Cadmium isotope composition of the Earth's mantle inferred from analysis of oceanic basalts and komatiites

• Published in: Chemical Geology
• Main Authors: Devos, Gabriel, Moynier, Frédéric, Creech, John, Liu, Deze, Puchtel, Igor S., Bizzarro, Martin
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2024
• Subjects: Bulk Silicate Earth; Cadmium double spike; Cadmium stable isotopes; Komatiites; MC-ICP-MS; Mid-oceanic ridge basalts; Fennoscandia
• Online Access: https://curis.ku.dk/portal/da/publications/cadmium-isotope-composition-of-the-earths-mantle-inferred-from-analysis-of-oceanic-basalts-and-komatiites(a058a67e-849c-4aec-b495-97d825d0077c).html; https://doi.org/10.1016/j.chemgeo.2024.121996

Cadmium stable isotope ratios can serve as valuable tracers for biological and geological processes, including nutrients sampling from the surrounding medium, volatilization events, and terrestrial differentiation. However, studies of the isotope fractionation occurring during geological processes require the characterization of the isotope compositions of the different terrestrial reservoirs. Notably, the cadmium isotopic composition of the Bulk Silicate Earth (BSE) is still a matter of debate. To address this issue, we present high-precision cadmium isotope data from a diverse set of samples, including mid-oceanic ridge basalts (MORB) and komatiites obtained using a 106 Cd[sbnd] 111 Cd double spike system. We processed nine biological and geological reference materials (AGV-2, BCR-2, BHVO-2, BIR-1, COQ 1, GSP-2, NIST 2711, NOD-A-1, ERM BB 186) and two pure Cd solutions (AAS and ChemLab) to assess the accuracy and precision of the method. Our study revealed that twenty-three MORB samples (including ten analyses from the literature) from the Atlantic, Indian, and Pacific oceans have a homogeneous cadmium isotopic composition. The weighted mean isotope composition for these samples, expressed as δ 114 Cd (the permil deviation of the 114 Cd/ 110 Cd ratio from the NIST SRM 3108 Cd standard), is 0.07 ± 0.10 ‰ (2SD, n = 23). We also studied twelve komatiite samples from the Abitibi (Superior Craton, Canada), Barberton (Kaapvaal Craton, South Africa), and Kostomuksha (Baltic Shield, Fennoscandia) greenstone belts to characterize the deep Archean mantle isotope composition. These samples, combined with four analyses from the literature (Pickard et al., 2022) exhibited isotopic compositions ranging from −0.33 ± 0.04 ‰ to 0.26 ± 0.08 ‰. We observed a negative correlation between cadmium abundances and the degree of secondary alteration (measured by loss on ignition-LOI). For samples with low LOI (≤ 6 wt%), the isotope compositions of komatiites are quite consistent. On the other hand, the samples with higher LOI (between ...