The molybdenum isotope composition of the Bulk Silicate Earth constrained from molybdenites and its implications for the nature of late-stage building blocks

The mass-independent Mo isotope composition of the Bulk Silicate Earth (BSE) bears great potential to investigate the origin of the Earth’s latest 10–20% planetary building blocks. However, currently different estimates for the Mo isotope composition of the BSE render constraints on the composition of late-stage accretionary materials difficult. To address this issue and to revisit the Mo isotope composition of the BSE, we report high-precision molybdenum isotope data for a comprehensive set of terrestrial molybdenites from different locations around the globe covering mineralization ages that extend from the Archean to the Phanerozoic. The molybdenite results are used to constrain the Mo isotope composition of the BSE as follows: ε 92 Mo = 0.04 ± 0.06, ε 94 Mo = 0.03 ± 0.03, ε 95 Mo = 0.01 ± 0.01, ε 97 Mo = 0.02 ± 0.02, ε 100 Mo = 0.05 ± 0.06 ( n = 16, 95% confidence interval, relative to the NIST SRM 3134 Mo standard). In contrast to previous studies, no resolvable ε 94 Mo and ε 95 Mo anomalies were observed, suggesting a BSE composition with predominantly non-carbonaceous chondrite provenance. Considering the analytical uncertainties of our new BSE estimate and literature data for carbonaceous and non-carbonaceous meteorites, it remains a viable option that 12±10% of the present-day Mo budget in the BSE derives from carbonaceous meteorite material delivered during late-stage accretion. This amount of Mo is consistent with the fraction of Mo that was delivered to Earth during its final 0.5% of accretion by the late veneer.