[meteorite-list] Meso Origins...was Mesosiderite?

From: Mr EMan <mstreman53_at_meteoritecentral.com>
Date: Sun, 18 Feb 2007 08:14:19 -0800 (PST)
Message-ID: <772429.7034.qm_at_web51013.mail.yahoo.com>

Hello Gary,List

As to the question of why mesosiderites don't contain
chondrules...

Mesosiderites are almost identical chemically. So much
so, we believe they came from a single incident; that
being the collision of two asteroids, at least one of
them (if not both) was "differentiated" to the extent
of having an iron core. The iron composition is that
of the IIIAB group of iron meteorites.

Being differentiated means they had sufficient size to
accumulate heat through the decay of isotopic Al26 and
hold that temperature high enough and long enough to
melt the original chondritic material. Over time this
separated into denser iron and lighter silicates, thus
the term "differentiate". This process is the origin
of asteroidal achondrites as well.

In the case of mesosiderites, current wisdom holds
this was a ballistic mixing of the silicates from one
or both asteroids into the probably molten core of the
target body. The disruption of the target body likely
stripped away the insulating layer (or induced a
centrifugal spin)which cooled too quickly to
completely rediffernetate.

Simple photo identification is dicey for there can be
round/oval blebs of iron similar to what is found in
the bencubbenites, but the silicate content shouldn't
have any identifiable chondrites. I have noticed
some NWA H-Chondrites which so much iron it covers as
much as 25-30% of the cut surface, and on casual
inspection looks not unlike some mesosiderites. On the
other hand the most mesos seem to have an irregular
surface which doesn't show the typical smoothness
typical of chondrites. I assume that Steve and I,
when we called your specimen an H, were going by the
apparent chondrules.

Elton

 
Received on Sun 18 Feb 2007 11:14:19 AM PST