Dear Jochen,
From my experience with zircon U-Pb geochronology I would conclude
from your info and image that the difference in the internal zircon
texture is the result of a difference in U content between the core
and younger rim. You would need to separate cor and rim, eg by using
ultrasonic to vibrate the crystal to pieces (time consuming and
doubtful) or you can use a SHRIMP to analyse core and rim separately.
High U concentration has probably resulted in radiation damage of the
crystal lattice of the core and consequently leakage of radiogenic
lead and discordant age results.
Best Regards
Dirk A. Nieuwland
NewTec
Sent from my iPhone
On 14 mei 2010, at 18:52, Jochen Mezger <[log in to unmask]
HALLE.DE> wrote:
> Apologies for multiple posting!
>
> Dear all,
>
> I am a casual applicant of U-Pb zircon geochronology, having just
> dated zircons from various granites and gneisses in the Pyrenees.
> Two of my samples, both Variscan (ca. 335 Ma) leucogranites, contain
> short prismatic, yellow-brownish zircons, with a rather high U-
> content: 2000-6000 ppm. What puzzles me is the internal texture of
> these zircons, revealed by CL imaging. While the rims show thin
> ocsillatory zoning, the central part has a very patchy appearance.
> Can this be damage caused by the high U-content? If so, it could
> explain the relatively low percentage of concordant ages, compared
> to zircons of the other samples. Is there any special term for this
> texture? I have looked into the literature, but did not find any
> comparable appearances.
>
> Thanks for any suggestions,
>
> Jochen
>
>
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> Dr. Jochen Mezger
> Lehrbeauftragter - Lecturer
> Allgemeine Geologie
>
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> ============================================================
>
> "Vous etes des assassins ! -
> You are murderers !" Octave Lapize, winner of the 1910
> Tour de France
> at the summit of the Tourmalet,
> Pyrenees
> to Henri Desgranges, organizer of the
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>
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> <Zircon_Soulcem.jpg>
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