[meteorite-list] NWA 7325 - Mercurian or not?

[Bob King]

Hi Melinda,
Wouldn't the later crystallization time for the moon's rocks be due to
it having formed after the Earth had accreted? I thought the Earth had
already formed before it was struck by the Mars-sized impactor that
initiated the moon's creation. If the impact melted the rocks,
wouldn't the future-lunar isotopic clocks have be re-set?
Thank you.
Bob

On Wed, Mar 20, 2013 at 1:36 PM, Melinda Hutson <mhutson at pdx.edu> wrote:
> Tony Irving has presented an abstract at the Lunar and Planetary
> Science Conference, describing NWA 7325 and suggesting it MAY be
> Mercurian.   Chemically speaking,   NWA 7325 is more plausible than
> the angrites (which Tony argued were Mercurian a few years ago) as a
> meteorite from Mercury, although there are some apparent mismatches.
> During the Q & A, Tim McCoy got up and argued the "con" position,
> stating that there are other  more likely origins for this unusual
> meteorite.  One big problem is the apparent crystallization age.
> There is some preliminary data (and I didn't get the isotopic system)
> suggesting the rock formed from a melt 4.5 billion years ago.  That
> argues against Mercury and for an asteroidal parent body.  McSween and
> others used the young crystallization ages of the SNC meteorites to
> argue they were Martian before we had proof in the form of trapped
> Martian atmosphere.  Mercury is larger than the Moon, and its surface
> looks somewhat younger than the older portions of the Moon.  Rocks
> from the Moon do not have the 4.5 billion year old crystallization
> ages we see in asteroidal samples.  Highlands rocks are generally
> 4.2-4.3 billion years old, and maria samples are distinctly younger.
> There is one old lunar rock, but the error bars are large on that
> date.  Mercurian rocks should have crystallized at or later than lunar
> highland rocks, and definitely later than asteroidal.   So maybe
> somewhere in the 4-4.3 billion year range would be expected.
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