Statistics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.304w&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 304
Statistics
Scientific paper
The origin of chondrules continues to be a problem resistant to detailed solution. Each passing year brings important new facts, but the small size of most chondrules (a 200- micrometer-diameter chondrule has a nominal mass of 12 micrograms) leads to difficulties for some kinds of studies, and the small size also makes the chondrules particularly vulnerable to alteration by metamorphism, even the mild reheating experienced by the least metamorphosed, type 3.0 chondrites. There is now a consensus within the community that the vast majority of chondrules formed in the solar nebula rather than in presolar or planetary processes. I therefore limit my discussion to nebular processes. Although one can find exceptions to all generalizations, the following features are general enough to require explanation by any successful model: (1) Chondrules have generally chondritic compositions; as an example, most chondrules have unfractionated rare-earth-element patterns and roughly constant Cr/Si and Mg/Si ratios. (2) Chondrules have variable compositions inconsistent with formation from a fine-grained, homogeneous precursor but consistent with precursors consisting of mixtures of cosmochemically plausible nebular components such as common silicates, refractory silicates, etc. (3) Chondrules were molten or partly molten for very brief periods. Depending on composition, chondrules are completely molten in the range 1400 to 1750 K; at these temperatures in a low-pressure (pH(sub)2 < 10^-3 atm) not only the volatile but also the common elements such as Mg, Si, and Fe would evaporate, yet most chondrules show no evidence for the loss of elements as volatile as Na and S. It follows that the period spent at the peak temperature was extremely brief, of order 1 s. (4) Many, perhaps most, chondrules were partly molten; the common porphyritic chondrules contain mineral fragments that survived the chondrule-producing heating events. The magnitude of the individual thermal events was only marginally sufficient to form chondrules. (5) Chondrule- forming events were relatively common; statistics based on compound barred-olivine chondrules indicate that most chondrules experienced two heating events of sufficient magnitude to melt a sizable fraction of the chondrule. (6) Coarse-grained rims on chondrules testify to additional brief heating events prior to particle agglomeration and thus removal of the chondrule from contact with the nebula. The evidence points to a series of flash heating events that occurred in a nebular region having a high dust/gas ratio. Impact heating seems implausible because the required interparticle velocities are too high. Large-scale heat sources such a shock fronts seem unable to provide the short durations of maximum heating. Magnetic field reconnection is only efficient at altitudes where vanishingly small amounts of dust are expected. Lightning powered by the differential rotation between the nebular gas and the thin (100-km) dusty midplane seems to be the most plausible heating mechanism, but the detailed physics is yet to be worked out.
No associations
LandOfFree
Constraints on Chondrule Origins does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Constraints on Chondrule Origins, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Constraints on Chondrule Origins will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1210007