Fw: [meteorite-list] CRE ages of Nakhlites and NWA 998

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sat Nov 11 17:16:46 2006
Message-ID: <00ce01c705df$0dc41a70$d97a4b44_at_ATARIENGINE>

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
>>
>
Received on Sat 11 Nov 2006 05:16:36 PM PST