The antimony isotope composition of Earth's mantle and crust

A new ion-exchange chromatography method was developed for the separation of Sb from different matrices to determine the δ¹²³Sb values of the bulk silicate Earth (BSE) and upper continental crust (UCC) relative to NIST SRM 3102a (δ¹²³Sb = [((¹²³Sb/¹²¹Sb)_sample/(¹²³Sb/¹²¹Sb)_{NIST SRM 3102}) - 1] × 1000). Using this method, we analyzed a wide variety of ultramafic to felsic igneous rocks, metamorphic, and sedimentary rocks. Ultramafic and mafic rocks, most closely representing the composition of the depleted upper mantle, display small variations in δ¹²³Sb from −0.11 to 0.11 ‰, yielding a rather homogenous Sb isotopic composition of 0.00 ± 0.04 ‰ (average, 2SE, n = 16). The average δ¹²³Sb value of intermediate and felsic igneous rocks (n = 10) and magmatic sulfides (n = 2) is nearly indistinguishable from that of the depleted upper mantle, indicating insignificant Sb isotope fractionation during formation of the continental crust. The depleted upper mantle may, thus, be considered to represent the BSE. By contrast, greater variability in δ¹²³Sb (−0.28 to 0.52 ‰) is observed for Mesoarchean to Paleozoic glacial diamictite composites, potentially recording the compositional evolution of the UCC. Mesoarchean diamictites tend to have slightly heavier Sb isotopic composition (average δ¹²³Sb = 0.31 ± 0.38 ‰, 2SD, n = 4), though they are within uncertainty of the average values of Paleoproterozoic (0.07 ± 0.46 ‰, 2SD, n = 7), Neoproterozoic (0.01 ± 0.29 ‰, 2SD, n = 9) and Phanerozoic (average 0.00 ± 0.14 ‰, 2SD, n = 3) diamictites. The reason for the large variability of δ¹²³Sb of the Precambrian UCC, as indicated by the diamictites, is uncertain, but may reflect weathering effects, an isotopically heterogeneous early UCC, changing behavior of Sb during crustal differentiation, or a sampling bias (e.g., all of the Archean diamictites are from the Kaapvaal craton). Combining data obtained for the diamictites with those of felsic and intermediate magmatic samples yields a δ¹²³Sb value of 0.07 ± 0.07 ‰ (2SE, n = 33), which provides an estimate for the average UCC. These results provide a framework for investigating the formation of Sb ore deposits, and deep- or near-surface bio-geochemical Sb cycling.