IMPLICATIONS OF POIKILITIC TEXTURES IN LL‐GROUP CHONDRITES

Abstract

Lithic fragments in LL‐group chondrites commonly have poikilitic textures, in part or in whole, where mainly olivine is enclosed by orthopyroxene. Partially poikilitic fragments also have grano‐blastic areas and anhedral olivine larger than the olivine enclosed by pyroxene. In analogy to lunar poikilitic rocks and lithic fragments, poikilitic lithic fragments in LL‐group chondrites, i.e., meteorites which are highly brecciated due to repeated impacts, are also interpreted as being related to impact events on meteorite parent bodies where melting and reheating of protolith occurred. Compositional characteristics of minerals in certain fragments, such as highly‐unequilibrated clinopyroxene (CaO, 14.5 to 17.3 wt %; Al2O3, 6.7%) and relatively high CaO (0.70 to 2.5 wt %) in orthopyroxene in a Ngawi fragment, seem to indicate a melt origin. However, as in the lunar case, it is difficult to decide whether the meteoritic poikilitic textures resulted from complete or partial melting or largely by solid‐state recrystallization, although the large olivines that may be relict crystals appear to indicate that at least partial melting was involved. In all probability, all three processes are responsible for the poikilitic textures in chondrites, since temperature regimes produced by impact processes are likely to range widely. These interpretations may also apply to the poikilitic‐textured Shaw chondrite, L‐group, which may owe its poikilitic texture to impact partial‐melting processes while in the parent body regolith.