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

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

Jun 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004pepi..143..185n&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 143, p. 185-199.

Physics

8

Scientific paper

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 660km 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 CaSiO3-rich perovskite at pressures of about 20-22GPa. Dissociation of ringwoodite to MgSiO3-rich perovskite and magnesiowüstite (Mw) was completed at 22.0+/-0.2GPa according to Matsui et al.'s periclase pressure scale, and at 21.7+/-0.1GPa according to Shim et al.'s gold pressure scale. Majorite garnet persisted to about 24GPa where pyrolite transformed to a lower mantle mineral assemblage, i.e. MgSiO3-perovskite, CaSiO3-rich perovskite, and magnesiowüstite. Thus, majorite garnet coexists with the lower mantle assemblage at pressures of about 22-24GPa. In the quench experiments, an assemblage of MgSiO3-perovskite, magnesiowüstite, CaSiO3-rich perovskite, and majorite garnet was synthesized at 22.5GPa and 1600°C, in which Mg-perovskite contained 2.8wt.% Al2O3, and was significantly poorer in Fe than coexisting magnesiowüstite. The Fe-Mg partition coefficient between Mg-perovskite and magnesiowüstite including ferric iron (Kapp=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 Kapp 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-Al2O3-SiO2 system.

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