Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.536l&link_type=abstract
Meteoritics, vol. 30, no. 5, page 536
Computer Science
2
Scientific paper
Whether or not chondritic metal results from equilibrium condensation in the nebula [1], or reflects reduction during chondrule formation [2] is still a matter of debate. Tiny metal globules are found both in matrix of primitive meteorites and in olivine-pyroxene bearing chondrules, suggesting that reduction reactions could have been established either before or during chondrule formation. The causes of reduction processes are also controversial, either controlled by the reduced protosolar atmosphere or due to the presence of reduced carbon in the chondrule precursors [3]. These issues are of fundamental importance in understanding chondrule formation processes and conditions. In addition, because chondrules are the raw material for the Earth accretion, the understanding of the processes controlling the formation of metal (and its compositions) might be also essential for the understanding of the early differentiation of the Earth. In order to shed light on metal formation in chondrules, a series of reduction experiments were carried out to establish 1. the timing of the reduction 2. the compositional effects of reduction and 3. the causes of the reduction. San Carlos olivines, Fa16 with trace amounts of Ni, Co, P, Mn, were used as starting material, this composition being close to that of chondrules on average. Olivines were ground to 50-100 m in order to be comparable to chondrule precursors. Experiments were run in a 1 atmosphere vertical furnace, in the temperature range 1550 to 1650 C and under different oxygen fugacities (IW-1 to C-CO buffer). These reduced atmospheres were imposed by a flux of different proportions of gases (CO, CO2, H2 and Ar) or by using graphite capsules with a flux of pure CO gas. For a given T and fO2, experiments were performed with time scales of 5 mn to 8 hours, and terminated by quenching the run products in dry conditions at 500 C/sec. Each experiment produced an assemblage of olivines (Fo-rich), silicate glass and metal globules (Fe-Ni) either as tiny blebs (< 1m) included in olivine or as globules (1 to 50 m) located in the silicate melt at the olivine grain boundaries. The textural features are very similar to those observed in natural unequilibrated ordinary chondrules. Indeed, olivines may or may not show a dusty appearance, with or without preferential alignment of metal blebs in the same run products. Owing to these experiments, it is also possible to specify unambiguously the mechanism for the reduction reaction: Olivine (Fa 16) > Olivine (< Fa16) + Si-glass + Fe metal + O2. In term of composition, olivine, Fe metal and glass are drastically dependent on the imposed oxygen fugacity, run duration and temperature. Within the experimental conditions, olivines vary from Fa 16 to Fa 0.15, Fe metal from 60 wt% Ni to 2 wt% Ni, and glass from silica-poor and iron-rich composition to silica-rich and iron-poor composition. In general, olivine becomes more forsteritic as oxygen fugacity decreases and run duration increases, and for a fixed oxygen fugacity, the Ni content of metal phases shows a drastic decrease as run duration increases. Moreover, these data show that the rate of this reduction process is strongly sensitive to the temperature and the nature of the reducing agent. In the light of these textural and compositional data, this study suggests that metal in chondrules can be produced on a time scale relevant for chondrule formation by reduction reactions and that these processes could also explain the main textural and compositional features of olivine and metal observed in natural chondrules. References: [1] Grossman L. and Olsen E. (1974) GCA, 38, 173-187. [2] Scott E. R. D and Taylor G. J. (1983) Proc. LPSC 14th, in JGR, 88, B275-B286. [3] Connolly H. C. Jr. et al. (1994) Nature, 371, 136-139.
Chaussidon Marc
Libourel Guy
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