Fw: [meteorite-list] CRE ages of Nakhlites and NWA 998
Sterling K. Webb
sterling_k_webb at sbcglobal.net
Sat Nov 11 17:16:36 EST 2006
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
------------------------------------------------------------------------
> ----- Original Message -----
> From: "Walter Branch" <waltbranch at bellsouth.net>
> To: "Meteorite List" <meteorite-list at 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: <bernd.pauli at paulinet.de>
> To: <Meteorite-list at 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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