“…The composition of mid‐ocean ridge basalts (MORB) reflects a variety of subsurface processes [e.g., Sinton and Detrick , ; Perfit and Chadwick , ; Klein , ; White and Klein , ]. In addition to fractional crystallization, reactive crystallization in dunite channels [e.g., Kelemen et al ., ; Collier and Kelemen , ; Liang et al ., ], melt rock reactions in the lithosphere [e.g., Drouin et al ., ; Sanfilippo et al ., ; Paquet et al ., ], or the gabbroic crust [e.g., Lissenberg and Dick , ; MacLeod et al ., ], mantle source heterogeneity [e.g., Sims et al ., ; Donnelly et al ., ; Shimizu et al ., ], variations in depths or extents of melting [e.g., Klein and Langmuir , ; Langmuir et al ., ; Perfit et al ., ], and crustal assimilation [e.g., Michael and Schilling , ; Wanless et al ., ] can all modify trace element contents and, depending on the phases involved, alter the CaO, Al 2 O 3 , and MgO content of erupted basalts [e.g., Michael and Shilling , 1989; Lissenberg and Dick , ; Paquet et al ., ]. For example, melt rock reaction in gabbroic rocks may involve the dissolution of olivine and crystallization of clinopyroxene [ Paquet et al ., ] changing the CaO/Al 2 O 3 of the melt.…”