A preferred method for the determination of bulk compositions of coarse-grained refractory inclusions and some implications of the results

摘要

Analyses of coarse-grained refractory inclusions typically do not have the solar CaO/Al2O3 ratio, probably reflecting nonrepresentative sampling of them in the laboratory. Many previous studies, especially those done by instrumental neutron activation analysis (INAA), were based on very small amounts of material removed from those restricted portions of inclusions that happened to be exposed on surfaces of bulk meteorite samples. Here, we address the sampling problem by studying thin sections of large inclusions, and by analyzing much larger aliquots of powders of these inclusions by INAA than has typically been done in the past. These results do show convergence toward the solar CaO/Al2O3 ratio of 0.792. The bulk compositions of IS coarse-grained inclusions determined by INAA of samples >2 mg have an average CaO/Al2O3 ratio of 0.80 +/- 0.18. When bulk compositions are obtained by modal recombination based on analysis of thin sections with cross-sections of entire, large, unbroken inclusions, the average of 11 samples (0.79 +/- 0.15) also matches the solar value. Among those analyzed by INAA and by modal recombination, there were no inclusions for which both techniques agreed on a CaO/Al2O3 ratio deviating by >similar to15% from the solar value. These results suggest that: individual inclusions may have the solar CaO/Al2O3 ratio; departures from this value are due to sample heterogeneity and nonrepresentative sampling in the laboratory; and it is therefore valid to correct compositions to this value. We present a method for doing so by mathematical addition or subtraction of melilite spinel, or pyroxene. This yields a set of multiple, usually slightly different, corrected compositions for each inclusion. The best estimate of the bulk composition of an inclusion is the average of these corrected compositions, which simultaneously accounts for errors in sampling of all major phases. Results show that Type B2 inclusions tend to be more SiO2-rich and have higher normative Anorthite/Gehlenite component ratios than Type B1s. The inclusion bulk compositions lie in a field that can result from evaporation at 1700-2000K of CMAS liquids with solar CaO/Al2O3, but with a wide range of initial MgO (30-60 wt%) and SiO2 (15-50 wt%) contents. Copyright (C) 2004 Elsevier Ltd.