Relative stability and contrasting elastic properties of serpentine polymorphs from first‐principles calculations

Abstract

AbstractWe conduct first‐principles calculations to determine the relative stability field and elasticity contrast of serpentine polymorphs at pressure and temperature conditions relevant to the oceanic lithosphere and subduction zone. At high pressures (>4 GPa), the corrugated form of serpentine polymorph (antigorite, Atg) plus brucite (Brc) assemblage is thermodynamically more favorable compared to its planar counterpart (lizardite, Liz). The phase boundary between Liz and Atg + Brc exhibits a negative slope in the whole P‐T range, indicating that this transition can be driven by increasing either pressure or temperature. Near 0 GPa, the slope is about −33 K/GPa. As pressure exceeds 1 GPa, the transition temperature starts to decrease more rapidly. Because of the corrugated nature of its constituent layers, Atg is more susceptible to intralayer deformations (7 to 36% smaller C11, C22, C12, and C66 at ambient conditions) while more resistant to interlayer shear deformation (25% larger C44, 36% larger C55) than Liz. In contrast, their responses to the interlayer compressive deformation (C33) are similar. For isotropic polycrystalline aggregates at pressures between 0 and 4 GPa, Atg exhibits a smaller bulk modulus (12 to 15%) and a larger shear modulus (6 to 11%) than Liz, while their density contrast is within 1%. Accordingly, a Liz/Atg transition is accompanied by a decrease in Vp (2 to 3%), an increase in Vs (3 to 5%), and a more pronounced drop in Vp/Vs (6 to 8%). These results may help to identify and characterize serpentine polymorphs produced under various geological settings.