Accretion and Preservation of D-Rich Organic Particles in Carbonaceous Chondrites: Evidence for Important Transport in the Early Solar System Nebula

Abstract

We have acquired NanoSIMS images of the matrices of CI, CM, and CR carbonaceous chondrites to study, in situ, the organic matter trapped during the formation of their parent bodies. D/H ratio images reveal the occurrence of D-rich hot spots, constituting isolated organic particles. Not all the organic particles are D-rich hot spots, indicating that at least two kinds of organic particles have been accreted in the parent bodies. Ratio profiles through D-rich hot spots indicate that no significant self-diffusion of deuterium occurs between the D-rich organic matter and the depleted hydrous minerals that are surrounding them. This is not the result of a physical shielding by any constituent of the chondrites. Ab initio calculations indicate that it cannot be explained by isotopic equilibrium. Then it appears that the organic matter that is extremely enriched in D does not exchange with the hydrous minerals, or this exchange is so slow that it is not significant over the 4.5 billion year history on the parent body. If we consider that the D-rich hot spots are the result of an exposure to intense irradiation, then it appears that carbonaceous chondrites accreted organic particles that have been brought to different regions of the solar nebula. This is likely the result of important radial and vertical transport in the early solar system.

Cometary Isotopic Measurements

“…This isotopic composition is a signature of a primitive organic matter from cold and dense regions of the protosolar cloud or protoplanetary disk. Remarkably, the extreme D excesses observed in UCAMMs are not limited to hotspots of 1-2 µm in size like usually observed in IDPs or meteoritic insoluble organic matter (e.g., Remusat et al 2010, and references therein). The Duprat et al (2010) association in UCAMMs of primitive organic matter with mineral phases (including crystalline minerals) that show evidence of nebular processing (Dobricǎ et al 2012), suggest that deuteration of the organic matter may have taken place in the outer regions of the protoplanetary disk rather than in the presolar molecular cloud core (Duprat et al 2010).…”

Organic material in meteorites and the link to the origin of life

“…Despite numerous measurements, interpretations of the isotopic composition of organics and hydrated minerals in chondrites are still a matter of intense debate. It must be noted that in all the hydrated chondrites, water is always depleted in D compared to organic matter, whereas the equilibrium requires the opposite, whatever the location of the H in the molecule [46,47]. This indicates that isotopic disequilibrium has been preserved during 4.56 billion years on the parent body of these carbonaceous chondrites and that parent body processes may not have erased the signatures of the synthesis.…”

“…not interstellar) for the D-rich hot spots. Then, if we can produce a large range of D/H in organics within the protosolar nebula, or the parent molecular cloud, a pre-accretion diversity in isotopic composition of organic precursors could also explain the multiscale diversity in chondrites [46]. It must be noted that the most D-rich organic grains in chondrites does not show significant isotopic exchange with the mineral matrix, questioning a parent body effect to account for the observed diversity [46].…”

“…Then, if we can produce a large range of D/H in organics within the protosolar nebula, or the parent molecular cloud, a pre-accretion diversity in isotopic composition of organic precursors could also explain the multiscale diversity in chondrites [46]. It must be noted that the most D-rich organic grains in chondrites does not show significant isotopic exchange with the mineral matrix, questioning a parent body effect to account for the observed diversity [46]. This assumption tackles the concept of a common precursor for all the chondrites and it implies that we should rather consider the organic matter in each chondrite group as a sampling of a range of isotopically and chemically distinct organic precursors.…”

Organic material and gases in the early Solar System : the meteorite record

“…Despite numerous measurements, interpretations of the isotopic composition of organics and hydrated minerals in chondrites are still a matter of intense debate. It must be noted that in all the hydrated chondrites, water is always depleted in D compared to organic matter, whereas the equilibrium requires the opposite, whatever the location of the H in the molecule (Sessions et al 2004;Remusat et al 2010). This indicates that isotopic disequilibrium has been preserved during 4.56 billion years on the parent body of these carbonaceous chondrites and that parent body processes may not have erased the signatures of the synthesis.…”