Scientists from CNRS, Muséum National d’Histoire Naturelle de Paris, Manchester University, Université Paris XIII, Sorbonne Université and Institut de physique du globe de Paris/Université de Paris, have confirmed by laboratory experiments the existence of a novel isotopic effect, discovered 35 ago during ozone synthesis. This effect - from its magnitude and properties - violates elementary rules governing the distribution of isotopes during chemical reactions. It could be relevant for many chemical elements and be part of some well-known isotopic variations, identified in the components of meteorites, and generally considered to reflect stellar nucleosynthetic processes.
In the Solar system, most chemical elements show isotopic variations, “anomalies”, which are not predicted by any theory and never observed in laboratory experiment and terrestrial rocks. These “anomalies” are classically considered to arise from the presence, in the protosolar disk, of grains condensed in the envelopes of dying stars, several millions or billion years before the formation of the solar system. These grains, called presolar, were isolated from meteorites and their diversity in isotopic compositions is understood as due to stellar nucleosynthesis of chemical elements.
At its formation, in its inner zones, close to the early Sun, the protosolar disk is a hot gas. During its slow cooling (in a few million years), microscopic grains appear by condensation. At a later stage, they will form the main constituents of the terrestrial planets. In the disk, the turbulence mixes these grains on large spatial scales with presolar grains. This mixture is preserved in a few classes of meteorites whose parent bodies have never been melted since their formation. Here is the schematic drawing of the standard model accounting for the isotopic anomalies found in meteorites and planets.
In the early 1980’s it has been experimentally shown that ozone synthetized in the laboratory by electric discharges, exhibits an oxygen isotopic effect similar to the isotopic anomaly found in meteorites. This effect is also present in atmospheric ozone. However, the chemical reactions responsible for the formation of ozone did not take place in the protosolar disk where ozone was absent. None of the theoretical studies done so far have suceeded to identify the physical mechanism at the origin of this isotope effect.
Following the path put forwards by a recent theoretical study offering a new solution to the ozone effect, a chemical reaction scheme that could yield isotopic anomalies was imagined: the surface of nanometric grains condensed from a gaseous plasma would favor the coupling between chemical and isotopic reactions.
Experiments have been realized with titanium, an emblematic refractory element whose isotopic compositions exhibit several isotopic anomalies in meteorites. These experiments have been realized in a plasma generated by a microwave excitation in a vapor of titanium chloride solubilized in an organic liquid.
Isotopic analyses of titanium in sub-micrometric grains condensed form the gas phase and where titanium is associated with organic carbon, exhibit systematic isotopic anomalies similar to those found in presolar grains. These variations are well accounted for by the model developed for ozone, suggesting that it could be applied to other chemical elements. In this respect, isotopic anomalies could also sign peculiar chemical reactions involving the reactive radicals produced by molecular dissociation in plasma.
Such a conclusion opens the door towards an unexplored domain: chemical reactions in natural plasma. These isotopic effects might offer new perspectives to study the conditions prevailing in the early solar system or in stellar atmospheres.
> Robert F., Tartèse R., Lombardi G., Reinhardt P., Roskosz M., Doisneau B., Deng Z. & Chaussidon M. (2020) Mass-independent fractionation of titanium isotopes and its cosmochemical consequences. Nature Astronomy - https://doi.org/10.1038/s41550-020-1043-1
Marc Chaussidon, équipe de cosmochimie, astrophysique et géophysique expérimentale