Abstract
Antigorite is the high-temperature member of the serpentine group
minerals and is broadly considered a primary carrier of water in
subducting oceanic lithosphere at the fore-arc. It has a wavy crystal
structure along the a-axis and several polysomes with different
$m$-values (m=13-24) have been identified in nature. The number (m) is
defined as the number of tetrahedra in one wavelength and is controlled
by the misfit between the octahedral and tetrahedral layers. The degree
of misfit mainly depends on the volumes of the MgO6 octehedra and SiO4
tetrahedra within the layers, which vary as a function of temperature
and pressure. However, it is not well understood which m-values of
antigorite are stable at different temperature and pressure conditions.
To investigate the pressure dependence of the stability of different
m-values in antigorite, we performed first-principles calculations for
several polysomes (m=14-19) at high pressure (0-14 GPa) and compared
their enthalpies (T: static 0 K). We found that although the energy
differences between polysomes are small, polysomes with larger m-values
are more stable at ambient pressure, while polysomes with smaller
m-values are more stable at elevated pressures. This suggests that the
structure of antigorite in oceanic lithosphere that has subducted into
the deep Earth may gradually evolve into a different polysome structure
than antigorite samples observed at ambient or near-pressure conditions.
This structural change may be related to the formation of the lower
plane of the double seismic zone, as changes in polysome m-values are
accompanied by a minor dehydration reaction.