Petrogenesis of alkaline basalts from Socorro Island, Mexico: trace element evidence for contamination of ocean island basalt in the shallow ocean crust

Postcaldera mafic volcanism on Socorro Island, Mexico, an island located in the eastern Pacific Ocean on a segment of a mid-ocean ridge spreading center abandoned at ~3.5 Ma, dominantly comprises alkalic basalt, hawaiite, and mugearite with subordinate mildly alkalic-transitional basalt. Most major element data are consistent with differentiation of alkalic basalt to hawaiite and mugearite by up to 50% fractionation of plagioclase + clinopyroxene + olivine ± Fe-Ti oxides. Sr and Nd isotopic ratios exhibit relatively narrow ranges. Despite this, a subset of the alkalic basalts has negative Ce anomalies and abundances of P₂O₅, Ba, Y, and some REEs in excess of those predicted by fractional crystallization models; these chemical features require open-system processes. The observed heterogeneities are perhaps best explained by assimilation of ocean crustal components by basalt in a shallow magma reservoir. -from Authors