Fig. 3: ⁸⁷Sr/⁸⁶Sr versus element concentration showing theoretical mixing curves of inferred endmember compositions.

The direction of fractional crystallization is denoted by arrows. The green star denotes the EM2-derived mixing endmember composition (SiO₂ = 65.5 wt%, MgO = 0.5 wt%, Al₂O₃ = 17.1 wt%, FeO = 3.8 wt%, CaO = 2.7 wt%, TiO₂ = 1 wt%, and Na₂O + K₂O = 11 wt%; see Supplementary Table 2), which is calculated in this study (see “Methods”); the black hatched line shows the path of the calculated mixing curves, where each hatch increases by 10% mixing from the geochemically depleted mafic endmember to the more evolved EM2-derived endmember (black hatched line is derived from the same methodology as described briefly in Fig. 2 and in the “Methods”). Mixing is clearly defined by a curve with a non-zero slope, and is unlike the lavas that have been affected only by fractional crystallization (which have zero-slope trends). The ALIA-D115 lavas are represented by red squares, the other Samoan whole rock lavas by orange diamonds, and Samoan pillow glasses by purple circles. See Fig. 1 caption for data references; Samoa pillow glass data can be found in Workman et al.⁶. All errors bars smaller than symbols.