COMPOSITIONAL CONTINUITY OF ENSTATITE CHONDRITES AND IMPLICATIONS FOR HETEROGENEOUS ACCRETION OF THE ENSTATITE CHONDRITE PARENT BODY

Abstract

Chemical compositions of eleven enstatite chondrites (1 EH5, 4 EH3, 3 EL3, 1 EL5, and 2 EL6) and their metal and nonmagnetic fractions determined by instrumental neutron activation analysis are reported. The abundances of nonvolatile lithophile elements, such as Al, Sc, and Mg, increase from EH to EL, while those of siderophile and moderately volatile elements decrease in the sequence of EH5, to EH3, EL3, and EL5,6. The continuity in compositions of enstatite chondrites and, in particular, the inverse variations of moderately volatile element abundances with petrographic type between EH and EL groups demonstrate that enstatite chondrites have been derived from a common parent body. The enstatite chondrite parent body formed by heterogeneous accretion of materials available in the accretional region; metal was effectively accreted into the core where EH5 chondrites were derived, and the abundances of metal decreased as accretion proceeded. In complement, silicate abundances gradually increased in the layers that accreted later.The lack of correlation between volatile element abundances and metamorphic degree demonstrates that losses of moderately volatile elements from enstatite chondrites cannot have resulted from parent body processes. Furthermore, it is expected that during accretion various components would mix and thus erase any early fractionation of moderately volatile elements of a single parent body. Variations of ambient gas temperatures during accretion are also impossible to produce the volatile element pattern of enstatite chondrites. It is suggested that moderately volatile elements in the enstatite chondrites were lost during local heating, the chondrule formation process. Formation of chondrules appears to have proceeded during the accretion of the enstatite chondrite parent body and lasted for a certain period.The mineralogical and textural features of enstatite chondrites can be explained in terms of two stages of metamorphism. The metamorphic trend from EH3 to EH5 resulted from internal heating whereas the trend from EL3 to EL6 was due to an external heat source. It seems very likely that the EH chondrites were metamorphosed during accretion, thus cooled rather rapidly. Activities of the early sun could serve as an energy source for the external heating.