Yong-Fei, Jim, Yuri, and all other interested parties,

The bottom line is that nobody has reliably measured the Sr diffusion
coefficient in micas. Empirical closure temperature estimnates from the Alps
provide some constraints and Giletti (1991) provides a some preliminary data
for biotite (800-900C).

For the purposes of carrying out the modelling I had to make some rather bold
assumptions in my 1997 paper. As Yong-Fei points out it is true that the D0
and E values I used for muscovite do not conform to a compensation
relationship (they cannot because E is assumed to be the same as biotite).
Whether he has a valid point or not rests on how much dependence one puts on
a compensation relationship - the idea being that if one plots E vs. log D0
for the same species diffusing in different minerals (or alternatively
different species diffusing in the same mineral) then you will, supposedly,
get a straight line relationship.

I am sitting here with a plot in front of me of log D0 and E for Sr
diffusion
in different minerals and, although one might be able to fit a line through
biotite (my estimated values), K-feldspar, hornblende and labradorite, other
minerals such as albite and garnet plot up to 2 log units in D0 away from the
line. Therefore I personally don't give much credence to the compensation
line approach - its too uncertain to make estimates of the diffusion
parameters that are better than anybody else's at the present time.

I end my 1997 paper with a plea that somebody actually goes and measures
these diffusion coefficients to save us all these speculative arguments. It
is astonishing that mica Rb-Sr dating (one of the major geochronological
systems used in dating uplift of mountain belts) has never been
experimentally calibrated. Any offers?

All the best,
Gawen Jenkin

For more information on diffusion, closure temperatures, ore deposits and a
nearly working web version of COOL see my web site (below).
.
Dr Gawen RT Jenkin
Lecturer in Applied Geology         direct dial:+44 (0116) 252 3934
Department of Geology            general office:+44 (0116) 252 3933
Leicester University                        fax:+44 (0116) 252 3918
LE1 7RH                           http://www.le.ac.uk/geology/grtj1