[meteorite-list] Let's talk about meteorites

[Galactic Stone & Ironworks]

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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