FW: RE: [meteorite-list] Ureilites/Venus - Ureilite carbon
From: almitt <almitt_at_meteoritecentral.com>
Date: Thu Apr 22 09:49:02 2004
Joseph and all,
This topic came up last July and I posted to the list the findings of our top
people on the Ureilite Parent Body. It's nice to romanticize about these coming from
Venus but one should always consider the facts as we know rather than speculate on
what we don't know about these. I think there is hard evidence showing these don't
come from Venus but then its always fun to debate the issue isn't it. See my comments
(very similar to what was posted in the thread this time) below from last July.
Here is what Harry Y. McSween has to say about these types of meteorites in his most
excellent book Meteorites and Their Parent Planets.
Ureilite were derived from a body with a particularly complex history. The interior
was partially melted and basaltic magma was extracted. The ureilite residue was then
held at a temperatures of at least 1,250 deg. C for some time, causing it to
recrystallize. The reduction of iron seen in the rims of sensitive silicate grains
suggests reaction with graphite, a reaction that is extremely sensitive to pressure.
Estimates of the pressure needed to account for the compositions of reduced silicates
correspond to depths of approximately 100 km, possibly a minimum size for the ureilite
parent asteroid. Various workers have called on explosive volcanism, shock heating by
impact, or the impact of a carbonaceous asteroid into an already molten body to
explain the curious characteristics of these meteorites. Because of their high carbon
content, ureilites are very dark, with albedos of only approximately 7%. The spectra
of low-albedo members for the S(I) subtype are dominated by olivine and S(II)
asteroids additionally contain a small amount of calcium rich pyroxene. Either of
these sub-classes might correspond to the mineralogy of brachinites.
He goes on to say that the S type asteroids are interpreted to represent potentially a
range of meteorite types from ordinary to primitive achondrites from a small degree of
melting. The diversity in these S type asteroids is probably a direct result of the
igneous processes that many of the these asteroids have experienced.
To me this suggests an asteroid parent body rather than a planetary type body.
The Ureilites contain over forty different finds/falls making it the second largest
achondrite group. This doesn't fit too well in the modeling of the number of Venus
like objects we might expect to find here (though I don't think I agree with modeling
too well). The oxygen isotope mixing line is what is typically found in and
of unprocessed nebular materials. There is also trapped nobel gasses in abundance
similar to the carbonaceous chondrites and the presence of carbon and the oxygen
isotope composition of these meteorites suggest a link with carbonaceous chondrites.
The age of these meteorites are 4.5 billion (for the olivine and pyroxene) but some
radiogenic isotopes were redistributed at 4.0 billion. Seeing how Venus is a very
dynamic planet even to this day it doesn't seem to fit the bill very well at all. I
that these are one of the more bizarre and perplexing of all meteorites and there is
to find out yet. Best!
Received on Fri 21 Sep 2001 08:08:07 AM PDT