Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.597w&link_type=abstract
Meteoritics, vol. 30, no. 5, page 597
Computer Science
1
Chondrites, Ordinary, Fayalite, Matrix, Meteorites, Chainpur, Kaidun, Krymka, Ngawi, Olivine
Scientific paper
INTRODUCTION. Matrix persists as one of the most poorly characterized chondritic components. Its aggregational nature makes it an excellent place to search for primitive chondritic components that prevailed in the nebula during and after chondrule formation as well as components recording processes that predated and postdated accretion. In this study we focus on the occurrence and formation of the fayalitic olivine in the matrix of the Krymka LL3.1 unequilibrated ordinary chondrite. RESULTS. We limited our study to matrix areas clearly sandwiched between chondrules and did not include chondrule rims. In Krymka, matrix is coarser-grained and more Fe-rich than the rim material. Matrix is also highly variable in the size, shape and composition of its components, whereas chondrule rims appear more uniform. Krymka matrix is an aggregation of diverse mineral and lithic components. Mineral components include olivine, enstatite, diopsidic pyroxene, Ti-Al-rich Ca-pyroxene, hedenbergite, amorphous silicate material, spinel, oxides, troilite, and metal. Olivine is clearly dominant (~75% normative) and occurs in a variety of textures and compositions. Fayalitic olivine (Fa(sub)(58-94), avg.=Fa(sub)(72)) is ubiquitous throughout the matrix and occurs as (1) Isolated platelets (typically 1-3 micrometers x 3-5 micrometers, with some up to 10 micrometers in length), (2) Platelet clusters, which include randomly oriented platelets and/or intergrown platelets, (3) Platelet overgrowths which are overgrowths of parallel platelets on surfaces of larger (10-300 micrometers), more magnesian (Fa(sub)(4-34), avg.=Fa(sub)(19)) olivine fragments, (4) Euhedral-subhedral crystals (1-10 micrometers) which are often associated with and compositionally similar to platelets, and (5) Fluffy aggregates - irregularly shaped porous aggregates of submicron crystals. TEM study of the overgrowths reveals that the direction of elongation of the fayalitic platelets is along the c axis corresponding with the c direction of the larger olivine substrate. The larger, more magnesian olivine fragments are generally single crystals, but in some cases are associated with pyroxene (Fs(sub)(5-27),Wo(sub)(0.5-2)) or high-Ca pyroxene. These larger olivines are compositionally similar to chondrule olivines, whereas the fayalitic platelets are texturally and compositionally unlike olivines in chondrules. Fayalitic olivine with morphologies similar to those in Krymka matrix occur in the Chainpur LL3.4 and Ngawi LL3 matrix, but are much less common. These textures may have been characteristic of all primitive ordinary chondrite matrix, but were generally overprinted by metamorphic recrystallization. DISCUSSION: Fayalitic olivine in Krymka matrix records a process that has important implications for understanding the evolution of ordinary chondrites. It may form under oxidizing nebular conditions through solid state reactions in the presence of free silica, or vapor-solid reactions in a gas with a high silica activity [1,2]. Textural observations have been used to support a nebular origin for similar fayalitic olivine in CV3 matrix, and thermodynamic calculations indicate it could form in a nebula with a supersolar H2O/H2 ratio [3]. Vaporization experiments show that at ~10^(-6) bar and ~1650K olivine evaporates incongruently to produce a fayalitic vapor [4]. However, textural arguments favoring post-accretion formation of the fayalitic olivine in ordinary chondrite matrix and in CV3 dark inclusions have also been presented [5,6]. Thus, we consider three hypotheses for the formation of the fayalitic olivine in Krymka matrix: (1) vapor-solid reactions between a silica-rich vapor and metallic Fe degrees in the nebula, (2) vaporization of olivine-rich material to produce a fayalite vapor, followed by recondensation, or (3) parent body heating/dehydration of pre-existing phyllosilicates. Although the platy layered structure of some of the fayalitic olivine is suggestive of the layered structures of phyllosilicates and some fayalitic olivine texturally resembles saponite replacing olivine (as observed in the Kaidun CR chondrite), the platelet overgrowths on larger olivine crystals probably represent growth features and not a replacement of phyllosilicates. The platy morphology of fayalitic olivine could also indicate growth from a vapor consistent with hypotheses 1 and 2. The wide range in compositions of associated fayalitic olivine platelets suggests that they did not all form in the same environment and were not in contact under high enough temperatures to result in equilibrium; therefore, we conclude that the fayalitic olivine formed in a nebular environment. The fayalitic olivine platelets and associated fayalitic olivine in Krymka matrix may record vapor solid reactions under oxidizing nebular conditions or partial evaporation of a more Mg-rich olivine to produce a fayalite vapor, followed by recondensation. Formation through heating/dehydration of phyllosilicates is less likely. References. [1] Nagahara H. (1984) GCA, 48, 2581-2595. [2] Nagahara H. and Kushiro I. (1987) EPSL, 85, 537-547. [3] Hua X. and Buseck P. R. (1995) GCA, 59, 563-578. [4] Nagahara H. et al. (1994) GCA, 58, 1951-1963. [5] Alexander C. M. et al. (1989) EPSL, 95, 187-207. [6] Kojima T. and Tomeoka K. (1994) Meteoritics, 29, 484.
Prinz Martin
Weisberg Michael K.
Zolensky Michael E.
No associations
LandOfFree
Fayalitic Olivine in Matrix of the Krymka LL3.1 Chondrite does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Fayalitic Olivine in Matrix of the Krymka LL3.1 Chondrite, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fayalitic Olivine in Matrix of the Krymka LL3.1 Chondrite will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-830994