High precision triple oxygen isotope composition of small size urban micrometeorites indicating constant influx composition in the early geologic past

Abstract

Abstract In this study, we present a method for high precision Δ′ 17 O (Δ′ 17 O RL = ln(δ 17 O + 1) – λ RL ln(δ 18 O + 1)) analysis of small mass silicate and oxide materials. The analyses were conducted by laser fluorination in combination with gas chromatography and continuous flow isotope ratio monitoring gas spectrometry. We could analyze the oxygen isotope composition of samples down to 1 μg, which corresponded to about 13 nmol O 2 . The analytical error (we report the 1 σ external reproducibility of a single analysis) in δ 18 O increases with decreasing sample sizes from ~0.2‰ for ~20 μg samples to ~0.9‰ for 1 μg samples. For Δ′ 17 O, we achieved an external reproducibility of 0.04‰ for a sample mass range between 1 and 27 μg. The uncertainty in Δ′ 17 O is smaller than the uncertainty in δ 18 O due to the correlated errors in δ 17 O and δ 18 O. We applied the method to urban micrometeorites, that is, small meteorites (<2 mm) that were sampled from a rooftop in Berlin, Germany. A total of 10 melted micrometeorites (S‐type cosmic spherules, masses between 11 and 22 μg) were analyzed. The oxygen isotope compositions are comparable to that of modern Antarctic collections, indicating that the urban micrometeorites sample the same population. No indication for terrestrial weathering had been identified in the studied set of urban micrometeorites making them suitable materials for the study of micrometeorite origins.