IN-SITU FORMATION OF PALISADE BODIES IN CALCIUM, ALUMINUM-RICH REFRACTORY INCLUSIONS

It has been suggested that palisade bodies-shells of spinel found with in some calcium, aluminum-rich inclusions (CAIs) and the phases the sh ells enclose-are intact mini-CAIs that predate and were captured by th eir current hosts while the latter were still molten. We present new d ata and observations that indicate that most palisade bodies formed in stead in situ while their host inclusions were crystallizing The evide nce includes observations of spinel-lined cavities and grass-filled, c ircular structures outlined by spinel in experimental run products cry stallized from melts, a partially formed palisade body in an inclusion ; a fassaite crystal that is optically continuous across a palisade wa ll; and similarity of unusual mineral compositions in some palisade bo dies and their hosts. Our observations can be used to refute arguments for exotic origin and are most consistent with a model for in situ fo rmation involving: (1) formation of vesicles in a largely molten inclu sion; (2) nucleation of spinel upon and/or adherence to vapor-melt int erfaces, forming spinel shells around vesicles; (3)leakage of vesicles and filling with melt while spinel shells remain largely intact; and (4) crystallization of melt inside shells. This model is similar to on e proposed for formation of segregation vesicles, which are partially- to completely-filled vesicles found in some terrestrial basalts. In a ddition, we interpret framboids (i.e., dense clusters of spinel with l ittle material between grains, found in most inclusions that contain p alisade bodies) as polar or near-polar sections through palisade bodie s and therefore do not make a genetic distinction between the two feat ures.