34. Mineralogic Distribution of Iron in the Upper Half of the Transition Zone in the Earth'S Mantle

  1. George H. Sutton,
  2. Murli H. Manghnani,
  3. Ralph Moberly and
  4. Ethel U. Mcafee
  1. Syun-Iti Akimoto,
  2. Masaki Akaogi,
  3. Kaoru Kawada and
  4. Osamu Nishizawa

Published Online: 17 MAR 2013

DOI: 10.1029/GM019p0399

The Geophysics of the Pacific Ocean Basin and Its Margin

The Geophysics of the Pacific Ocean Basin and Its Margin

How to Cite

Akimoto, S.-I., Akaogi, M., Kawada, K. and Nishizawa, O. (1976) Mineralogic Distribution of Iron in the Upper Half of the Transition Zone in the Earth'S Mantle, in The Geophysics of the Pacific Ocean Basin and Its Margin (eds G. H. Sutton, M. H. Manghnani, R. Moberly and E. U. Mcafee), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM019p0399

Author Information

  1. Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-Ku, Tokyo 106, Japan

Publication History

  1. Published Online: 17 MAR 2013
  2. Published Print: 1 JAN 1976

ISBN Information

Print ISBN: 9780875900193

Online ISBN: 9781118663592

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Keywords:

  • Geophysics—Pacific area—Congress;
  • Woollard, George Prior, 1908

Summary

Chemical 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.