Large zinc isotope variations in eastern Pacific seamount basalts as a result of mantle metasomatism

In recent years, zinc (Zn) isotope systematics has been proposed as a novel tracer for additions of near-surface materials to the mantle, while Zn isotope fractionation during magmatic processes was considered insignificant. Here we show that lavas from seamounts near the East Pacific Rise (5°–15°N) define the largest δ 66 Zn variation (from 0.19 ± 0.03 ‰ to 0.52 ‰ ± 0.02 ‰) observed in unaltered oceanic basalts. The high-δ 66 Zn lavas have low CaO contents and CaO/Al 2 O 3 ratios, clearly arguing against an origin involving additions of recycled carbonates. Furthermore, high-δ 66 Zn lavas have higher abundances of highly incompatible elements (e.g., high La/Sm and Sm/Yb) as well as more enriched radiogenic Sr-Nd-Pb isotope compositions. These characteristics link their origin with a compositionally enriched mantle component of ancient low-degree melt metasomatic origin. We suggest that the low-degree melt metasomatism in the seismic low-velocity zone beneath ocean basins may cause significant chemical and isotopic (e.g., Zn and Fe) variations in the upper mantle at present and in the past. Our findings further indicate that the high-δ 66 Zn signatures in terrestrial basalts most likely originate from additions of low-degree metasomatic melts to their mantle sources. We also demonstrate that widespread interpretation that invokes subducted carbonates to explain mantle-derived melts with heavy Zn isotope compositions is inconsistent with multiple observations.