Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.497c&link_type=abstract
Meteoritics, vol. 30, no. 5, page 497
Other
1
Beta-Phase, Exsolution Lamellae, Gamma-Phase, Meteorites, Mbale, Olivine, Veins, Shock
Scientific paper
Mbale is a shocked L6 chondrite [1]. Some shock-induced veins of 0.1 to 2 mm width intersect the meteorite. The veins consist of diaplectic plagioclase glass, unmelted silicate fragments including olivines and pyroxenes, and the shock-induced melt materials consisting of metal-troilite intergrowths, microcrystalline pyroxene and olivine, and silicate glass. Here we report the first finding of shock-induced exsolutions in olivine that appear as compositionally different lamellae which presumably formed at high pressure by inversion of olivine (alpha) to the beta+gamma polymorphs. Olivine compositions in the chondritic part of Mbale range from Fa(sub)22.5 to Fa(sub)26 (average Fa(sub)24). In contrast, the exsolved olivines in the shock-induced veins (30 to 100 mm in diameter) consist of alternating Fe-rich (Fa(sub)22-32) and Fe-poor (Fa(sub)9.9-18.5) lamellae that range in width from less than 0.5 micrometers to up to 10 micrometers. Each individual lamella actually consists of microcrystals of less than 0.5 micrometers in diameter with rather diffuse borders within the lamella. The lamellae occur in ~30 micrometer segments which were formed by faulting and displacement. Micro-Raman spectroscopy confirms that both types of exsolution lamellae currently have the olivine (alpha) structure. The partially melted veins of Mbale may have reached a post-shock peak temperature of 1473 K >= T <= 1873 K. Based on the phase diagrams of the olivine(alpha)-wadsleyite(beta)-ringwoodite(gamma) transformations in the system Mg(sub)2SiO(sub)4-Fe(sub)2SiO(sub)4 at 1473 K [2], olivine of composition ~Fa(sub)24 should exsolve at equilibrium conditionsins to beta- + gamma-phases at P >12.7 GPa, and transform to a single gamma-phase at >15 GPa. Figure 1 shows that the determined compositions of the Fe-rich and Fe-poor lamellae in Mbale match perfectly the compositions at the beta + gamma/gamma and the beta + gamma/beta phase boundaries, respectively. Since these grains depict well ordered lamellar structure displaced by several faults, the exsolution lamellae of olivine must have nucleated before faulting. TEM studies of ringwoodite in black veins in other shocked chondrites indicate that the original olivine has transformed into a mosaic of ringwoodite grains in nano-meter size range [3,4]. The observed lamellar structure in Mbale olivines is indicative of exsolution transformation from single grains before faulting of the olivines and definitely before solidification of the silicate melt to glass in the vein. We therefore consider it unlikely that the shock-induced olivine lamellae were produced via retrograde exsolution from gamma to gamma + beta. Our findings strongly suggest that the exsolutions were produced from an originally homogeneous olivine (alpha) by the following mechanism: Original homogenous olivine exsolved at P>12.7 GPa and 1473 K >=T <= 1873 K into Fe-poor beta- and Fe-rich gamma-phases possibly along [001] which is the predominant dislocation orientation in olivine during deformation [4,5]. The dislocations in olivine may have provided the nucleation sites for the beta- and gamma-phases due to their reduced activation barrier to nucleation. The grains were then faulted to several segments of 30 micrometers in diameter. A cooling rate of >= 260 K/s during the solidification of the shock-induced melt was estimated in the interval of 1223-1673 K, based on the metallic dendrite arm spacing or cell width of ~14 micrometers [6]. The cooling of the veins after the solidification of metal and silicate melts should have been much slower than 260 K/s because of the homogeneous Ni-concentration within the metallic dendrites. We interpret the survival of the lamellar structure as due to a nonlinear cooling rate: (1) very fast cooling after pressure release thus inhibiting obliteration of the lamellae by diffusion; followed by (2) slow cooling <1223 K thus leading to inversion of gamma and beta lamellae to alpha without chemical homogenization. References: [1] Jenniskens P. et al. (1994) Meteoritics , 29, 246-254. [2] Akaogi M. et al. (1989) JGR, 94, 15671-15685. [3] Madon M. and Poirier J. P. (1983) Phys. Earth Planet. Inter., 33, 31-44. [4] Price G. D. et al. (1979) Contrib. Mineral. Petrol., 71, 211-218. [5] Ashworth J. R. and Barber D. J. (1975) EPSL, 13, 43-50. [6] Scott E. R. D. (1982) GCA, 46, 813-823. Fig.1. Olivine (alpha)- wadsleyite (beta)- ringwoodite (gamma) relations in the system Mg(sub)2SiO(sub)4-Fe(sub)2SiO(sub)4 at 1473 K (after Akaogi et al. [2]). The compositions of the Fe-poor and Fe-rich lamellae of a representative olivine grain are plotted as open circles. Note that in all cases the compositions of the lamellae plot exactly at the the beta + gamma/beta and beta + gamma/gamma phase boundaries.
Chen Michelle
El Goresy Ahmed
Wopenka Brigitte
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