Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmmr54a..08e&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #MR54A-08
Physics
3900 Mineral Physics, 3944 Shock Wave Experiments, 5420 Impact Phenomena, Cratering (6022, 8136), 5709 Composition (1060), 6022 Impact Phenomena (5420
Scientific paper
Olivine crystals entrained in shock-melt veins in chondrites display partial to complete transformation to ringwoodite and wadsleyite [1-4]. A crucial issue is if the olivine-ringwoodite inversion was exclusively incoherent by grain boundary nucleation and growth, or if coherent intracrystalline mechanisms were also active. The incoherent mechanism is at least three orders of magnitude faster than the coherent [5,6]. We report two ringwoodite growth textures in chondritic olivines entrained in shock-melt veins in Sixiangkou chondrite (1) polycrystalline grain-boundary growth in small olivines [1], and (2) intracrystalline ringwoodite lamellae in large olivines. The first type is reminiscent of the incoherent mechanism [5,6]. (2) TEM investigations reveal that ringwoodite lamellae in olivine entrained in veins are polycrystalline. Some ringwoodite crystallites at the two-phase boundary depict coherent olivine/ringwoodite intergrowth: (3-11) ringwoodite (130) olivine. This is the first report of coherent intergrowth in meteoritic olivine. Olivine grains entrained in melt veins of Peace River meteorite (Fa22-25) depict phase transition to ringwoodite and wadsleyite with spectacular textures and stark contrasting compositions: (1) Ringwoodite (Fa32-36) occurs as grain boundary crystallites in the outer regions of olivine, whereas wadsleyite (Fa12-14), occupies the olivine cores. (2) Two well-oriented sets of zoned ringwoodite lamellae (<12 μm; Fa30- 39) intersect large olivines and are overgrown by wadsleyite crystals (<8 μm; Fa17-19). Residual olivine (Fa18-20) displays in its interior two sets of wadsleyite lamellae. SEM, Raman, and synchrotron XRD mapping reveal clear evidence for back transformation of ringwoodite to both secondary wadsleyite and olivine, respectively. Synchrotron XRF mapping shows depletion of both ringwoodite and wadsleyite in Mn and Zn due to back diffusion to olivine. References: [1] Chen M. et al. (1996) Science, 271, 1570. [2] Chen M. et al. (2004) PNAS, 101, 15033. [3] Ohtani E. et al. (2004) EPSL, 227, 505. [4] Beck P. et al. (2005) Nature, 435, 1071. [5] Kerschhofer L. (2000), Phys. Earth Planet. Interiors, 121, 59. [6] Mosenfelder J. L. et al. (2001) Phys. Earth Planet. Interiors, 127, 165.
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