APPLICATION TO THE TECTONIC DISCRIMINATION OF BASALTS

An extensive dataset of 756 samples was compiled from the online PETDB and GEOROC databases (www.petdb.org and georoc.mpch-mainz.gwdg.de). The dataset can be downloaded from electronic annex EA-1. It contains:

• 256 Island arc basalts (IAB) from the Aeolian, Izu-Bonin, Kermadec, Kurile, Lesser Antilles, Mariana, Scotia and Tonga arcs.
• 241 Mid-ocean ridge (MORB) samples from the East Pacific Rise, Mid Atlantic Ridge, Indian Ocean, and Juan de Fuca Ridge.
• 259 Ocean-island (OIB) samples from St. Helena, the Canary, Cape Verde, Caroline, Crozet, Hawaii-Emperor, Juan Fernandez, Marquesas, Mascarene, Samoan and Society islands.

Duplicate analyses were excluded from the database to avoid potential bias towards overrepresented samples. 51 geochemical features were used:

• Major element concentrations (in weight percent): SiO$_2$, TiO$_2$, Al$_2$O$_3$, Fe$_2$O$_3$, FeO, CaO, MgO, MnO, K$_2$O, Na$_2$O, P$_2$O$_5$
• Minor and trace element concentrations (in ppm): La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Sn, Cs, Ba, Hf, Ta, Pb, Th, U
• Isotopic ratios: $^{143}$Nd/$^{144}$Nd, $^{87}$Sr/$^{86}$Sr, $^{206}$Pb/$^{204}$Pb, $^{207}$Pb/$^{204}$Pb, $^{208}$Pb/$^{204}$Pb

The fact that different measurement units are mixed and that for none of the 756 samples all 51 features were measured is not a problem for the construction of the trees, once again illustrating the robustness of the method. The classification trees discussed next were constructed using the rpart library of the statistical software package R, which can be downloaded free of charge from www.r-project.org. The actual R-code is provided in electronic annex EA-2.