Insights into the origin of carbonaceous chondrite organics from their triple oxygen isotope composition

Romain Tartèse, Marc Chaussidon, Andrey Gurenko, Frédéric Delarue, and François Robert

PNAS August 6, 2018. 201808101; published ahead of print August 6, 2018.


“Refractory organic matter found in volatile-rich asteroidal materials essentially comprise the elements C, H, O, N, and S, which are thought to be important building blocks for life. Characterizing the origin(s) of these organics thus constitutes a key step to constrain the origin of life on Earth and appraise the habitability potential of other worlds. However, how and where these organics formed are still highly debated. In this study, we have determined the oxygen isotope composition of refractory organics from two families of carbonaceous chondrites. These data suggest that these organics formed in the nascent Solar System, possibly through chemical reactions occurring in the disk surrounding the young Sun.”

“Dust grains of organic matter were the main reservoir of C and N in the forming Solar System and are thus considered to be an essential ingredient for the emergence of life. However, the physical environment and the chemical mechanisms at the origin of these organic grains are still highly debated. In this study, we report high-precision triple oxygen isotope composition for insoluble organic matter isolated from three emblematic carbonaceous chondrites, Orgueil, Murchison, and Cold Bokkeveld. These results suggest that the O isotope composition of carbonaceous chondrite insoluble organic matter falls on a slope 1 correlation line in the triple oxygen isotope diagram. The lack of detectable mass-dependent O isotopic fractionation, indicated by the slope 1 line, suggests that the bulk of carbonaceous chondrite organics did not form on asteroidal parent bodies during low-temperature hydrothermal events. On the other hand, these O isotope data, together with the H and N isotope characteristics of insoluble organic matter, may indicate that parent bodies of different carbonaceous chondrite types largely accreted organics formed locally in the protosolar nebula, possibly by photochemical dissociation of C-rich precursors.”