Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..503e&link_type=abstract
Meteoritics, vol. 30, no. 5, page 503
Mathematics
Logic
1
Acapulco, Chromite Zoning, Graphite, Isotopes, Carbon, Nitrogen
Scientific paper
Acapulco is considered to be a link between primitive chondritic meteorites and the differentiated achondrites. Its parent body presumably formed by accretion of material of chondritic compositions at an fO2 that lies between that of H- and enstatite chondrites [1]. The accreted chondritic material was subjected 4.557 Gyr ago to peak temperatures close to 1200 degrees C that lead to partial melting and extensive recrystallization [1, 2]. Seven morphologically different types of graphite with large variations in C- and N-isotopic compositions were recently reported from Acapulco [3, 4]. At least four distinct isotopic reservoirs are required to explain the C- and N-isotopic compositions of these graphites [3, 4]. While the silicate minerals in Acapulco have isotopically heavy N (delta^(15)N = + 15 per mil) chromites were found to be isotopically light (delta^(15)N = _ 75 to _ 82 per mil). Chromite occurs in Acapulco in six different assemblages: (1) as inclusions in silicates, (2) in FeNi, (3) in troilite, (4) with FeNi and troilite, (5) with FeNi and silicates, and (6) with troilite and silicates. It is also rarely present as small idiomorphic inclusions in plagioclase. Chromites in contact with silicates display no chemical zoning for Cr, Al, Ti, Fe, Mg, Mn, or Zn to the silicate borders thus indicating high degree of equilibration with the silicate neighbours. The MgO-contents of chromites in metals and troilites (4.74 to 7.2 %) are relatively lower and their compositional ranges are relatively wider than those in contact with silicates (6.1 to 7.69 %). Zoning profiles of MgO and FeO in chromites in all assemblages are quite flat. Chromites in contact with metals and troilite display a variety of zoning patterns of Cr, Al, Ti, and Zn. All these chromite types , however, depict the same MnO zoning trends with low MnO-contents in their cores (0.96 to 2.14 %) than in their rims to metal or troilite (1.7 to 3.1 %). With few exceptions, the zoning behaviour of Cr, Al, and Ti does not follow a substitutional scheme. Chromites with reverse Cr-zoning (61.3 wt. % Cr2O3 in the cores and 63.2 Wt. % Cr2O3 at the rims ) may have either flat Al2O3 - patterns (5.46 - 5.53 wt. %) or normal zoning trends (5.6 wt. % in the core and 4.81 wt. % at the rim). Some grains display prominent complementary Cr2O3- and Al2O3- zoning patterns (62.2 % wt. Cr2O3 and 2.9 wt. % Al2O3 in the Core; 58.9 wt. % Cr2O3 and 5.7 wt. % Al2O3 at the rim). In those grains the zoning profiles of TiO2 and ZnO (Figure 1) are similar to those of Al2O3 (in the core 1.33 wt. % TiO2, 1.63 wt. % ZnO; at the rim 0.67 wt. % TiO2, 1.24 wt. % ZnO). The well developed zoning of Cr, Al, Ti, Mn, and Zn from the cores of chromites to their borders to FeNi and troilite and the variability of the zoning patterns in assemblages containing FeNi and troilite indicate that the encountered zoning types reflect the primordial chemistry of these chromites in the parental material before melting. We have delineated six different types of zoning in Acapulco chromites so far. None of the encountered zoning patterns could have developed by crystallisation from a chondritic melt. The present results support the previous findings [3, 4] that several sources must have had contributed to the parental material of Acapulco. However, genetic correlations between the isotopically different graphite morphologies and the various chromites in Acapulco could not be established so far. References: [1] Zipfel et al. (1995) GCA, in press. [2] G"pel D. et al. (1992) Meteoritics, 27, 226. [3] El Goresy A. et al. (1995) Nature, 373, 496-499.[4] El Goresy A. and Zinner E. K. (1995) LPS XXVI, 367-368. [5] Sturgeon G. and Marti K. (1991) Proc. LPS, Vol. 21, 523-525. [6] Kim Y. and Marti K. (1994) LPS XXV, 703-704. Fig.1. Zoning profiles for Cr2O3, Al2O3, MnO, ZnO, and TiO2 in chromite # 1 enclosed in troilite.
El Goresy Ahmed
Janicke J.
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