The olivine-wadsleyite phase transformation in mantle peridotite

One of the most important goals in the study of the olivine (α) - wadsleyite (β) transformation is to understand the seismic discontinuity observed near the depth of 410 km in the Earth’s mantle. Although one school of thought attributes this discontinuity to radical chemical changes from lherzolite to picritic eclogite[1], the α-β transformation in an isochemical peridotitic mantle has widely been accepted as the underlying cause. If the latter interpretation is valid, the composition and temperature can be inferred by the depth and magnitude of the discontinuity, providing useful information for understanding the present state of the Earth’s transition zone. In spite of extensive experimental investigations of this transformation, most studies were conducted in the simple system Mg2SiO4-Fe2SiO4 and/or using the quenching technique[2]. No efforts have been directed to study the kinetic barrier of the α-β transformation under normal mantle conditions. In addition, recent studies have demonstrated increasing needs for the study of the α-β transformation in multi-component systems relevant to the Earth’s mantle[3,4]. We choose the KLB-1 spinel lherzolite, a xenolith from Kilborne Hole Crater in New Mexico USA, as starting material because it represents one of the most undepleted mantle compositions. The high P-T x-ray diffraction experiments were conducted using a two-stage multi-anvil press on beamline 13-BM-D of Advanced Photon source, Argonne National Laboratory. Phase identifications were made by observing the first appearance of a phase that was absent in the starting sample and by closely monitoring complete disappearance of a low-pressure phase from the assembly. In the presence of both α and β phases, transformation direction was identified by observing a significant change in the relative intensity between the two phases. Our results demonstrate the experimental feasibility of studying α-β transformation in complex systems and of resolving pressure differences of less than 0.4 GPa for the two-phase loop, even with the effect of kinetic barrier being taken into account. References: [1] Anderson and Bass, Nature 320, 321, 1986; [2] Katsura and Ito, J. Geophys. Res. 94, 15663, 1989; [3] Stixude, J. Geophys. Res. 102, 14835, 1997; [4] Irifune and Isshiki, Nature 392, 702, 1998.