Compositional Heterogeneity Metal Grains of Primitive Chondrites: Onset of Metamorphism

Details Compositional Heterogeneity Metal Grains of Primitive Chondrites: Onset of Metamorphism Compositional Heterogeneity Metal Grains of Primitive Chondrites: Onset of Metamorphism

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.327b&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 327

Other

Bishunpur, Chondrites, Chondrules, Iron-Nickel, Metamorphism, Murchison, Renazzo, Semarkona

Scientific paper

Previous studies have shown that unequilibrated chondrites, Cr-, P- and/or Si-rich metal were present in small oxidized inclusions in the metal grains [1-3]. This was interpreted as the result of oxidation and exsolution from solid solution during metamorphism, an assumption largely based on the apparent absence of such inclusions in the metal of Semarkona [3] and Renazzo [4], the least metamorphosed of the chondrites studied. To better characterize the most primitive chondritic metals, we have continued further the study of Fe,Ni grains in these latter chondrites. Careful observations of Semarkona metal with backscattered electrons in the SEM reveal that it is not as homogeneous as we first thought. In addition to the rounded SiO2 inclusions described earlier, [3] it generally contains inclusions of varying abundance, in the size range 0.1-0.5 micrometers, whose constituents are Cr, S, and P (and possibly Fe and Ni). The variations of the P/Cr ratio (P is even absent in some inclusions) let us think that we are dealing with the association of two minerals, possibly a Cr sulphide (daubreelite?) and schreibersite. Less frequent inclusions, found in Si-bearing Fe,Ni, have no detectable S and may be made of a Cr phosphide similar to that observed in high-Si metal of ALH85085 [5,6]. Larger schreibersite inclusions (~2 micrometers) have been found in the metal of one chondrule. In the rare Si-rich metal grains, Si is also found in inclusions, which may consist in perryite, associated or not with Cr and P-bearing minerals. It is not clear at the moment whether Cr, P, or Si are also partly in solid solution in Fe,Ni. Metal inhomogeneity in Semarkona is not restricted to minor elements. Small, often elongated zones of Ni-rich metal are common, with varying abundance and sizes in the range 0.33 micrometers. Their composition is ~50 wt% Ni, while the metal in which they lie has 3-4 wt% Ni. As already noted for Renazzo, [4] the properties of the metal appears to be largely determined by the chondrule formation. The composition, size and abundance of the inclusions, as well as the bulk composition of the Fe,Ni grains (measured with a defocused beam in the EMP), are very similar in all grains of a given chondrule, while the variations are large from chondrule to chondrule. Large metal grains outside chondrules have comparable properties, which supports the idea that they, too, were once processed in chondrules. The survey of Renazzo metal has been less extensive up to now. Comparable inclusions have been seen, but are much less common. On the other hand, we have not found anything similar in the metal of either Murchison or Bishunpur. It is too early to draw definitive conclusions from our observations. It seems however likely that we are seeing the effects of a low temperature metamorphism on metal grains whose compositions were fixed at the time of chondrule formation. It is thus understandable that these effects are less pronounced in Renazzo than in Semarkona, and absent in Murchison. This mild metamorphism has not had the oxidizing effects on the metal seen in slightly more metamorphosed chondrites like Bishunpur. Conversely, the stronger metamorphism suffered by the latter erased or bypassed the effects seen in Semarkona, and led to the formation of different, oxidized, inclusions. In both chondrites, larger rounded SiO2 inclusions are interpreted as being due to oxidation at high temperature during chondrule cooling [7]. As progress is made in the small scale observation and analysis of the metal of chondrites, it becomes clearer and clearer that this phase bears the marks of at least two processes: chondrule formation, and metamorphism. Comparative studies of chondrites that suffered different conditions of metamorphism undoubtedly helps decipher the effects related to each of these processes. References: [1] Zanda B. et al. (1990) Meteoritics, 25, 422-423. [2] Perron C. and Bourot-Denise M. (1992) LPS XXIII, 1055-1056. [3] Perron C. et al. (1992) Meteoritics, 27, 275. [4] Zanda B. et al. (1991) LPS XXII, 15431544. [5] Kimura M. and El Goresy A. (1989) Meteoritics, 24, 286. [6] Zanda B. (1992) LPS XXIII, 1569-1570. [7] Hewins R. H. and Zanda B. (1992) Meteoritics, 27, 233.

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