The oxygen isotope effect in the earliest processed solids in the solar system: is it a chemical mass-independent process?

Astrochemistry Laboratory, Solar System Exploration Division, NASA's Goddard Space Flight Center, Greenbelt, Maryland 20771, USA e-mail: Ashraf.Ali@gsfc.nasa.gov

Received: 12 December 2006
Accepted: 20 March 2007

Abstract

Aims.An anomalous effect in the abundances of oxygen isotopes in the most refractory calcium-aluminum-rich inclusions (CAIs) was discovered some thirty years ago. The origin of these oxygen isotopic anomalies has hitherto remained unexplained. The origin is neither nuclear, nor has the recent photochemical self-shielding explanation been proven to be valid. We discuss a possible chemical mechanism to resolve these observed effects.

Methods.By uniting the most recent laboratory observations of nanoclusters of silicates in beams and the first principles theoretical studies of their structure and properties with a major dynamical constraint recently described as the surface non-RRKM effect during SiO₂ formation on the growing grain, we show that the origin of the anomalous isotopic effect in high-temperature minerals in CAIs is chemical and strictly mass-independent.

Results.We report that the surface non-RRKM effect would represent a major process in the formation of our own solar system and observable protoplanetary accretion disks, and the mass-independent isotope effects are directly associated with the formation of primary grains in the high temperature nebular environment. We expect that this chemical reaction mechanistic approach combined with future time-resolved studies on the kinetics of growth of silicates and a precise knowledge of the oxygen isotopic abundances of the sun would provide a very detailed understanding of the origins of formation of our solar system.