Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..199b&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 199
Mathematics
Logic
Scientific paper
Achondrites have been generally assumed to be derived from magma formed from some chondritic melts or mixtures and the genetic model often proposes specific crystallization sequences to account for particular achondrite properties. However, some achondrites such as primitive achondrites (Acapulco-type chondrites and/or Lodranites) and ureilites still preserve primitive signatures of chondritic source materials (Palme et al., 1981). To account for formation of such meteorites, a model involving formation of ultramafic meteorites without invoking large-scale melting such as magmatic process has been proposed (Takeda, 1989). The process to form extensively recrystallized chondrites (H7,L7,LL7) should be investigated as a first step to make such meteorites. We investigated the textures and chemical compositions of two L7 chondrites from Frontier Mountains by electron probe microanalysis (EPMA) and scanning electron microscope (JEOL 840A SEM) with chemical mapping analysis (CMA) utility, and compared them with other type 7 chondrites, primitive achondrites, and ureilites. Two polished thin sections, FRO90058,001 and FRO90066,001 have been supplied from the EUROMET. The textures of FRO90058 and FRO90066 are very similar. Textural characteristics of some parts of the samples are rather similar to primitive achondrites than chondrites. However, a few chondrules still remain in FRO90058. Two samples are composed of subangular recrystallized clasts and much comminuted grains of clasts and minerals filling interstices between the clasts. The facts indicate that they are partly brecciated. The recrystallized parts show coarse-grained textures of orthopyroxene(opx), olivine(oliv) and opaque minerals. In fine- grained portions, silicate minerals exhibit granoblastic texture. Plagioclase is present as elongated, irregular grains in the fine-grained portion, while in coarser regions, plagioclase fills interstices or channels and is intruded by opx. Olivine grains are compositionally uniform with mean Fa=25. Opx of FRO90058 and FRO90066 are similar and their chemical compositions are nearly uniform from grain to grain and within the range of Mg# (=Mg x 100/(Mg+Fe) mol%) from 79 to 80. In the present L7 chondrites, the Ca content of opx is lower and the Ca-content of augite is higher than that of Y74160(LL7) and that of augite is higher, indicating that temperature of equilibration is lower than that of Y74160(LL7) (Takeda et al., 1984). The plagioclase compositions in the LL7(Y74160) chondrite show chemical zoning and their ranges differ from one place to another. The plagioclase compositions of FRO L7 chondrites also show minor chemical variation (Ab80-87), though it is not as large as Y74160. We interpret these zonings as resulted from plagioclase growth in localized areas where compositions of Ca-Na-Al-rich materials in their regions are slightly variable. For an internal heat source, such as radioactive decay, temperature increase and decrease would require longer time and plagioclase compositions would be homogenized. Impact process seems more likely heat source although shock features seem to have been erased by subsequent recrystallization. From mineralogical and chemical observations, these chondrites appear to show L6 chondrite-like features, although their textures are rather similar to those of some primitive achondrites. This type of chondrite may be on the road to becoming achondrites, but loss of low-temperature melts are not clearly shown in these samples. We thank EUROMET for the meteorite samples. References: Takeda H. (1989) Earth Planet. Sci. Lett. 93, 181- 194; Takeda H., Huston T.J., and Lipschutz M.E. (1984) Earth Planet. Sci. Lett. 71, 329-339; Palme H., Schultz L., Spettel B., Weber H.W., Wanke H., Michel-Levy M.C., and Lorin J.C. (1981) Geochim. Cosmochim. Acta 45, 727-752.
Baba Takayasu
Takeda Hidenori
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