The Similar Reduction Processes of Primitive Achondrites from Different Parent Bodies

Details The Similar Reduction Processes of Primitive Achondrites from Different Parent Bodies The Similar Reduction Processes of Primitive Achondrites from Different Parent Bodies

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..605y&link_type=abstract

Meteoritics, vol. 30, no. 5, page 605

Mathematics

Logic

2

Acapulcoites, Achondrites, Primitive, Cooling Rates, Meteorites, Allan Hills 81187, Tierra Blance, Winonaites

Scientific paper

Winonaites and acapulcoites are subgroups of the primitive achondrites, but their oxygen isotopic compositions indicate that they are from different patent bodies [1]. Winonaites generally have smaller Fe/(Fe+Mg) atomic ratios (fe#) of mafic silicates than most acapulcoites [2]. Although these two groups are from different parent bodies, we find some records of common processes such as Mg/Fe zoning at the rims of mafic silicates. Tierra Blanca (TB) is a Fe-rich member of winonaites and Allan Hills 81187 (ALH) is Mg-rich member of acapulcoites [3]. The mafic silicates of both TB and ALH show reverse zoning of Mg/Fe at the rims of grains. We studied TB and ALH81187 by mineralogical techniques to gain better understanding of the two stage reduction processes of their parent bodies. The composition of the TB orthopyroxene (Opx) is almost constant, Ca2.2Mg90.7Fe7.1 and fe# = 7.4, but the fe#s in olivine grains show reverse zoning. The cores of olivine grains have fe# = 5.4 and the rims have fe# = about 3.5. These values are larger than the data from King et al. [4] and some other winonaites [2]. The fe# of the ALH olivine is almost constant (fe# = 3.9) and fe#s of the ALH Opx (fe# = 6.5, core) decrease at the rims. The fe#s of rims of ALH Opx grains are similar to the fe# of the ALH olivine. The fe#s of mafic silicates of TB are larger than those of ALH. The equilibrated temperature 900 degrees C-1100 degrees C was estimated from the Opx composition of TB by pyroxene geothermometry [6]. The temperature 1200 degrees C by two pyroxene geothermometer has been reported [5]. The cooling rate of TB and ALH were calculated by using the same method as that of Miyamoto and Takeda [7]. The linear cooling rates of 200 degrees C/year (900 degrees C-600 degrees C), 800 degrees C/year (1000 degrees C-600 degrees C) and 3000 degrees C/year (1100 degrees C-600 degrees C) give the best fit to the Fe-Mg profile at the rims of the TB Opx. The cooling rates of 1.5 degrees C/year (1000 degrees C-600 degrees C), 3.5 degrees C/year (1100 degrees C-600 degrees C) and 20 degrees C/year (1200 degrees C-600 degrees C) give the best fit to the Fe-Mg profile of the ALH Opx. The cooling rate of TB is larger than that of ALH. Considering the facts that TB cooled faster than ALH and that diffusion of Fe in olivine is faster than that of Opx, we suggest that the difference between fe#s of Opx and olivine and the reverse zoning of mafic silicates in TB and ALH can be explained by the difference in reduction in the solid state. The reduction of TB was terminated in an earlier stage of the reduction process than ALH because olivine is more likely to be reduced than pyroxene. Winonaites and acapulcoites were formed in different parent bodies, but it seems that they were suffered from similar reduction processes. The fe#s of winonaites are variable and fe#s of most winonaites (fe# < 5), are smaller than those of most acapulcoites (fe# = about 10-11) [2, 8]. The difference may have been formed during the recrystallization involving reduction before the solid state reduction occurred. It indicates that there have been two kinds of reduction, one is reduction during the recrystallization and the other is reduction in the solid state at the final stage of cooling when the planetesimals were disrupted or broken up. It is possible to envision that the winonaites with very small fe#s of the cores of mafic silicates are products of reduction in the solid state terminated later stage than ALH. We thank the Meteorite Working Group and Planetary Materials Database Collections of University of Tokyo for samples. References: [1] Clayton R. N. et al. (1992) LPS XXIII, 231-232. [2] Kimura M. et al. (1992) Proc. NIPR Symp. Antarc. Meteorites, 5, 165-190. [3] Yugami K. et al. (1993) 18th NIPR Symp. Antarc. Meteorites, 34-37. [4] King E. A. et al. (1981) Meteoritics, 16, 229-237. [5] Benedix G. K. et al. (1995) LPS XXVI, 99-100. [6] Lindsley (1983) Am. Mineral., 68, 477-493. [7] Miyamoto M. and Takeda H. (1994) JGR, 99, 5669-5677. [8] Graham A. L. et al. (1977) Mineral. Mag., 41, 201-210.

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