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
Fischer et al (2010, Ann Rev)
lithosphere-asthenosphere boundary (LAB) properties
crustmantle
w/ melt
Eaton et al (2009, Lithos)
Mantle
Crust
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
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
LIDChemicalMechanicalThermalSeismological
Tectosphere
Bottom: asthenosphere (LAB)
Top: MOHO (seismic)petrologic break
Oceanic Continental: craton vs off-craton
The different Lithospheresone example
Where are the cratons and off-cratons
Pearson and Witting (2008, GSL)
Where are the cratons and off-cratons
Lee et al (2011, Ann Rev)
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
Oceanic & Continental
Crusts
60% of Earth’s surface consists of oceanic crust
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)
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
Continental Lithospheric MantleCLM growth models
Lee et al (2011, Ann Rev)
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?
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
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
- 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)
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)
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
Lee et al (2011, Ann Rev)
Fischer et al (2010, Ann Rev)
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)
“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
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
Sm-Nd isotopes do not tell you about the age of the CLM
McDonough (1990, EPSL)
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
*
Mineralogy of the Lithospheric Mantle
Olivine
ClinopyroxeneOrthopyx
mafic
ultramafic
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
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)
Spinel- facies mineralogy
(<70 km)
Garnet- facies mineralogy
(>70 km)
Lee et al (2011, AnnRev)
Olivine is important
MassifOff-craton
On-craton dunite
Prim. Mantle
meltingtrend
Secular decrease in the ambient mantle temperature – resulted in lower degrees of depletion in the CLM
Lee et al (2011, AnnRev)
Mafic Lithologies
pyroxenites eclogites
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
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?
Characterization of elements in peridotites
Compatible to mildly incompatible elements
Di = Ci in residue/Ci in melt
Di > 1, compatible element
Di <1, incompatible element
Highly incompatible elements
K, in Peridotites:Lithospheric Mantle
Heat Producing Elements
McDonough (1990, EPSL)
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
McDonough (2000, EPSL)
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
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
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
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)
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.
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