The influence of crustal recycling on the zirconium isotope composition of ocean island lavas

Crustal recycling into Earth’s mantle influences mantle composition and contributes to chemical heterogeneities in Earth’s interior. Zirconium is a refractory and immobile element in fluids and its stable isotopes are fractionated during magmatic differentiation. Therefore, Zr isotopes have the potential to be used to trace crustal recycling into the mantle. Here, we provide a survey of Zr isotopic systematics in some oceanic island lavas erupted at intraplate volcanic hotspots, collected from geochemically and geologically diverse mantle end members. These samples document either extremely high- 3 He/ 4 He ratios, HIMU (high ‘μ’, or high 238 U/ 204 Pb), EM1 (enriched mantle I) or EM2 (enriched mantle II) mantle sources. Most ocean island lavas display homogeneous δ 94/90 Zr (permil deviation of the 94 Zr/ 90 Zr ratio from the IPGP-Zr standard) values, especially for high- 3 He/ 4 He, HIMU and EM1, with δ 94/90 Zr indistinguishable within error relative to the bulk silicate Earth estimate (i.e., 0.041 ± 0.041‰). In contrast, Samoan EM2 lavas display higher δ 94/90 Zr (up to 0.194 ± 0.019‰). Additionally, the δ 94/90 Zr values of the Samoan EM2 lavas show co-variation with radiogenic 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios. The elevated δ 94/90 Zr signatures are not the result of zircon saturation, mantle melting or shallow assimilation. Instead, the Samoan δ 94/90 Zr- 87 Sr/ 86 Sr and δ 94/90 Zr- 143 Nd/ 144 Nd trends are consistent with binary mixing between a depleted mantle source with lower δ 94/90 Zr and a recycled component originating from the upper continental crust (UCC) (∼6%) with higher δ 94/90 Zr. The δ 94/90 Zr of the recycled UCC material in the Samoan mantle source is calculated to be 0.363‰, falling within the δ 94/90 Zr range of intermediate-felsic continental rocks. Based on the δ 94/90 Zr- 87 Sr/ 86 Sr array formed by EM2 lavas, a felsic continental protolith from UCC is the most likely origin for the incompatible element isotopic composition of the Samoan EM2 mantle.