Tin isotope compositions of ocean island basalt mantle sources

Ocean island basalts (OIB) provide geochemical information on the evolution of Earth’s mantle and the role of crust and mantle lithosphere recycling. Atlantic OIB from the Canary Islands, Azores, Cape Verde and Tristan da Cunha have long-lived radiogenic isotope compositions indicating various contributions of recycled crust and lithospheric components in their mantle sources. We report tin and strontium isotopes and trace element abundance data for lavas from these islands revealing greater than 0.6‰ variability in δ 122/118 Sn (−0.15 to +0.46‰, relative to NIST3161a). Some of this Sn isotope variability can be accounted for by post-crystallization alteration, where the most altered lavas have anomalously radiogenic 87 Sr/ 86 Sr, low Sn contents and enrichment in the heavier isotopes of Sn compared with fresh OIB lavas. Magmatic differentiation can also result in higher δ 122/118 Sn due to fractionation of Sn-compatible iron–titanium oxides. Fresh Atlantic OIB with similar MgO contents, estimated extents of partial melting and redox states of their mantle sources to Hawaiian OIB or mid-ocean ridge basalts are, on average, ∼0.3‰ lower in δ 122/118 Sn. Possible causes of the lower δ 122/118 Sn signature based on mass balance mixing constraints are from a combination of recycled crust and low-δ 122/118 Sn highly melt-depleted peridotite, or from hydrothermally altered oceanic lithosphere with low-δ 122/118 Sn mixing with ambient peridotite. Distinguishing between these two possibilities is currently hampered by a lack of available Sn isotope data on well-characterized highly melt-depleted peridotites or hydrothermally altered oceanic lithosphere components, including crust and altered peridotites. It is notable, however, that an isotopically light Sn isotope signature exists in arc lavas, consistent with dewatering of hydrothermally altered oceanic lithosphere. A key feature of low-δ 122/118 Sn in Atlantic OIB is that they require a proportionally high percentage of recycled Sn. The most plausible explanation for the recycled Sn isotope signature is due to partial melting of hybridized pyroxenite-peridotite mantle sources generated by mixing of recycled crust into peridotite.