Extreme oxygen isotope anomaly with a solar origin detected in meteoritic organics

Materials formed in the early Solar System generally exhibit a characteristic oxygen isotopic signature known as the non-mass-dependent oxygen isotope anomaly, the origins of which are unclear. The anomalies are thought to reflect isotopic fractionation in the chemical reaction that first formed solid material from the gaseous medium, but the proposed mechanism and environment of formation are the subject of debate. Here we analyse micrometre-sized grains of acid-insoluble organic matter from a carbonaceous chondritic meteorite recovered in Antarctica. We find that the organic matter has the highest 18 O/ 16 O and 17 O/ 16 O ratios known in planetary material, except for pre-solar grains. The oxygen ratios are enhanced by up to 53±11% and the ¹³C/ ¹²C values by 29±5% relative to terrestrial values. We suggest that the coherent enrichments of ¹⁷O, ¹⁸O and ¹³C in the organic matter can best be explained by its formation being due to the photodissociation of carbon monoxides in a gas medium at temperatures of about 60 K or higher. These conditions are equivalent to those expected at the envelope of the proto-solar nebula, and we suggest the organic matter formed there.