Mineralogic Distribution of Iron in the Upper Half of the Transition Zone in the Earth'S Mantle
2011; American Geophysical Union; Linguagem: Inglês
10.1029/gm019p0399
ISSN2328-8779
AutoresSyun-iti Akimoto, Masaki Akaogi, Kaoru Kawada, Osamu Nishizawa,
Tópico(s)earthquake and tectonic studies
ResumoChemical equilibrium partitioning of Mg2+ and Fe2+ between (Mg, Fe)2Si04 olivine or spinel and (Mg,Fe)3 Al2Si3O12 garnet was investigated at pressures of 40 to 100 kbar and at temperatures of 1000° to 1200°C, using tetrahedral- and cubic-anvil types of high pressure apparatus. Relative enrichment of Fe2+ in garnet compared to olivine was observed at 40 kbar and 10/50°C. When olivine transforms to spinel at the higher pressures, however, the partition relation reverses; relative concentration of Fe2+ in spinel is remarkably high. Apparent partition coefficient K′={XSp Fe)(1-XSp Fe)}/{XGa Fe/ (1-XGa Fe)}, where XSp Fe and XGa Fe denote the mole fraction of Fe2Si04in (Mg,Fe)2Si04 spinel and that of Fe3Al2Si3012 in (Mg,Fe)3Al2Si3012 garnet, ranges from 2.7 to 3.1 at 80–100 kbar and 10000°–1200°C. Based on these partition data and on Ringwood's pyrolite model, chemical composition of (Mg,Fe)2SiO4 modified spinel or true spinel in equilibrium with complex garnet solid solution in the mantle transition zone was estimated to be (Mg0.87Fe0.13)2Si04. It is concluded that 2% increase in Fe2Si04 component in (Mg,Fe)2Si04 solid solution is expected in the transition zone as a coupling effect of the olivinemodified spinel-true spinel transformation and the pyroxene-garnet transformation. By combining the present mineralogical model with the olivine-spinel transformation diagram of the Mg2Si04-Fe2SiO4 system and with seismic profiles in the mantle, temperatures at depths of 380 and 500 km were provisionally estimated as 1400° and 1620°C, respectively.
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