At 01:51 PM 3/5/97 GMT, David J. Tyler wrote:
>GM: "While this is true, the high temperatures of the water allow
>them to carry lots of minerals in solution. As these hot waters
>rise, they deposit some of these minerals along the pathways,
>effectively sealing these passages off by means of
>mineralization."
>
>True. There will be a zonation in the mineralisation, as
>minerals come out of solution differently according to the
>pressure/temperature conditions. Sealing may occur, but in the
>neo-catastrophist scenario, continuing tectonic disturbance will
>occur to reopen channels or to create new pathways.
>
I guess I don't fully understand your neocatastrophist views. The
continuing tectonic activity may or may not continue. Whether or not it
does continue can be determined by field evidence. For instance, continued
emplacement of magma would continue to fracture the region, allowing water
to flow doen near (but not to) the magma chamber. This continued activity
continues to bring heat up from depth and so the tectonic phase is not the
phase of final cooling. But eventually, the magma runs out of its puch and
the tectonic activity ceases. At this time, the mechanisms we have
discussed, like the scaling problem and the solidified rind around the magma
chamber come into effect. What continues the tectonic push?
>DT: > The shapes of the metamorphic aureoles were inconsistent
>> with purely convective heat loss, and yet could be explained
>> by invoking convective activity.
>
>GM: "I think you mean "inconsistent with conductive heat loss".
>But once again as the batholith center begins to cool it is not
>in convective communication with the surface and is only in
>conductive communication with a convective system."
>
>Yes, I do mean "inconsistent with conductive heat loss". My
>apologies for the typo. I accept that in the upper lithosphere,
>there will be a solid boundary to the magma chamber - although
>fracturing may be repeatedly occurring. However, at higher
>temperatures and pressures, systems may not behave in ways that
>are "obvious" to us on the surface. Research in this area does
>not have a catastrophist emphasis, so those of us who are
>sympathetic with this approach have to wait for occasional papers
>to appear. An example is Shen and Keppler (1997): "Direct
>observation of complete miscibility in the albite-H2O system",
>Nature, 385(20 February), 710-712. They note: "With increasing
>pressure, the solubility of water in silicate melts and the
>solubility of silicate materials in hydrous fluids increase....
>The existence of complete miscibility between fluids and silicate
>melts has profound consequences for the phase relations in the
>mantle. At conditions below the critical point, a hydrous fluid
>phase can coexist with solid mantle minerals, until the water-
>saturated solidus is reached. At this temperature, a hydrous
>silicate melt forms which coexists with the hydrous fluid and
>mantle minerals. Beyond the critical point, however, fluid and
>melt can no longer coexist as two separate phases." This
>implications of this for the topic we are discussing is
>uncertain, but I cite Shen and Keppler to demonstrate that
>alternative perspectives on problems exist and ought to be
>explored.
But I am not sure that this increase in solubility of silica with high
pressure water is anything new. At one time I worked directly with three
of the men instrumental in the discovery of Prudhoe bay oil field. They
told me lots about silica dissolution at depth. Prudhoe Bay was drilled
with the Lisburne limestone as the target. The Triassic Sadlerochit
sandstone was not even considered as a possible reservoir. In the Brooks
Range to the south of Prudhoe, the Sadlerochit sand is tightly cemented by
hydrothermal silica. At the surface, there is almost no porosity in this
rock. But due to the history of the area, the Sadlerochit sand had been
deeply buried and the silica cement dissolved just as the oild was migrating
through the rock. This meant that there was lots of porosity for the oil.
It was a big surprise to those men to find oil where they did. My point is
that silica dissolution may work at depth, but once again as the silica rich
waters are carried to the surface, the silica scaling should occur at a
shallower level sealing the deeper part off.
>
>At this point, perhaps I should breathe a sigh of relief: I'm not
>under pressure to defend my views! As a geophysicist, Glenn
>should be excused for his dislike of mineralogy!
Yeah and my friend who knows all this stuff has been busy with lease sale so
I can't get his attention yet.
>Regarding models of batholith formation
>DT:
>>> ... Large magma
>>>bodies moving upwards through the crust of the earth have the
>>>problem of "What creates the space into which they move?"
>GM: "melting and incorporation of the rock into the magma. as
>>well as uplift. Mt. St. Helen underwent an uplift and even an
>>expansion of the mountain prior to its eruption."
>DT:
>>Are you serious?
>
>GM: "Yes I am serious, .... [stuff re Mt St Helens deleted]
>... Like a balloon, the mountain had expanded several meters
>horizontally. So yes I am serious."
>
>I think we have a misunderstanding here. Yes, Mt St Helens did
>expand by several metres before it went off. Yes, many volcanoes
>around the world are fitted with arrays of markers so that
>changes in shape can be monitored. The point I was making is
>that movements of a few metres are totally inadequate to solve
>the space problem when magmas are intruded. My question should
>have read: "Are you serious in asserting that such tiny movements
>can solve the batholith emplacement space problem?". I also
>pointed out that if you extend the argument to allow for large
>displacements, then you are moving into tectonic adjustment and
>catastrophism, both of which undermine points you had been making
>earlier in your post.
My points are not undermined at the point that the batholith quits moving
and begins cooling. At that point the tectonic motion ceases. If the water
was contacting the magma, the same thing that happens to lava when it
contacts the sea should happen. Immediately, the outer portion cools
creating a rind around it. This can be seen in pillow lavas.
>
>GM: "I have no doubt that water could flow through the cracks
>that must have existed in the rocks of St. Helens. But you have
>still not dealt with the conductive zone which grows as the
>magma chamber cools. The solid lava rind will not have cracks
>through which water can flow. On a mile wide batholith, as it
>cools, the water will not be able to flow to the actual lava
>chamber."
>
>Some of my earlier comments did respond to this. In addition,
>I would refer again to Cann and Stiens (1982). They argue that
>black smokers must be tapping the heat of magma chambers. We may
>not understand the physics of the heat flows - but to insist that
>the magma chamber is rapidly insulated from further heat loss
>does not accord with observations today regarding hot oceanic
>ridge systems.
>
Tapping the HEAT of a magma chamber is quite different from tapping the
MAGMA CHAMBER itself. I would fully agree that the hydrothermal waters are
tapping into the heat of the magma chamber. The low temperatures of the
hydrothermal waters argue against the tapping into the magma. Apparently
most of the vents are between 200 and 500 deg C ( and I am being generous on
the upper end. (see Heinrich Holland, The Chemistry of Atmospheres and
Oceans, p. 196). since this is far below the temperature of the magma, it
would seem to argue no contact. the May 1981 Scientific American article you
cite, gives a temp of 350 C for the east pacific rise smokers. (p. 101) If
this were actually in contact with molten rock, 900+ C,, I would think the
temperature would be higher.
glenn
Foundation, Fall and Flood
http://www.isource.net/~grmorton/dmd.htm