Magmatic iron meteorites (including the large IIIAB group) are thought to
have formed by fractional crystallization within the cores of differentiated
asteroids, layered by silicate mantles. Asteroidal collisions can eventually
expose the cores (which in many or most cases have already crystallized) and
send some of the pieces on their way to the inner solar system. Nonmagmatic
irons (such as IAB) are more controversial. Some think that they also
formed in cores; others that they formed as metal melt pools at the bottoms
of impact craters on chondritic asteroids.
Alan Rubin
Institute of Geophysics and Planetary Physics
University of California
3845 Slichter Hall
603 Charles Young Dr. E
Los Angeles, CA 90095-1567
phone: 310-825-3202
e-mail: aeru...@ucla.edu
website: http://cosmochemists.igpp.ucla.edu/Rubin.html
----- Original Message -----
From: "Richard Montgomery" <rickm...@earthlink.net>
To: "Alan Rubin" <aeru...@ucla.edu>; <meteorite-list@meteoritecentral.com>
Sent: Wednesday, December 15, 2010 4:47 PM
Subject: Re: [meteorite-list] iron meteorite cooling rates and Meteorite Men
Hi List. (ot a chemist, me, just a collector, not ametorologist, just a
passionate meteorite guy.
This is mostly a question from Allan's post just now: I was always under
the impression that iron meteorites resulted from colliding
differentiated parent-bodies, and that the crystallization sequence was
achieved after an impact that exposed a core, molten NiFe suddenly ejected
into space without the shield of its former silicate mantle. Am I way off
base? Does Thompson structure develope within?
----- Original Message -----
From: "Alan Rubin" <aeru...@ucla.edu>
To: <meteorite-list@meteoritecentral.com>
Sent: Wednesday, December 15, 2010 4:21 PM
Subject: Re: [meteorite-list] iron meteorite cooling rates and Meteorite
Men
The iron meteorite cooling rates generally range from about 1 -
100ºC/Myr.
The reason for such slow rates is that the metal cores are buried deeply
within silicate mantles and heat cannot readily escape. The coarseness
of
the Widmanstatten pattern is a function of cooling rate -- more slowly
cooled irons will develop thicker kamacite lamellae. But there are two
other factors that govern the coarseness of the structure -- the Ni
concentration and the nucleation temperature. The lower the Ni
concentration in the metal, the more kamacite will develop upon cooling.
Metal that begins to nucleate at a higher temperature will have a longer
period within which kamacite can grow.
Alan Rubin
Institute of Geophysics and Planetary Physics
University of California
3845 Slichter Hall
603 Charles Young Dr. E
Los Angeles, CA 90095-1567
phone: 310-825-3202
e-mail: aeru...@ucla.edu
website: http://cosmochemists.igpp.ucla.edu/Rubin.html
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