Antimony isotope composition of the bulk silicate Earth

Antimony is an incompatible chalcophile element. Its concentration in the Earth’s mantle was estimated to ~0.14 μg g-1 Sb [3], while the Sb estimates for the upper continental crust scatter between 0.2 to 0.8 μg g-1 [4]. Recent analytical advances in the precision and accuracy of stable Sb isotope analyses allowed to assess the potential of δ123SbNIST3102a as a proxy for ore forming [1] and oxidation/weathering [2] processes. Application of this proxy, however, is limited by the lack of knowledge of the Sb isotope composition of the Earth and its fractionation during high-temperature cycling. Here, we present bulk rock Sb isotopic compositions of a range of basic and ultrabasic USGS reference materials (n = 12) from different settings and a suite of basic rocks (n = 3) from Hawaii, aiming to estimate the Sb isotopic composition of the bulk silicate Earth (BSE).  The sample suite shows an isotopic range (all values expressed hereafter as δ123SbNIST3102a) from -0.10 to +0.27 ‰. Two of the fifteen samples display an isotopic composition of > +0.24 ‰ which are, however, marked by low Nb/U, Ce/Pb, and elevated Zr/Nb, indicating crustal contamination in the magmatic source [5]. Excluding these two samples our analyzed sample suite shows an average δ123SbNIST3102a value of +0.01 ± 0.08‰ that we suggest as a preliminary reference value for the Sb isotope composition of the bulk Earth. Our findings furthermore provide first insights for Sb isotope variations in high-temperature rocks that may be related to recycling processes in the mantle. [1] Zhai et al. (2021), GCA. 3016, 84-97. [2] Ferrari et al. (2023), Chem. Geol. 641, 121788. [3] McDonough and Sun (1995), Chem. Geol. 120, 223-253. [4] Majzlan and Filella (2023), Geochem. In press, 126072. [5] Hofmann et al. (1986), EPSL. 79(1-2), 33-45.