What is the Lithosphere: it is not the asthenosphere Lithosphere: mechanical boundary layer, dry-mostly, stable for 10 8 -10 9 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 10 4 -10 5 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).

lithosphere-asthenosphere boundary (LAB) properties crust mantle w/ melt Fischer et al (2010, Ann Rev)

Eaton et al (2009, Lithos)

Mantle Crust

Composition of the lithospheric mantle

Approaches

geophysics: seismology, gravity, heat flow, tectonics (rheology, deformation, uplift, erosion) geochemistry: petrography, elemental, isotopic Sampling the lithospheric mantle

Approaches

geophysics: 10 3 – 10 6 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

one example

The different Lithospheres

LID Chemical Mechanical Thermal Seismological Tectosphere Bottom: asthenosphere (LAB) Top: MOHO (seismic) petrologic break Oceanic Continental: craton vs off-craton

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 Depth D(m) = 2500 +350t 1/2 q = 480/t 1/2 Heatflow Wei and Sandwell 2006 Tectonophysics

Continental Lithospheric Mantle CLM 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?

40,000 data points Earth’s Total Surface Heat Flow Conductive heat flow measured from bore-hole temperature gradient and conductivity Surface heat flow 46  3 TW (1) 47  2 TW (2) mW m -2 (1) Jaupart et al (2008)

Treatise of Geophys.

(2) Davies and Davies (2010)

Solid Earth

Earth’s surface heat flow 46

±

3 (47

±

2)

Mantle cooling (18 ± 10 TW) Crust R* (7 ± 3 TW) Core (9 ± 6 TW) Mantle R* (13 ± 4 TW) *R radiogenic heat

after Jaupart et al 2008 Treatise of Geophysics

(0.4 TW) Tidal dissipation Chemical differentiation ±

are 1s.d. estimates

- linear relation between heat flow and radioactive heat production - characteristic values for tectono-physiographic provinces. 180 160 (b) 140

Q = Q

0

+ Ab

120 100 80 (Q 0 ) 40 20 60 0 0 Birch et al., (1968) 2 4 6

uW m -3

(A) 8 EUS SN 10 B & R 12

Q = Q 0 + Ab 1 Baltic Shield 2 Brazil Coastal 3 Central Australia 4 EUS Phanerozoic 5 EUS Proterozoic 6 Fennoscandia 7 Maritime 8 Piedmont 9 Ukraine 10 Wyoming 11 Yilgarn Mahesh Thakur & David Blackwell (in press)

Archean lithosphere is thick & cold

0

Kalihari Slave

50 2 100 4 6 8 Lesotho Kimberley Letlhakane

Best Fit

10 0 200 400 600 800 1000 1200 1400 1600

Temperature ( o C)

150 Jericho Lac de Gras Torrie Grizzly 200

Kalihari

250 200 400 600 800 1000 1200 1400 1600

Temperature ( o C)

300

From Rudnick & Nyblade, 1999

Lee et al (2011, Ann Rev)

Fischer et al (2010, Ann Rev)

Age of CLM

Isotope systems NO: U-Pb, Sm-Nd, Rb-Sr, Lu-Hf (incompatible element systems) YES: Re-Os (compatible element systems) Lee et al (2011, AnnRev) Pearson and Witting (2008, GSL)

187 Os/ 188 Os

“Alumina-chron”

Yangyuan Peridotites, North China Craton PUM T RD 0.5

(Ga) 1.0

1.5

2.0

2.5 Al 2 O 3 (wt. %) Data filter: - No peridotites with less than 0.5 ng/g Os plotted - No samples analyzed by sparging.

J.G. Liu et al., 2009; 2011

Hannuoba Peridotites,Central Zone: 1.9 Ga lithosphere

0.132

PUM

2 sigma error < spot size

0.128

187 Os/ 188 Os

0.124

0.120

0.116

Gao et al., 2002, EPSL

0 0.1

0.2

Age = 1.94 ± 0.18Ga

Initial = 0.1155 ± 0.0008

Initial g Os = 0 MSWD = 23 0.4

187 Re/ 188 Os

0.3

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 - Off-Craton xenoliths* - Massif peridotites - Archean - post-Archean - post-Archean - Abyssal peridotites - Oceanic Massifs - Phanerozic - Phanerozic

*no compositional distinction in Protoerzoic and Phanerozoc Off-Craton

Mineralogy of the Lithospheric Mantle Olivine

*

ultramafic Orthopyx Clinopyroxene mafic

Mafic assemblages in the

CLM

Pyroxenites versus Eclogites - Archean roots have distinctive assemblages - Diversity of d 18 O 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)

Olivine is important Lee et al (2011, AnnRev)

Massif Off-craton On-craton dunite Prim. Mantle

melting trend Secular decrease in the ambient mantle temperature – resulted in lower degrees of depletion in the CLM

Mafic Lithologies pyroxenites eclogites Lee et al (2011, AnnRev)

Median composition of the

CLM

*

* 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

OPX-enrichment is secondary: melt addition or cumulate control

Composition of the CLM: trace elements

Treatment of data: non-gaussian distribution average (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 sampling sampling by geologists infiltration 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 D i = C i in residue/C i in melt D i > 1, compatible element D i <1, incompatible element

Highly incompatible elements

Heat Producing Elements

K, in Peridotites: Lithospheric Mantle

McDonough (1990, EPSL)

REE composition of CLM

(median values only)

Primitive mantle normalized

LREE-enrichment not strong MREE ~ Primitive Mantle Cratons are strongly HREE-depleted Most depleted is most enriched – not explained feature

McDonough (2000, EPSL)

Incompatible elements in CLM

(median values only) K-depletion - low % partial melt metasom.

~ Primitive Mantle

Primitive mantle normalized

We can build a complete picture of elements in CLM!

Incompatible element Budget in CLM

Places limits on heat production in CLM

two-stage production of composition

compatibles, never >factor 2 times PM

Primitive mantle normalized

degree of depletion Constrained from Ca, Al & Ti Th Nb La Nd Zr Ti Yb Ca Sc Al Ga Re Si Fe Mn Integration of major , minor and trace elements Mg Ni Ir

Attributes of Continental Crust and Lithospheric Mantle Reservoir Continental crust Cont. Lithospheric Mantle Mantle (all else down there) Silicate Earth Thickness (km) Mass (10 22 kg) Mass % U (ng/g) ±U (ng/g) % U (%) 40 2.17

0.54% 1300 30% 35% ~160 2695 2895 8 395 404.3

2% 98% 100% 30 13 20 50% 20% - 3% 62% 100%

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.