Re: Seismic waves, density and temperature.

On Apr 1, 11:46 am, auxotectonics_deletethis@xxxxxxxxxxxxxxxxxx
(Florian) wrote:
Stuart <bigdak...@xxxxxxx> wrote:
On Mar 30, 2:28 am, auxotectonics_deletethis@xxxxxxxxxxxxxxxxxx
(Florian) wrote:
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.

Stuart, it is the opposite, these 5-8 km of crust are supposed to
entrain the whole lithosphere down when they get eclogitized.

They certainly would help. But who suggests that as the only source of
buoyancy?


Absurd isn't it?

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?

He did not miss it, he argues that the crust has a different composition
than the asthenosphere, so that temperature is a marginal parameter
considering density. Composition has a larger impact.

Maybe. Maybe not. I agree that how subduction gets going is an
interesting problem. What keeps it going is a less interesting one.


"The oceanic lithosphere is a frozen shallow (30-100 km deep)
asthenosphere, previously depleted below ridges. Therefore, the oceanic
lithosphere is the differentiated lighter upper part of the mantle: then
why should it be heavier a priori than the undepleted deeper (100- 300
km) asthenosphere lying beneath the old oceanic lithosphere where a
pyrolite density of 3400 kg/m3(Jordan, 1988; Kelly et al., 2003) is
inferred? Moreover, hydrothermal activity generates serpentinization of
the mantle along the ridge that decreases the density even more."

Th question is, does it compensate for the cooler temperatures. If
it does, than this raises more issues for subduction initiation.




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

Hmmm, when something is depleted, its density is always decreased
compared to the original material.

Reference?


You should revise the notion of
solvent, solute and solution


Basalt is extracted from mantle at low pressure. Basalt is less dense
than (mantle - basalt) at
low pressure. Basalt has a density of 2.7. Peridotite is at least >
3.0




and lower speed according to a naive interpretation of
that formula.

The impact of a change in density alone is not much significant. A 10%
increase in density would account for only a 5% decrease in Vp at
constant modulus.


This is very simple. The Moho represents a sharp increase in body wave
speed.

No increase, no Moho.The reason for the increase is because of a
change in moduli.

And of course the key-phrase here "at constant modulus".

As I've said before density and the moduli have a complex
relationship.


[...]

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.

Or the presence of water.

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.

Deep-focus seismicity is related to phase changes and those changes have
certainly an impact that can't be neglected.

Its affected by the phase changes, but not because of them. There are
plenty
of earthquakes within the slab far from the phase changes.



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.

That's not the lower temperature of a slab that is questionable, but its
higher density compared to surrounding material. The point is that cold
does not mean denser than surrounding material. 600 K cooler means what?
2% denser?

That would be all the mantle needs to convect vigorously.


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

That's a dead war to me. The one-layer/two-layer war does make sense in
the context of convection with ascending and descending flows, not when
there is only ascending flows.

Its dead to you because of an overcommitment to a non-scientific
hypothesis.


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?

Nothing. It was just to point that tomographies are misleading because
they can't help to make the difference between composition,density or
temperature.

A blue area in a tomography can mean a stiff/cold region and at the same
time, a red area in the same tomography can mean a dense area.


Depends on the circumstances. Anderson doesn't agree with you about
slabs.



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

I think he means that the incompressibility and rigidity are always much
larger than the static pressure, so that the static pressure has no
significant influence compared to the modulus. Hu???

You have no idea what he's talking about either.


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?

Actually, it does not make sense to me either :-)

Well, when you figure it out explain it to me.

Stuart
.

Relevant Pages

  • Re: Seismic waves, density and temperature.
    ... the whole lithosphere! ... cooler than the upper mantle. ... meaning cooler temperature displace the ... and at temperatures above 800 oC rigidity diminishes rapidly. ...
    (sci.geo.geology)
  • Re: Seismic waves, density and temperature.
    ... entrain the whole lithosphere down when they get eclogitized. ... This is the mainstream explanation for the slab pull. ... lithosphere is the differentiated lighter upper part of the mantle: ... You said that the change of density due to temperature is rather ...
    (sci.geo.geology)
  • Re: Seismic waves, density and temperature.
    ... entrain the whole lithosphere down when they get eclogitized. ... lithosphere is the differentiated lighter upper part of the mantle: ... You said that the change of density due to temperature is rather ... Its affected by the phase changes, ...
    (sci.geo.geology)
  • Re: Magnetic field
    ... ago claimed that lithostatic pressure is an impediment to ... Now, regarding subduction, the issue is that of the density of the slab ... relatively to that of surrounding mantle. ... these issues are irrelevant because the lithosphere is not ...
    (sci.geo.geology)
  • Re: Magnetic field
    ... ago claimed that lithostatic pressure is an impediment to ... Now, regarding subduction, the issue is that of the density of the slab ... relatively to that of surrounding mantle. ... these issues are irrelevant because the lithosphere is not ...
    (sci.geo.geology)