A partial melting control on the Zn isotope composition of basalts

Abstract

Basaltic partial melts are produced in a range of tectonic settings, including fluid-assisted melting above subduction zones, decompression melting at ridges and thermally driven melting above mantle plumes. To examine the role of partial melting on Zn, isotope and abundance data are reported for modern large-degree partial melts of the mantle represented by 22 mid-ocean ridge basalts (MORB) from three ocean basins and the first data for boninites. Boninites have some of the lowest Zn abundances of all terrestrial basalts and Zn isotope compositions ( δ 66 Zn = þ 0.21 ± 0.06 ‰ ), generally lighter than for MORB ( δ 66 Zn = þ 0.28 ± 0.06 ‰ ). Accounting for partial melting, komatiites, boninites and MORB derive from mantle sources with δ 66 Zn of ∼ 0.16 ± 0.06 ‰ . Lower-degree partial melts, such as alkali basalts, can have higher δ 66 Zn, with up to ∼ 0.4 ‰ variation possible from partial melting of distinct peridotite mantle sources. Partial melting of fertile lherzolitic and depleted harzburgitic mantle sources can generate significant Zn isotope variability and should be evaluated prior to ascribing crustal, enriched or lithological components to mantle reservoirs from Zn compositions of planetary basalts.