[meteorite-list] Let's talk about meteorites

[Alan Rubin]

Sorry,, but I guess I was not clear.  The only group of silicate-bearing 
irons widely agreed to have come from an asteroid core is the IVA group. 
This group has little silicate, mainly small grains of silica, which some 
think may have been vapor deposited in the core.  There are no collisions 
involved in forming the IVA irons except the one or ones that shattered 
their parent differentiated asteroid and liberated them.  The other 
silicated irons, i.e., the IAB, IIICD and IIE groups, may be from 
chondritic, not-differentiated asteroids, that never experienced global 
melting.  These irons may have formed after an impact into the chondritic 
surface of these bodies involving local melting and separation of the 
metallic and silicate liquids because they were immiscible.  The metal 
liquid sank to the crater floor, incorporated some rapidly melted silicate 
debris and cooled.  This is a controversial model and not universally 
accepted.  Mesosiderites are differentiated rocks consisting of roughly half 
metal and half silicate.  The silicate is basically basalt and 
orthopyroxenite, i.e., eucrite and diogenite material. The metal is similar 
to that of the IIIAB iron meteorites (a differentiated iron group) and so is 
most likely from the core of a differentiated (i.e., globally melted) 
asteroid.  My model from some years ago was that the iron core (plus 
overlying mantle) of the projectile impacted the basaltic/orthopyroxenitic 
surface of another (target) asteroid and formed the mesosiderites.  The 
large gabrroic clasts in many of the mesosiderites seem to have formed by 
two or more episodes of impact melting, and grain settling.  Their origin 
appears rather different from that of the silicated irons.
Alan

----- Original Message ----- 
From: "Carl 's" <carloselguapo1 at hotmail.com>
To: <meteorite-list at meteoritecentral.com>
Sent: Thursday, September 03, 2009 4:42 PM
Subject: Re: [meteorite-list] Let's talk about meteorites


>
>
>
>
> OK, I think I see the subtle difference. I have had to re-read Dr. Rubin's 
> post several times to get the picture (Thanks to MikeG, too). Simplified, 
> an asteroid slams into another planetary body right to the iron core and 
> forms mesosiderites. That part I knew, but when smaller iron asteroids 
> slams onto larger rocky asteroids the surface layer  forms the silicated 
> irons. I would have thought that would also form mesosiderites, too. Hmmm. 
> Very interesting.
>
> Thank you, Dr. Rubin.
>
> Carl
>
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