Mathematics – Logic
Scientific paper
May 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000m%26ps...35..561l&link_type=abstract
Meteoritics & Planetary Science, vol. 35, no. 3, pp. 561-568 (2000).
Mathematics
Logic
3
Scientific paper
We report the results of a mineralogical and oxygen isotopic study of 362 chondrules disaggregated from the Bo Xian chondrite. The range of mineral compositions (Fa 0.8 - 31.2 %, mean = 23.5 %, mode = 27-28%) are consistent with a reclassification of this meteorite from LL4 to LL3.9. Chondrule diameters range 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 partly reflects the high proportion of chondrule fragments. The chondrule size distribution, which is approximately log-normal, is consistent with size sorting mechanisms. This sorting could be linked to the fragmentation of many chondrules on the parent body. However, in detail, the variation in diameter 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 being determined by the chondrule formation mechanism. Seven % of the sectioned chondrules (102) contain chemically fractionated mineral assemblages - cristobalite-bearing and Al-rich. This significant degree of chemical heterogeneity probably resulted from both igneous and volatility controls. Oxygen isotope compositions were determined on mineral separates and 16 of the sectioned chondrules. Three separate isotopic exchange events have been identified. The dominant one is a low temperature hydrous gas-solid exchange event between 16O-rich solid and 16O-poor gas reservoirs which lay along a slope 1.0 line on 3-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 this exchange, no relationship between isotopic composition and chondrule texture or size was identified. The feldspar separate and cristobalite-rich chondrules have the most 16O-poor compositions. Subsequently, thermal metamorphism in the parent body led to partial isotopic equilibration between the different mineral phases. A third exchange event, predating the other 2, is probably shown by one of the Al-rich chondrules. This has an 16O-rich composition, lying below the terrestrial fractionation line. Another Al-rich chondrule has a normal OC isotopic composition. It is not clear whether the isotopic fractionation recorded in some Al-rich chondrules can be achieved by the dominant gas-solid exchange. Instead, the precursor oxygen to the mineral phases may have become 16O-rich during an earlier phase of mass independent fractionation.
Bridges John C.
Franchi Ian A.
Hutchison Robert
Li Chunlai
Ouyang Ziyuan
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