## 4 Results  
### Major Elements  
The major and trace-elemental compositions of the samples are presented in Table 1. Based on the SiO\({}_{2}\) vs. K\({}_{2}\)O + Na\({}_{2}\)O classification diagram of [PERSON] et al. (1986) [37], the volcanic rock samples were classified as basaltic trachyandesites, trachyandesites, tephrites, tephriphonolites, and trachytes (Figure 2). Half of the samples were basaltic trachyandesites. All of the analyzed samples were in the alkaline region of the diagram, even though two chemically distinct (alkaline and sub-alkaline) magma series are present on Jeju Island (Figure 2) [14,16,18]. The basaltic trachyandesites (Samples 1, 2, 6, 8, and 9) contained SiO (49.73-51.95 wt.%), Na\({}_{2}\)O (6.23-5.89 wt.%), and K\({}_{2}\)O (2.19-1.38 wt.%). The tephrite (samples 3 and 7) exhibited the lowest SiO\({}_{2}\) contents (47.82-48.21 wt.%), Na\({}_{2}\)O contents (5.99-6.49 wt.%), and K\({}_{2}\)O contents (1.39-2.19 wt.%). The trachybasalt (sample no. 5) had an SiO\({}_{2}\) content of 50.126 wt.%, a Na\({}_{2}\)O content of 5.32 wt.%, and a K\({}_{2}\)O content of 0.89 wt.%. The tephriphonolite (sample no. 10) contained SiO\({}_{2}\) (55.21 wt.%), Na\({}_{2}\)O (7.87 wt.%), and K\({}_{2}\)O (3.45 wt.%). Trachyte, which was the most felsic sample (Sample No. 4), had an SiO\({}_{2}\) content of 61.94 wt.%, a Na\({}_{2}\)O content of 8.79 wt.%, and a K\({}_{2}\)O content of 5.01 wt.%. All of the samples fell along the Na-series, since the Na\({}_{2}\)O/K\({}_{2}\)O values were \(\sim\)1.76-6.01. In an AFM diagram, the samples plotted near the boundary between the calc-alkaline and tholeiitic fields (Figure 3). The AFM diagram displays the fractional crystallization trend, which will be explained in detail later. Figure 1: Geologic map of Jeju Island (modified from [PERSON] et al., 2000) [36] with the sampling locations of trachytic rocks (1–10) and mantle xenoliths (X1–2). \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**Sample** & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **10** & **X1** & **X2** \\ \hline
As depicted in Harker diagrams of the variations in major oxide and SiO\({}_{2}\) contents (Figure 4), the basaltic trachyandesite samples had low MnO, Na\({}_{2}\)O, and K\({}_{2}\)O contents, but it had high Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), CaO, P\({}_{2}\)O\({}_{5}\), and MgO contents. The P\({}_{2}\)O\({}_{5}\) content decreased as the SiO\({}_{2}\) content increased and started to decrease at 53% of SiO\({}_{2}\) (Figure 4), suggesting the crystallization of apatite. The tephrite and trachybasalt samples had major oxide contents similar to those of the basaltic trachyandesites, except that trachybasalts had lower P\({}_{2}\)O\({}_{5}\) and Al\({}_{2}\)O\({}_{3}\) contents. However, the tephriphonolite and trachyte contained high Na\({}_{2}\)O and K\({}_{2}\)O contents and pronounced high MnO contents. The Na\({}_{2}\)O, K\({}_{2}\)O, MnO, and SiO\({}_{2}\) contents of the more felsic rocks, such as tephriphonolites and trachytes, were positively correlated. These two rock types also exhibited negative correlations of Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), P\({}_{2}\)O\({}_{5}\), MgO, CaO, and TiO\({}_{2}\) contents with SiO\({}_{2}\) contents. The fractional crystallization of clinopyroxene and plagioclase is indicated by the negative correlation