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GEO-METAMORPHISM 2001

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Subject:

Re: More metam facies confusion

From:

Bruce Bathurst <[log in to unmask]>

Reply-To:

No title defined <[log in to unmask]>

Date:

Thu, 15 Nov 2001 10:12:25 -0800

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (76 lines)

Jürgen Reinhardt wrote:

<SNIP, SNIP>

> If I understand this correctly: Facies has no implications of P or T.
> Yet, from facies (= an association of ...) we infer P and T (i.e., physical >conditions). I have a problem with logic here. It may have been that at the
> time the facies concept had been devised, P and T could not be quantified
> so easily, but this has changed. Just about every modern textbook in Metamorphic
> Petrology has a facies diagram which is a quantitative P-T diagram (not that
> they would look the same, but still...).

I found no problem with logic; but yes,
I think I can see how it could appear
illogical. The word 'implies' has two
slightly different meanings. Here's how
I read Howard's note.

Goldschmidt's work with the phase rule
predicted that, within a mineral facies,
two intensive thermodynamic measures
could vary. Eskola adjusted Osann's
calculation in an attempt to project
from H2O and CO2, so that these would be
pressure and temperature. Scientists
have argued about his success. This was
an attempt, using chemical theory, to
correlate a laboratory scale (T, p, &c)
with a natural scale (chemical
reactions). This was 'tacked on' to the
facies classification, and isn't
necessary for its use.

>To me, the facies concept is a convenient subdivision of the P-T plane... .

Mineral facies were meant to be an
actual classes of rocks. The facies
concept (or 'notion', as described in
Paper #3) was meant to be a new
classification of (igneous and)
metamorphic rocks. Subdivisions of the
p,T-plane change year to year, but the
facies classification does not. It is an
elegantly natural classification.

A naturally found assemblage of fossil
shells has no implication of geologic
age, without radioactive dating. From a
single assemblage, however, we might
infer all similar assemblages indicate
the same age. From the stratigraphic
placement of sets, we might infer that a
particular order of assemblages
indicates an increasing or decreasing
age. Using a mass spectrometer, we might
correlate the assemblage of shells with
a age scaled in years.

>The facies concept had been developed when quantification of P-T-X relations was not >the standard procedure it is today. Whatever the historical background is, we need >facies for mostly practical purposes and not much else.

Statigraphers accomplished much using
the more primitive measures, which they
could employ personally. These didn't
need fancy instruments. It was my
impression that facies were meant to be
used analogously, but haven't been.

> Some of the ongoing discussion is a bit too abstract for my taste.

Sorry about any unnecessary abstraction
from me. Those who read the SCMR's paper
#3 can judge whether such distinctions
as natural and artificial are warranted.


Bruce

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