Re: Earth Matters

Jack Dominey <look@xxxxxx> eyed the audience and in choked emotion intoned:
news:hgff04l5iq60uil2l0gmlr8ihc868dt3au@xxxxxxx:

In <invalid-9F0A25.11210517042008@[74.223.185.199.nw.nuvox.net]>, Ymir
<invalid@xxxxxxxxxxxxxxx> wrote:

I don't know off hand how much new water is produced by natural
processes on earth, but those processes which I can think of all tend to
be biogenic and thus wouldn't have been available during the Earth's
early history.

I believe there is still a great deal of original water bound up in
the rocks of the crust. Volcanoes push out a helluva lot of steam.
The Wikipedia articles I just checked didn't give any estimates of the
proportions of magmatic water versus ground water, but I was under the
impression a great deal of the steam is released directly from the
magma.

There is a significant amount of water in both crustal and mantle rocks.
Just look at any whole rock analyses published in geological journals.
Sometimes water is reported specifically along with OH and CO2 and some
other volatiles but often the sum of all volatiles is reported simply as
LOI (loss on ignition). LOI is usually a few wt. %. Analytical method is a
factor, XRF for example cannot really measure the lighter elements though
they are cometimes calculated anyway using so-called correction algorithms.

Phase equilibria are also significantly affected by the presence of water.
The main effect is that it lowers the melting point. The mantle, though
significantly "dryer" than the crust represents a vastly greater reservoir
of primoridal water because of its vastly greater volume. Roughly 3/4 of
the earth's surface is composed of mantle derived basalts (ocean floor) 10
to 20 km thick covered by a veneer of water 2-3 km thick. Most continental
volcanism occurs where oceanic crust is being subducted beneath continental
margins, thus lowering the melting point of the water saturated rocks and
subsequently lowering the melting points of the overlying continental rocks
as the saturated magmas rise due to bouyancy.

The short answer is, with respect to volatiles and specifically water, the
earth is still degassing from its post accretionary state when it formed in
the solar nebula. At a first approximation, degassing and loss of volatiles
to space have been in equilibrium for +- 4 by.

HTH

--
Here we may reign secure, and in my choyce
To reign is worth ambition though in Hell:
Better to reign in Hell, than serve in Heav'n.

.

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