[meteorite-list] Ablation Zone 5 Layers...was Fusion Crust on Irons--Not

From: MEM <mstreman53_at_meteoritecentral.com>
Date: Thu, 19 Nov 2009 20:11:21 -0800 (PST)
Message-ID: <153797.41038.qm_at_web55201.mail.re4.yahoo.com>

Hello Martin,Graham, Gregg and List,

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 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 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". 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.

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

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.

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.

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":

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.

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)

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.

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.

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.


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!
 

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.

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.

Elton

--- 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
Received on Thu 19 Nov 2009 11:11:21 PM PST