Hi, Walter, List
There are disputes about the K-Ar dating of
Martian rocks because of the high level of 36Ar
in the atmosphere and the likelihood of losing
40K from the surface rocks. One group suggests
the 38Ar dates are distorted (see reference below).
"because atmospheric and cosmogenic 40Ar and
36Ar would confound accurate measurements and
calibration, and because 40Ar may be lost from
the sample over time..." One problem could be
that the isotope levels of the Martian rock are
whacky even before it's blasted off the planet.
-------------------------------------------------------------
Free-browsing copy of a good source of
information on the arguments on Martian geology:
http://fermat.nap.edu/books/0309089174/html
--------------------------------------------------------------
It would be difficult to explain the 38Ar discrepancies
as a loss of the gas from the sample in transit, because
the crystal lattice "gaps" that would let some of the 38Ar
escape from the rock would let ALL the 3He out, since
the He atom is much smaller than the 38Ar atom and
"leaks out" much more readily (as much as I hate to
disagree with Bernd). Heating would allow the 3He to
escape first.
Sterling K. Webb
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----- Original Message -----
From: "Walter Branch" <[EMAIL PROTECTED]>
To: "Meteorite List" <meteorite-list@meteoritecentral.com>
Sent: Saturday, November 11, 2006 2:15 AM
Subject: [meteorite-list] CRE ages of Nakhlites and NWA 998
Hello Everyone,
I have been studying nakhlites in general and martian meteorite NWA 998
in particular. This paper,
http://www-curator.jsc.nasa.gov/antmet/mmc/XXV_NWA998.pdf
quotes a CRE of 9.3 m.y. using 38Ar. This paper
http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1137.pdf
quotes a CRE age of 9.4 m.y, again using 38Ar. However, the paper
further points out CRE ages using 3He and 21Ne are in better agreement
(12.2 and 11.7 m.y., respectively) and are in agreement with other
nakhlite CRE ages. The authors state, "All nakhlite CRE ages based on Ar
are significant (sic) younger than those based on He and Ne, an
unexplained characteristic also observed among some shergotite CRE ages."
Does anyone know why this is?
-Walter Branch
----- Original Message -----
From: <[EMAIL PROTECTED]>
To: <Meteorite-list@meteoritecentral.com>
Sent: Saturday, November 11, 2006 2:39 PM
Subject: [meteorite-list] CRE ages of Nakhlites and NWA 998
Maybe the nakhlites underwent a(nother) high temperature event
(collision?)
while floating in space. Maybe this event influenced the cosmogenic
nuclides
of 38Ar in a different way than those of 4He and 21Ne. Maybe one of the
radio-
genic clocks was thus (partially) reset. I don't know if such a *p a r t
i a l*
resetting is possible. Maybe this high temperature event led to a
preferential
loss of argon while the NWA 998 nakhlite was able to retain its amount of
radio-
genic helium and neon. Nothing to back it up with, ... only guessing.
Bernd
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