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sci.geo.geology |
On Mar 30, 2:28 am, auxotectonics_deletethis@nachon_andthis.net > > The lithosphere gets to be about 100km think. > Hu? Only basalt and gabbro Eclogitized, so it concerns only 5-8 km not > > Also, it is on average roughly 600-700K I mean how does Doglioni miss that? > > > <http://tetide.geo.uniroma1.it/sciterra/sezioni/doglioni/Publ_download/W > > Some things in here may make sense, but then... > > "The high velocity of slab detected by tomography could be related not > 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. > > The celerity of a P-wave is given by [ (K+4/3U)/R) ]^1/2 where U is the > Just a reminder, the celerity of S-waves is given by [U/R]^1/2. > > So one wonders how in the world high velocity of the slab could *ever* be > So actually, the higher velocity in the slab could mean that the slab is > > Indeed, the speed of seismic waves increases in slabs because they are > Not so sure. Don L. Anderson has a discussion about tomography problems, > See paragraph: "Seismic velocity is not a thermometer" Elsewhere on that page Don talks about the effects of slabs in the Sorry. If you think slabs don't have a significant negative thermal I don't see Don arguing against this point. But welcome to the one- > By the way, I note the same confusion, i.e., denser material <=>higher > "Nevertheless, tomographic images are often interpreted assuming that a > Weird, isn't it? > Especially, that a few lines further: > "The African and south Pacific "superplumes" are vast, low-velocity > thus suggesting again that low velocities region are associated to high > > Maybe they meant to say "lower temperature" and "not higher density" but > > I also take issue with his statement that mantle densities are "quite > Actually, the density profile raises another issue according to Stavros: What a surprise. > "Then seismology tells us that the velocity of both P and S waves There is no other physically For example according to the Preliminary > "The implication is quite clear: Depth and therefore static There should be no problem finding references to experiments For seismic wave > So, where is the flaw? Stuart
(Florian) wrote: No kidding. But 5-8km of oceanic crust isn't going to stop 100km of
> Stuart <bigdak...@aol.com> wrote:
> > (Florian) wrote:
> > > Not without full eclogitization. Eclogitization occurs only at depth >45
> > > km and is only partial.
> the whole lithosphere!
lithosphere.
No response?
> > cooler than the upper mantle. That buys you another 1.5%- 2%.
> > > See Doglioni et al "What moves slabs": Its done with surface waves too. Among the first studies to fully
> > > hatMovesSlabs.pdf>
> > Statements like the below give me pause with respect to this paper.
> > to its higher density but to its higher rigidity and viscosity"
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.
I hadn't forgotten, but thanks.
> > shear rigidity, K is the incompressibility and R is the density.
Yes, according to him, the oceanic moho should not exist. If lighter
> > do to a simple change in density? Greater density by itself means slower.
> 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.
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.
I agree it can be overdone. This is because velocity variations can be
> > 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.
> and notably the tendancy to wrongly use it as a kind of thermometer:
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.
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.
anomaly you
are in serious denial.
layer vs. two-layer wars.
Or both density and the low velocities have something to
> velocity:
> 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)."
> 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)"
> density.
with a change in chemistry with respect to nominal mantle.
And this has what to do with slabs?
Oh great another EEer....
> > it still wouldn't make sense. What would imbue these roughly tabular
> > regions with special stiffness and viscosity?
> > 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.
So far so good.
> 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.
> possible way for seismic wave velocity to increase with depth but for Define incompressibility and rigidity thresholds
> 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.
> Reference Earth Model - PREM the density of crustal rocks at ~10 km Not to me. Maybe you should ask Don Anderson if the above
> 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."
> stress-pressure has no effect on elastic moduli.
is clear to him?
in the peer-reviewed literature which show the elastic moduli are
invariant under pressure. Does he have any?
> velocity to increase with depth in the mantle rigidity has to increase Sounds like a bunch of BS to me.
> 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."
You're joking right?