Berry, FJ

Bland, PA

Menzies, ON

Spiral - Imperial College Digital Repository

  AN 57FE MÖSSBAUER STUDY OF THE METAMORPHIC SEQUENCE IN UNEQUILIBRATED ORDINARY CHONDRITES.  O. N. Menzies1,  P. A. Bland1 and F. J. Berry2. 1Planetary and Space Sciences Re-search Institute, The Open University, Milton Keynes MK7 6AA, UK, 2 Department of Chemistry, The Open Uni-versity, Milton Keynes MK7 6AA, UK.    Introduction:  Unequilibrated ordinary chondrites (UOCs), along with carbonaceous chondrites, are some of the most primitive materials in the solar system [1]. However, heterogeneic silicate compositions and fine-grained matrix make analysis difficult by conventional means.  Previous work [2] has shown that 57Fe Mössbauer spectroscopy can be used to identify chemical compo-sitional differences in equilibrated ordinary chondrites, particularly by looking at Mössbauer parameters as-cribed to olivine. The technique also provides informa-tion on the abundance of Fe-bearing phases in chon-drites. Therefore we chose to use Mössbauer spectros-copy to study mineral abundance, olivine chemistry and equilibration in UOCs. Methods:  A series of 18 UOCs collected in the Antarctic, and also several falls were examined by Mössbauer spectroscopy. Approximately 100mg of powdered meteorite was placed in a lead sample holder. The spectra were recorded at 298K with an 57Fe Mössbauer spectrometer using a 57Co/Rh source. Spec-tra were fitted with a constrained non-linear least squares fitting programme of Lorentzian functions.  Results and Discussion:   Mössbauer parameters. Three parameters reflect-ing the chemical environment of electrons at the nu-cleus are observed by Mössbauer analysis. One, quad-rupole splitting (QS), gives an indication of the sym-metry of the Fe site [3] and we have shown it to be correlated with silicate chemistry in chondrites [2].  The QS value for each chondrite was plotted against the degree of equilibration, or class. QS values increase with metamorphism. Figure 1. shows this rela-tionship strengthened by the addition of equilibrated ordinary chondrites and UOC falls.  The widths of olivine doublets in Mössbauer spectra can be correlated with degree of homogeneity [4]. We would expect a large line width for more unequili-brated samples. This observation is observed when line width is plotted against degree of equilibration, i.e. the graph is inverse to Figure 1.  Mineralogy trends.  Mössbauer spectroscopy gives information about the proportion of total Fe found in each Fe-bearing mineral component. These figures can be roughly correlated to wt% values although the ma-jor trends can still be observed from the Mössbauer values [5].                Figure 1. Graph showing how the Mössbauer parame-ter, quadrupole splitting, varies with increasing equili-bration. Included are UOCs (both falls and Antarctic finds) and EOC falls.  Graphs plotting the amount of each mineral against degree of equilibration for the Antarctic UOCs show some interesting relationships. The data was con-firmed once again by looking at UOC falls, which show similar trends. The percentage of silicates increases with equili-bration and varies inversely with a paramagnetic Fe3+ component, probably Fe-bearing clay. FeNi metal and troilite remain relatively constant. Antarctic H chon-drites do not show these trends possibly due to in-creased terrestrial alteration [6].  Ferric Fe is indicative of oxidative weathering in a hydrous environment [7]. It is important to be aware that these Antarctic meteorites are all finds and have therefore been undergoing alteration for some time. However, the fact that clear trends in the mineralogy are observed with increasing equilibration and are con-firmed by analysis of falls, suggests pre-terrestrial al-teration.  It indicates that there is significant primary phyllosilicate in most UOCs. During parent body metamorphism, phyllosilicates were dehydrated and converted to ferromagnesian silicates. References:  [1] Dodd, R. T. et al. (1967) GCA, 31, 921-951. [2] Menzies, O. N. et al. (2001) LPS XXXII Abst., # 1622. [3] Berry, F. J. (1983) Physics Bulletin 34, 517-519. [4] Mitra, S. (1992) Perg. Press, p9. [5] Bland, P. A. et al. (1997) LPI Technical Report, 97-02, 3. [6] Bland, P. A. et al. (1998) GCA, 62, 3169-3184. [7] Burns, R. G. in Pieters, C. M. and Englert, P. A. J. (1993) Cambridge Uni. Press. p539-556. 2.942.962.9833.022.5 3 3.5 4 4.5 5 5.5 6 6.5HLLLOlivine quadrupole splitting (mm/s)EquilibrationEquilibrated ordinaryUnequilibratedordinary chondrites chondrites/ Class64th Annual Meteoritical Society Meeting (2001) 5294.pdf

An 57Fe Mössbauer study of the metamorphic sequence in unequilibrated ordinary chondrites.

Abstract

Similar works

Full text

Available Versions

Spiral - Imperial College Digital Repository