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.

> > > 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?

> > 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.

> > 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.

> > 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?

> > 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?