The Apollo 17 drill core: Comparative modal petrology and phase chemistry of the > 20µm and <20µm soil fractions

We have determined the mode and phase chemistry of the 20‐10µm soil fraction for three levels in the Apollo 17 drill core and compared them to the > 20µm fraction. The results indicate that the 20‐10µm fracton is enriched in Al2O3‐rich clear glass and to a minor extent in olivine and depleted in pyroxene and ilmenite relative to the >20µm fraction. Plagioclase and pyroxene of highland‐type compositions are relatively more abundant in the 20‐10µm fraction as well. These contrasts in mode and phase chemistry explain the enrichment in Al2O3 and depletion in FeO and TiO2 in the <20µm fraction which are observed in bulk chemical analyses of soil fractions. The contribution of highland‐derived material increases with decreasing grain size. Highland material contributes 14% to 30% to the mode of the fused soil‐free 200‐20µm fraction, but 25% to 55% of the fused soil‐free 20‐10µm fraction. The enrichment of highland‐derived material in the finer size fractions can be explained by the finer grain‐size distribution of the more mature highland soil derived from the local massifs. A more effective transport of finer‐grained particles may also contribute to this enrichment but we have no compelling evidence for this effect at the present time.