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

From: Melinda Hutson <mhutson_at_meteoritecentral.com>
Date: Wed, 20 Mar 2013 11:36:08 -0700
Message-ID: <CANP=7y8O7zepNs917uWtzjHKjXewxPiWjRHjFvME7v=1_45AQg_at_mail.gmail.com>

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.
Received on Wed 20 Mar 2013 02:36:08 PM PDT