Re: Yixian Dating Again

[Josh Smith <smithjb@sas.upenn.edu>]

Quoting David Marjanovic <david.marjanovic@gmx.at>:

> ----- Original Message -----
> From: "Jaime A. Headden" <qilongia@yahoo.com>
> 
> > <Incidentally, the Tuchengzi Formation is Late Berriasian
> (Swisher et al.,
> > 2002).>
> >
> >   This annoys me ... Swisher says this ... There are Ar/Ar
> dtaes by Lo that suggest Jurassic for Sihetun. Which are younger.
> 
> Meanwhile revised AFAIK. Weren't they whole-rock ages while the
> K dates are based on individual crystals, or am I mixing this up with
> something? There are other radiometric dates (also Ar/Ar IIRC) that
> suggest Barremian for Sihetun.
> 
Yeah, this is essentially correct, although any dating related to Ar is 
more suspect than using, say, U-Pb, because of the tendancy for Ar to 
be driven off during subsequent reheating events, such as would have 
been common place in Liaoning during Yixian time given the tectonic 
regime.  But yeah, whole rock Ar-Ar dating isn't a particularly good 
way to try and obtain a date for Yixian rocks.

-j

----
Josh Smith
Department of Anthropology
Harvard University
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Director, Bahariya Dinosaur Project http://egyptdinos.org

After 1 August:
Josh Smith
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Washington University
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