Stable Sn isotope signatures of Mid-ocean ridge basalts

Abstract

The constraint of Sn isotope composition of the Earth's mantle is the prerequisite for understanding planet formation with Sn isotopes and Sn isotope fractionation in terrestrial reservoirs. However, previous estimates of the Sn isotope composition of the Earth's mantle are hampered by the small number of mantle-related samples. In this study, we analyzed Sn stable isotope compositions of 27 basalts from geochemically and geologically diverse mid-ocean ridge segments, to constrain the Sn isotope composition of the depleted mantle source. The spread of Sn isotope compositions for the 27 different mid-ocean ridge basalts (MORBs) is limited, with δ122/118SnBHVO-2 (delta notation of 122Sn/118Sn ratios relative to USGS BHVO-2 basalt standard) values ranging from −0.051 ± 0.015‰ to 0.209 ± 0.016‰ (δ122/118Sn3161a from 0.280 ± 0.015 to 0.540 ± 0.016‰), despite great geochemical and geographical diversities of the samples. The slightly higher δ122/118SnBHVO-2 value (up to 0.209 ± 0.016‰ (δ122/118Sn3161a up to 0.540 ± 0.016‰)) of one MORB sample appears to be caused by post-eruptive alteration of the basalts. Other than that, the δ122/118SnBHVO-2 values of the MORB samples do not show correlations with sample latitude, spreading rate of mid-ocean ridge, MgO content of the bulk rock samples, and partial melting index Na8.0 (defined as [Na2O] + 0.373 × [MgO] − 2.98), implying that Sn isotopes do not fractionate significantly during MORB melt generation and evolution processes. This study confirms the limited Sn isotopic variability between fresh MORBs globally, pointing to the Sn isotopic homogeneity of the depleted mantle source. Using these new MORB data, we proposed an estimate of the δ122/118SnBHVO-2 value for the Earth's depleted mantle to be 0.036 ± 0.087‰ (δ122/118Sn3161a of 0.367 ± 0.087‰) (2SD, N = 12). This value provides a reference point for understanding the planetary and magmatic processes of Earth from a Sn isotope perspective.