Seismic waves, density and temperature. - sci.geo.geology

Message from discussion Seismic waves, density and temperature.

Stuart

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More options Mar 31 2008, 5:06 pm

Newsgroups: sci.geo.geology

From: Stuart <bigdak...@aol.com>

Date: Mon, 31 Mar 2008 00:06:30 -0700 (PDT)

Local: Mon, Mar 31 2008 5:06 pm

Subject: Re: Seismic waves, density and temperature.

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On Mar 30, 2:28 am, auxotectonics_deletethis@nachon_andthis.net

(Florian) wrote:
> Stuart <bigdak...@aol.com> wrote:
> > (Florian) wrote:
> > > Not without full eclogitization. Eclogitization occurs only at depth >45
> > > km and is only partial.

> > The lithosphere gets to be about 100km think.

> Hu? Only basalt and gabbro Eclogitized, so it concerns only 5-8 km not
> the whole lithosphere!

No kidding. But 5-8km of oceanic crust isn't going to stop 100km of
lithosphere.

> > Also, it is on average roughly 600-700K
> > cooler than the upper mantle. That buys you another 1.5%- 2%.

No response?

I mean how does Doglioni miss that?

> > > See Doglioni et al "What moves slabs":

> > > <http://tetide.geo.uniroma1.it/sciterra/sezioni/doglioni/Publ_download/W
> > > hatMovesSlabs.pdf>

> > Some things in here may make sense, but then...
> > Statements like the below give me pause with respect to this paper.

> > "The high velocity of slab detected by tomography could be related not
> > to its higher density but to its higher rigidity and viscosity"

> Indeed, that statement looks incorrect.

> > For starters tomography of slabs is done using P-waves.

> Nowadays, tomography is done using S-waves and P-waves.

Its done with surface waves too. Among the first studies to fully
document the
velocity highs associated with slabs were those back in the
early 80's using what was then called residual sphere analysis, even
before the
use of tomography was wide spread.

> > The celerity of a P-wave is given by [ (K+4/3U)/R) ]^1/2 where U is the
> > shear rigidity, K is the incompressibility and R is the density.

> Just a reminder, the celerity of S-waves is given by [U/R]^1/2.

I hadn't forgotten, but thanks.

> > So one wonders how in the world high velocity of the slab could *ever* be
> > do to a simple change in density? Greater density by itself means slower.

> So actually, the higher velocity in the slab could mean that the slab is
> simply less dense than surrounding material. Interesting.
> Doglioni argues that oceanic basalt is depleted and serpentinized which
> support this interpretation but is at odd with the quote above.

Yes, according to him, the oceanic moho should not exist. If lighter
density components
are extracted from the upper mantle, that leaves the lithosphere
denser and lower speed
according to a naive interpretation of that formula. But density and
the elastic moduli
have a complex relationship.

> > Indeed, the speed of seismic waves increases in slabs because they are
> > stiffer; temperature has a greater effect on rigidity than it does on
> > density. You'll also notice that there is no term for viscosity in the
> > wave speed formula. The slab has higher viscosity because it is cooler.
> > The viscosity increases with decreasing temperature. It is the higher
> > stiffness that allows one to
> > ( That should read "larger rigidity", for that matter the
> > incompressibility also increases with decreasing temp.)
> > put bounds on the density through an appropriate equation of state. There
> > is simply no question that the slab is cooler than the surrounding mantle
> > and hence more dense. How much more may be arguable, but there is no
> > question that it is more.

> Not so sure. Don L. Anderson has a discussion about tomography problems,
> and notably the tendancy to wrongly use it as a kind of thermometer:

> See paragraph: "Seismic velocity is not a thermometer"

> <http://www.mantleplumes.org/TomographyProblems.html>

I agree it can be overdone. This is because velocity variations can be
due
to chemical effects, i.e., changes in composition or the presence of
melt. Partial melt is not an issue here
nor is it an issue for the vast bulk of the mantle. However, I
strongly disagree that this is a
problem with respect to slabs. Slabs which we can trace all the way
the
to the surface, seismically, mechanically, and just about any other
way you can
think off. As Doglioni is forced to admit, they are stiffer. Moreover
we knew this before
anyone did seismic tomography. We knew this because of deep-focus
seismicity. And
since there are not profound major element chemical changes in a slab,
the fundamental cause for
this stiffness is that on average they are hundreds of degrees cooler
than the ambient mantle.

Elsewhere on that page Don talks about the effects of slabs in the
transition zone. It is
thicker in regions where there is slab, and thinner where there's not.
This is a fundamental
prediction satisfied. The olivine-Spinel transition at around 400km
depth is an exothermic reaction
meaning the transition is displaced upwards to lower pressure by
cooler termperatures. The spinel olivine
to perovskite-olivine transition at around 650km is endothermic,
meaning cooler temperature displace
the transition lower. Furthermore, seismologists have measured the
upward deflection of olivine ->
spinel olivine transition.

Sorry. If you think slabs don't have a significant negative thermal
anomaly you
are in serious denial.

I don't see Don arguing against this point. But welcome to the one-
layer vs. two-layer wars.

> By the way, I note the same confusion, i.e., denser material <=>higher
> velocity:

> "Nevertheless, tomographic images are often interpreted assuming that a
> simple velocity-density-temperature correlation exists. High velocity
> (blue) is generally attributed to cold, dense, sinking material, and low
> velocity (red) as hot, low-density, rising material (Figures 4 & 5)."

> Weird, isn't it?

> Especially, that a few lines further:

> "The African and south Pacific "superplumes" are vast, low-velocity
> bodies in the lower mantle whose low velocities have been shown to
> result from anomalously high density and not to high temperature
> (Trampert et al., 2004)"

> thus suggesting again that low velocities region are associated to high
> density.

Or both density and the low velocities have something to
with a change in chemistry with respect to nominal mantle.
And this has what to do with slabs?

> > Maybe they meant to say "lower temperature" and "not higher density" but
> > it still wouldn't make sense. What would imbue these roughly tabular
> > regions with special stiffness and viscosity?

> > I also take issue with his statement that mantle densities are "quite
> > speculative". I disagree, they are well constrained by modeling the
> > earth's free oscillations. In fact people are using free oscillations to
> > work out some of the three dimensional structure of density variations in
> > the mantle. As far as I know, the mean radial density profile for the
> > Earth is known to a couple of percent if not better.

> Actually, the density profile raises another issue according to Stavros:

Oh great another EEer....

What a surprise.

> "Then seismology tells us that the velocity of both P and S waves
> increases with depth in the mantle. Density also increases
> with depth. So, if it was only for density the velocity of
> seismic waves should decrease with depth.

So far so good.

There is no other physically

> possible way for seismic wave velocity to increase with depth but for
> the elastic moduli to increase with depth at a faster rate than density.
> This is impossible though if temperature increases with depth.
> Experiment has shown that the elastic moduli are very sensitive to
> temperature; rigidity decreases as temperature increases, and vice
> versa, and at temperatures above 800 oC rigidity diminishes rapidly.
> This decrease cannot in any way be compensated by pressure-depth,
> because the pressure at any depth is below the rigidity and
> incompressibility thresholds.

Define incompressibility and rigidity thresholds

For example according to the Preliminary

> Reference Earth Model - PREM the density of crustal rocks at ~10 km
> depth, is ~2900 kg/m3, the pressure ~0.3 GPa, whereas rigidity mu, and
> incompressibility K, are ~26.6 and ~52 GPa, respectively. At 77 km the
> density is ~3375 kg/m3, the static stress ~2.45 GPa, the rigidity ~67.4
> GPa, and the incompressibility modulus ~130 GPa. At the depth of 667 km
> the corresponding values for density rho, pressure, rigidity mu, and
> incompressibility K are ~4381 kg/m3, ~23.8, ~155 and ~300 GPa, and at
> 2888 km they are ~5566 kg/m3, ~136, ~294, ~656 GPa."

> "The implication is quite clear: Depth and therefore static
> stress-pressure has no effect on elastic moduli.

Not to me. Maybe you should ask Don Anderson if the above
is clear to him?

There should be no problem finding references to experiments
in the peer-reviewed literature which show the elastic moduli are
invariant under pressure. Does he have any?

For seismic wave

> velocity to increase with depth in the mantle rigidity has to increase
> with depth; as a result temperature cannot increase with depth.
> Therefore the seismic wave velocity profile is an approximation of the
> actual, whereas the conventional temperature profile is not based on
> observation, experiment and logic, i.e., it is not an approximation of
> the actual."

Sounds like a bunch of BS to me.

> So, where is the flaw?

You're joking right?

Stuart

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