Dear All
I think that the use of Th/U ratios in zircon alone to deduce
petrogenetic information must be treated with caution. Whilst there is a
suggestion that Th/U in zircon from felsic rocks should fall in the
range 0.15-0.85 (e.g. Williams and Claesson 1987) there is no reason why
metamorphic growth would not entirely overlap this range of values. It,
of course, depends upon the Th/U ratio of the metamorphic fluid. I would
argue that the Th/U ratio should only be used in combination with
petrographic information (e.g. CL and BSE imaging of zircons) and
preferably other mineral chemistry data such as REE to make such
statements.
All the best
Craig
Dr Craig Storey
Dept. of Earth Sciences
The Open University
Walton Hall
Milton Keynes
MK7 6AA
UK
Tel +44 1908 652558
Fax +44 1908 655151
http://www3.open.ac.uk/Earth-Sciences/people/154.shtml
-----Original Message-----
From: Metamorphic Studies Group [mailto:[log in to unmask]]
On Behalf Of Eric Essene
Sent: 27 July 2005 13:29
To: [log in to unmask]
Subject: Re: Metamorphic vs magmatic zircon
All,
I would think that granitic zircon metamorphosed in the granulite
facies would have the same chemistry as an unmetamorposed granitic
zircon.
eric
On Jul 27, 2005, at 5:28 AM, Dr H.MOURI wrote:
> Dear friends:
> I read somewhere (I think in Hoskin & Schaltergger, 2003 - apologies
> if I
> give the wrong reference - I don't remember the right one) that in
> general
> the Th/U content of igneous zircons is something like Th/U > 0.5 and
> in
> metamorphic zircons it is < 0.05.
> I would like to ask if this is what you see in general (from your
> experience
> with zircon dating) and how much we can rely on this compositional
> feature
> to discriminate between what is "metamorphic" and what is "magmatic"
> in the
> absence of clear textural evidence (such as zoning in igneous
zircons).
> I thank you very much in advance for your reply.
> Wishing you all the best,
> hassina
>
>
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