In situ formation of palisade bodies in Ca-,Al-rich refractory inclusions

Details In situ formation of palisade bodies in Ca-,Al-rich refractory inclusions In situ formation of palisade bodies in Ca-,Al-rich refractory inclusions

Jan 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997m%26ps...32...61s&link_type=abstract

Meteoritics, vol. 32, pages 61-70

Computer Science

5

Scientific paper

It has been suggested that palisade bodies (shells of spinel found within some CAIs and the phases the shells enclose) are intact mini-CAIs that predate and were captured by their current hosts while the latter were still molten. We present new data and observations that indicate that most palisade bodies instead formed in situ while their host inclusions were crystallizing. The evidence includes observations of spinel-lined cavities and glass-filled, circular structures outlined by spinel in experimental run products crystallized from melts; a partially formed palisade body in an inclusion; a fassaite crystal that is optically continuous across a palisade wall; and similarity of unusual mineral compositions in some palisade bodies and their hosts. Our observations can be used to refute arguments for exotic origin and are most consistent with a model for in situ formation involving: 1) formation of vesicles in a largely molten inclusion; 2) nucleation of spinel upon and/or adherence to vapor-melt interfaces, 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 one proposed for formation of segregation vesicles, which are partially- to completely-filled vesicles found in some terrestrial basalts. In addition, we interpret framboids (dense clusters of spinel with little material between grains, found in most inclusions that contain palisade bodies) as polar or near-polar sections through palisade bodies, and therefore do not make a genetic distinction between the two features.

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