[meteorite-list] Let's talk about meteorites

From: Alan Rubin <aerubin_at_meteoritecentral.com>
Date: Thu, 3 Sep 2009 14:52:06 -0700
Message-ID: <AB579AD21A894F428DF0452AE16B0645_at_SINOITE>

There are three principal goups of silicated iron meteorites: the IAB-IIICD,
IIE and IVA groups. Recent work seems to indicate that the IAB and IIICD
groups are related. Iron meteorites that are thought to have formed by
fractional crystallization processes have certain slopes on element-element
diagrams (e.g., Ir-Ni) that match those expected by fractional
crystallization, presumably located in cores. The IVA irons exhibit such
trends and so are presumably from the core of a differentiated asteroid. A
few IVA irons contain small grains of silica. The IAB-IIICD irons do not
exhibit trends on element-element diagrams consistent with fractional
crystallization. Some researchers (e.g., John Wasson) believe that they are
not from cores but are rather from impact pools on chondritic asteroids.
Their silicates are basically chondritic in bulk composition and even
contain the so-called "planetary gases" as are found in chondrites but not
differentiated meteorites. The IIE irons have alkali-rich silicates. These
silicates are not chondritic in composition but do have planetary gases.
The metal portion of these iron meteorites also do not conform to the slopes
expected for fractional crystallization. They also may have formed as
impact pools on chondritic asteroids. The alkali-rich silicates, I believe,
formed because feldspar has a low impedance to shock compression and hence
melts first during shock events. The IIE silicates are similar in
composition to impact-melt pockets in ordinary chondrites.
    Mesosiderites are differentiated meteorites. Their metal may have been
derived from a core. Their silicates are basically eucrite and diogenite
material. I modeled mesosiderites as having formed via the collision of a
core (with some overlying mantle) to the basaltic surface of another
asteroid.
Alan Rubin
Received on Thu 03 Sep 2009 05:52:06 PM PDT