[meteorite-list] Ablation Zone 5 Layers...Not

From: Jason Utas <meteoritekid_at_meteoritecentral.com>
Date: Fri, 20 Nov 2009 03:58:19 -0800
Message-ID: <93aaac890911200358i28916c75y6ce699e18422829b_at_mail.gmail.com>

Hello Elton, All,
I'm going to go through this bit by bit to do it justice...


> Yes on a freshly recovered iron, there appears to be a "film" of what we believe is "magnetite-like oxide/nitride micro-crystals, probably including some sulfide and phosphide minerals" which form through interaction with hot atmospheric plasma. ?Even though some of it is magnetic, some of it is easily dislodged with a wipe of the finger.


I assume this coating is relatable to the iridescent film which often
coats stony meteorites - the film that often disappears within days of
a fall.


>I surmise that this rapidly goes to hematite or limonite but I've not thought through the chemistry and I suspect a valence discrepancy that makes this type meteoric "magnetite" unstable. The mineral assemblage in the coating/film is a result of passage through the atmosphere and not per se the resulting changes that occur with the passage of time on the surface.


I shall point you toward this photograph of the external surface of a
Sikhote-Alin. This iron was found ~50+ years after falling and still
retained its exterior surface. The features you see are not made of
melted Fe, but of an outer coating of iron oxide which formed during
atmospheric descent.

http://www.flickr.com/photos/cameteoritefinder/2335664239/sizes/l/


> ?I think I can proffer an argument for what is and is not a scientifically underpinned definition of "crust" but I'll work on that later. For the time being the use of "crust" by present definition involves glass and last time I checked there is no such thing as "iron glass".


Where did you get this definition? Why is it more valid than the one
accepted by Buchwald, Nininger, Krinov, and the folks at the USNM?
Why does fusion crust *have* to have glass in it? Honestly, this
whole thing seems like a semantics battle on your part.


>We expect to find something analogous to "crust" so we call what we see "crust"-- I understand that. But when we stray too far everything including dust mites, rust and, fungus gets called fusion crust.


Right.
Here:

http://www.aerolite.org/catalogue/sikhote-alin-aaa-33-2.htm

What you're looking at is the original external surface of the iron,
made of that FeO layer that you keep insisting isn't fusion crust.
It's perfectly analogous to the crust of stony meteorites, except, of
course, in that it contains no glass.


> As there is also a surface bluing occasionally observed (much like after welding) this may be a directly formed oxide/nitride layer of chemically altered meteorite while emerging from the incandescent phase of flight but since I am unsure of the composition I'll leave it out of the below discussion.


I've seen that on stony meteorites as well. But since you're leaving
it out, there's no real need to address it.


>?Chances are it is also quickly lost to weathering on the surface--even in the museum drawer.


Maybe. I saw some pretty Oum Dreyga's with such a film still present
as of this winter - in a drawer in Alain Carion's shop in Paris. As
such, I have the feeling that such layers may not simply sublimate
with time, but they do seem to disappear rather rapidly when
meteorites are left in the field.


>The bottom line here is: we have to accept the probability of an ever-evolving surface on our meteorites. ?Some happen quickly and may be gone in a flash and some oh so slowly. This should not deter us however from discussing the basis for each step that comes and goes.


This also has nothing to do with our argument, for the most part. I
don't think there's anyone here who denies such a fact, so stating it
is somewhat superfluous.


> I believe to discuss meteorite surface features e.g crust, non crust, flow lines, ripples, regmaglypts, pits, bubbles, and all the variations, we should come to a working definition in general principle of what to call them so we know we are discussing the same thing.


Sounds good to me.


> Part of that is acceptance that there is an "ablation/ablated zone" generally 2-6mm from the physical surface where the meteoroid last interacted with the heat of reentry. This zone my eventually be proved a new type of "rind", geologically speaking.


Unfortunately, it's hard to gauge whether or not such a feature truly
exists on stony meteorites because, due to their decreased
conductivity, this heating does not occur over distances quite so
large.

See page two.

http://adsabs.harvard.edu/full/1982Metic..17...27R

So there's kind of an "ablationary rind," but it really only exists to
the extent that you just noted in iron meteorites...

> Of the layers physically present, I see two branches/variations which we may reintegrate but for ease of discussion the first is mostly the non-silicate bearing iron branch of "layers":

So we *are* talking about irons' fusion crusts...ok...

> The ablated/ablation zone includes amongst it layers:

> 1)Lost Layer/ Null layer: The material which is missing, includes ALL the material which is no longer there which we may conclude was lost from its pre-entry form due to atmospheric interaction. ?It may be marginal but may be needed to discuss surface depth in relation to cosmic ray tracks 14C concentrations, etc.

O...k....the stuff that's no longer there. A wordy description, but
sure. Call it what you will.

> 2) Oxide Film or Coating: There is a layer of non-physically/non-chemically, bonded oxide film which is not persistent, subject to rapid erosion/weathering, abrasion etc. This represents a condensation coating which is applied after ablation stops. (See bluing discussion also)

Such a coating forms on irons and stones alike, though - iridescent
films have been reported on many freshly fallen meteorites, regardless
of type.

> 2a) This is where fusion crust might be found if there were normally crystalline molecules that melted and quickly quenched leaving an anamorphic solid. But what we know as true fusion crust is more complicated than that and is largely governed by the composition of the meteoroid.

You breeze over it so nicely, without addressing the issue. Hum.
Well, again, I don't know why you insist on the glassy nature of a
fusion crust: I really can't fight your definition of it, because it
simply doesn't make any sense. There's no reason to draw the line
there, and I can't think of a single reason why fusion crust should
*have to* contain glass. Knowing meteorites, I would define the
fusion crust as the layer of meteoric material transformed into melt
during a meteorite's ablative stages of flight, which later solidifies
into a solid coating of material on the surface of the stone, iron,
what have you. I see no reason to insist on glass - I agree that
making a distinction between the properties of stones' versus irons'
fusion crusts and their structures might well be a worthwhile
endeavor, but insisting on calling the crust that forms on irons 'not
a fusion crust' seems a pointless task.

> 3)Flow/ Thermodynamic Features:

> 3a)There may be a layer of flow streams/esker-like inverted stream channels where molten material, which escaped evaporation and,, was displaced from one spot to another where it may have been redeposited. ?Regardless it is an artifact of reentry and we may also include it in the subset of features we refer to as "flight markings" This is a gray area also because this is more akin to a surface feature than a true layer but I throw it out on the table for discussion. There will be occasional features which represent movement of material over top of a previously ablated surface and time and consensus will determine if it merits a layer designation.

I disagree; such structures are merely features of the aforementioned
fusion crust layer, and should be deemed synonymous with said layer.
They are, after all, composed of the same materials, and one is not
below or above the other layer; you're talking about the same stuff
here.
These features are made of the fusion crust noted above, so calling
them a distinct layer seems pointless.
You're not even arguing the difference between icing and frosting.
You're arguing a difference between frosting and thick frosting. It
just doesn't make sense.

> 3b)There is also the occasional surface feature (semi flow) (which may or may not be a layer) of plastically deformed "ripple-marks" which give a satiny, wavy, micro "ridge and valley" pattern not unlike the depth and texture of fingerprints (NOTE this is not the same as "thumb-print" regmyglipts) Not all irons have this very thin layer. These ripple marks appear to form via fluid dynamics. ?I surmise (but have yet to prove) these are ripple marks of a extremely short-lived state where semi-molten metal is plastic enough to deform along lines where superheated gas eroding gas passing in both laminar and turbulent flow over the continuously evolving surface of the meteoroid. ?It leaves, a row and furrow/valley and ridge/ripple-like marking, submilimeter in depth. ?This results in that "less than glossy","satin-like" sheen seen on some meteorites--This is legitimate flight marking and therefore may actually be a surface feature and not a true layer but, a
> ?variation on the ablation surface. I am leaning toward this being a surface feature vs an independent layer.

See the specimen in my flickr stream pictured above. This "layer," as
the one before it, is synonymous with fusion crust.

> 4)Ablation surface: It is included to distinguish from the newly fallen surface any weathered/flaking/rusting surface all too frequently mistakenly called "fusion crust" on Canyon Diablos, Natans ?etc. Crust if present sits atop the ablation surface as it represents incorporated atmospheric gasses and possible re-deposition of Physically and chemically altered material from another location on the meteoroid, etc. ?Surface features can be in the ablation surface or above it depending on their origins. The ablation surface is a distinct demarkation between what was removed and what remained even if subsequently it bubbled into fusion crust or represents a redeposition of condensate from this ablative/ heating/ shearing process--which also needs a generic but descriptive name!

False. Completely and utterly incorrect.
You're talking about the surface of the iron meteorite itself, below
the fusion crust.
How on earth can you put this "layer" between the fusion crust and the
reheated rim when many Canyon Diablos and Nantans have seen so much
weathering so as to lose any trace of their original reheated rims!?
The only irons I have *ever* seen to possess such a surface are desert
irons, where the crust has been stripped from the fresh metal,
allowing for a thin coating of desert varnish on the exposed iron (any
substantial oxidation would destroy this "layer"), and on antarctic
irons, where a similar process often occurs.
Canyon Diablo and Nantan are examples of irons where the crust has
been removed - along with countless mm or cm of material. This
"layer," as you define it, does *not* exist on such irons.

Oh - Sikhotes sometimes exhibit such a surface as weathering has
removed patches of fusion crust while leaving the surface of the iron
relatively unaltered. It's a good thing they're so fresh or this
wouldn't be true...

> 5)TAZ: ?Thermally Altered Zone: in this zone is the material which was not displaced nor reformed, per se-- but was thermally altered to a major or minor degree. Some volatile gases my have been out gassed but a major effect would be resetting magnetic orientation within the zone. ?There are means to analyze how deeply and to what range of elevated temperatures this zone was subjected to.

Well, the major effect generally noted is the recrystallization of the
meteoric material, but sure - this is a legitimate "layer."

> 6) All the remaining material largely unaffected by the change in address from solar orbit to our collections. A place holder for the time being but all that which is not a part of the ablation zone.
> I'll leave it there for tonight and for a straw man suggest there are 5 layers(on irons at least) in the "Ablation Zone". These layers are thick or thin; regions of original material which were in someway altered /affected by the dynamics of passage through the atmosphere.

Right, well...you have my point of view. It's based on the fact that
the fusion crusts on iron meteorites and on stony meteorites form
through the same general processes into analogous structures and
function in the same way on both types of meteorites. Your insisting
on glass being a component of fusion crust seems as likely to be
present due to a misinterpretation of some archaic article, as best I
can tell, so I really don't know what to say. You keep stating the a
fusion crust must contain glass 'because it is defined that way,' but
when I stand back and ask the obvious question - 'why is it defined as
such, and does that make sense,' all I get in response is a
reiteration of your conviction that fusion crust is defined as such,
and the definition cannot be changed.

Science is change, Elton.

The trouble with this point, though, is that you've taken up the
conservative mantle of "no change" when I cannot find a single
reference anywhere that states that fusion crust *must* contain glass.
 All of the literature from NIninger to Buchwald, to Krinov, to
modern-day descriptions of meteorites entering the USNM from
Antarctica - they all state that irons have fusion crusts.

In other words, you're saying the definition shouldn't be changed from
one in which glass is an indisputable component of fusion crust when
that's not stated anywhere in the first place.
You can't advocate *maintaining* a definition when it's *never* been
accepted as fact, because that's not how definitions work. It needs
to be accepted before you can try to "keep it accepted." Otherwise
you're just advocating a new theory based on the historical merit of
the theory - which, if it has never been accepted in the first place,
is simply circular reasoning.

You're the one advocating a backwards sort of change, Elton. We're
going along with accepted meteoritics. And unless you have a reason
to say that glass is an inherent component of what we are to deem
fusion crust, I suggest that you come up with a better reason than
"it's defined that way," because it's getting old.

Regards,
Jason

> --- On Thu, 11/19/09, Martin Altmann <altmann at meteorite-martin.de> wrote:
>
>> From: Martin Altmann <altmann at meteorite-martin.de>
>> Subject: Re: [meteorite-list] Fusion Crust on Irons--Not
>> To: Meteorite-list at meteoritecentral.com
>> Date: Thursday, November 19, 2009, 7:21 AM
>> Unlike in politics and public opinion
>> (and sometimes in science),
>> in meteoritics it sometimes can be more difficult to adhere to
>> theories/legends,if one gets samples in ones very hands, which exhibit the opposite of that, the theory postulates.
>>
>> If you ever had an early picked Sikhote at hand,
>> or if you had taken from Andi Gren's Boguslavka slices
>> (a fall, who simply hadn't enough time in field, to develop
>> a magnetite, wuestite, limonite or whatever -ite weathering crust),
>> you would be very surprised.
>>
>> Cause they don't display that ominous blue-ish flimsy
>> luster, which is often reported as fusion crust, but a thick and fat layer of a discernibly different matter than the material beneath, of a dark color and rough to silky surface.
>>
>> I never believed in iron fusion crusts neither, but when I
>> got in these freshly picked up observed falls, I was disabused.
>>
>> Main problem in that question is, as it was correctly
>> mentioned here, that we simply have so few pristine samples of fresh iron falls and that most irons we get in our collections arrive with weathered or artificially cleaned surfaces.
>>
>> Now you may argue about the word "crust" as a
>> (pseudo-)scientific term...well for me scientific terms are best, when they keep most of their meaning they have in their common use in the language. And there crust - meant for me a layer on the outside of an object.
>
>> .....and we have the problem, that there exist these
>> freshly fallen lumps with that strange crust. Shall we hide them in the deepest corner of our drawers, cause they don't fit in the axiom, that fusion crusts are fusion crusts only, when silicates are melting?
>>
>> Sometimes, if the results don't fit into a theory, one has
>> to think about modifying the theory,
>>
>> Else there wouldn't be no meteorites in our sense at all,
>> Nada, Niente, Nix, Nimic, cause we all would know that they are products of our Aristotelian atmosphere, solidfied accumulations of terrestrial vapours and probably created by lightning strokes,wouldn't we?
>>
>> Best!
>> Martin
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Received on Fri 20 Nov 2009 06:58:19 AM PST