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Layered Convection

Layered Convection. Jill Schleicher, Jonathan Sparks, Carl Ulberg ESS502 2/16/12. A Reason for Choosing Layered Convection. Layered convection means no simple boundaries between convection layers We argue for complexity within the chemistry of the Earth, which points to ‘layered’ convection.

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Layered Convection

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  1. Layered Convection Jill Schleicher, Jonathan Sparks, Carl Ulberg ESS502 2/16/12

  2. A Reason for Choosing Layered Convection • Layered convection means no simple boundaries between convection layers • We argue for complexity within the chemistry of the Earth, which points to ‘layered’ convection

  3. Three arguments for complexity within the Earth • MORB vs OIB- evidence for compositional complexity (different source regions) • Seismic discontinuity at 660 km could be explained by property changes (Silver et al 1988)- Density barriers prevent whole-mantle convection but allow for subducting slabs and hot spots to cross • Mineral physics shows a 2% change in density between upper and lower mantle, which can lead to formation of separate convection cells (Knittle et al 1986).

  4. Compositional Heterogeneities (MORB vs OIB) Sun and McDonouch, 1989 Earth Ref Database • Similar major element compositions. • Differences in trace elements, isotopes, gases (depleted vs. enriched)

  5. Seismic Discontinuities PREM model, Dziewonski and Anderson, 1981 • Several discontinuities, e.g. 410/660 km (change in phase/density/chemistry/viscosity/temperature, all of which are sufficient for splitting convective cells) Silver et al, 1988

  6. Density Contrasts in the Mantle • Mineral physics analysis shows a 2% change in intrinsic density between the upper and lower mantle (Knittle et al 1986) • Change probably due to enrichment in Fe or Si • Density contrast would create separate convective cells but still allow for penetration of slabs Kellogg et al, 1997 Change in Vs due to enrichment of Fe (l) or Si (r)

  7. Possible Models • Silver et al made a model for 2 layer convection divided at the 660 km seismic discontinuity that still allows hot spots and subducting slabs to cross the barrier. • Kellogg model Kellogg et al 1999 Silver et al 1988

  8. Conclusion: Complexity in Mantle Suggests Complexity in Convection • Chemical heterogeneities throughout the mantle sourcing basalts • Seismic heterogeneities • Density heterogeneities • Further studies needed to constrain mantle convection dynamics • Whole mantle convection is a good first approximation

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