Re: To John Harshman -- re Isochrons


"Zoe" <muze10@xxxxxxx> skrev i en meddelelse
news:8u7vv1tp028fi4chudm60toph3euoodbaj@xxxxxxxxxx
On 23 Feb 2006 03:58:41 -0800, "John Drayton"
<bitbucket55@xxxxxxxxxxx> wrote:

Zoe wrote:

On Wed, 22 Feb 2006 09:31:22 -0600, "Dan Luke"
<c172rg@xxxxxxxxxxxxxxxxx> wrote:

<snip>

3. The earth's spin is not a mechanism for mixing materials on this
earth. Otherwise I could put a bucket of paint on my porch, add some
blue paint, and expect it to eventually become well mixed through the
spinning of the earth.

The earth's crust is not homogenous.

I agree with you.

As it the planet spins on its axis, it
reacts differently to the tidal pulls of sun & moon.

agreed, but these pulls are not sufficient to mix anything to the
point of being homogenous, are they?

The earth is about 26 miles higher at the equator than it
is at the poles. This is is because of the rotational and
gravitational forces, which you so readily dismiss as
ineffectual.

if you want to credit rotational and gravitational forces with mixing
magma to the point of being homogenous, then you need to explain why
these same forces do not mix my can of blue and white paint, sitting
in the hot sunshine on my porch.

Off topic: gravitation drives erosion (water running from mountain to sea).
In a million years you'r paint will be intermixed due to erosion. The issue
of 'mixing' as a general term is 'the cooling planet' ... temperature
induced density differences, for very short. Back to topic

This may become a large post.
As to the isochrons this part of the discussion has derailed pretty far. To
recapitulate the fluid/solid earth: the different behavior of seismic
shear-waves and pressure-waves showes that the outer core is liquid (does
not pass shear-waves), the rest of the earth is solid, becourse it does pass
both waves. Both solids and liquids has the posibillity of convecting (ala
ye old one's example with the lava lamp). You seem very surprised that this
convection happens (see note *).
You've got links to phase diagrams. The difference between the crust and the
mantle is marked by a seismic difference (see note **) showing two different
types of rock - a solid to solid fase-change is thought to happen.
High-pressure rocks change to low-pressure rock. Closer inspection can be
found in mantle-ophiolites.
The convections makes possible some rock or fluid mixing all depending on
what's convecting. A magma is a liquid, some extract of the mantle not
neseccarily representing it's whole rock composition - you'll need a
specialist for details. Sucessfull generic dating methods (not isochrons)
may distinguish different intermixing magmas, but we know the errors prone
to these methods. Whatever magma you have at hand, mixed or just some
extract, it will solidify according to fase-rules and in an order that gets
careful attention from endogen geologists. Different crystals will emerge
from this liquid on cooling (see note***). A 'rock' is a collection of
different crystals. It is these single crystals that constitute 'a sample'
on the isochron pages thus a rock gets more than one sample-point and they
are all representing the rock that evolves from the original fluid.
This discussion of 'mixing' could shape up with references to what's mixing
and the chosen scale (rock, bed, formation, region, convection-cell, mantle
or core - solid or fluid) - and I think that you still owe us a rigorous
explanation of why it has impact on the isochron method.

http://aerospace.wcc.hawaii.edu/shape.html

Having said that there's two questions you need to ask
yourself:

1. What size are the samples which are used for
isochron dating?

very small.

2. Is it reasonable to expect that a local homogeneity
migh be present within a sample of that size, considering
the time available?

why would you expect this?

Becourse the start-out material is fluid magma in the chamber. (remember the
cream/coffee mixing)

If I pull a sample from a universe of
similar material, should it not reflect the characteristics of its
little sampling universe?

Maybe if you make sure that you sample some sort of unmineralized fluid like
glass/obsidian. I'm not sure what you mean.

For the entire rock to be non-homogenous,
but each little sample to be homogenous is a startling new turn in
chemistry that emerged last night or the day before.

Correct the syntax - the error is unfortunate becourse you may be right on
top of a key question. To recapitulate: the fluid magma is homogeneous
(atleast on a local scale), the rock evolved from it is not becourse it is
build of different minerals (and perhaps have a 'super-cooled' liquid fase
like obsidian too) - but anywhere Sr incorporates in these minerals it does
so in the isotopic proportions present in the magma. If one type of crystal
with a high meltingpoint consumes Sr leaving less for a lower meltingpoint
crystal in the same batch of magma - they still incoorporate Sr in
eqvivalent isotopic proportions.


You still seem to be under the illusion that you need
homogeneity across a vast area.

what I said was that the kind of mixing that would cause homogenous
magma, even locally, is not evident in the natural world. And my
question is, what mechanisms are there that would cause this supposed
homogeneity in any particular magma?

I think that you confuse 'magma' for molten mantle. Only the latter is
matrix-supported.
'Unhomogenized rock' is an error-factor from ie the local pipe that the
magma passes through (zenoliths).

note *
About convection: You do know why water in a pot on a stove whirls round?
Hot water is lighter than cold water. There's not much difference when
solids in the mantle transfer heat from the core or responds on cooling on
it's surface - or meterology.
I've mentioned the imperical basis for dicerning between fluid or solid
earth. This part is my own speculation: Seismic shear-waves travel perfectly
through sand when it's matrix-supported (each grain resting upon another
grain). This doesn't prevent water from moving through the sand. I cann't
tell weather some eqvivalent process happens in the mantle, but it wouldn't
be inconsievable.

note **
A seismic reflector reveals an acoustic impedance contrast (acoustic
impedance = 'Rock density' * 'Rock_Acoustic_Velocity').
'Rock_Acoustic_Velocity' is an inherent material/rock constant as is 'Rock
density'.

note***
This part is prone to 'turn out your light' becourse of it's complexity if
you want to consider 'details' as you sometimes seems to cherish.
What exactly kind of mineral would grow from a soup of elements or small
constituents? ... well, you can profit a lot of understanding if you accept
that two different minerals may form, one with a need for a divalent ion
(Ca++ or Sr++, if you look in the periodic table, you'll find that they
share a lot of similar qualities), and another one with a need for a
monovalent ion (K+ or Rb+, also neighbours in the periodic table). I don't
need to say that it's particularly the valency that determines their
chemical behaviour, do I?
The silicates that evolves have the unfortunate character of using both
types of alkali metals in the same mineral, but pyroxenes and felspars use
them in different amounts. Finally, a fairly narrow range of
mineral-composition characterize a type of rock that is present in several
beds and extensive areas.

No, I'm not an endogen geologist, and the typhoes are all mine.

Carsten


.

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