The Origin of Chondrules and Refractory Inclusions in Chondritic Meteorites

Abstract

Examples of calcium-aluminumÈrich inclusions (CAIs) surrounded by thick chondrule mantles have been found in chondritic meteorites and cast doubt on the conventional belief that CAIs and chondrules possessed di†erent spacetime origins in the primitive solar nebula. We study speciÐc processes by which such objects, and the more common ordinary CAIs and chondrules, might have formed by Ñare heating of primitive rocks interior to the inner edge of a gaseous accretion disk that has been truncated by magnetized funnel Ñow onto the central proto-Sun. Motivated by the appearance of the chains of Herbig-Haro knots that deÐne collimated optical jets from many young stellar objects (YSOs), we adopt the model of a Ñuctuating X-wind, where the inner edge of the solar nebula undergoes periodic radial excursions on a timescale of D30 yr, perhaps in response to protosolar magnetic cycles. Flares induced by the stressing of magnetic Ðelds threading both the star and the inner edge of the Ñuctuating disk melt or partially melt solids in the transition zone between the base of the funnel Ñow and the reconnection ring, and in the reconnection ring itself. The rock melts stick when they collide at low velocities. Surface tension pulls the melt aggregate into a quasi-spherical core/mantle structure, where the core consists mainly of refractories and the mantle mainly of moderate volatiles. Orbital drift of rocks past the inner edge of the disk or infall of large objects from the funnel Ñow replaces the steady loss of material by the plasma drag of the coronal gas that corotates with the stellar magnetosphere. In quasiÈsteady state, agglomeration of molten or heat-softened rocks leads to a di†erential size-distribution in radius R pro- portional to R~3e~Lt@tLR, where tLD20 yr is the drift time of an object of Ðducial radius L 41cmand t is the time since the last inward excursion of the base of the funnel Ñow and X-wind. Thus, during the D30 yr interval between successive Ñushing of the reconnection ring, Ñash-heated and irradiated rocks have a chance to grow to millimeter and centimeter sizes. The evaporation of the moderately volatile mantles above large refractory cores, or the dissolving of small refractory cores inside thick ferromag- nesian mantles before launch, plus extended heating in the X-wind produce the CAIs or chondrules that end up at planetary distances in the parent bodies of chondritic meteorites.