Precise determination of phase relations in pyrolite across the 660 km seismic discontinuity by in situ X-ray diffraction and quench experiments

Abstract

Mineral assemblage changes in a pyrolite composition with increasing pressure were observed by in situ X-ray diffraction and quench experiments at pressures near that of the 660 km seismic discontinuity and at a fixed temperature of 1600 °C. According to results obtained by in situ X-ray diffraction experiments, ringwoodite (Rw) was observed with majorite garnet and CaSiO 3-rich perovskite at pressures of about 20–22 GPa. Dissociation of ringwoodite to MgSiO 3-rich perovskite and magnesiowüstite (Mw) was completed at 22.0±0.2 GPa according to Matsui et al.’s periclase pressure scale, and at 21.7±0.1 GPa according to Shim et al.’s gold pressure scale. Majorite garnet persisted to about 24 GPa where pyrolite transformed to a lower mantle mineral assemblage, i.e. MgSiO 3-perovskite, CaSiO 3-rich perovskite, and magnesiowüstite. Thus, majorite garnet coexists with the lower mantle assemblage at pressures of about 22–24 GPa. In the quench experiments, an assemblage of MgSiO 3-perovskite, magnesiowüstite, CaSiO 3-rich perovskite, and majorite garnet was synthesized at 22.5 GPa and 1600 °C, in which Mg-perovskite contained 2.8 wt.% Al 2O 3, and was significantly poorer in Fe than coexisting magnesiowüstite. The Fe–Mg partition coefficient between Mg-perovskite and magnesiowüstite including ferric iron ( K app=0.27±0.06) is very close to that in the Al-free system, which suggests that these P– T conditions are in the vicinity of those of ringwoodite decomposition. Both the results of in situ X-ray diffraction and quench experiments in the present study yield a convergent result that ringwoodite decomposes into Mg-perovskite and magnesiowüstite before the garnet-to-perovskite transition at 1600 °C in pyrolite. The relation between the Al content in Mg-perovskite and K app in pyrolite is non-linear, which is consistent with the Fe–Mg partitioning between Mg-perovskite and magnesiowüstite previously reported for a simpler MgO–FeO–Al 2O 3–SiO 2 system.