Chondrule formation models involving precursors of granoblastic olivine aggregates (GOA) of either planetesimal or nebular origin have recently been proposed. We have therefore conducted chondrule simulation experiments using mixtures of 100 h-thermally annealed GOA and An + En to test the viability of GOA as predecessors of porphyritic olivine (PO) chondrules. Isothermal runs of less than 5 min at 1350-1550 °C result in GOA disaggregation and Fe-Mg exchange; runs of 0.5-4 h show textures superficially similar to granular and PO chondrules, but with reversely zoned olivine. Charges isothermally heated at 1550 °C for 1 and 4 h before being cooled at 10 and 100 °C/h undergo olivine crystallization and yield classical PO textures. Although most evidence of origin from GOA is erased, the cores of normally zoned euhedral crystals are relict. As 'phenocrysts' in Type I chondrules can be relict such chondrules could have experienced similar peak temperatures to those of Type II chondrules. Chondrules containing GOA with olivine triple junctions resemble experimental charges heated for minutes at temperatures between 1350 and 1450 °C and Type I chondrules with subhedral to anhedral olivine plus GOA relicts resemble charges heated at the same temperatures but for longer duration. Type I chondrules with a mass of granular olivine or irregular, anhedral olivine grains in the center, and much glass nearer the margin, on the other hand, require limited heating at high temperature (1550 °C) while Type I chondrules with euhedral olivines, resemble charges heated at 1550 °C for 4 h. The majority of Type I chondrules in CV chondrites display evidence of derivation from GOA. Many finer-grained chondrules in CR and UOC on the other hand, could not have been derived from such coarse-grained precursors, but could have formed from fine-grained dustballs as stipulated in the standard paradigm. Thus, both GOA and dustballs represent viable chondrule precursors of coarser and finer-grained Type I PO chondrules, respectively.
Bibliographical noteFunding Information:
We would like to thank P. Jones for access to and assistance with the JEOL 6400 SEM in the Department of Earth Sciences at Carleton University, and Frédéric Couffignal for help with the Cameca SX100 at the Université Paris VI. J. Boesenberg is thanked for the maintenance and frequent repairs of the furnace and related lab materials and instruments. We are indebted to Brigitte Zanda and Bertrand Devouard for permission to use their unpublished BSE images of chondrules, and to R. Jones, S. Krot, G. Libourel, G. Lofgren and H. Nagahara whose published micrographs we have reproduced. Reviews by G. Libourel and H. Palme aided us greatly in improving the manuscript. This work was funded by NASA grants NNG05GK11G and NNX08AG62G (Cosmochemistry) awarded to R.H.H.
ASJC Scopus subject areas
- Geochemistry and Petrology