A Tree of HFS Elements and Isotopic Ratios Only

Even though the classification tree that was built in the previous section (Figure 4) performs very well for fresh basalts, this might not necessarily be the case for weathered or metamorphosed samples. For example, a lot of the power of this tree depends on Sr, which is considered a mobile element (e.g., Rollinson, 1993). Also MgO, Ni and Rb are species used in Figure 4 that are mobile to some degree. Performance of our classification, which was based on fresh samples of basalt, cannot be guaranteed if used for the tectonic discrimination of samples that have undergone alteration. Therefore, an alternative tree was built that only uses the so-called high field strength (HFS) cations, which are characterized by an ionic potential greater than two (Rollinson, 1993), as well as the isotopic ratios of Sr, Nd and Pb, because these are considered less prone to change during alteration than the concentrations themselves. The following 28 features were used: TiO$_2$, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y, Zr, Nb, Hf, Ta, Pb, Th, U, $^{143}$Nd/$^{144}$Nd, $^{87}$Sr/$^{86}$Sr, $^{206}$Pb/$^{204}$Pb, $^{207}$Pb/$^{204}$Pb and $^{208}$Pb/$^{204}$Pb. The resulting pruned tree is shown in Figure 5, while its surrogate splits are given in Table 2. The first two splits, both on TiO$_2$, contribute 85% of the discriminative power of the tree. The cross-validated misclassification error of the optimal tree is 16%, i.e., there is 84% chance of a correct classification.