[meteorite-list] Let's talk about meteorites

[Alan Rubin]

Several folks have brought up interesting questions and I'll try to answer 
them.  But, first of all, I have to admit that I'm not an expert on iron 
meteorites.  There are many other researchers, including a few members of 
this list, who are far more knowledgeable than I am.
        John Wasson has recently grouped various IAB and IIICD irons and 
some ungrouped irons into a "IAB complex."  These may be all from one 
asteroid, or perhaps from several.  They all have broadly similar metal 
compositions and do not display element-element (e.g., Ir-Ni) concentration 
plots that appear consistent with the fractional crystallization processes 
that are believed to occur in the cores of molten asteroids. The silicates 
in these irons also have the planetary-type rare gas abundances that we see 
in chondrites but not in eucrites, presumably because they were volatilized 
during the extensive melting that eucrites experienced.  This suggests that 
the silicates in these irons were rapidly cooled.  This is consistent with 
the model that they were derived from chondrites as is their approximately 
chondritic bulk compositions.  Now, the question of why these irons display 
nice Widmanstatten patterns that appear consistent with slow cooling over 
millions of years...  I suspect that this is not due to monotonic cooling 
but rather to annealing, perhaps induced by impact heating processes.  If an 
impact on a chondritic asteroid causes localized melting, metal and silicate 
segregation and metal pooling on the crater floor (as in this model), then 
the slow cooling indicated by the metal might be due to burial beneath 
well-insulated debris (perhaps impact ejecta); such material would have a 
low thermal diffusivity and would promote relatively slow cooling.  Could it 
be slow enough to cause a Widmanstatten pattern?  I don't know, but repeated 
impacts over the course of millions of years could cause periodic episodes 
of annealing.  This might (or might not) work.  Although there may seem to 
be an inconsistancy between the fast cooling of the silicates and the slow 
cooling of the metal, this can be readily accommodated.  Once the silicates 
quench and the planetary gas is essentially sealed in, then they could be 
annealed without much of the gas leaking out.
        It is important to note that not everyone agrees with the 
impact-melting model for the IAB-IIICD and IIE irons.  Others would argue 
that these irons did form in differentiated asteroids, perhaps in cores, 
perhaps in isolated pods in the mantle that never sank to the asteroid 
center.  I'm not convinced by these models, but perhaps this exchange will 
prompt one of the advocates to explain it.
Alan



----- Original Message ----- 
From: "Mr EMan" <mstreman53 at yahoo.com>
To: "Carl 's" <carloselguapo1 at hotmail.com>; 
<meteorite-list at meteoritecentral.com>; "Alan Rubin" <aerubin at ucla.edu>
Sent: Friday, September 04, 2009 6:36 AM
Subject: Re: [meteorite-list] Let's talk about meteorites


--- On Thu, 9/3/09, Alan Rubin <aerubin at ucla.edu> wrote:
<<snip... The metal liquid sank to the crater floor, incorporated some 
rapidly melted silicate debris andcooled. This is a controversial model and 
not universally accepted.>>

I think this theory has a potential fatal flaw if what we think we know 
about taenite/kamacite growth is valid.  Without an insulating blanket the 
molten pool will not exist in a molten state long enough to permit 
crystallization aka Widmanstatten patterns.

Be it remembered that Widmanstatten pattern/crystal growth is very very slow 
on the order of 10's of degrees cooling per million years. It is difficult 
to develop a scenario that integrates a large crater on an Goldilocks 
Asteroid which works.

Goldilocks: Not too small as escape velocity is so low there is no fall 
back/re-accretion to bury the melt; Not too large as the asteroid would have 
already differentiated into a metallic core...so it has to be just right, at 
the threshold of the larger size with sufficient gravitational acceleration 
to not just recapture ejecta but to do it rapidly enough to insulate the 
molten metal.  I envision a steeply conical deep crater which could minimize 
the amount of fall back ejecta to cover the surface. keep the pool--if in 
fact, such one exists.  This scenario also requires to nearly identical 
impacts; one down the throat of another, millions of years apart.  This 
tends to disfavor the crater floor theory on just the statistics. It would 
be interesting to locate a crater on an asteroid that fits the definition of 
Dewar flask.

Popigai, here on earth had the depth and fall back to insulate a 600m melt 
on the crater floor and it only stayed molten for "a few thousand years" Not 
millions! This was a scenario that was given all benefit of favorable 
condition and still could not stay molten long enough.

I can see why this theory has some doubters.  Were we able to find a rapidly 
quenched FeNi meteorite without the Widmanstatten marker than I could see a 
scenario for this theory, but to my meager knowledge of irons I can't recall 
one. One caveat, I can not positively confirm any silicated iron (e.g. 
Miles) shows or doesn't show a pattern when etched.  Ergo, I may have made 
the case for validating or invalidating the theory.

As far as impact-induced melting and melt pockets scattered around the 
interior, meeting the insulation demands, I find much more reasonable.  A 
vignette example would be Portales Valley as it proves a process on a micro 
level indicating the possibility that it has operated on a macro level.

Elton
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