[meteorite-list] Chondrule formation mechanism (Info Please)

[Warin Roger]

Hi, all,

I am surprised that nobody evoked the theory following which chondrules were formed in relatively very few privileged zones of space. They would then form through one or more impacts of relatively large asteroids, onto the parent body covered with regoliths (and even with megaregoliths). 
The excellent book of Derek Sears, entitled “The origin of chondrules and chondrites” (Cambridge Planetary Science, 2004) supports this hypothesis. In corollary, ordinary chondrites (85% on Earth) would be quite rare in cosmos, and only few parent bodies would produce chondrites.

Glad to hear some comments on the above assumptions.

Thanks,

Roger Warin



----- Message d'origine ----
De : Sterling K. Webb <sterling_k_webb at sbcglobal.net>
À : meteorite-list at meteoritecentral.com
Cc : E.P. Grondine <epgrondine at yahoo.com>
Envoyé le : Dimanche, 22 Octobre 2006, 20h38mn 55s
Objet : Re: [meteorite-list] Chondrule formation mechanism (Info Please)


Hi, Ed, Rob,

    This scenario (Ed's) would require that we would
find a chondrule with a formation age of 3.9 Gya, I
think. As far as I know, that has never happened.

    All chondrites (so called because they contain
chondrules) are the same age: "about" 4.555 Gya.
Chondrules are the same age (2 to 5 million years 
variation among chondrules) as the chondrites they 
occur in. The "about" is because the dating methods 
have a limit to how precisely they can resolve 
small age differences.

    Dating by lead isotopes says the solar system
is 4.560 +/- 0.005 Gya old. Other systems of isotope 
measurements (like 147Sm/143Nd) give 4.553 +/- 0.003,
and so forth. Within the limits of measurement, all
chondrites are the same age, a hair younger than the
solar system itself, the Class of Zero, and so are their
chondrules.

    Meteorites that do not (never did) contain chondrules
have varying ages. Lunaites are the age of that portion
of the lunar crust they came from, generally quite old
compared to Martians which have the "formation age" 
of the basalt flow they were chipped off of for the long 
haul to Earth. Irons, which formed inside a differentiating 
body, have younger ages; some very much younger if 
the differentiation took a long time (Weekeroo Station IIe 
is 4.340 Gya, Kodaikanal IIe 3.800 Gya, many IAB irons 
the same).

    I'm thinking that before you need to develop a theory
to explain a 3.9 Gya chondrule, you'd have to actually
have a 3.9 Gya chondrule. As far as I know, none with 
discordant ages have ever been found. In certain solar 
circles it would be Big News.

    Oddly, if you Google for "oldest chondrule," you get
the oldest chondrules, and if you Google for "youngest 
chondrule," you get the oldest chondrules... on the grounds
that it is "young" as the solar system. If you Google for 
"discordant chondrule age," you get arguments over 2 or 3
million years in the age of something 4-1/2 billion years old. 
    

Sterling K. Webb
--------------------------------------------------------------------
----- Original Message ----- 
From: "E.P. Grondine" <epgrondine at yahoo.com>
To: <meteorite-list at meteoritecentral.com>
Sent: Sunday, October 22, 2006 10:24 AM
Subject: Re: [meteorite-list] Chondrule formation mechanism (Info Please)


> Hi Rob - 
> 
> You noticed the contradiction in cooling periods as
> well.
> 
> What I am thinking is that there was at least one
> larger parent body which was "disrupted" about 3.9 Gya
> (at time of LPBE).  When this larger parent body was
> disrupted, then the "effervescent" "foaming" that led
> to some chondrules occured - sudden cooling, as
> gravitation pressure had been released, and much lower
> local gravity. Local processes suddenly take over - a
> sharp gravitational and pressure transition, and a
> sudden cooling. Gross processes - perhaps sufficiently
> gross to overwhelm other small forces.
> 
> Through collisions of the resulting fragments, we see
> some of the meteorite types we see today.
> 
> good hunting, 
> Ed
> 


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