Replacement textures in CAI and implications regarding planetary metamorphism

Abstract

Formation by a secondary metamorphic event, rather than primary crystallization from a melt or a sequential nebular condensation, is indicated by textural and chemical features of five coarse grained, Ca- and Al-rich inclusions (CAI) from the Allende meteorite which contain embayed pyroxene surrounded by melilite. It is suggested that the most probable environment for a metamorphic process (requiring the addition of Ca derived from calcite or from the introduction of a fluid phase) is that of a small planetary body, rather than the solar nebula. These results are compatible with O isotopic heterogeneities within CAI, and offer a mechanism for the production of lower temperature alteration phases, together with the rim phases found in these inclusions.