[meteorite-list] NWA 5400 Age & Origin Processes

From: Greg Hupe <gmhupe_at_meteoritecentral.com>
Date: Wed, 29 Sep 2010 12:08:09 -0400
Message-ID: <36D99BC54A3F4FC6A6E6C800AD6FE5F1_at_greg323a964987>

Dear List Members,

As promised, here is a basic summary of conversations I have had with Dr.
Tony Irving who is the lead investigator of NWA 5400 and possible pairings.
Also keep in mind that analysis is ongoing by a number of scientists and
labs from around the world, which will eventually produce informative and
thought-provoking publications.

The distinction must be made between formation age on some probably
long-destroyed "planetary" body, and the time of resampling of any leftover
 chunks that presumably are still parked in orbit somewhere in the main
asteroid belt. This distinction applies to many ancient achondrites,
 including "typical" brachinites, NWA 5400 and angrites. All have very
ancient formation ages >4.5 billion years, but the small samples we now have
 in our hands could not have spent the past 4.5 billion years traveling in
space - long ago they would have accreted to another large body or been
 consumed by the Sun.

The cosmic ray exposure ages (29 million years for NWA 5400, up to 70
million years for angrites) indicate how long ago small meteoroids were
 liberated from the leftover "storage bodies" parked in the asteroid belt
(or somewhere else subject to recent collisions). So it is important to
realize
 that this is a multi-stage process: accretion and at least partial
differentiation very early in solar system history, followed by catastrophic
collisions stripping off
 exterior portions of or completely disintegrating the body, trapping of any
asteroid-sized surviving remnants in some orbit with transfer potential to
 Earth, and finally recent "chipping off" of bits of these storage bodies to
yield the meteorites we find.

So NWA 5400 was not derived from our modern planet Earth, nor are the
angrites most likely derived from the modern planet Mercury. Instead, if
there is a connection between NWA 5400 and Earth (or other former bodies
accreted in near-Earth orbit) then it is a very ancient one. Likewise, any
connection
 between angrites and Mercury must be a very ancient one. One possibility is
that angrites might represent ancient lithosphere stripped off an originally
 larger planet (leaving the unusually large core and relatively thin modern
lithosphere of Mercury). This would also mean that the near-surface
 materials on Mercury today would represent the former deep lithosphere, and
so may not be expected to match exactly with angrites. An alternative is
 that angrites (and NWA 5400) are not specifically from proto-Mercury (or
proto-Earth), but from other now-destroyed bodies that had accreted in their
respective vicinities of the solar nebula.

The giant collision hypothesis for the origin of the Moon, and the fact that
we even give a name (Theia) to a planetary body that no longer exists (but
 is strongly indicated), highlight the importance of inferred collisions
early in solar system history. Perhaps we are lucky enough to have in our
 hands a few pieces sampled more recently from some fortuitous leftovers. I
hope this helps lead future discussions of NWA 5400 in the direction that
this meteorite dictates, not those of pairings or supposed pairings, none of
which matter when considering the origin of NWA 5400.

Best regards,
Greg

====================
Greg Hupe
The Hupe Collection
NaturesVault (eBay)
gmhupe at htn.net
www.LunarRock.com
IMCA 3163
====================
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Received on Wed 29 Sep 2010 12:08:09 PM PDT