Pursuing a program of analysis of chondritic Fe-Ni and inclusions therein, we have examined the metal of the unequilibrated chondrites Bishunpur (L3.1) and Semarkona (LL3.0). Inclusions (1 to 30 micrometers) of chromite, phosphate, silica, and silicates were found in the metal of moderately metamorphosed chondrites (Perron et al., 1990). In more primitive chondrites, tiny inclusions (<1 micrometer) are frequent and spread over most or all of the metal grains (Zanda et al., 1990; Perron and Bourot-Denise, 1992). They are made of chromite and, less frequently, silica. In Leoville (CV3), small sarcopsides (Fe phosphate) are also common. In Tieschitz (H3.6) and Krymka (LL3.1), sarcopsides are rare, but larger merrillites (Ca phosphate, a few micrometers across) are found, particularly at grain boundaries, in FeS or at the border between FeS and Fe-Ni. Apparently all the Cr, P, and Si in the metal of these chondrites reside in small, oxidised, inclusions. We believe inclusions formed by oxidation and exsolution from solid solutions in Fe-Ni, and are larger in equilibrated chondrites due to growth and chemical evolution during metamorphism. To determine when and where the exsolution occurred, we extended our work to Bishunpur and Semarkona, two of the least metamorphosed ordinary chondrites. Our electron microprobe analyses of the metal of Bishunpur agree with those of Rambaldi and Wasson (1981). However, from optical and SEM observations, we conclude that Cr and Si, when present in Fe-Ni, are totally gathered in small inclusions. Although P also forms small inclusions, generally along with Cr, most is in veins, or--in a few chondrules--in schreibersite patches. As noted by Rambaldi and Wasson and as we observed in other chondrites, the Si-bearing grains are inside chondrules and rimmed with troilite. The overall picture for Bishunpur metal thus fits well with that in the chondrites studied previously. Semarkona, on the other hand, appears very different. Its metal contains measurable amounts of Cr, P, or Si but probably in solid solution, as we did not see the corresponding inclusions. Metal grains in 6 chondrules were analysed: all contain Cr and P (up to 1 wt%), and those in one chondrule Si (up to 0.3 wt%). Within each chondrule, the Cr concentration varies little from grain to grain, much less than from chondrule to chondrule, as observed for Renazzo (Zanda et al., 1991). The concentrations of P and Si are much more variable. As in Leoville, none of the three elements has been detected in metal grains outside chondrules. If Cr, P, and Si are indeed in solid solution in Semarkona metal, then most probably, the inclusions seen in the metal of other chondrites were exsolved during metamorphism. These observations leave open the question of where and when Cr, P, and Si were incorporated into Fe-Ni. Moreover, if no inclusions were detected in Cr-, P-, and Si-bearing metal in Semarkona, scarce inclusions nevertheless exist in some metal grains of this meteorite. These are large (up to 30 micrometers), made of silica or silicate, and round as if made in the liquid state. Similar inclusions were also observed in Bishunpur and Renazzo (Zanda et al., 1991). Such inclusions, as well as the integration of dissolved elements in chondritic metal may be the result of an earlier, high temperature episode (Hewins and Zanda, 1992). References: Perron C., Bourot-Denise M., Pellas P., and Marti K. (1990) Meteoritics (abstract) 25, 398. Perron C. and Bourot-Denise M. (1992) Lunar Planet. Sci. (abstract) XXII, 1055. Rambaldi E.R. and Wasson J.T. (1981) Geochim. Cosmochim. Acta 45, 1001-1015. Zanda B., Bourot-Denise M., and Perron C. (1990) Meteoritics (abstract) 25, 422. Zanda B., Bourot-Denise M., and Perron C. (1991) Lunar Planet. Sci. (abstract) XII, 1543. Hewins R. H. and Zanda B. (1992) Meteoritics (abstract), this volume.
- Pub Date:
- July 1992