Re: ALL the oceanic crust is recycled in 180 My: unrealistic


"Florian" <auxotectonics_deletethis@xxxxxxxxxxxxxxxxxx> wrote in message
news:1imz3q1.1ogy3fnq9qyd2N%auxotectonics_deletethis@xxxxxxxxxxxxxxxxxxxxx
brad <lbjohnson1949@xxxxxxxxx> wrote:

On Sep 7, 5:33 pm, auxotectonics_deletethis@xxxxxxxxxxxxxxxxxx
(Florian) wrote:
brad <lbjohnson1...@xxxxxxxxx> wrote:
Please tell me how you can get Si rich magma by adding water.

Addition of volatile (like water) leads to partial melting =>
fractional
crystallization => silica rich magma.

This is the primary mechanism of andesite formation at "subduction
zones".

No.
Weathered siliceous material is subducted and resulting
melt , added to basaltic basement is Si enriched. Water only lowers
the melting point, does not break Silicate bonds.

You missed one or two trains.
Slab melting is one possible mechanism but is not assumed to be the
primary one. The primary mechanism is slab dehydration, with released
water/volatiles inducing partial melting of peridotite in the mantle
wedge, then fractional crystallization leading to the formation of
silica rich magma.

Umm, andesite formation is a result of "slab dehydration, with released
water/volatiles inducing partial melting of peridotite in the mantle
wedge, then fractional crystallization leading to the formation of
silica rich magma"?

We've already established that andesite cannot form from peridotite.
Secondly you've repeatedly said that "upduction of deep mantle material is
responsible for arc volcanics", and have stressed that peridotite doesn't
exist below 300km. But it's good to know that you believe that slabs are
getting down there in order to partially melt.

In plate tectonics, water can only come from slab dehydration.
By contrast, in auxotectonics, water can come from dehydration of the
overthrusted (understand buried) oceanic crust OR from the deep
interior. But it is more likely to come from the deep interior because
surfaceward migration of volatile can also explain the deep earthquakes.

Ditto. See above.

So why is nothing older ? Some seafloor ( somewhere ) should be very
old.

Why is nothing older in the modern ocenic basins?
Nothing is older because the current large oceans did not exist 200 My
ago. There were only narrow basins in between cratons, about the width
of the Red Sea. Yep, the Red Sea *is* an ocean despite its name.

So why no old sediment preserved in those narrow basins ?

Weird question. Those old basins were closed by upductions and are now
under the form of ophiolites. The old sediments are present above the
ophiolites.

Umm, the Michigan basin is closed? It certainly is buried by kms of
sediments, but it is STILL a basin. Where are the ophiolites in the
Michigan basin?

George


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