Hello Melinda, List,
thank you for your interesting post. Is the possible age of NWA 7325 really
a contradiction? The MIT published a report a few weeks ago, in which a very
old surface when mercury is suspected.
http://web.mit.edu/newsoffice/2013/mercury-may-have-harbored-ancient-magma-ocean-0221.html
"...scientists at MIT have proposed that Mercury may have harbored a large,
roiling ocean of magma very early in its history, shortly after its
formation about 4.5 billion years ago...
"The thing that's really amazing on Mercury is, this didn't happen
yesterday," says Timothy Grove, a professor of geology at MIT. "The crust is
probably more than 4 billion years old, so this magma ocean is a really
ancient feature." "
For me, it is not unlikely that there are large areas with 4.5 billion year
old rocks on mercury (mixed with younger lava plains). But of course, as Dr.
Irving wrote in his abstract, Mercury is "only" a possible candidate for the
origin of this absolutely unique meteorite, hopefully further research will
tell us more about it. But it seems definitely a planetary rock! There are
already some newer results as in the abstract, which are very amazing and
soon will be presented by Dr. Irving. In general, the research on NWA 7325
ist still in its beginning.
By the way, I have spoken to many scientists and none of them has ever seen
a similar meteorite. It is an absolutely unique and intriguing find. And
yes, it´s a possible piece of Mercury!:-)
Best regards,
Stefan
www.sr-meteorites.de
----- Original Message -----
From: "Melinda Hutson" <[email protected]>
To: <[email protected]>
Sent: Wednesday, March 20, 2013 7:36 PM
Subject: [meteorite-list] NWA 7325 - Mercurian or not?
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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