> 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.

> > 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 :-)