What is the Lithosphere: it is not the asthenosphere

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.