of antigorite at the micrometer-scale is rare, with important implications for the interpretation of geochemical signatures obtained by in situ techniques.