Spinel group minerals in LL3.00–6 chondrites: Indicators of nebular and parent body processes

Abstract

We carried out a systematic study of spinel group minerals in LL3.00–3.9 and LL4–6 chondrites. With increasing petrologic type, the size and abundance of spinel increase. The compositions of spinel group minerals in type 3 chondrites depend on the occurrence; Mg–Al-rich spinel occurs mainly in chondrules. Some chromite occurs in chondrules and matrix, and nearly pure chromite is exclusively encountered in the matrix. The occurrence of nearly pure chromite and the wide compositional variations distinguish spinel group minerals in types 3.00–3.3 from those in the other types. Spinel group minerals in types 3.5–3.9 show a narrower range of compositions, and those in types 4–6 are homogeneous. The changes in composition and abundance of spinel in type 3 chondrites are most likely due to thermal metamorphism. Therefore, the chemistry of spinel group minerals could be used as a sensitive indicator of metamorphic conditions, not only for type 3–6, but also 3.00–3.9. They can be applied to identify the most primitive (least metamorphosed) chondrites. The bulk compositions of spinel-bearing chondrules and the textural setting of the spinel indicate that most spinel group minerals crystallized directly from chondrule melts. However, some spinel grains, especially those enclosed in olivine phenocrysts, can not be explained by in situ crystallization in the chondrule. We interpret these spinel grains to be relic phases that survived chondrule melting. This is supported by the oxygen isotopic composition of a spinel grain, which has significantly lighter oxygen than the coexisting olivine. The oxygen isotopic composition of this spinel is similar to those of Al-rich chondrules. Our discovery of relic spinel in chondrules is an indication of the complexities in the early solar nebular processes that ranged from formation of refractory inclusion, through Al-rich chondrule, to ferromagnesian chondrules, and attests to the recycling of earlier formed materials into the precursors of later formed materials. The characteristic features of spinel group minerals are not only sensitive to thermal metamorphism, but also shed light on chondrule formation processes.