[meteorite-list] Questions about accretion. Part 1 Aluminium 26, and Asteroid ages

From: Mr EMan <mstreman53_at_meteoritecentral.com>
Date: Tue, 7 Apr 2009 19:19:56 -0700 (PDT)
Message-ID: <357368.63438.qm_at_web55207.mail.re4.yahoo.com>

My ISP continues to lose much of my email else send them in huge batches.

Some additional points to what was discussed thus far:

Iron migration to the core of a heat building/holding sized body is a buoyancy issue and gravity driven so long as the iron remains molten.

Accretion probably had an electrostatic component which may be an anti accretion force, there was some covalent molecular bonding but as strange as it seems the primary attractant has to be gravity yes molecule to molecule-- chondrule to chondrule. Chondrule formation is a whole other treatise not covered here.

After accretion:
Aluminum 26 is a radioactive isotope with half life of .73(?)million years which decays to Magnesium 26. The bulk occurrence of Al26 in the early solar system had to be ejected from a solar fission furnace. When we find magnesium within a crystal matrix where aluminum should be, we know it started out as an atom of Al26. The heat of that Al26 decay is widely believed to be the driver for differentiating in asteroids accreted from chondrules and non-chondrule particles. Except for the planetary meteorites and Impact Melt Breccias(IMB) all original common chondrite to achondrite parent body conversion appears to have taken place in the approximate 15-20 Million years starting with the formation of the current solar system. The first 5 million being the time when accretion was ongoing.

There are two theories of H Chondrite parent body formation. Both include zones. One is that there were multiple H class parents of different sizes yielding different petrological classes. The other is that there were but one or very few H parent bodies and what started off as H3 and melted from heat distributed inside to out. As the heat source ran lower and lower, the chondrite "cake" was left partially uncooked resulting in an "onion layer" set of zones with H3 on the surface and H7/achondrite toward the center(yep with an iron core)

Either way, there is a successive fall off of formation/cool-off ages in H Class formation ages and that is to be expected. H3 chondrite zones/bodies ran out of heat earlier than H5s so fewer chondrules were melted (thermally metamorphosed). As a class, H3s zones congealed a bit earlier than the other H4,H5,H6 zones. Because Al26 was more or less uniformly distributed, we may infer that H3s either came from smaller bodies which were barely large enough to hold some heat but not large enough to let the full melting cycle run to achondrite sizes. And/or They come from the crustal regions of a substantial sized asteroid. Either way they were liberated in a major disruption that exposed them down to their cores. From Widmanstatten studies we know that the cooling at the metallic core was a very slow rate of a a couple to a few tens of degrees per million years. I am sure somewhere someone has cross referenced these rates to improve on what we believe we
 know about asteroid formation ages.

For more reading:
<http://www.psrd.hawaii.edu/Sept02/Al26clock.html>
(See the last chart on the above link for asteroid/meteoroid formation ages)
<http://www.thefreelibrary.com/Aluminum+emerges+as+early+timekeeper-a018639626>

Elton

Note that Formation age, Cosmic Ray Exposure age(CRE) are not the same. The formation age of meteoric material may or not be the same age as when it was liberated/ejected from the parent body depending if the shock was sufficient to reset the atomic clocks.
Received on Tue 07 Apr 2009 10:19:56 PM PDT