Figure 20. Nb/Ce, Nb/Zr, and Zr/Ti ratios vs. Zr/Y ratio or Zr content. Basalts from Site 1136 are shown as solid symbols; a highly altered sample is shown by the open circle. The se ratios are sensitive to the presence of a component from the continental crust. Note that b asalts from Site 738, which are thought to contain a continental component, have the lowest Nb/Ce and Nb/Zr and highest Zr/Ti values. Basalts from Site 1136 most closely resemble the upper composition al group from Site 749. Ratios are normalized using estimated primitive mantle abundances of Sun an d McDonough (1989). Numbers indicate ODP site numbers.