On 09/01/2013 07:39, Steve Willner wrote:
In article ,
Juergen Barsuhn writes:

Martian origin of some meteorites is proven by the oxygen
isotope ratios in small gas bubbles

I think that's only part of the story. In any case, the Martian
orgin of NWA 7034 seems to be based mainly on Fe versus Mn, but there
are lots of other ratios that match Mars. Interestingly, NWA 7034
matches results of Mars rovers and orbiters much better than it
matches SNC meteorites, suggesting that it is more representative of
Mars' current surface than the SNCs are.

I didn't see any mention of gas bubbles, but I might have missed it.

The abstract says they determined the oxygen stable isotope ratio from
the bulk mineral and from the heat releasable water content of 0.6%
(with different delta_O17 values 0.58%o and 0.33%o respectively).

Please excuse the use of %o for per mil.

Why should one expect that the isotope ratio in the atmosphere has
remained unchanged through more than 2 billion years?

I don't know of any evidence one way or the other, but I'm not a
planetary scientist.

They are weakly dependent on the temperature and so the amount of water
locked up at the poles. Heavier isotopes of water have a slightly lower
vapour pressure and gradually concentrate in the liquid phase.

But the point here is probably that it is different enough from the
Earth's oxygen isotope ratio to rule out a terrestrial water source.

I presume that when Mars lost its magnetic field and atmosphere the
molecules containing light isotopes were more vulnerable to being swept
away by the solar wind so that what remains is enriched in heavy oxygen.

Especially, if it is claimed that the meteorite left the
Martian crust during a wet period of Mars?

The data show that the meteorite contains lots of water, hence the
"wet period." Isotope ratios give its age, which I _think_ refers to
the time since the rock solidified. I don't know how that epoch
relates to other studies of Mars' climate, but the name "Amazonian
epoch" does suggest water.

Correct. Geological dating methods depend on the radioactive decay of
certain species since the rock last solidified. The abstract doesn't say
which decay system they used to date it but Nd and Pb are popular.

The OP might find the following URL helpful as an introduction:

http://geology.cr.usgs.gov/capabilit.../geochron.html

The rare earth signature is another way to determine the type of a
meteorite with a reasonable degree of confidence. This can be done using
laser ablation with only tiny amounts of the specimen used up.

I am not a planetary scientist either but I know one or two and have
worked on software for mass spectrometry for a few decades.

--
Regards,
Martin Brown

[Mod. note: some special characters removed from no-spam From: address
in an attempt to get this posted -- mjh]