Barred olivine chondrules in ordinary chondrites

Abstract

Barred olivine chondrules were studied in H‐L‐LL3‐6 chondrites. No important differences were found between barred olivine chondrules in H3, L3, and LL3 chondrites. However, comparison of type 3 barred olivine chondrules with all chondrule types in H‐L‐LL3 ordinary chondrites show that barred olivine chondrules are enriched in FeO, Al2O3, Na2O, and K2O and are depleted in SiO2 and MgO. Mineral abundances of barred olivine chondrules show that they are enriched in the feldspathic component compared to most chondrules. Pyroxene is rarely absent and is sometimes a major component. Pyroxene‐rich barred olivine chondrules in type 3 chondrites correlate negatively with feldspathic glass‐rich barred olivine chondrules, indicating that pyroxene formed at the expense of olivine bars. Barred olivine chondrules in ordinary chondrites decrease systematically in Cr2O3 and TiO2 with increasing petrologic type. FeO, however, shows a systematic decrease with petrologic type in H chondrites, no change in L chondrites, and a systematic increase in LL chondrites. The conclusions of this study are: (1) Barred olivine chondrules are mainly intermediate in major element composition between the abundant Fe‐Mg chondrules and the rare Al‐rich chondrules. (2) Systematic compositional changes in barred olivine chondrules from type 3–6 in H‐L‐LL chondrites may be the result of primary differences. (3) Precursor components that melted to form barred olivine chondrules were enriched in olivine‐rich and volatile (feldspar and feldspathoid‐rich) components. The alkalies may have been condensed where alumina was already available.