Bo Xian (LL3.9): Oxygen-isotopic and mineralogical characterisation of separated chondrules

We report the results of a mineralogical and O-isotopic study of 362 chondr ules disaggregated from the Bo Xian chondrite. The range of mineral composi tions (Fa = 0.8-31.2%, mean = 23.5%, mode = 27-28%) are consistent with a r eclassification of this meteorite from LL4 to LL3.9. Chondrule diameters ra nge from 0.20 to 3.40 mm (mean = 0.74 mm) in the disaggregated population. A lower mean diameter (0.64 mm) calculated from thin-section measurements p artly reflects the high proportion of chondrule fragments. The chondrule si ze distribution, which is approximately log-normal, is consistent with size -sorting mechanisms. This sorting could be linked to the fragmentation of m any chondrules on the parent body. However, in detail, the variation in dia meter of different chondrule types and a hiatus in the size distribution at 0.6 mm indicate that there may have been complex controls perhaps partly b eing determined by the chondrule formation mechanism. Seven percent of the sectioned chondrules (102) contain chemically fractionated mineral assembla ges: cristobalite-bearing and Al-rich. This significant degree of chemical heterogeneity probably resulted from both igneous and volatility controls. Oxygen-isotopic compositions were determined on mineral separates and 16 of the sectioned chondrules. Three separate isotopic exchange events have bee n identified. The dominant one is a low-temperature hydrous gas-solid excha nge event between O-16-rich solid and O-16-poor gas reservoirs that lay alo ng a slope 1.0 line on three-isotope plots. Partial equilibration with the gas by feldspar and cristobalite, which exchanged more rapidly than olivine or pyroxene, led to formation of a slope 0.77 mixing line for Bo Xian and other LL chondrites. Mineralogy is the dominant control on the extent of th is exchange; no relationship between isotopic composition and chondrule tex ture or size was identified. The feldspar separate and cristobalite-rich ch ondrules have the most O-16-poor compositions. Subsequently, thermal metamo rphism in the parent body led to partial isotopic equilibration between the different mineral phases. A third exchange event, predating the other two events, is probably shown by one of the Al-rich chondrules. This has an O-1 6-rich composition, lying below the terrestrial fractionation line. Another Al-rich chondrule has a normal ordinary chondrite isotopic composition. it is not clear whether the isotopic fractionation recorded in some Al-rich c hondrules can be achieved by the dominant gas-solid exchange. Instead, the precursor O to the mineral phases may have become O-16-rich during an earli er phase of mass-independent fractionation.