[meteorite-list] NWA5400 redux - a long explanation

From: drvann at sas.upenn.edu <drvann_at_meteoritecentral.com>
Date: Tue, 28 Sep 2010 23:57:21 -0400
Message-ID: <1285732641.4ca2b921495aa_at_webmail.sas.upenn.edu>

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
>

>
Received on Tue 28 Sep 2010 11:57:21 PM PDT