I am wondering about the part of the garnet you describe as transitional (between about 1060 and 1500 microns, if I understand this correctly). It doesn't look to me as being entirely due to diffusional exchange between two compositionally different growth zones. The Ca profile in the transitional zone shows a variation in concentration gradients that indicates rather original growth zoning (not excluding minor diffusional adjustment, of course). Without seeing the garnet in thin section, I'd rather put a boundary at about 1400, where, incidentally, there is also a little "bump" in the Mn profile which may(!) be the remains of a former Mn-rich rim on your garnet-1 indicating an intermittent period of minor resorption. But that is now hardly above the "noise level" in the Mn profile, so I wouldn't put money on that one. In any case, your Ca profile is the closest you have with regard to the original growth zoning. Mn is nearly flat and Mg+Fe give you a mirror copy of the Ca profile, which is a situation we see in many garnets that have experienced partial diffusional homogenisation. At that stage, the Mg and Fe profile may have been thoroughly modified (apart from Mn). The element levels indicated by you for the two garnet generations need not be the original ones when garnet grew. My guess is that Mn would have looked quite different for a start.
J. Reinhardt
>>> Richard Palin <
[log in to unmask]> 5/21/2011 2:07 PM >>>
Dear group,
I have an aluminosilicate garnet from a metapelitic schist which has
experienced two metamorphic episodes, the first being MP-HT and the
latest being MT-HP (kyanite-grade, ~680C and 11kbar). Subsequently,
the garnet profile currently shows a core-rim chemical relationship
with a transition region between; the shape of this profile strongly
suggests that this transition is the result of intracrystalline
diffusion between these two initially separate chemical regions (see
attached zoning profiles).
Given that conditions of peak metamorphism are well constrained, I was
wondering if anyone knew of a program which could simply model the
diffusion ("relaxtion") of divalent cations along this line. The main
aim is to extract an order-of-magnitude estimation of the time spent
at peak T; i.e. with an initial step-function, I want to see how long
the garnet would need to stay at 680C to match what is now seen.
A literature search brought up Theria_G (Gaidies et al. 2008) and,
although it comes close, I couldn't see how it could answer this
particular question. I should not that this is in no way an in-depth
study of diffusion in garnet; I am assuming that variables which are
normally considered (such as pressure, temperature and radius) are
fixed. The objective is to extract an estimate of time from this
profile, and any suggestions would be greatly appreciated.
Thanks,
Richard Palin
D.Phil Research Student,
Himalayan Studies Group
University of Oxford,
Department of Earth Sciences,
South Parks Road,
Oxford,
OX1 3AN,
United Kingdom
http://www.earth.ox.ac.uk/~richardp/----------------------------------------------------------------
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