Dear all,
I would not really agree that the world does not give a toss about the REE. I wish I would right now own a high grade high tonnage REE deposit (ideally enriched in HREE), given the current demand for those goodies. And the technical demand will, as far as metal market analysts are concerned, keep increasing in the coming decade. This economic importance should justify that research aimed at improving our understanding of source, transport and enrichment of REE is being continued. Geochronologists that deal with monazite might subscribe to this latter statement as well.
Cheers,
Thomas
------------------------------------------------
Dr. Thomas Wagner
Institute for Geochemistry
and Petrology
ETH Zürich
NW F 82.4
Clausiusstrasse 25
CH-8092 Zürich
Switzerland
phone: ++41-(0)44-6328019
e-mail: [log in to unmask]
web: http://www.geopetro.ethz.ch/people/twagner
________________________________
From: Metamorphic Studies Group on behalf of Bruce Yardley
Sent: Sat 07.08.2010 20:29
To: [log in to unmask]
Subject: Re: Major element closure temperatures in high-grade garnet
All - What is new nowadays is that we are able to measure zoning of trace elements such as the REE. I do wonder if part of the problem lies in the fact that we invariably plot zoning profiles so that the concentration range we choose for the y-axis more or less matches the range in the natural crystal. That way we can see what is going on, but the profiles and particularly the gradients look similar whether it is a major or trace component. I suspect that diffusion is being driven by gradients in the number of atoms of an element per formula unit, not by relative concentrations. Looked at in these terms (atoms per formula unit per micron), the gradients in REE in Nigel's paper in Lithos look trivial compared to the major element gradients that Woodward and I independently deduced were being obliterated by diffusion in the upper amphibolite facies all those years ago. Is it possible that, instead of possessing special properties not shared by major cations, the REE and other trace elements appear to retain their zoning simply because there is so little of them there? Would we still see a ghost of bell-shaped zoning in Mn if we blew up the y-axis to the same scale we use for REE in a high-T homogeneous garnet? Or do the REE never begin to diffuse just because there is not enough of a gradient to get them started? In Nigel's paper it is only right at the edges of resorbed grains that the gradients get steep enough to be comparable to the gradients that drive major element diffusion in the same rim regions, and that fits with the argument that they diffuse back into the resorbing grain.
I have never been a fan of REE and I find the thought that maybe the world does not give a toss about them either, particularly gratifying......
Bruce
Professor Bruce Yardley
School of Earth and Environment
University of Leeds
Leeds LS2 9JT
UK
Tel. +44 (0)113 343 5227
Currently at: Sektion 3.3, Deutsches GFZ, Telegrafenberg, 14473 Potsdam, Germany
________________________________________
**********************
|