what is the lithosphere: it is not the asthenosphere
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Lithosphere: mechanical boundary layer, dry-mostly, stable for 108-109 a, possessing a steady-state conductive geotherm with base in cratons at 4-7 GPa (170–250 km), shallower (ca 100-150km) in off-cratons, and shallower still in oceans (<100 km)
Asthenosphere: weak layer underneath the lithosphere, area with pervasive plastic deformation deforming over 104-105 a. It is a region with small scale partial melt and is electrically conductive (c.f., lithosphere).
LAB: Lithosphere-asthenosphre boundary, a transition region of shear stress and anisotropic fabric, perhaps a transition between diffusion vs dislocation creep. The transition may or may not be sharp (up to tens of km).
What is the Lithosphere: it is not the asthenosphere
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Fischer et al (2010, Ann Rev)
lithosphere-asthenosphere boundary (LAB) properties
crustmantle
w/ melt
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Eaton et al (2009, Lithos)
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Mantle
Crust
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Composition of the lithospheric mantleApproaches
geophysics: seismology, gravity, heat flow, tectonics
(rheology, deformation, uplift, erosion)
geochemistry: petrography, elemental, isotopic
Sampling the lithospheric mantleApproaches
geophysics: 103 – 106 meters
geochemistry: 10-3 – 10-6 meters
- 6 to 12 orders of magnitude difference
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Why study composition of the CLM?
- Place constraints on the timing and tectonic setting for the formation of continents & their roots
- Examine consequences of the Earth’s secular evolution
- Test models of basaltic source regions
- Characterize the inventory of elements in an Earth reservoir
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LIDChemicalMechanicalThermalSeismological
Tectosphere
Bottom: asthenosphere (LAB)
Top: MOHO (seismic)petrologic break
Oceanic Continental: craton vs off-craton
The different Lithospheresone example
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Where are the cratons and off-cratons
Pearson and Witting (2008, GSL)
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Where are the cratons and off-cratons
Lee et al (2011, Ann Rev)
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Growth of Lithospheric Mantle (LM)
- Mostly linked to crust production- Different in oceanic vs continental setting- Oceanic: crustal growth in divergent margin
settings, with LM growth via lateral accretion of refractory peridotite, followed by conductive cooling of deeper lithosphere
- Continental: mostly convergent margin tectonic growth, with some intraplate contributions, LM grows by accretion of refractory diapirs
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Oceanic & Continental
Crusts
60% of Earth’s surface consists of oceanic crust
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Oceanic lithosphere cools, thickens and increases in density away from the ridge
Increasing density of lithosphere with age leads to progressive subsidence (age-depth relationship)
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Seafloor subsidence & heatflow reflect progressive thickening of lithosphere with age
D(m) = 2500 +350t1/2
q = 480/t1/2
Depth
Heatflow
Wei and Sandwell 2006 Tectonophysics
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Continental Lithospheric MantleCLM growth models
Lee et al (2011, Ann Rev)
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Heat production in the Lithosphere
- Heat Producing Elements (HPE): K, Th, U
- Continental Surface heat flow (Q) Craton 40 mW m-2 Off craton 55 mW m-2
- Near surface heat production
- Heat production versus depth
- Concentration of HPE in Lithospheric Mantle?
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Earth’s Total Surface Heat Flow
Conductive heat flow measured from bore-hole temperature gradient and conductivity
Surface heat flow 463 TW (1)
472 TW (2)
(1) Jaupart et al (2008) Treatise of Geophys.(2) Davies and Davies (2010) Solid Earth
mW m-2
40,000 data points
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after Jaupart et al 2008 Treatise of Geophysics
Mantle cooling(18±10 TW)
Crust R*(7±3 TW)
Mantle R*(13±4 TW)
Core(9±6 TW)
Earth’s surface heat flow 46 ± 3 (47 ± 2)
(0.4 TW) Tidal dissipationChemical differentiation
*R radiogenic heat
± are 1s.d. estimates
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- linear relation between heat flow and radioactive heat production- characteristic values for tectono-physiographic provinces.
Q = Q0 + Ab
0 2 4 6 8 10 120
20
40
60
80
100
120
140
160
180
EUS SN B & R
uW m-3
mW
m-2
Birch et al., (1968) (A)
(b)
(Q0)
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Q = Q0 + Ab
1 Baltic Shield2 Brazil Coastal3 Central Australia4 EUS Phanerozoic5 EUS Proterozoic6 Fennoscandia7 Maritime8 Piedmont9 Ukraine10 Wyoming11 Yilgarn
Mahesh Thakur & David Blackwell (in press)
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Kalihari Slave
Pre
ssur
e (G
Pa)
Lesotho
Kimberley
Letlhakane
JerichoLac de GrasTorrieGrizzly
Depth (km
)
Best Fit Kalihari
50
100
150
200
250
300
0
2
4
6
8
100 200 400 600 800 1000 1200 1400 1600 200 400 600 800 1000 1200 1400 1600
Temperature (oC)Temperature (oC)
Archean lithosphere is thick & cold
From Rudnick & Nyblade, 1999
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Lee et al (2011, Ann Rev)
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Fischer et al (2010, Ann Rev)
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Age of CLM
Lee et al (2011, AnnRev) Pearson and Witting (2008, GSL)
Isotope systems
NO: U-Pb, Sm-Nd, Rb-Sr, Lu-Hf (incompatible element systems)
YES: Re-Os (compatible element systems)
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“Alumina-chron”
Data filter: - No peridotites with less than 0.5 ng/g Os plotted- No samples analyzed by sparging.
Al2O3 (wt. %)
187Os/ 188Os
PUM
J.G. Liu et al., 2009; 2011
TRD (Ga)0.5
2.5
1.0
1.5
2.0
Yangyuan Peridotites, North China Craton
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Hannuoba Peridotites,Central Zone:1.9 Ga lithosphere
PUM
0.116
0.120
0.124
0.128
0.132
0 0.1 0.2 0.3 0.4
2 sigma error< spot size
Age = 1.94 ± 0.18GaInitial = 0.1155 ± 0.0008
Initial gOs = 0MSWD = 23
187Re/188Os
187Os/ 188Os
Gao et al., 2002, EPSL
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Sm-Nd isotopes do not tell you about the age of the CLM
McDonough (1990, EPSL)
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Lithospheric Mantle samples: Oc. vs Cont.
- On-Craton xenoliths - Archean
- Off-Craton xenoliths* - post-Archean
- Massif peridotites - post-Archean
- Abyssal peridotites - Phanerozic
- Oceanic Massifs - Phanerozic
*no compositional distinction in Protoerzoic and Phanerozoc Off-Craton
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*
Mineralogy of the Lithospheric Mantle
Olivine
ClinopyroxeneOrthopyx
mafic
ultramafic
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Mafic assemblages in the CLM
Pyroxenites versus Eclogites
- Archean roots have distinctive assemblages
- Diversity of d18O values (evidence for recycling)
- Probably ~5% by mass in CLM (…squishy #)
- Which ones are lower crustal vs those resident in the CLM? …. what is the Moho?
Mafic lithologies are there, but what to do with them? – they do not dominant CLM chemical budget
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Significant findings:
- Cratonic roots are melt residues of circa ≤ 30% depletion
- Off-cratonic regions are dominantly post-Archean, with no chemical distinction in suites over the last 2.5 Ga
- Melt depletion occurred at <3 GPa in all regions
- Re-Os system yield robust ages for the CLM that can be correlated with the ages of local surface rocks
- No evidence for vertical compositional gradients in the CLM
- CLM growth during crustal genesis via residual diapiric emplacement (conductive cooling additions – negligible)
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Spinel- facies mineralogy
(<70 km)
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Garnet- facies mineralogy
(>70 km)
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Lee et al (2011, AnnRev)
Olivine is important
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MassifOff-craton
On-craton dunite
Prim. Mantle
meltingtrend
Secular decrease in the ambient mantle temperature – resulted in lower degrees of depletion in the CLM
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Lee et al (2011, AnnRev)
Mafic Lithologies
pyroxenites eclogites
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Median composition of the CLM
OPX-enrichment is secondary: melt addition or cumulate control
* In Kaapvaal, less so Siberian, much less elsewhere is the CLM OPX-enriched
*
- System is modeled w/ differ ratios of “basalt” + residue = PM- Fe-depletion @ hi melt depletion most bouyant residues
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Composition of the CLM: trace elementsTreatment of data:
non-gaussian distributionaverage (not a good measure) median (better) log-normal avg (better, will equal mode)
Sampling biases:fraction of ultramafic to mafic analytical (below detection (reported?), not measured)geological samplingsampling by geologistsinfiltration by host magma, weathering of xenoliths
Is it an enriched mantle region?- mantle metasomatism?- source of basalts?
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Characterization of elements in peridotites
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Compatible to mildly incompatible elements
Di = Ci in residue/Ci in melt
Di > 1, compatible element
Di <1, incompatible element
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Highly incompatible elements
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K, in Peridotites:Lithospheric Mantle
Heat Producing Elements
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McDonough (1990, EPSL)
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REE composition of CLM (median values only)
LREE-enrichmentnot strong
MREE ~ Primitive Mantle
Cratons are strongly HREE-depleted
Most depleted is most enriched – not explained feature
Primitive mantle normalized
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McDonough (2000, EPSL)
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Incompatible elements in CLM (median values only)
K-depletion - low % partial melt metasom.
~ Primitive Mantle
We can build a complete picture of elements in CLM!
Primitive mantle normalized
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SiFeMn
MgNiIr
YbCaSc
NdZrTi
ThNbLa
AlGaRe
Incompatible element Budget in CLM
Places limits on heat production in CLM
degree of depletionConstrained from Ca, Al & Ti
Integration of major, minor and trace elements
compatibles, never >factor 2 times PM
Primitive mantle normalized
two-stage production of composition
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Reservoir Thickness (km)
Mass (1022 kg) Mass % U (ng/g)
±U (ng/g)
%U (%)
Continental crust 40 2.17 0.54% 1300 30% 35%
Cont. Lithospheric Mantle ~160 8 2% 30 50% 3%
Mantle (all else down there) 2695 395 98% 13 20% 62%
Silicate Earth 2895 404.3 100% 20 -- 100%
Attributes of Continental Crust and Lithospheric Mantle
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For cratonic & off-cratonic regions- melt depletion is a continuum with no significant differences in time or space (also cannot identify regional distinctions*)
- OPX-enrichment is an overprinted feature found in some cratons and is dominant in the Kaapvaal cratonic and immediate off-cratonic area
- residual peridotites were produced at <3 GPa and have been overprinted by low degree undersaturated melts
- CLM is not a significant chemical reservoir, for the Earth’s budget its compositional contribution = mass contribution
(*Large scale perspective, regional features not highlighted)
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For cratonic & off-cratonic regions- elements show a non-normal log distribution
- median composition characterizes the abundances of the moderately to highly incompatible trace elements in the Lithospheric Mantle (Oceanic and Cont.)
- absence of chemical signature in CLM for growth in convergent margin settings
- the absence of this signature does not mean the CLM was not developed dominantly in such a tectonic setting
- Stability of CLM…. this is another lecture, but let’s discuss!
Thank you.