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

Chemical compositions of eleven enstatite chondrites (1 EH5, 4 EH3, 3 EL3, 1 EL5, and 2 EL6) and their metal and nonmagnetic fractions deter mined by instrumental neutron activation analysis are reported. The ab undances of nonvolatile lithophile elements, such as Al, Sc, and Mg, i ncrease from EH to EL, while those of siderophile and moderately volat ile elements decrease in the sequence of EH5, to EH3, EL3, and EL5,6. The continuity in compositions of enstatite chondrites and, in particu lar, the inverse variations of moderately volatile element abundances with petrographic type between EH and EL groups demonstrate that ensta tite chondrites have been derived from a common parent body. The ensta tite chondrite parent body formed by heterogeneous accretion of materi als available in the accretional region; metal was effectively accrete d into the core where EH5 chondrites were derived, and the abundances of metal decreased as accretion proceeded. In complement, silicate abu ndances gradually increased in the layers that accreted later. The lac k of correlation between volatile element abundances and metamorphic d egree demonstrates that losses of moderately volatile elements from en statite chondrites cannot have resulted from parent body processes. Fu rthermore, it is expected that during accretion various components wou ld mix and thus erase any early fractionation of moderately volatile e lements of a single parent body. Variations of ambient gas temperature s during accretion are also impossible to produce the volatile element pattern of enstatite chondrites. It is suggested that moderately vola tile elements in the enstatite chondrites were lost during local heati ng, the chondrule formation process. Formation of chondrules appears t o have proceeded during the accretion of the enstatite chondrite paren t body and lasted for a certain period. The mineralogical and textural features of enstatite chondrites can be explained in terms of two sta ges of metamorphism. The metamorphic trend from EH3 to EH5 resulted fr om internal heating whereas the trend from EL3 to EL6 was due to an ex ternal heat source. It seems very likely that the EH chondrites were m etamorphosed during accretion, thus cooled rather rapidly. Activities of the early sun could serve as an energy source for the external heat ing. Copyright (C) 1997 Elsevier Science Ltd.