Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p32a0548x&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P32A-0548
Physics
1025 Composition Of The Mantle, 3662 Meteorites, 3924 High-Pressure Behavior, 3944 Shock Wave Experiments, 5410 Composition
Scientific paper
Fe2SiO4 with the spinel structure was synthesized by Ringwood [1] and inferred to be important in Earth's upper mantle [1,2], but it has not previously been found in nature. Umbarger is an L6 chondrite, which contains a series of high-pressure phases: ring-woodite, akimotoite, augite and hollandite-structured plagioclase [3-6]. Transmission electron microscopy also revealed a Fe2SiO4-spinel and stishovite assem-blage in a Fe-rich zone of a melt pocket. Phyllosili-cates and glass are also associated with the Fe2SiO4-spinel, indicating post-shock aqueous glass alteration. The Fe2SiO4-spinel grains are equant to irregular, with sizes from 100 to 400 nm. Diffraction patterns can only be indexed as a spinel structure, but the unit cell is a little large (a= 8.46 Å) relative to synthetic Fe2SiO4-spinel (a=8.234 Å [1]). EDS analyses show that the spinel has a fayalite compositon with Fe/(Fe+Mg) ratio ranging from 62% to 97%. Stishovite occurs as elongated prismatic crystals up to 1 mm long and up to 100 mm wide. The alteration products in Umbarger are phyllosilicates rich in Fe and Si, with a basal layer spacing of about 9.7 Å. Crystallization of melt veins began with akimotoite and ringwoodite at pressures up to 25 GPa and ended with augite at pressure less than 18 GPa [3, 4]. Mg2SiO4-spinel and stishovite are stable in the pres-sure range from 15 GPa to 25 GPa in the enstatite-forsterite system [7], while Fe2SiO4-spinel and stishovite is stable at lower pressure. Fe2SiO4-spinel and stishovite assemblage, like that of Akimotoite plus ringwoodite, is a sub-solidus assemblage, indicating crystallization of a supercooled liquid during rapid cooling and decompression associated with shock pres-sure release. Reference: [1] Ringwood A.E. (1958) GCA,15, 18-29. [2] Irifune T. (1993) The Island Arc, 2, 55-71. [3] Xie Z. and Sharp T. G. (2000a) LPS XXXI, 2065.pdf. [4] Xie Z. and Sharp T. G. (2000b) MAPS, 35 A172. [5] Xie Z. et al.(2001) LPS XXXII, 1805.pdf. [6] Sharp T.G. (2001) LPS XXXII, 2159.pdf. [7] Gasparik T. (1992) JGR, 97, 15181-15188.
Sharp Thomas G.
Xie Zhaohua
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