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). 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: 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 The different Lithospheres one example 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 +350t1/2 q = 480/t1/2 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? Earth’s Total Surface Heat Flow 40,000 data points Conductive heat flow measured from bore-hole temperature gradient and conductivity Surface heat flow 463 TW (1) 472 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) Mantle R* (13±4 TW) *R radiogenic heat after Jaupart et al 2008 Treatise of Geophysics Core (9±6 TW) (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 (b) 160 Q = Q0 + Ab 140 mW m-2 120 100 80 EUS SN B&R 60 (Q0) 40 20 0 0 2 4 6 uW m-3 Birch et al., (1968) 8 (A) 10 12 Q = Q0 + 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 Slave 50 2 100 4 150 6 Lesotho Jericho Kimberley Lac de Gras Letlhakane 8 Best Fit 200 Depth (km) Pressure (GPa) Kalihari Kalihari Torrie 250 Grizzly 300 10 0 200 400 600 800 1000 1200 1400 1600 Temperature (oC) 200 400 600 800 1000 1200 1400 1600 Temperature (oC) 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) “Alumina-chron” Yangyuan Peridotites, North China Craton PUM TRD (Ga) 0.5 187Os/ 1.0 188Os 1.5 2.0 2.5 Al2O3 (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 187Os/ 0.124 188Os 0.120 Age = 1.94 ± 0.18Ga Initial = 0.1155 ± 0.0008 Initial g Os = 0 0.116 MSWD = 23 0 Gao et al., 2002, EPSL 0.1 0.2 187Re/188Os 0.3 0.4 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 * ultramafic mafic Orthopyx Clinopyroxene 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) Olivine is important Lee et al (2011, AnnRev) melting trend Massif Off-craton On-craton dunite Prim. Mantle 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 Di = Ci in residue/Ci in melt Di > 1, compatible element Di <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 two-stage production of composition Places limits on heat production in CLM 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 Mg Ni Ir Integration of major, minor and trace elements Attributes of Continental Crust and Lithospheric Mantle ±U U (ng/g) (ng/g) % Thickness (km) Mass (1022 kg) Mass % 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% Reservoir U (%) 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.