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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.

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Presentation on theme: "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."— Presentation transcript:

1 Lithosphere: mechanical boundary layer, dry-mostly, stable for a, possessing a steady-state conductive geotherm with base in cratons at 4-7 GPa (170–250 km), shallower (ca km) in off-cratons, and shallower still in oceans (<100 km) Asthenosphere: weak layer underneath the lithosphere, area with pervasive plastic deformation deforming over 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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3 Fischer et al (2010, Ann Rev) lithosphere-asthenosphere boundary (LAB) properties crust mantle w/ melt

4 Eaton et al (2009, Lithos)

5 Mantle Crust

6 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: – meters - 6 to 12 orders of magnitude difference

7 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

8 LID Chemical Mechanical Thermal Seismological Tectosphere Bottom: asthenosphere (LAB) Top: MOHO (seismic) petrologic break Oceanic Continental: craton vs off-craton The different Lithospheres one example

9 Where are the cratons and off-cratons Pearson and Witting (2008, GSL)

10 Where are the cratons and off-cratons Lee et al (2011, Ann Rev)

11 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

12 Oceanic & Continental Crusts 60% of Earth’s surface consists of oceanic crust

13 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)

14 Seafloor subsidence & heatflow reflect progressive thickening of lithosphere with age D(m) = t 1/2 q = 480/t 1/2 Depth Heatflow Wei and Sandwell 2006 Tectonophysics

15 Continental Lithospheric Mantle CLM growth models Lee et al (2011, Ann Rev)

16 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?

17 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) (1) Jaupart et al (2008) Treatise of Geophys. (2) Davies and Davies (2010) Solid Earth mW m -2 40,000 data points

18 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 dissipation Chemical differentiation *R radiogenic heat ± are 1s.d. estimates

19 - linear relation between heat flow and radioactive heat production - characteristic values for tectono-physiographic provinces. Q = Q 0 + Ab Birch et al., (1968) (A) (b) (Q 0 )

20 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)

21 KalihariSlave Pressure (GPa) Lesotho Kimberley Letlhakane Jericho Lac de Gras Torrie Grizzly Depth (km) Best Fit Kalihari Temperature ( o C) Archean lithosphere is thick & cold From Rudnick & Nyblade, 1999

22 Lee et al (2011, Ann Rev)

23 Fischer et al (2010, Ann Rev)

24 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)

25 “Alumina-chron” Data filter: - No peridotites with less than 0.5 ng/g Os plotted - No samples analyzed by sparging. Al 2 O 3 (wt. %) 187 Os/ 188 Os 188 Os PUM J.G. Liu et al., 2009; 2011 T RD (Ga) Yangyuan Peridotites, North China Craton

26 Hannuoba Peridotites,Central Zone: 1.9 Ga lithosphere PUM sigma error < spot size Age = 1.94 ± 0.18Ga Initial = ± Initial  Os = 0 MSWD = Re/ 188 Os 187 Os/ 188 Os 188 Os Gao et al., 2002, EPSL

27 Sm-Nd isotopes do not tell you about the age of the CLM McDonough (1990, EPSL)

28 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

29 * Mineralogy of the Lithospheric Mantle Olivine ClinopyroxeneOrthopyx mafic ultramafic

30 Mafic assemblages in the CLM Pyroxenites versus Eclogites - Archean roots have distinctive assemblages - Diversity of  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

31 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)

32 Spinel- facies mineralogy (<70 km)

33 Garnet- facies mineralogy (>70 km)

34 Lee et al (2011, AnnRev) Olivine is important

35 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

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

37 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 hi melt depletion most bouyant residues

38 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?

39 Characterization of elements in peridotites

40 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

41 Highly incompatible elements

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43 K, in Peridotites: Lithospheric Mantle Heat Producing Elements

44 McDonough (1990, EPSL)

45 REE composition of CLM (median values only) LREE-enrichment not strong MREE ~ Primitive Mantle Cratons are strongly HREE-depleted Most depleted is most enriched – not explained feature Primitive mantle normalized

46 McDonough (2000, EPSL)

47 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

48 Si Fe Mn Mg Ni Ir Yb Ca Sc Nd Zr Ti Th Nb La Al Ga Re Incompatible element Budget in CLM Places limits on heat production in CLM degree of depletion Constrained 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

49 Reservoir Thickness (km) Mass (10 22 kg) Mass %U (ng/g) ±U (ng/g) % U (%) Continental crust %130030%35% Cont. Lithospheric Mantle ~16082%3050%3% Mantle (all else down there) %1320%62% Silicate Earth % % Attributes of Continental Crust and Lithospheric Mantle

50 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)

51 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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