Cosmic Ray Induced Mass-Independent Oxygen Isotope Exchange: A Novel Mechanism for Producing ^16O Depletions in the Early Solar System

A fundamental puzzle of our solar system's formation is understanding why the terrestrial bodies including the planets,comets,and asteroids are depleted in ^16O compared to the Sun. The most favored mechanism,the selective photodissociation of CO gas to produce ^16O depleted water,requires finely tuned mixing timescales to transport ^16O depleted water from the cold outer solar system to exchange isotopically with dust grains to produce the ^16O depleted planetary bodies observed today. Here we show that energetic particle irradiation of SiO_2 (and Al_2O_3) makes them susceptible to anomalous isotope exchange with H_2O ice at temperatures as low as 10 K. The observed magnitude of the anomalous isotope exchange (D^17O) is sufficient to generate the ^16O depletion characteristic of the terrestrial bodies in the solar system. We calculated the cosmic-ray exposure times needed to produce the observed ^16O depletions in silicate (SiO2) dust in the interstellar medium and early solar system and find that radiation damage induced oxygen isotope exchange could have rapidly ( 10-100 yrs) depleted dust grains of ^16O during the Sun's T-Tauri phase. Our model explains whythe oldest and most refractory minerals found in the solar system, the anhydrous Calcium with Aluminum Inclusions (CAIs),are generally ^16O enriched compared to chondrules and the bulk terrestrial solids and provides a mechanism for producing ^16O depleted grains very early in the solar system's history. Our findings have broad implications for the distribution of oxygen isotopes in the solar system, the interstellar medium, the formation of the planets and its building blocks as well as the nature of mass-independent isotope effects.