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

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 degreesC. According to results obtained by in situ X-ray diffraction experiments, ringwoodite (Rw) was observed with majorite garnet and CaSiO3-rich perovskite at pressures of about 20-22GPa. Dissociation of ringwoodite to MgSiO3-rich perovskite and magnesiowustite (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 old pressure scale. Majorite garnet persisted to about 24 GPa where pyrolite transformed to a lower mantle mineral assemblage, i.e. MgSiO3-perovskite, CaSiO3-rich perovskite, and magnesiowustite. Thus, majorite garnet coexists with the lower mantle assemblage at pressures of about 22-24 GPa. In the quench experiments, an assemblage of Mg2SiO3-perovskite, niagnesiowustite, CaSiO3-rich perovskite, and majorite garnet was synthesized at 22.5 GPa and 1600degreesC, in which Mg-perovskite contained 2.8 wt.% Al2O3, and was significantly poorer in Fe than coexisting magnesiowustite. The Fe-Mg partition coefficient between Mg-perovskite and magnesiowustite 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 magnesiowustite before the gamet-to-perovskite transition at 4600 degreesC 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 magnesiowustite previously reported for a simpler MgO-FeO-Al2O3-SiO2 system. (C) 2004 Published by Elsevier B.V.