Message from discussion
Seismic waves, density and temperature.
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Newsgroups: sci.geo.geology
Subject: Re: Seismic waves, density and temperature.
From: auxotectonics_deletethis@nachon_andthis.net (Florian)
Date: Tue, 1 Apr 2008 23:46:14 +0200
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Stuart <bigdak...@aol.com> wrote:
> On Mar 30, 2:28 am, auxotectonics_deletethis@nachon_andthis.net
> (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.
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.
"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."
> > 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. You should revise the notion of
solvent, solute and solution.
> 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.
[...]
> > 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.
> 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?
> 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.
> > 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.
> > 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???
> > 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 :-)
--
Florian
"Toute vérité passe par trois phases. D'abord, elle est ridiculisée;
ensuite, elle rencontre une vive opposition avant d'être acceptée comme
une totale évidence" - Arthur Schopenhauer
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