Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30s.577s&link_type=abstract
Meteoritics, vol. 30, no. 5, page 577
Mathematics
Logic
1
Chondrites, Iron-Nickel Metal, Meteorites, Krymka, Phosphates, Shock Metamorphism, Troilite
Scientific paper
Shock effects of various intensities are common in all kinds of meteorites, but, in ordinary chondrites, the most severe ones are observed mostly in metamorphosed chondrites (petrologic types 5 and 6), and they are rare in type 3 [1]. However, we report here observations of strong shock effects in a specimen of the Krymka (LL3.1/S3) chondrite, one of the three most primitive ordinary chondrites. Examination of various samples of Krymka in the Ukrainian meteorite collection has already revealed evidences of shock effects, indicative of pressures of 25-45 GPa (assuming non-porous material) and temperatures <= 500 degrees C, with local excursions to >=988 degrees C (melting of Fe-FeS eutectic) [2]. Sample N1290/29 (134 g), from the same collection, contains light-colored, friable, completely melted zones. Observations were made on 6 polished sections from that specimen, with a total area of 14 cm2. In 4 of these, 4 melted regions occupy a total of ~3 cm2. They are chondrule-free and consist of porphyritic and skeletal silicates (mainly olivine) in a cryptocrystalline mesostasis, along with metal-troilite mixtures with dendritic and cellular structures. Olivine composition is variable (Fa(sub)10-26) but to a lesser extent than in Krymka chondrules (Fa(sub)0-35). The mesostasis is also inhomogeneous. Apart from a few notable exceptions, olivine crystals are normally zoned, with FeO increasing from core to rim. Fe-Ni cells are zoned as well, consisting of a core with 11.2-22.6 wt%Ni and 0.83-0.96 wt%Co (probably martensite), and a Ni-rich rim (up to 51.3 wt%Ni, probably tetrataenite). The metal contains P (0.12-0.43 wt%), which seems to be, at least in part, in tiny schreibersite inclusions, and the troilite contains Ni (0.05- 4.2 wt%Ni). Metal-troilite mixtures contain abundant globules (up to 15 microns in diameter) of a Fe-Na phosphate (maybe maricite). These globules are usually rimmed with numerous euhedral micrometer-sized chromites. Larger euhedral chromites also occur isolated in metal-troilite. The melted regions are surrounded by a transition zone with chondritic texture, containing completely melted troilite and partially melted metal. The highly variable Co concentration (0.2-13.0 wt%) of the metal grains in this zone reflects the high degree of unequilibration of the Krymka chondrite. Shock pressure in the range 75-90 GPa is normally required to induce local complete melting of chondritic matter, but, in the present case, pressure as low as 30 GPa may be responsible for the observed effects, because of the porous nature of type 3 material [1]. In any case, the temperature must have been larger than 1450 degrees C. Experimental data on zoning trends in olivine [3] and Ni content in troilite [4] suggest a cooling rate of at least 100 degrees C/h. The association of P and Cr with metal-troilite reflects their association with metal in the host: Ca- phosphates are common at the boundaries of Krymka metal grains and many of these contain myriads of tiny chromites [5]. P and Cr were probably reduced at high temperature and they dissolved in metal-troilite liquid. Upon cooling, they reoxidized and crystallised. The unusual phosphate may result from higher volatility and mobility of Na relative to Ca. Mechanical deformations (shear) of some of these phosphate globules and of melted troilite attest that, after complete solidification of the melted regions, Krymka was subjected to other, less intense shock(s). References: [1] St"ffler D. et al. (1991) GCA, 55, 3845. [2] Semenenko V. P. et al. (1987) The Meteorites of Ukraine, 218 pp., Naukova Dumka (in Russian). [3] Radomsky P. M. and Hewins R. H. (1990) GCA, 54, 3475. [4] Smith B. A. and Goldstein J. I. (1977) GCA, 41, 1061. [5] Perron C. and Bourot-Denise M. (1992) LPS XXIII, 1055.
Perron Claude
Semenenko Vera P.
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