  Relative to calcium, both strontium and barium are markedly enriched in
Earth's continental crust compared to the basaltic crusts of other
differentiated rocky bodies within the solar system. Here, we both re-examine
available archived Keck spectra to place upper bounds on n(Ba)/n(Ca) and
revisit published results for n(Sr)/n(Ca) in two white dwarfs that have
accreted rocky planetesimals. We find that at most only a small fraction of the
pollution is from crustal material that has experienced the distinctive
elemental enhancements induced by Earth-analog plate tectonics. In view of the
intense theoretical interest in the physical structure of extrasolar rocky
planets, this search should be extended to additional targets.
