1. Institute of Geophysics and Planetary Physics,
2. Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA

Correspondence to: J. R. Lyons^1,2 Correspondence and requests for materials should be addressed to J.R.L. (Email: jrl@ess.ucla.edu).

Abstract

The abundances of oxygen isotopes in the most refractory mineral phases (calcium-aluminium-rich inclusions, CAIs) in meteorites¹ have hitherto defied explanation. Most processes fractionate isotopes by nuclear mass; that is, ¹⁸O is twice as fractionated as ¹⁷O, relative to ¹⁶O. In CAIs ¹⁷O and ¹⁸O are nearly equally fractionated, implying a fundamentally different mechanism. The CAI data were originally interpreted as evidence for supernova input of pure ¹⁶O into the solar nebula¹, but the lack of a similar isotope trend in other elements argues against this explanation². A symmetry-dependent fractionation mechanism^{3, 4} may have occurred in the inner solar nebula⁵, but experimental evidence is lacking. Isotope-selective photodissociation of CO in the innermost solar nebula⁶ might explain the CAI data, but the high temperatures in this region would have rapidly erased the signature⁷. Here we report time-dependent calculations of CO photodissociation in the cooler surface region of a turbulent nebula. If the surface were irradiated by a far-ultraviolet flux 10³ times that of the local interstellar medium (for example, owing to an O or B star within 1 pc of the protosun), then substantial fractionation of the oxygen isotopes was possible on a timescale of 10⁵ years. We predict that similarly irradiated protoplanetary disks will have H₂O enriched in ¹⁷O and ¹⁸O by several tens of per cent relative to CO.

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