2019 GEO 56690 Geodynamics Last Time The Lithosphere Revisited There are several different processes and associated observations that researchers may actually mean when they refer to ID: 809263
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Slide1
22
Apr 2019
© A.R. Lowry 2019
GEO 5/6690 Geodynamics
Last Time: The Lithosphere Revisited
There are several different processes and associated
observations that researchers may actually mean when they
refer to
“
the lithosphere
”
:
Thermal Boundary Layer
is the uppermost region where
heat transfer is dominated by conduction with some advection
Seismic Lid
lies above a mantle negative velocity gradient
that is sharp enough to generate wave conversions, called by
seismologists
“
lithosphere-asthenosphere boundary
”
(
LAB
)
Original
Lithosphere
is the layer that supports stress on long
timescales; often modeled as an elastic plate but really a
rheological definition involving dynamically maintained stress
Seismogenic
layer is the brittle-field portion of the dynamical
lithosphere
Slide2Next up: Exercises
II due Mon April 29
at 5 pm;Heather’s Final Project presentation will be Mon @ 9:00 am!
Slide3Tom Jordan [1975, 1981] examined chemistry, density,
seismic velocity of mantle xenoliths and found that “
fertile” mantle lithosphere (garnet lherzolite) is more dense, has lower seismic velocity than residuum
mantle after melting (peridotite).Expressed density in terms of amolar ratio of Fe to Mg, R,and molar fraction Al2O3 (
X
Al
) as:
Using measurements of xenoliths
(at surface P,T), he empiricallyfound the partial derivative termsto be –0.70 and 0.32,respectively.
“
Tectosphere
”:
Slide4Melt relations (e.g. compilation of
McKenzie & Bickle, J. Petrol.,1988; subsequent incarnations such as MELTS, pMELTS)
suggestXAl drops,XFe stays~constant,XMgincreaseswithincreasing
degree ofmelting:
solidus
liquidus
(in
melt)
(in
melt)
(in
melt)
(GPa)
Slide5Melt depletion also
may
increasethe P-wave velocityof mantle rock (possiblesensitivity in VS also, butthought to be much lessso).Hence, high-velocity“
rolls” north and south ofthe Snake River plain werehypothesized to be residueof melting in the center,which is low density because of bothtemperature andcomposition…
Humphreys et al., GSA-Today, 2000
partial
melt
residuum
Slide6Isopycnic hypothesis: Rudnick et al 1998; Lee and Rudnick
1999; Shapiro et al 1999; Forte and Perry 2000:
Suggests that isostatic equilibrium is partially maintained by melt depletion offsetting the negative buoyancy associated with low temperature cratonic lithosphere…
Slide7Problem though: The bulk composition that results from
melting depends on pressure as well as degree of melting
.Above 100 km depth (where most melting derives) thebuoyancy effect is small; it peaks ~150 km… Theseare derived from mineral physics + melt experiments.
Note the effect on velocity is even smaller!
Schutt & Lesher, JGR 2006
Slide8Wang et al. EPSL 2014
Most-recent dynamical
modeling suggests
compositional buoyancyhelps resist RT-instabilitybut does not preventdeformation…And no compositionalbuoyancy is needed tokeep cratons stable if
there is a 10x difference
in compositional
viscosity
!
Suggesting?
No viscosity difference; yes chemical buoyancy
= 3x; yes chemical buoyancy
= 3x; no chemical buoyancy
= 10x; no chemical buoyancy
Slide9A bit more on
geophysical
LAB: It’sobserved bothin seismic andelectricalconductivity
structure (butare they thesame thing?)Here, MT imagesuggests partialmelt &/or freewater undersubductingCocos plate.
Naif et al. Nature 2012
Slide10There are as many (or more)
candidate mechanisms and
related processes as thereare observations that relate
to them…Leaving the answer somewhatnebulous.
Eaton et al. Lithos 2009
Mierdel et al. Science 2007
Slide11Foster et al. EPSL 2013
WY
NE
IA
This image
shows
two
negative velocity
gradients! One
is referred to as
mid-lithosphericdetachment orMLD... Here,the impedancechangecorrelates withan anisotropygradient…
Which also maybe a mechanism.
Slide12Isotope
Geochemistry
Isotope geochemistry quantifies concentrations of radioactive isotopes & their daughter products in rocks. It has proven a very useful tool for:
• Geochronology (probably most familiar) •
Analyses of
chemical differentiation
processes
Radioactive isotopes are especially useful because
they can be used to constrain time.Important to know: •
Initial isotopic concentrations!!! • Radioactive decay constants • Reservoir affinities and transport processes
Slide13(Note these are mostly taken from notes by J. Shervais
© 2007)
Slide14Slide15Slide16Hence significance
Sr 0.706 isopleth…
Represents mixing
with crust derived
from continental
melting < ~0.5 Ga.
Slide17(CHUR: Chondritic Uniform Reservoir)
Slide18A standard notation:
where:
= measured ratio
s
= ratio expected for bulk Earth
Slide19N-MORB = Normal Mid-Ocean Ridge Basalt;
E-MORB = Enriched in incompatible elements
(represents plume perturbation?)
Slide20Slide21DMM = Depleted MORB Mantle; EM1 and
EM2 are enriched end members
Slide22Slide23Slide24Slide25Sub-Continental
Lithospheric
Mantle
OR?
Ancient Recycled
Oceanic Crust
& Sediments
OR?
Continental
Lower Crust
MORB Melt
End-Member
MORB Residuum
End-Member?
(HIMU = High
where
=
238
U/
204
Pb
)
Slide26Slide27Slide28Slide29