Oxygen Isotopic Evolution in the Early Solar Nebula:Interpretation of the Small Oxygen Isotopic Variation of Chondrules
T. Fukui and K. Kuramoto
Department of Cosmosciences, Hokkaido University, Japan

Oxygen isotopic systematics observed in chondritic components provides important clues toelucidate physical and chemical processes occurred in the early solar nebula. Chondrules haveO-isotopic composition similar to those of terrestrial materials with small variation. In
contrast, CAIs, the oldest minerals formed in the early solar nebula, have anomalously16O-rich composition [1]. A plausible explanation for the origin of the systematics isenhancement of 16O-poor H2O concentration in the initially 16O-rich inner solar nebula [2].
The 16O-poor H2O is produced by isotope-selective photodissociation of CO in the parent
molecular cloud and stored as icy mantle on silicate dust in the cold outer part of the nebula.
The ice-coated dust particles drift inward due to nebula gas drag [3] and releases water vapor
at the hot inner nebula. As a result, the 16O-poor H2O concentrates with time and the meanO-isotopic composition of the inner nebula evolves to be 16O-poor. This model is consistent
with the large variation between CAIs and chondrules, however, not likely to explain thesmall O-isotopic variation of chondrules because H2O concentration is expected to be
temporally and spatially heterogeneous during chondrule formation.
In this study, we suggest a process which is overlooked in the previous study and possibly
explains the small variation of chondrules; i.e., enhancement of silicate dust concentration in
the inner nebula. Collision experiment of the nebula dust analogous implies that the typicalsize of dust particles would be determined by the adhesive properties of them [4]. Because
silicate is less sticky than H2O ice, dust particles would be refined after evaporation of icy
mantle. The size of silicate dust is theoretically estimated to be ~ sub-millimeter, which isinterestingly comparable to that of typical chondrules. Such small dust particles are wellcoupled with the nebula gas motion and hardly drift inward. Thus concentration of silicate
dust would also be enhanced almost same degree as that of H2O in the inner nebula. The mostimportant consequence of this model is that, in case the concentration of H2O and silicate dust
is enhanced >~ 5 times larger than the solar abundance, the mean O-isotopic composition of
the inner nebula is almost determined as the weighted mean of O-isotopic composition of
H2O and silicate in the solar abundance ratio; i.e., independent for their concentration. Thissuggests that temporal and spatial variation of concentration of H2O little affects on the meanO-isotopic composition of the inner nebula as long as the enhancement is sufficiently large.
Numerical simulation on the evolution of concentration of H2O and silicate dust using typical
properties of observed protoplanetary disks confirms that such large enhancement is sustained
during chondrule formation in the inner nebula. Thus, the small O-isotopic variation ofchondrules would be reproduced in our model.
References
[1]
[2]
[3]
[4]

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