Compositional Stratification in the Deep Mantle

1. Literature Review of Compositional Stratification in the Deep Mantle Louise H. Kellogg, Bradford H. Hager, Rob D. van der Hilst

2. Say What? Model Constraints MORB/OIB Plumes Isotopic Ratios Subduction Depleted/Enriched Mantle Heat Flux D” Region D” Region Transition Zone “Dense” Layer Mineral Physics Heat Flux Model Constraints Density Contrasts

3. Ahh!…I think I see…. Nope…..I lost it.

4. Compositional Boundary • Modeling Constraints • Account for global heat flux. • Produce rich variety of mantle-derived basalts. • Consistent with “inferred” seismic tomography. • Long wavelength structure of lower mantle. • Complex relations between bulk sound and shear wavespeed. • Dynamically consistent.

5. Inferred Seismic Tomography From Van der Hilst et al., 1997

6. MORB Isotope Review • Recall that the parent isotope for 87Sr is 87Rb. • In depleted mantle the ratio of Rb to Sr will be low because Rb is more incompatible than Sr. • Over time a depleted mantle will produce less 87Sr than would an undepleted mantle. • Depleted mantle would be expected to show relatively low  87Sr/86Sr ratios. • Mantle with the composition of the Bulk Earth would be expected to have a 87Sr/86Sr ratio of about 0.7045. • MORBs have 87Sr/86Sr ratios in the range between 0.7020 and 0.7025 • Consistent with idea that MORB represents depleted mantle.

7. MORB Isotopic Review • 144Nd is a stable, nonradiogenic isotope of Nd. • Amount of 144Nd in any rock does not change with time. • Depleted mantle will produce more 143Nd than original bulk earth composition. • In magmas derived from melting of depleted mantle the 143Nd/144Nd ratio will increase to higher values than in an undepleted mantle of Bulk Earth composition. • 143Nd/144Nd ratio of the Bulk Earth is expected to be about 0.51268. • 143Nd/144Nd ratios of MORBs show higher values, ranging from 0.5130 to 0.5133.  • Also consistent with idea that MORB represents depleted mantle.

8. OIB Isotopic Review • Show higher values of 87Sr/86Sr and lower values of 143Nd/144Nd than MORB. • This indicates that the mantle source for the OIB is enriched in Rb relative to Sr and Nd relative to Sm. • Mantle source is considered to be “enriched” relative to MORB

9. MORB vs. OIB • MORB represents further depletion of a depleted mantle due to recycling in the shallow mantle. • OIB represents upwelling of heated, “enriched” mantle. • “Enriched” mantle the product of continental derived oceanic sediment which is subducted. • Cold, subducted slab sinks to depth near CMB, mixing with mantle.

10. Mass of Depleted Mantle • Mass of the depleted component of the mantle is estimated by isotopic comparisons of the crust and MORB. • Estimates range from 25% - 90%

11. Earth’s Heat Flux • Extra heat source is required to account for current heat flux. • 6 TW generated by the crust. • 38 TW through either heat generation or cooling. • D” Region • Anomalous seismic velocities several hundred kilometers thick. • Accounts for only small fraction. • Must look at transition in seismic heterogeneity.

12. 1600 km vs. 660 km

13. 1600 km vs. 660 km • In many models, 660 km boundary is interpreted to indicate phase change. • 660 km boundary has also been interpreted to represent convection boundary. • Convection boundary models are not supported by seismological evidence.

14. 1600 km vs. 660 km • Kellogg et al. proposes ~1600 km depth is transition zone of depleted MORB and enriched mantle. • OIB is product of mixing within the transition zone. • Plumes generated from thermal boundary between transition zone and “dense” layer.

15. 1600 km vs. 660 km From Van der Hilst and Karason, 1999

16. 1600 km vs. 660 km From Kellogg et al., 1999

17. Modeling of Mantle Convection • Viscosity is P/T dependent. • Heating of mantle from below. • Ra = 2 * 107 • Upper and lower mantle enriched with radiogenic elements. • Ra = 1.16 * 108 + 5.12 * 108

18. Modeling of Mantle Convection Temperature Model from Kellogg et al., 1999

19. Modeling of Mantle Convection Viscosity model from Kellogg et al., 1999

20. Modeling of Mantle Convection Density change due to Composition Density overestimation model

21. Limitations in Modeling • Difficult to resolve density variations over distances of <1000 km. • Temperature and thermal expansion coefficient of lower mantle needed. • Model is incompressible and has constant thermal expansivity.

22. Fe Enrichment vs. Si Content • Fe enrichment does not affect elastic modulus. • Main effect is wave speed reduction due to increased density. • Altering Si content changes mineral proportions. • Seismically fast even though it is hot. • Adding Si to Fe enriched region would reduce large velocity variations.

23. Fe Enrichment vs. Si Content Fe Enrichment Si Alteration

24. Mass of Undepleted Layer? • Difficult to determine due to tomography of higher density layer. • Mass will also be affected by tomography of underlying D” region. • More study is needed to analyze relationship between dense layer and D” layer.

25. Slab Penetration • Temperature and composition indicate slab reaching near the CMB • Difficult to image in the dense layer. • Slab may break up into “drips” at this depth. • Slab penetration to CMB is interpreted to be relatively rare, short-lived event.