On Thu, 06 Mar, Glenn Morton wrote:
DT:
>I think your argument is viewing the intrusion of a
>magma as a closed system - whereas I would want to explore the
>thought that the system is open and there are many inter-related
>and interdependent disturbances.
GM:
"I am aware of this non-magmatic tectonism but that is irrelevant
to the problem of cooling a batholith because we find batholiths
in tectonically inactive areas like the East Coast of the United
State."
I would hesitate to describe ANY area as "tectonically inactive":
everything is relative. Within a Plate Tectonics framework, you
OUGHT to have plenty of tectonic activity wherever you have
batholiths emplaced.
GM: "The Great Stone Dome that I mentioned a while back as well
as diabase sills found on Georges Bank offshore Massachusetts,
are in areas that did not suffer from much regional tectonism.
The entire problem with the oil potential of the east coast was
that there was too little faulting, which means too little
tectonism, yet there are still some batholiths."
Maybe this is a sign that conventional models are inadequate to
explain the observations ... IF they did not form in the
"normal" way, I would be very cautious about an argument that
assumed they did.
DT:
>Again, you appear to regard the system as closed: an assumption
>I would not wish to defend. The analogy of pillow lavas is a
>good one as far as the chilling of the magma is concerned, but
>pillow lavas also illustrate aspects of open systems: as the
>magma moves inside the pillows, the "rind" may swell and
>fracture, or may collapse and implode.
GM: "But at the point that the magma chamber ceases motion, the
rind can no longer swell. If the magma chamber never went
through this phase, there would be no cool batholiths. At some
point the rind must never again swell and break. this is the
point at which the long cooling time begins to happen.
Whilst I am tempted to agree with you, I am actually going to
disagree - to make a point. I am arguing that closed systems
cannot be assumed, and that there is plenty of field evidence to
support the idea of openness. Do you not have faulted granite
plutons like us in the UK? Even when solid, fracturing can
continue, opening up new channels. Solid granites have no
shortage of channels for water movement. One of the reasons the
Hot Dry Rock project failed (in Cornwall, UK) is that the
injected water leaked away too quickly, and the research team
could not confine the steam. There were just too many routes for
water to escape.
Regarding water emerging from black smokers:
DT:
>On this point, I think we are in basic agreement. I would not
>use the 350 deg C temperature in the way that you have, because
>a distance of 2-3 km of rock is between the black smoker and the
magma chamber.
GM: "Here is my reasoning. Since the solid rock above the magma
chamber has a low thermal conductivity, 1000 degree water should
quickly heat the walls of the conduit to near 1000 deg,so that
there would then be little means for the water to cool on the way
up. The water travels so fast that it would have little time to
cool on its way up. It moves at 1-5 m/s (Mcdonald et al, Earth
and Planetary Science Letters 48:1-7 p. 2) At these speeds, the
water could come from a depth of 1 kilometer in 16 minutes."
Here is my reasoning. As the hot water rises, it mixes with
cooler water that is circulating within the convection cell. The
temperature of the emerging water is considerably cooler than the
water at depth.
[snip]
GM: "I will agree that hydrothermal convection will cool an area
more rapidly than an area cools without it. The reason is that
the water circulation avoids part of the conduction pathway. But
there still would be some of the conduction pathway required, and
that would make the cooling quite long. Hydrothermal activity
effectively acts to remove SOME of the overburden through which
conduction must take place otherwise."
Sorry, Glenn. My mind is not as sharp as it should be. I do not
understand this paragraph - but it looks as though there's a
measure of agreement, so I'll move on.
On Thu, 06 Mar 1997 Steven M. Smith wrote:
"I have recently resubscribed to this list and have been
following this discussion of cooling intrusives with interest."
Welcome back to the list - and glad to know this exchange is of
wider interest.
SS: "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)...."
SS: "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. .... 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
several 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!"
Thanks for drawing attention to this: very interesting! I have
two comments.
1. The model must show a significant reduction in timescales over
conduction-only heat loss.
2. The results must be dependent to some extent on the model
specification. For example, the pluton is intruded into water-
saturated fractured rock - but what are the assumptions about
water entering and leaving the system? Also, what tectonic
disturbances took place during cooling, and how did these affect
the outcomes?
And thanks for the additional references.
On Sun, 09 Mar 1997, Glenn Morton wrote:
"Hi David, I got several of the articles that you suggested.
I have learned a couple of things. First an active hydrothermal
system which is removing heat from a batholith also removes
oxygen-18 from the rocks below...."
This is true. Isotopic fractionation mechanisms are linked, for
example, to chemical reactions, boiling, and filtration through
shales.
GM: "Intrusions which did not undergo a hydrothermal cooling are
also found. They have a normal isotopic signature because the
O-18 couldn't escape. one such conductively cooled intrusion is
the Santa Rosa intrusion."
This does seem significant. I would be happier if this were one
of several complementary lines of evidence, all pointing in the
same direction. The details of the Santa Rosa pluton warrant
closer examination, but for the present, in this case, I accept
that the evidences for convection are lacking.
GM: "This is an 8 x 16 km wide intrusion so conductive cooling
would take a long long time.
While I know that you, David, have stated that you don't need to
have all the intrusions cooled within a year, there are those on
the list who need to have everything cool within a 4-6000 year
period. Intrusions lacking hydrothermal activity present a
greater problem for their chronology since these things remain
hot for hundreds of thousands of years."
The "convective cooling" scenario was not presented (in my posts)
as a panacea to permit short timescales, but where it is
applicable, it can dramatically shorten timescales. I would not
want to claim it is always applicable: if geology is to be a
science, theory must be sensitive to the data. This is,
ultimately, my reason for challenging the "conduction-cooling"
model. It is also my justification for being interested in some
of the other "unorthodox" theories mentioned earlier in this
thread. I have come to the conclusion that there is far more
catastrophism in the geologic record than can be accommodated by
the current "orthodoxy" - despite its protestations that it can
accommodate catastrophes. When you get down to the
interpretation of field evidences, the legacy of Lyell is still
very strong. There is a real need to identify more clearly the
assumptions in models; to develop ways of testing those
assumptions; and to be more alert to alternative paradigms of
interpretation.
Best wishes,
David J. Tyler.