Tim Bell wrote:
> Dugald
> I would have to go back to the thinnies to check, but as I remember it, the
> lawsonite was isolated in side the feldspar. Of course always in the 3rd
> dimension there could be a connection to quartz but I doubt it. So if this
> was so, you could accept a contribution fom the strain energy resulting fom
> the dislocation density. What did you think of Wintsch's attempts to
> quantify it for coesite? I am sure Bob will not mind if you speak freely. I
> always thought he was onto something there and so I am prejudiced in his
> favour. Nothing seems to have come of it over the intervening years though.
> Cheers
> Tim
Tim et al.
I’ve been staying out of this over pressure discussion, but I thought
I’d come in this morning with my two sense worth (so to speak), only to find
you (Tim) remembering some of my work on strain energy ‘overloading.’ Thanks
for that. By the way, we have applied these ideas to slates (J. Struct.Geol. 8,
767-780, 1986), and to high grade ‘stress minerals’ and myrmekite problems: (J.
Geol. 96, 143-161, 1988, J. Meta. Geol. 7, 261-275. 1989), and no, we have not
pursued this as hard as it probably warrants.
I wanted to come into the discussion of using thermobarometry to
identify overpressures with a pet caution of mine. Prograde metamorphic mineral
growth is so complicated, and can be driven by so many variables, that I think
that it is operationally impossible to assign any particular local equilibrium
(garnet composition in a core to rim gradient) to any point on a
P-T-t-overpressure path with enough precision to define where on this path this
garnet grew. This already ignoring the assumptions that we have to make about
if equilibrium existed, and if it can exist in non-hydrostatic stress, and if
so which grains or which portions of grains might have been part of an
equilibrium.
To expand on my thinking, metamorphosed rocks exist at P-T conditions
conducive to mineral growth for a long, but poorly quantified time (>10 or 10s
of m.y.?). Tim – you have been particularly good at pointing out to us that
foliations develop potentially many times during the ‘life cycle’ of a growing
garnet or staurolite. Each time the foliations and cleavages redevelop, they
record a deformation event. Isn’t the strain recorded in these cleavage events
reflective of some sort of tectonic overpressure? Surely rocks can not be very
strong, supporting large differential (tectonic?) stresses if they strain
(produce cleavages) so (apparently) easily? A bigger question – if for
instance, garnet were to grow during a cleavage-forming event, how would we
know whether it grew during the loading (tectonic overloading) portion of the
event, or during the relaxation portion of the event? Could garnet growth keep
pace with, and actually be ‘driven by’ tectonic overpressure? If the stresses
rise, then the strain rate must not rise with it. When stresses relax, does the
strain rate go up? A faster strain rate would produce grains with higher strain
energy (see above). The dissolution of these grains might well promote more
garnet growth – that growth eventually occurring as a consequence of ‘tectonic
overpressures,’ but the growth happens as the overpressures grade back to
normal ‘lithostatic’ pressure.
I agree with Eric that the geologic evidence for overpressures is
small. Thinking about whether minerals in rocks could ever ‘remember’ in their
compositions an overpressure is a terrific question, and its answer will drive
us to understanding metamorphic rocks more thoroughly. But we have to think
about what drives reactions, and confirm that the event we are looking for does
drive grain growth/re-equilibration that is quenchable, and thus has at least
the chance of remembering the P-T-u of that event.
bob
--
Robert Wintsch
Department of Geological Sciences
Indiana University
Bloomington, IN 47405
Tele: (812) 855-4018
Fax: (812) 855-7899
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