Compositional transition in natural alkaline lavas through silica-undersaturated melt-lithosphere interaction

Natural alkaline lavas have diverse compositions-varying widely from nephelinite through basanite to alkali olivine basalt-the origin of which is controversial. In particular, identifying the roles of recycling carbonates in the source and evolution of natural alkaline lavas is commonly difficult. Zinc isotope ratios (delta Zn-66) have great potential due to the strong delta Zn-66 contrast between marine carbonates and the mantle. Here we present a systematic variation of Zn isotopes with Sr-Nd isotopes and incompatible elements (e.g., Nb, Th, and Zn) in nephelinites, basanites, and alkali olivine basalts from eastern China. The elevated delta Zn-66 of nephelinites and high-alkali basanites relative to the mantle demonstrates that the silica-undersaturated melts were derived from a carbonated mantle. Alkali olivine basalts and low-alkali basanites show a gradual decline of delta Zn-66 with SiO2 and have Zn-Sr-Nd isotopic and chemical compositions shifted toward that of an enriched lithospheric mantle. Infiltration of silica-undersaturated basanitic melts and reaction with the lithospheric mantle account for the transition of strongly alkaline melts into weakly alkaline melts via consumption of orthopyroxene and mixing with silica-rich melt derived from lithospheric mantle. High-delta Zn-66 wehrlite xenoliths found in these alkaline lavas record metasomatism of the lithospheric mantle by basanitic melts. Thus, silica-undersaturated melt-lithosphere interaction could be one of the most common causes of compositional diversity in natural alkaline lavas.