1 / 47

Messages from the core-mantle boundary? The role of D'' in layered-mantle evolution

Messages from the core-mantle boundary? The role of D'' in layered-mantle evolution. Igor Tolstikhin, Ian Kramers & Albrecht W. Hofmann. Prologue Message from cosmochemistry: Please upgrade your pyrolite composition. Ringwood‘s method:.

jela
Download Presentation

Messages from the core-mantle boundary? The role of D'' in layered-mantle evolution

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. Messages from the core-mantle boundary?The role of D'' in layered-mantle evolution Igor Tolstikhin, Ian Kramers & Albrecht W. Hofmann

  2. PrologueMessage from cosmochemistry:Please upgrade your pyrolite composition

  3. Ringwood‘s method: • Mantle peridotites have been depleted by the removal of melt.Original mantle composition can be reconstituted by recombining the melt with the depleted peritotite.Idea basically sound, but afflicted by the uncertainty of the exact compositions and the required proportion of peridotite and melt • Ringwood‘s pyrolite is obsolete

  4. Cosmochemical Estimates of Mantle Composition • The silicate Earth consists of carbonaceous chondrites • Minus the elements now residing in the core • Minus the elements lost by volatilization

  5. Method of Palme & O‘Neill,2003, Treatise on Geochemistry FeO in mantle peridotite nearly constant: FeO = 8.1% Because FeO is not significantly affected by melt extraction

  6. MgO in Silicate Earth

  7. The five major oxides

  8. Pyrolite versus Bulk Silicate Earth Recommended pyrolite upgrades: McDonough, W. F. and S.-S. Sun (1995). The composition of the Earth. Chem. Geol.120, 223-253. Palme, H. and O‘Neill, H. St.C. (2003) Cosmochemical estimates of mantle composition. In Treatise on Geochemistry. Vol. 2.01.

  9. FUNDAMENTAL UNSOLVED PROBLEM OF EARTH SCIENCE

  10. Role of mineral physics

  11. Standard 2-layer model preferred by geochemists(Allègre version)

  12. Smith &Sandwell Map of the Oceans Old problem: Geochemical contrasts between ridges and plumes

  13. Geochemical Evidence Favoring 2-Layer Mantle

  14. The geochemical constraints are not new, but they have not gone away, even though mineral physics no longer requires separate mantle reservoirs.

  15. Nd-Sr Isotope Comparison: MORB versus OIB MORB data: PetDB (Su, Y.J. Dissertation Columbia Univ., 2002 OIB data: Albarède, Introduction to Geochemical Modeling, 1995.

  16. 4He/3He distributions in Plumes and MORB Tolstikhin 3He/4He frequency 3He = primordial, solar 4He = radiogenic, a particles MORB-source low in 3He: „relatively degassed“ OIB-source high in 3He: „relatively undegassed“ Tolstikhin & Hofmann, (2005) PEPI, 148, 109-130.

  17. Neon isotopes in MORB and OIB Graham, D.W. 2003, in Porcelli et al. Noble Gases in Geochemistry and Cosmochemistry. Reviews in Mineralogy and Geochemistry. Min. Soc. Am. 47: 247-317

  18. Xenon isotope production from extinct radioactivity

  19. 129Xe/130Xe - 136Xe/130Xe in MORB and OIB The 129Xe excess requires very early mantle degassing and preservation of a very ancient noble gas reservoir

  20. Contradictory closure ages for 2-stage Xe degassing models Assume Complete Xe loss until T(clo) Complete retention after T(clo) Closure age for 129Xe production ≤ 150 Ma Closure age for 136Xe production ≥ 550 Ma This discrepancy requires existence of two Xe reservoirs

  21. Three-Reservoir Mass Balance

  22. Two-layer noble gas degassing evolution model Variations of this model developed by several authors, e.g. Tolstikhin, Kellogg, Porcelli, Allègre et al. Almost all of the action takes place in the upper mantle. Tiny whiffs of primordial noble gas from the nearly undegassed lower mantle are entrained in mantle plumes, and they leak into the upper mantle.

  23. O‘Nions & Tolstikhin, 1996, Results Result of the two-layer degassing model: Mass transfer from the lower mantle to the upper mantle,described by the coefficient a, is less than 1% of the mass of the lower mantle per Ga.This is at least 50 times less than the present-day subduction flux.Results provide strong support for two-layer convection with little mass exchange between layers But they are based on the assumption that the deep reservoir represents the entire lower mantle below 670km

  24. EVIDENCE FAVORING WHOLE-MANTLE CONVECTION

  25. Grand & van der Hilst (1997) Seismic tomography: Strong evidence for whole-mantle Circulation. Conventional two-layer convection is no longer tenable Seismic Tomography: Deep subduction of Farallon Slab Figure 1. Cross sections of mantle P-wave (A) and S-wave (B) velocity variations along a section through the southern United States. The endpoints of the section are 30.1°N, 117.1°W and 30.2°N, 56.4°W. The images show variations in seismic velocity relative to the global mean at depths from the surface to the core-mantle boundary. Blues indicate faster than average and reds slower than average seismic velocity. The large tabular blue anomaly that crosses the entire lower mantle is probably the descending Farallon plate that subducted over the past ~100 m.y. Differences in structure between the two models in the transition zone (400 to 660 km depth) and at the base of the mantle are probably due to different data sampling in the two studies.

  26. Finite Frequency Tomographic Images of Deep-Mantle Plumes Montelli et al., 2004, Science 303, 338-343

  27. Larson: God‘s kitchen

  28. How can we reconcile the contradictory evidence for whole-mantle and layered-mantle convection?

  29. Kellogg‘s Stealth Layer Kellogg et al. 1999, Science, 283, 1881-1884: A smaller, seismologically invisible „stealth layer“

  30. VLVP Seismic Characteristics • A very broad region. • A large negative shear velocity gradient (-2% - -12%). • Steeply dipping edges. • Rapidly varying thickness and geometry over small distances. • The maximum P velocity reduction is -3%.

  31. BUT WE NEED A VERY ANCIENT RESERVOIR THAT PRESERVES PRIMORDIAL (≥4.5 Ga) COMPOSITION

  32. Rationale for Model of Tolstikhin & Hofmann(PEPI, 148, 109-130, 2005)

  33. New Model: Noble gases stored in primitive D‘‘ layer Tolstikhin model-2

  34. Creation of Ancient Chondritic Reservoir in D‘‘

  35. Evolution of Nd Isotopes in the Mantle

  36. New Evidence from 142Nd, the decay product of extinct 146Sm (t1/2 = 100 Ma) Measurable differences in the abundance of 142Nd can only be produced in the first 0.5 Ga of Earth evolution.

  37. 142Nd produced by extinct 146Sm Non-chondritic 142Nd Isotopes in Crust-Mantle Requires ancient hidden reservoir (D‘‘ ?) with low Sm/Nd ratio. Science, in press

  38. Consequence for 143Nd isotope mantle evolution Hidden reservoir (D‘‘ ?) with low Sm/Nd ratio must be ancient and small Boyet & Carlson, Submitted to Science The rest of the mantle is depleted (=high Sm/Nd) and MORB-like

  39. Conclusions - 1 • D‘‘ formed about 4.5 Ga ago, containing: • (1) Subducted basaltic crust (80 %) containing heat-producing Th, U, K, plus other incompatible elements • (2) Fe-rich chondritic material (20 %) causing high density • (3) Implanted solar noble gases trapped in this layer

  40. Dense Dregs at Base of the MantleChristensen & Hofmann, 1994, JGR

  41. Subsequent evolution • Very slow loss by entrainment in plumes • 80% remains in D‘‘ • 20 % of original D‘‘ reservoir has been • entrained by uprising mantle flow • mixed with mantle

  42. Evolution of Mass Fluxes

  43. D‘‘- Mantle - Crust - Atmosphere Mass Balance

  44. Role of melting at the base? • Hotter early mantle + superheated core • leads to partial melting in lower mantle. • If melt is denser than solid, it will migrate downward and create a chemically dense layer at the base of mantle.

  45. D‘‘ Eutectic (Boehler) Dense Melt T Magnesiowüstite (Mg,Fe)O Perovskite MgSiO3

  46. Conclusions - 3 • Partially molten lowermost mantle • High density because of melt composition • Melt must segregate downward to create denser bulk composition in D‘‘ • May have solidified or still be molten in part

  47. Three Wise Men (Jacoby) Wolfgang Jacoby

More Related