La pierre de Sena : des informations sur les conditions de formation des chondrites à bronzite

Résumé En Fr

The Sena stone, an equilibrated bronzite chondrite with well delineated chondrules, has not suffered any shock after its agglomeration and therefore is suitable for the study of formation process. Most of the isolated olivine and orthopyroxene crystals have a rim grown on a fragmented core. The defects in the olivine cores are those of crystals broken at high temperature while those of the rim are growth dislocations (Poirier, oral commun.). Olivine composition is Fa17_18 with 0.02 CaO ; bronzite is Fs15_17 ; the minor element distribution shows a greater homogeneity in the rim than in the cores of both olivines and orthopyroxenes. All the free surfaces of crystals (silicates, FeNi, FeS) show crystallographic planes and growth steps. Some droplet chondrules are sticking on rock fragments. These observations appear to be compatible with the formation of chondrules by fragmentation of a preexisting rock at high temperature (1300-1000° C) followed by a rapid agglomeration in a transient atmosphere where the crystals have achieved their present aspect beginning with a rapid cooling (in the range 1000-700° C) followed by a much slower one. Some of the Sena characteristics can be recognized in many H4-H5 chondrites though more or less obscured by subsequent shock event(s) . The fragmentation at high temperature followed by a rapid agglomeration in these stones contrasts with the much slower agglomeration of the type 3 chondrites where each chondrule cools independently and collects low temperature fines on its surface before accretion.

La chondrite de Sena est un type H 4-5 qui n'a pas subi de choc après son agglomération. Elle est donc favorable à l'étude des processus de formation. On étudie particulièrement la croissance orientée des cristaux d'olivine et d'orthopyroxène et les défauts de l'olivine. La formation des chondres pourrait s'expliquer par la fragmentation à chaud d'une roche préexistante ; l'agglomération lui succéderait immédiatement dans une atmosphère transitoire, et le refroidissement — d'abord rapide — deviendrait progressivement très lent dans une couche du corps parent peu éloignée de la surface.