Phase transitions of serpentine in the lower mantle

Abstract

Phase transformations in natural serpentine and chlorite have been studied at 25–50GPa and 900–1280°C by in-situ X-ray diffraction measurements using the multianvil apparatus with sintered diamond anvils. Antigorite was found to transform to a phase assemblage including phase H, at pressures above about 35–40GPa corresponding to those in the upper part of the lower mantle. The zero pressure bulk modulus K0 of phase H was determined to be 160GPa, assuming its pressure derivative K′=4. The thermal stability of phase H is significantly enhanced by a solid solution with a δ-AlOOH component, suggesting that aluminous phase H in cold slabs would deliver a certain amount of water into the deepest part of the lower mantle.