Granoblastic olivine aggregates (GOA) have been discovered in some Type I magnesian chondrules within carbonaceous chondrites by Libourel and Krot [Libourel, G., Krot, A.N., 2007. Evidence for the presence of planetesimal material among the precursors of magnesian chondrules of nebular origin. Earth Planet. Sci. Lett. 254, 1-8], who proposed an origin from pre-existing planetesimals. Amoeboid olivine aggregates (AOA), generally considered as aggregates of solar nebula condensates and found within similar carbonaceous chondrites, display similar equilibrium texture, though on a finer scale. For these reasons, we conducted experiments to determine if annealing of olivine required time scales appropriate to planetesimal or nebular heating. Pressed < 43 μm and < 63 μm San Carlos olivine powder (Fo88.4) was isothermally heated at temperatures ranging from 1350-1550 °C for 1-100 h. The 100 h runs yield olivine aggregates with well-developed granoblastic texture at all temperatures, manifest as a network of randomly-oriented and sutured olivine grains with 120° triple junctions. Individual olivine grains are 4-6 sided and polygonal by 1450 °C and equigranular texture is developed at high temperature (1500-1550 °C). Melting of olivine commences at 1450-1500 °C and aids in 'ripening' and suturing (grain coarsening and grain boundary migration). Textural equilibrium is clearly met at 1550 °C. A planetesimal origin cannot be ruled out; however, the experimental evidence reveals that granoblastic texture can be reproduced in an interval not inconsistent with heating times for nebular objects. GOA may have experienced higher degrees of thermal processing than the finer-grained AOA. If the precursors were the same, grain coarsening would have to be accompanied by modification to bulk and isotopic compositions. However, the precursors could have been olivine condensates formed later than AOA. Annealing may have been a widespread process operating in the primordial solar nebula responsible for thermal processing and formation of GOA prior to their incorporation into chondrules.
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We would like to thank two anonymous reviewers for supportive, constructive reviews. We also thank P. Jones for the use and assistance in operation of the JEOL 6400 SEM in the Department of Earth Sciences at Carleton University and T. Wang and L. Kleine for their help in the operation of the JEOL JSM-5600LV SEM at Rutgers University. J. Boesenberg and J. Delaney are thanked for their help in the maintenance of the furnace and related lab materials and instruments and interpretation of data. P. Bland, L. Howard, A.N. Krot, G. Libourel, and B. Zanda are thanked for various insights. This work was funded by NASA grant NNG05GK11G (Cosmochemistry) awarded to RHH.
- Granoblastic olivine aggregates (GOA)
- magnesian chondrules
- solar nebula
- thermal annealing