[meteorite-list] NWA5400 redux - a long explanation

From: Galactic Stone & Ironworks <meteoritemike_at_meteoritecentral.com>
Date: Wed, 29 Sep 2010 11:35:30 -0400
Message-ID: <AANLkTikmp6T2H34E66wdz8Vf0K+FBPusqXLxjru7pbXE_at_mail.gmail.com>

Hi David, Carl and List,

Thanks for the detailed explanation David. :)

I recall reading something about catastrophic events that can "reset"
the CRE of some materials. Maybe I am not remembering correctly. Is
it possible that a cataclysmic event could shock/alter a material to
such a degree that it would scramble the isotopes contained within? I
also seem to recall something about the radioactive isotopes of
Aluminum (?) somehow figuring into this "reset" equation. Can someone
clear me up on this?

What I am getting at, in relation to NWA 5400/Thea impact theories is
- the event in question was catastrophic on every level. Would this
complicate the analysis of determining any age or origin for this
material?

Personally, I find any meteorite that plots along the terrestrial
fractionation line to be interesting, in that it stands apart from
most other types of meteorites which do not. So NWA 5400 is
interesting, regardless of what it is or is not paired with.

So, if NWA 5400 is a brachinite, or is related to brachinites, then it
visually looks quite different from the majority of brachinites I have
seen. Going strictly by aesthetics, 5400 seems to have more in common
with enstatite meteorites than brachinites. Is it just me?

Best regards,

MikeG

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On 9/28/10, drvann at sas.upenn.edu <drvann at sas.upenn.edu> wrote:
> Carl:
> I am glad that I provoked thought - that is in my mandate as an
> educator...;)
>
> I will try to answer your questions, albeit perhaps not in order, and I hope
> I
> can explain.
>
> First, you ask about Mbarak's box of rocks, aren't they likely paired? Well,
> it
> is very unlikely that two different brachinites fell in the same spot. Not
> impossible, but very unlikely. Therefore, it seems likely that the rocks are
> from the same fall if found in the same area.
>
> Second, you have several questions about O isotopes. In the case of NWA5400
> pairings, it is important because all agree that it is a brachinite (more on
> this below); what makes it unusual is that is has different O isotopes than
> other brachinites, so any rocks that have similar O isotopes are likely from
> the same meteoroid.
>
> Oxygen isotopes haven't really taken over the pairing question; as has
> already
> been noted, many different parameters must converge before two rocks can be
> paired.
>
> What's up with the O isotope thing anyway? Oxygen has three 'isotopes' - it
> has
> three different weights, based on the number of neutrons in its nucleus. The
> weights, relative to hydrogen, are 16, 16 and 18. Theoretically, as oxygen
> is
> formed in the fusion reactions of the Sun and expelled, or trapped from
> molecules drifting in interstellar space, these three isotopes begin to sort
> out in the solar wind. The gravitational attraction of the lighter isotope,
> 16O, is, naturally, less than the others. Thus, the solar wind can more
> easily
> push the lighter isotope farther out into space. Consequently, there is a
> gradient of increasing amounts of 16O relative to 18O as you go farther out.
> (same logic appllies to 17O, of course). Since the sun continues to form
> oxygen, the system is continually replenished, and is thus arguably at a
> steady
> state (or there would be the complication that we don't know the gradient
> 4.5
> billion years ago). This theoretical concept is borne out by spectrographic
> measurements in space, so it seems to work. Within these gradients, planets
> formed. When, for instance, magnesium reacts with silicon and oxygen to form
> magnesium silicate (e.g. enstatite), it clearly would condense with the
> distribution of oxygen isotopes where it condensed. This is the basis for
> the
> idea that oxygen isotopes record how far away from the Sun the matter
> condensed. Naturally, there are complications, which I may gert back to
> before
> I finish here. Almost startlingly, when the first bunch of meteorites were
> analyzed, they showed a pattern consistent with this expectation. THus,
> oxygen
> isotopes are used to *infer* whereabouts the sample originated, at least
> within
> a few million miles or so.
>
> Now, as to NWA5400; maybe only two abstracts have been publshed. Keep in
> mind
> that it can take a while for things to get published, and it can take quite
> a
> while to complete these analyses and get them right. But, I would like to
> say,
> the two abstracts published say quite a lot, and reflect a great deal of
> analyses already performed. Tony Irvings group has, in my opinion produced
> as
> musch useful information as most of what gets published in the magazine
> "Science". The problem is that the scientific community does not know enough
> about the genesis of the Solar system to do much more than speculate about
> the
> meaning of the results. But what results they are:
>
> NWA 5400 is a Brachinite. What this means, is that it consists primarily of
> olivine (peridot) and is classified as an "ultramafic" rock - one high in
> Magnesium and iron and low in silicates (compared to crustal rocks of
> Earth).
> It is dense. In geology, one might call this rock a dunite or dunitic
> wehrlite;
> we find rocks like this on Earth (I have a few on my desk), and the
> compostion
> resembles the upper mantle of the Earth. The mineralogy of the rock isn't
> actually particularly rare. Because the isotope resemble Earth's, it has
> been
> suggested that NWA5400 is a remnant of the putative Earth-Theia impact. For
> a
> number of reasons, Theia probably formed near Earth's orbit, thus had an
> oxygen
> isotopic distribution similar to Earth's. The collision was more than
> powerful
> enough to exhume portions of Earth's mantle, particularly since, at this
> time,
> Earth wasn't yet exactly solid in the way we perceive it today.
>
> The metal content isn't really an issue. There isn't actually very much, and
> most resides in sulfides. It is also quite conceivable that, this early in
> Earth's planetogenesis, substantial amounts (by this I mean, say, 2-3%) of
> iron
> and nickel had not yet migrated to the core. Thus, a piece of the upper
> mantle
> knocked into space 4.5 billion yrs ago might have more metal than one might
> expect based on today's observations of the Earth. Do keep in mind, though,
> that we do fiond metal-rich rocks on Earth, even at the surface; the Plato
> Putorano basalt comes to mind. What is clear is that the rock had formed on
> a
> body big enough to differentiate.
>
> BTW, the there is an age on NWA5400; age of formation is consistent with
> Theia
> time frame. CRE ages are being done, and may point to the time of ejection;
> this will be quite interesting.
>
> Why are aubrites not Earthites? They are made of enstatite, a magnesium
> silicate
> related to, but different from, olivine. The chemical difference is that
> olivine
> contains more oxygen in its structure than enstatite, and consequently
> formed
> under more oxidizing conditions. Early in the Sun's formation, the fusion
> reactions did not produce enough oxygen to form olivine as a condensate in
> the
> planetary disk. Aubrites are thought to have been formed during this period.
> It
> is worth noting that the *vast* majority of the Earth is enstatite; the
> lower
> mantle is probably entirely enstatite (with metals migrating through it
> toward
> the core), albeit in a different structure due to the pressure. The Theian
> impact could have excavated some of this, too - we don't know, but maybe.
> More
> likely, there were enstatite planetismals left over after Earth began
> condensing, and the Sun began producing more oxygen. Some of these are in a
> stable (?) orbit near Earth, such as the asteroid Eger, which may be the
> source
> of Aubrites. So, yes, the Earth and the aubrites share a common origin, but
> aubrites are probably not from Earth's mantle, whereas NWA5400 may be.
>
>
> Does all this help? Hopefully, I haven't added to your confusion.
>
> DRVann
>
>
> Quoting cdtucson at cox.net:
>
>> David,
>> You make some very interesting and thought provoking points here.
>> I'm sure I will be pondering this for some time but what immediately hit
>> home
>> was your point about NWA 5400 and how it may be an Earthite.
>> As I have followed meteoritic's for the past 20 years in a very novice
>> capacity. The one thing I've noticed is it's study has had an evolution of
>> it's own.
>> By that I mean it started out rather simple by type. Then the types grew.
>> By
>> now there are a bunch of different types. Almahata Sitta itself has taught
>> us
>> a bunch and really changed my thinking especially as it relates to pairing
>> in
>> that pairing is very odd. Nothing matches and yet they must be paired
>> because
>> they fell together.
>> But NWA 5400 seems to be a brachinite with Earths O isotopes. It seems
>> like
>> lately these O isotopes have taken over in terms of Categorizing these
>> little
>> aliens. The part that makes this confusing is it seems that many types of
>> meteorites are turning out to have the Earths O isotopes. The Moon, NWA
>> 5400
>> and Aubrites just to name a few. So how then will this ever sort out?
>> What makes NWA 5400 more of an Earthite than an aubrite?
>> Additionally, I understand that this may be due to a zoning of some kind
>> whereby anything that formed within a certain zone is going to have the
>> same
>> O isotopes as Earth.
>> This I ask because Aubrites seem to be more like Earth than the
>> Brachinite
>> -like class of NWA 5400.
>> The metal alone found within NWA 5400 seems to rule Earth out as it's
>> possible origin? And unless the Earth was hit by a body that also had
>> Earth's
>> same O isotopes , wouldn't the O isotopes within NWA 5400 be different
>> that
>> Earths? I mean it should have a mixture of Earth and the body that hit it
>> as
>> an end result.? This too makes it very hard to understand why these
>> assertions are made.
>> As was pointed out earlier so far there are only two abstracts about NWA
>> 5400
>> . If you don't mind my asking, What are your thoughts on this?
>> And to throw one more question in there.
>> I have continually made the point that NWA 5400 has pairings based on the
>> fact that Mbarak had a box full of the same rocks. It seems to me it would
>> be
>> very difficult to believe that two brachinite like meteorites would have
>> been
>> found and were being sold but were from two different falls.
>> Common sense tells me that these have got to be the same. Why would this
>> fall
>> be any different than any other? Like rocks fall together in a fall.
>> Thank you.
>> Carl
>> --
>> Carl or Debbie Esparza
>> Meteoritemax
>>
>
>>
>
>
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--
Received on Wed 29 Sep 2010 11:35:30 AM PDT


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