[meteorite-list] Let's talk about meteorites

From: Galactic Stone & Ironworks <meteoritemike_at_meteoritecentral.com>
Date: Fri, 4 Sep 2009 12:28:32 -0400
Message-ID: <e51421550909040928p5b020d3m75d684f221a8e030_at_mail.gmail.com>

Hi Alan, Carl and List,

WOW! Thanks Alan! This is what I joined the Met List for. :)

Best regards and clear skies,

MikeG


On 9/4/09, Alan Rubin <aerubin at ucla.edu> wrote:
> 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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Received on Fri 04 Sep 2009 12:28:32 PM PDT