[meteorite-list] Oxygen Isotope Ratios was Lunar velocities...

From: Kelly Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:44 2004
Message-ID: <3AC63E0E.85BE1FF3_at_bhil.com>

Dear Elton, Darryl, Robert, and List,
    Oxygen isotope ratios are indicative of the composition of the
original material from which a body condensed and accreted. The values
for material from the Earth and the Moon, on a log (or sigma) graph,
occupy a little common island centered on the terrestrial fractionation
line. Oxygen isotope ratios cannot distinguish between lunar and
terrestrial materials.
    EH and EL meteorites are also on the terrestrial fractionation line
in an island that overlaps the Earth/Moon island. SNC's have their own
island of values just above that line; achondrites and stoney-irons just
below that line. The H, L, and LL chondrites each have an island of
values further above the terrestrial line. Carbonaceous chondrites have
a fractionation line all their own, with a different slope entirely.
    All this "proves" is that the Earth and the Moon condensed and
accreted from the same region of the forming solar system. It also
strongly implies that the original material of the inner (at least, and
probably outer) solar system was not well-mixed, was strongly zoned by
composition, and that the inner system bodies were rapidly assembled
from fairly narrow accretion zones.
    If, as is currently believed, the Moon was formed in a low-speed
impact (<5000 m/sec) with the Earth, the two bodies would have had to
have similar orbits before collision (otherwise the collision wouldn't
have been so low-speed). This would fit the scenario above.
    Oxygen ratios identify lunar achrondrites because their values are
not "meteoritic." The values for tektites are earth/moon-like, but fall
in a very narrow range (+/- 4%) of values, a much smaller range than
terrestrial surface rocks, for example. This points to a) an unique
source material, or b) formation by an unique process. Of course, we
knew that already. We just don't know the what, where, when, how, and
all those other little details that make life interesting.
    The big picture: this is all based on data from physical samples
that a) we have in hand and b) whose origin is known. When you consider
that, you realize that we're operating in near darkness here. Make a
list of all the solar system bodies and then check off whether or not we
have a sample of them. Mercury, no. Venus, no. Earth, yes (duh). Moon,
yes. Near Earth asteroids (Atens, Apollos, Amors), no (?). Mars, yes
(but not enough). Comets, no. Some asteroids (Vesta, M, C, E, ?), yes.
Other classes of asteroids, no (uncertain). From here on out, it's all
no, no, no. It's like being given ten random words from a full page of
text and being asked to reconstruct that text.


Sterling K. Webb
--------------------------------------------
"E.L. Jones" wrote:

>> I ask again , are the oxygen isotope ratios (O16-O17
>> -O18) in tektite glass indicitive of a lunar origin or an
>> Earthly origin? If this analysis hasn't been done ..Why
>> Not? It is good enough for evidence of lunar origin in
>> meteorites-- why not tektites?
>>
>> Regards,
>> Elton
>>
Received on Sat 31 Mar 2001 03:29:03 PM PST