[meteorite-list] Nwa5400 rdux - oops!

From: David R. Vann <drvann_at_meteoritecentral.com>
Date: Wed, 29 Sep 2010 12:54:40 -0400
Message-ID: <C549C823E9FA4D24824A6F41C755E6DB_at_sas.upenn.edu>

To all:

In response to Greg C. - yes you can repost it, and I take responsbility for any
errors or confusions arising. I would like to correct an error I made writing
this late last night after a tiring day rebuilding parts of my mass
spectrometer:

The Sun is not currently making oxygen; there is a sentence that effectively
states it does below - I think I crossed two sentences in my head as I was
writing. So, again, the Sun is not the source of the oxygen, or for that matter,
any of the 'heavier' elements - yet. Once it depletes enough hydrogen, it will
swich to Helium fusion, and begin synthesizing carbon. Conceivably, some of this
does occur within the core, but that would not escape yet. The source of the
'heavier' elements is cosmic debris, likely from a nearby supernova explosion
that has enriched our star in these elements relative to others that have a
similar age and type. [also, a minor typo - the weights of O isotopes are 16,17
and 18, not 16,16,and 18 as noted below- but then, I'm sure you'all figured that
one out for yourselves...;) - must stop writing things late at night...]


So, the sentence should read:
Theoretically, as oxygen is formed in the fusion reactions of stars and expelled
when they die, or trapped from molecules drifting in interstellar space, these
three isotopes begin to sort out in the solar wind

Later, another sentence should read:

 Since the sun continues to expel 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)

And add the following to clarify:
The source of this oxygen is largely particles that fall into the Sun's
photosphere due to gravitational attraction; molecules such as enstatite are
blown apart in the plasma, back into their consituent atoms.

In the aubrite discussion: Conventional thinking on their formation is that they
formed under "reducing" conditions, i.e. a lack of oxygen. I was trying to
invert this explanation and got it tangled up with solar oxygen, completely
wrong. During this period n the formation, the area was very rich in hydrogen
and the protosolar disk was forming and 'igniting'. In this area, the amount of
oxygen available, relative to heavier elements such as silicon and magnesium,
was too low to form much beyond enstatite (this is past, or near the end of,
carbonaceous chondrite formation). A better way of looking at this, is that the
materials that condensed into larger bodies also had, say, a lot of iron oxides
as well as enstatite. As the planetoid formed, the heat allowed transfer of
oxygen from metal iron.nickel oxides to the enstatite, forming olivine, whereas
the now-reduced metals suck into the core - same as you would make iron today
from e.g. hematite, leaving behind an oxygen-enriched silicate slag.

I think, trying to make the story seem simple when it is really far more
complicated that I put it, I went astray.

There, I think I got it right this time, to the best of my current
understanding.

Sorry about any confusion or inconvenience this caused - next time, I promise to
better proofread what I write...



I also missed the reference that says CRE has been done on the sample; thanks to
Sterling and Richard for explaining the reason for the apparent discrepancy, as
I was about to do just this, and therefore they saved me the trouble, so that I
can continue to do battle with the MS...

DRVann



| -----Original Message-----
| From: Greg Catterton [mailto:star_wars_collector at yahoo.com]
| Sent: Wednesday, September 29, 2010 10:44 AM
| To: drvann at sas.upenn.edu
| Subject: Re: [meteorite-list] NWA5400 redux - a long explanation
|
|
| Awesome info! do you mind if I repost some of this?
|
| Greg Catterton
| www.wanderingstarmeteorites.com
| IMCA member 4682
| On Ebay: http://stores.shop.ebay.com/wanderingstarmeteorites
| On Facebook: http://www.facebook.com/WanderingStarMeteorites
|
|
| --- On Tue, 9/28/10, drvann at sas.upenn.edu
| <drvann at sas.upenn.edu> wrote:
|
| > From: drvann at sas.upenn.edu <drvann at sas.upenn.edu>
| > Subject: [meteorite-list] NWA5400 redux - a long explanation
| > To: cdtucson at cox.net
| > Cc: meteorite-list at meteoritecentral.com
| > Date: Tuesday, September 28, 2010, 11:57 PM
| > 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 12:54:40 PM PDT