1 / 51

What is the Lithosphere: it is not the asthenosphere

What is the Lithosphere: it is not the asthenosphere.

taariq
Download Presentation

What is the Lithosphere: it is not the asthenosphere

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. 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 104-105a. It is aregion 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).

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

  3. Eaton et al (2009, Lithos)

  4. Mantle Crust

  5. 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

  6. 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

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

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

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

  10. 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

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

  12. 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)

  13. Depth Seafloor subsidence & heatflow reflect progressive thickening of lithosphere with age D(m) = 2500 +350t1/2 q = 480/t1/2 Heatflow Wei and Sandwell 2006 Tectonophysics

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

  15. 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 mWm-2 • - Near surface heat production • - Heat production versus depth • - Concentration of HPE in Lithospheric Mantle?

  16. Earth’s Total Surface Heat Flow 40,000 data points 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

  17. 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 (0.4 TW) Tidal dissipation Chemical differentiation after Jaupart et al 2008 Treatise of Geophysics ± are 1s.d. estimates

  18. linear relation between heat flow and radioactive heat production • - characteristic values for tectono-physiographic provinces. (b) Q = Q0 + Ab (Q0) (A) Birch et al., (1968)

  19. Q = Q0 + Ab 1 Baltic Shield 2 Brazil Coastal 3 Central Australia 4 EUS Phanerozoic 5 EUS Proterozoic 6Fennoscandia 7 Maritime 8 Piedmont 9 Ukraine 10 Wyoming 11 Yilgarn Mahesh Thakur & David Blackwell (in press)

  20. Jericho Lac de Gras Torrie Grizzly 0 200 400 600 800 1000 1200 1400 1600 200 400 600 800 1000 1200 1400 1600 Archean lithosphere is thick & cold 0 Kalihari Slave 50 2 100 4 Pressure (GPa) 150 Depth (km) 6 Lesotho 200 Kimberley Best Fit Kalihari Letlhakane 8 250 300 10 Temperature (oC) Temperature (oC) From Rudnick & Nyblade, 1999

  21. Lee et al (2011, Ann Rev)

  22. Fischer et al (2010, Ann Rev)

  23. 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)

  24. “Alumina-chron” YangyuanPeridotites, North China Craton PUM TRD (Ga) 0.5 187Os/ 188Os 1.0 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

  25. g Os Hannuoba Peridotites,Central Zone: 1.9 Ga lithosphere 0.132 PUM 2 sigma error < spot size 0.128 187Os/ 188Os 0.124 0.120 Age = 1.94 ± 0.18Ga Initial = 0.1155 ± 0.0008 Initial = 0 0.116 MSWD = 23 0 0.1 0.2 0.3 0.4 187Re/188Os Gao et al., 2002, EPSL

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

  27. Lithospheric Mantle samples: Oc. vs Cont. • On-Cratonxenoliths - 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

  28. Mineralogy of the Lithospheric Mantle Olivine * ultramafic mafic Orthopyx Clinopyroxene

  29. 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? Maficlithologies are there, but what to do with them? – they do not dominant CLM chemical budget

  30. 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)

  31. Spinel- facies mineralogy (<70 km)

  32. Garnet- facies mineralogy (>70 km)

  33. Olivine is important Lee et al (2011, AnnRev)

  34. Massif melting trend Off-craton dunite On-craton Prim. Mantle Secular decrease in the ambient mantle temperature – resulted in lower degrees of depletion in the CLM

  35. MaficLithologies pyroxenites eclogites Lee et al (2011, AnnRev)

  36. System is modeled w/ differ ratios of “basalt” + residue = PM • Fe-depletion @ hi melt depletion most bouyant residues Median composition of the CLM * * In Kaapvaal, less so Siberian, much less elsewhere is the CLM OPX-enriched OPX-enrichment is secondary: melt addition or cumulate control

  37. 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?

  38. Characterization of elements in peridotites

  39. Compatible to mildly incompatible elements Di = Ci in residue/Ci in melt Di> 1, compatible element Di<1, incompatible element

  40. Highly incompatible elements

  41. Heat Producing Elements K, in Peridotites: Lithospheric Mantle

  42. McDonough (1990, EPSL)

  43. 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

  44. McDonough (2000, EPSL)

  45. 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!

  46. 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

  47. Attributes of Continental Crust and Lithospheric Mantle

  48. 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 Kaapvaalcratonic 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)

More Related