Layered ConvectionJill Schleicher, Jonathan Sparks, Carl UlbergESS502 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

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).

Compositional Heterogeneities (MORB vs OIB)

Similar major element compositions. Differences in trace elements, isotopes,

gases (depleted vs. enriched)

Sun and McDonouch, 1989 Earth Ref Database

Seismic Discontinuities Several discontinuities, e.g. 410/660 km

(change in phase/density/chemistry/viscosity/temperature, all of which are sufficient for splitting convective cells)

PREM model,Dziewonski and Anderson, 1981

Silver et al, 1988

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)

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 1999Silver et al 1988

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