The origin of organic matter found in water- and carbon-rich meteorites, known as carbonaceous chondrites, which formed during the birth of the Solar System around 4.55 billion years ago, may provide important clues to understanding how life originated here on Earth. This is because carbonaceous chondrites organics are rich in elements such as carbon, hydrogen, oxygen, and nitrogen (the CHON elements) that are the most common in living organisms. Yet whether these organics were inherited from the interstellar medium, were synthesised by chemical reactions in the protosolar nebula, or formed by fluid-rock interactions on asteroidal and cometary parent bodies still remain highly debated.
To provide further constraints on this issue, with colleagues from the Muséum National d’Histoire Naturelle in Paris, the Institut de Physique du Globe in Paris, and the Centre de Recherches Pétrographiques et Géochimiques in Nancy, we have analysed the triple oxygen isotope composition of insoluble organic residues isolated from three emblematic carbonaceous chondrite meteorites, the CI chondrite Orgueil (see photo), and the CM chondrites Murchison and Cold Bokkeveld (you can read more about oxygen isotopes here).
To do so we used two complementary secondary ion mass spectrometry techniques, the CAMECA 1270/80 instruments in Nancy France allowing us to measure oxygen ratios at high precision while the CAMECA NanoSIMS instrument in Manchester provided high spatial resolution information.
Previous studies on carbonaceous chondrite organics have mostly focused on isotope systematics of two elements abundant in these organics – hydrogen and nitrogen. Oxygen is also fairly abundant in organics, comprising 10-20% of their mass. In addition, oxygen isotopes offer a crucial advantage compared to elements such as hydrogen and nitrogen, related to the fact that oxygen is made of three different stable isotopes, while hydrogen and nitrogen only have two stable isotope varieties. Indeed, having three stable isotopes allows testing whether processes by which carbonaceous chondrite organics formed have modified the oxygen isotope ratios 17O/16O and 18O/16O according to the mass differences between these three isotopes or not, providing key insights into their origin.
The results of our study, which has recently been published in PNAS, provide the first in situ, high-precision, triple oxygen isotope analysis of carbonaceous chondrite organics. These results show that the oxygen isotope systematics of the studied carbonaceous chondrites is similar to the relationship linking the composition of the Sun, of asteroid components, and of terrestrial planets. This suggests that organic materials accreted in chondritic asteroids probably formed through chemical reactions in the protosolar nebula at the beginning of Solar System evolution, rather than having been inherited from the interstellar medium or formed by fluid circulations on asteroidal bodies.
Full paper citation: R. Tartèse, M. Chaussidon, A. Gurenko, F. Delarue and F. Robert (2018) Insights into the origin of carbonaceous chondrite organics from their triple oxygen isotope composition. Proceedings of the National Academy of Sciences USA, doi: 10.1073/pnas.1808101115.
You can download a non-journal-formatted copy of the paper here