At 01:24 PM 3/6/97 GMT, David J. Tyler wrote:
>This exchange has, I hope, reinforced the thought I expressed
>earlier - "calculated" cooling rates are model dependent. I am
>not saying these magma bodies can cool within one year, but I am
>suggesting that assertions of long cooling times based on
>conduction represent the extreme upper-limits of possible values
>and, because convective cooling is so much more efficient than
>conduction, the actual times of cooling are orders of magnitude
>lower.
Because of the large number of metal deposit types associated with
intrusives, economic geologists have also spent a great deal of time
studying the effects of cooling magmas. One of the "landmark" papers
in this field was that of Cathles (1977).
Cathles, L.M., 1977, An analysis of the cooling of intrusives by
ground-water convection which includes boiling: Economic Geology,
v. 72, p. 804-826.
Similar to Glenn's _cool_ program (described in an earlier post from
this thread), Cathles created a cooling model. The first paragraph
of the abstract reads:
"A finite difference model of the cooling of an igneous intrusive of
limited volume is developed and used to investigate the relation
between igneous intrusion, the formation of liquid and vapor dominated
geothermal systems, and the formation of porphyry-type ore deposits.
The model takes into account the properties of pure water and
accomodates the phenomena of boiling and condensation. Permeability,
level of intrusion, and pluton volume are systematically varied.
Pressure, temperature, and fluid velocity are computed as functions of
time."
Cathles starts with a pluton at 700 deg. C intruded suddenly into a
water-saturated fractured formation of uniform permeability. The
initial model pluton is 1.5 km wide, 2.25 km high and the top is 2.75
km beneath the surface. (This size is typical of small plutons
associated with porphyery copper and molybdenum deposits found
throughout the southwest U.S., Mexico, and the S. American Andes.)
Cathles model also takes into account the heat generated by exothermic
reactions (alteration of minerals) that occur as convecting fluids
interact with and cool the pluton.
In order to keep this post short, I'll skip over the rest of the
details and jump to the point I wish to make. The main focus of this
research was not to investigate the length of time necessary to cool a
pluton, but to study the effects through time. Cathles' diagrams show
computed isothermic contours for sevaral variations of his model at
times of 1K, 5K, 10K, 20K, and 100K years after intrusion. All of his
various models show that the pluton would still be radiating some heat
after 100,000 years - even if it had developed a Yellowstone Park
geothermal system at the surface!
If you are interested in this subject, I strongly recommend reading
Cathles (1977) even though the mathematics get a little intense at
times. Some other references to summaries of similar work is given
below. (Sorry that my refs are also 15 to 20 years out of date. As
Glenn and David lamented, I haven't kept up with the current
literature on this topic :-( )
Burnham, C.W., 1979, Magmas and hydrothermal fluids, in Barnes,
H.L., ed., Geochemistry of Hydrothermal Ore Deposits: New York,
Wiley, p. 71-136.
Norton, D. and Cathles, L.M., 1979, Thermal aspects of ore
deposition, in Barnes, H.L., ed., Geochemistry of Hydrothermal Ore
Deposits: New York, Wiley, p. 611-631.
Burnham, C.W., 1981, Physicochemical constraints on porphyry
mineralization, in Dickinson, W.R. and Payne, W.D., eds., Relations
of tectonics to ore deposits in the southern Cordillera: Arizona
Geological Society Digest, v. 14, p. 71-77.
Burnham, C.W. and Ohmoto, H., 1981, Late magmatic and hydrothermal
processes in ore formation, in Mineral Resources: Genetic
Understanding for Practical Applications: National Academy Press,
p. 62-72.
Cathles, L.M., 1981, Fluid flow and genesis of hydrothermal ore
deposits, in Skinner, B.J., ed., Economic Geology 75th Anniversary
Volume: El Paso, Texas, Economic Geology Publishing Co., p.
424-457.
White, D.E., 1981, Active geothermal systems and hydrothermal ore
deposits, in Skinner, B.J., ed., Economic Geology 75th Anniversary
Volume: El Paso, Texas, Economic Geology Publishing Co., p.
392-423.
Steve Smith
[The opinions expressed here are my own
and should not be attributed to my employer.]
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