Paradigms versus paradoxes: Developing a new paradigm for

the mantle

Attreyee Ghosh, Ricardo Arevalo Jr., Ved Lekic, and Victor Tsai

withAdam Dziewonski, Barbara Romanowicz,

Louise Kellogg, and Wendy Panero

(names arranged in alphabetical order by first name)

Geochemistry likes a layered mantle

• DMM cannot account for the planet’s budget of:– Incompatible elements

• UDMM + UCC ≠ UBSE

– Radiogenic heat production• HDMM + HCC ≠ HBSE

– Noble gas abundances• 40ArDMM +

40ArCC + 40Aratm ≠ 40ArBSE

Seismology finds a variety of behaviors for slabs in the transition zone

• Tomographic images illustrate mass flux across the 660 km discontinuity

Li et al. (2008)Van der Hilst et al. (1998)

Li et al. (2008)

Geodynamics like well-mixed mantle reservoirs

• Mantle layering difficult to maintain for multiple Ga without significant mixing

Naliboff and Kellogg (2007)

Hofmann (1997)

2 layers traditionallimited exchange

Hybrid limitedexchange

1 layer whole-mantle convection

2 layers isolated upperand lower mantle reservoirs

Sobolev et al. (2005)

“Marble-cake mantle”

Morgan & Morgan (1999)

“Plum-pudding mantle”

Becker et al. (1999)

“Blob mantle”

What about this ‘D”’?

• Several geochemical studies have called upon an early, differentiated reservoir that has remained “hidden” at the core-mantle boundary

Boyet and Carlson (2006)

• Seismology and mineral physics observations indicate a heterogeneous layer at the core-mantle boundary

What about this ‘D”’?

Power Spectra

Lee et al., (2007)

1000 1500 2000 2500 3000

5

10

15

20

25

Temperature oC

Zone of neutrally or negatively buoyant melt

Lee and Luffi

Transition Zone

150 km

410’

660’

Pres

sure

(GPa

)

Tolstikhin and Hofmann (2005)

What about the role of thermochemical piles/superplumes?

• Increasing the volume of a deep mantle reservoir (e.g., including superplumes) dilutes the required incompatible/ radioactive element budget of this reservoir

2800 km depth seismic profiles

Kustowski (2006)

Upper mantle:Q - lower mantle: VshDegree 2 only

Romanowicz and Gung (2002)

Romanowicz and Gung (2002)

Defining the volume of a superplume

superplumes in S362ANI (1% slow anomaly)

Defining the volume of a superplume

Area (sq km)

Dep

th (

km)

S362ANI (0.6% contours)

Area (sq km)

Dep

th (

km)

SAW24B16 (1% contours)

Defining the volume of a superplume

Conservative estimates only consider depths >1000 km

Geochemical implications

• If we know the composition of the Continental Crust (CC; e.g., Rudnick and Gao, 2003), the Depleted MORB Mantle (DMM; e.g., Su, 2002) and the Bulk Silicate Earth (BSE; McDonough and Sun, 1995)…– The size of the DMM dictates the required

composition of a deep, Enriched Mantle Reservoir (EMR)

Geochemical implications

*Concentration range calculated from uncertainties in compositional models of CC and DMM

Thermal implications

Table 8 from Van Schmus (1995)Element Potassium ThoriumIsotope 40K 232Th 235U 238UIsotopic Abundance (wt%) 0.0119 100 0.71 99.28Decay Constant, λ (yr-1) 5.54E-10 4.95E-11 9.85E-10 1.55E-10Total Decay Energy (MeV/decay) 1.34 42.66 46.40 51.70Beta Decay Energy (MeV/decay) 1.181 3.5 3 6.3Beta Energy Lost as Neutrinos (MeV/decay) 0.65 2.3 2 4.2Total Energy Retained in Earth (MeV/decay) 0.69 40.4 44.4 47.5Specific Isotope Heat Production (cal/g-year) 0.22 0.199 4.29 0.714Specific Isotope Heat Production (μW/kg) 29.17 26.38 568.7 94.65Specific Elemental Heat Production (cal/g-year) 2.60E-05 0.199Specific Elemental Heat Production (μW/kg) 3.45E-03 26.38

Uranium

0.7498.1

Thermal implications

Maintaining neutral buoyancy

• Assume:– Fe is most important chemical variant– Fe has no effect on modulus or

thermal expansion– Thermal and chemical effects are

linear wrt velocity– Fe partitioning between mw and pv– Fe has a linear effect on density:

Chemical

Thermal

•Solve:

Chemical

Thermal

0

lnvs

Chemical lnvs

Thermal lnvs

Seismic

(Mg ,Fe )O (xFe)x

FeFeO (1 x

Fe)MgO

Stixrude & Lithgow-Bertelloni, 2005

What does this mean?

• Uncertainty in partitioning behavior has a first order effect

• Velocity drop at base of the mantle is >2.5%– Additional 1.5% Fe (reasonable)– Excess temperature of 450-700 K

• Velocity drop in mid-mantle is ~1%– Additional 0.5% Fe– Excess temperature of 180-275 K

• Super piles are neither on constant adiabat or isochemical if they are neutrally buoyant

Future questions to address

• How stationary are these superplumes?– Do surface tectonics dictate the large scale

flow in the mantle, or vice-versa?– Slab reconstructions (over the last 200 Ma)

and degree-2 signals are well correlated

Slab model of L-B & R (1998) Vs model S362ANI (U

pper mantle

comparison)

(Mid-mantle depths)

Slab model of Lithgow-Bertelloni & Richards (1998)

Vs model S362ANI

• Slab model of Lithgow-Bertelloni & Richards (1998)

• Vs model S362ANI

• The degree-2 velocity anomalies at the CMB are extremely well correlated with the integrated slab signal: the sum of all the slabs deposited during the last 200 Ma.

(Comparison at CMB)

Future questions to address

• How long could such a thermochemical reservoir be dynamically stable for?– “Bottom-up” dynamical test

• Starting conditions: 2 rigid conical masses attached to the CMB - representative size of superplumes

• The transition zone must be able to arrest, at least temporarily, sinking subducted materials

• The convection experiments, spanning a sufficiently large parameter space would give us insight into lower mantle mixing and return flow

H=150 km B=2

H=500 km B=0.7

H=1000 km B=1

H=1600 km B=0.7

Some typical snapshots at t ~ 4.55Ga

Dynamic criteria: stability over several Ga, topography of the interface, net density, and magnetic field

Kellogg and Ferrachat

Future work/questions

• What is the mass flux of material into the lower mantle? Reaching D”?– How much becomes incorporated in our deep

reservoir?

Fukao et al. (2001)

A new paradigm

• We propose that the lowermost mantle pattern of the two chemically and thermally distinct super-plumes dictates the planform of mantle dynamics for at least the last 200 Ma.

• The superplumes may have stable locations for at long periods of time, anchoring mantle plumes and influencing the paths of Wilson cycles.

• The transition zone plays an important role in the interaction between subducted slabs and the superplumes

A new paradigm

• Transition zone may be a “leaky” boundary layer– Subducted slabs pond in the transition zone, with

sufficient residence time for some oceanic crust to be re-circulated in the upper mantle

– Ponded material breaks through the 660 km discontinuity in avalanche-like events and is deposited around the upwellings giving rise to the ring of fast velocities girdling the Pacific

– Low-pass filter removes high wavenumber features from slab signal

– Temperature contrast sufficient to produce plumes at the 660 km discontinuity

Questions?HawaiiGeneric

72 km

362 km

652 km

942 km

1377 km

2102 km

2827 km

Slab integration model of

Lithgow-Bertelloni and

Richards (1998)

Further Required Assumptions

• Fe partitioning between MgO and MgSiO3

Andrault, 2001

Assume D=5

Andrault D

Badro D (HS->LS)

• 40Ar produced by decay of 40K (t1/2 = 11.93 Gyr)– Too heavy to be lost from atm– >99.9% Ar is 40Ar

• We know:– 280 ppm K in equals >150 Eg (1018 g) of 40Ar produced over 4.5 Ga– 66 Eg in atm, 10-20 Eg may be in crust, the rest must reside in the mantle– 40ArBSE = 40Aratm + 40ArCC + 40ArDMM +

40ArEMR – 40Ardegassed

EMR = Enriched Mantle Reservoir

CC = Continental Crust