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GEO-METAMORPHISM  2001

GEO-METAMORPHISM 2001

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Subject:

Re: piezotherm?

From:

Ben Harte <[log in to unmask]>

Reply-To:

No title defined <[log in to unmask]>

Date:

Tue, 16 Jan 2001 10:27:43 +0000

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Dear All,
I am sorry to be a bit behind hand, but I have only
just read the correspondence on ‘piezothermal
arrays’, and there are some points of information I
can contribute and viewpoints I would like to
express. ‘Piezothermal array’ has been used with a
more precise meaning than just pressure-
temperature array, and I suspect it may have been
this more restricted meaning that Kurt Stuewe was
referring to in his original question.

To my knowledge the first modern use of the term
‘piezothermal array’ or ‘piezothermic array’, with clearly
expressed meaning, was by Richardson and England (1979,
EPSL, 42, especially pages 187-188) and Thompson and
England (1984, J. Petrology, 25, especially page 932). They
use the term for the set of pressure-temperature conditions at
which maximum temperature (or the peak of metamorphism)
was reached at each depth in a thickened pile of rocks.

In terms of modelled pressure-temp-time conditions during
regional scale metamorphism, the piezothermic array (as
defined above) therefore corresponds to the array of pressure-
temperature conditions at which maximum temperature was
reached on each of a set of pressure-temp-time loops (with
each loop corresponding to a different depth in the pile).
Richardson and England (1979), and Thompson and England
(1984) explicitly suggest the use of piezothermic array in this
sense to replace the term ‘metamorphic geotherm’ as used by
England and Richardson (1977, J. Geological Society, 134,
especially pages 203-204). Although they are admittedly
diffident about the term ‘piezothermic array’ itself, they
consider it essential to define a term of this type without the use
of the word geotherm, since “these arrays bear no relation to
any one geotherm that existed during metamorphism”.

The piezothermic array definition given above has application
when considering either a calculated (thermal modelled) set of
pressure-temperature conditions, or an actual natural set of
pressure-temperature points determined from a metamorphic
terrain (or terrane). The various Richardson-England-
Thompson papers were obviously concerned with comparing
modelled conditions with natural data, and thus they compared
calculated piezothermic arrays with the sets of pressure-
temperature conditions obtained from metamorphic rocks in
each of a series of metamorphic terranes. For the natural rock
data, Thompson and England used the term ‘PT array’, but it is
obviously more explicit to use a term like ‘metamorphic field
gradient’ for the array of pressure-temperature data obtained
from a specific field area.

To my mind, it seems imperative that we should distinguish the
concept of ‘a set of pressure-temperature conditions at which
maximum temperature was reached at each depth in a
thickened pile of rocks’, from both:
thatof a geotherm
thatof set of pressure-temperature conditions determined
from a natural metamorphic rock sequence.
Distinction from (a) is essential for the reasons noted by
Richardson-England-Thompson and Frank Spear. To what
extent a piezothermic array (as defined above) resembles (b),
depends upon the causes and manners of preservation of
regional metamorphic zonation, and may well vary between
different metamorphic terranes. I believe it is also important to
note that thermal models used to calculate piezothermic arrays
are essentially one dimensional, and nature certainly is not! In
the absence of any other term to distinguish what Richardson-
England-Thompson called a ‘piezothermic array’, I therefore
argue that we should retain the use of the term explicitly for
what those authors defined it to be.

Of course, I wouldn’t be writing this piece if I hadn’t been
concerned about the importance of these distinctions before ( -
Harte and Dempster, 1987, on the interpretation of regional
metamorphic zones in Phil. Trans. R. Soc. Lond, A321, 105-
127).

Ben Harte


Ben Harte
Dept Geology & Geophysics,
University of Edinburgh, King's Buildings,
Edinburgh EH9 3JW,
Scotland, UK
(FAX 44 131 668 3184)
FAX: [44] (0)131 668 3184

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