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:

• 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• Ringwood‘s pyrolite is obsolete

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

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

MgO in Silicate Earth

Olivines from fertile peridotites have Mg-numbers(Mg# = molar MgO/(MgO+Feo) = 0.888 to 0.896.

Assume:Average whole-rock Mg# = olivine Mg# for fertile peridotite

Mg# = 0.890 ± 0.001

Then:MgO = (FeO x 0.5610 x Mg#)/(1 – Mg#)

MgO = 36.77 ±0.44 wt%

The five major oxides

Only 5 oxide components make up 98.4 % of the total mantle

MgO + Al2O3 + SiO2 + CaO + FeO = 98.41 wt%

SiO2 = 45.4% obtained from SiO2 – MgO correlation

Al2O3 from mass balance and the chondritic ratio CaO/Al2O3 = 0.813

Al2O3 = (98.41 – MgO – SiO2 – FeO)/(1 + 0.813)

Al2O3 = 4.49%

CaO = 3.65 % obtained from chondritic CaO/Al2O3 = 0.813

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.

FUNDAMENTAL UNSOLVED PROBLEM OF EARTH SCIENCE

MANTLE CONVECTION IN SINGLE-LAYER OR TWO-LAYER MODE?

GEOPHYSICAL EVIDENCE FAVORS “WHOLE-MANTLE” CONVECTION

GEOCHEMICAL EVIDENCE FAVO RS TWO-LAYER MANTLE)

RELATED QUESTION: ORIGIN OF PLUMES:

FROM BASE OF THE MANTLE?

OR

FROM BASE OF UPPER MANTLE?

Role of mineral physics

Potentially fundamental differences in mineral properties between upper and lower

mantle that would prevent or inhibit convective exchange between the two regions:

Major-element chemical differences causing differences in chemical

density

Clapeyron slope of phase transition at 660 km

Where are they now?

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

Smith &Sandwell Map of the Oceans

Geochemical contrasts between ridges and plumes

Old problem:

(1) Isotopic differences between plumes and MORB: Different source reservoirs

(2) Mass balance for incompatible-element budgets between continental crust and

depleted mantle:

Requires additional (primitive or enriched) reservoir

(3) MORB source produces no significant heat, but heat budget requires deep source.

Related old problem: Imbalance between helium and heat delivered by MORB

(4) Primordial (or solar) He and Ne isotopes in plumes, but not in MORB

(5) Xenon isotopes derived from short-lived, extinct radioactivity of 129I and 244Pu,

and from fission of long-lived 238U require 2 separate xenon reservoirs

Geochemical Evidence Favoring 2-Layer Mantle

The geochemical constraints are not new, but they have not gone

away, even though mineral physics no longer requires separate mantle

reservoirs.

Nd-Sr Isotope Comparison: MORB versus OIB

-5

0

5

10

0.7020 0.7025 0.7030 0.7035 0.7040 0.7045 0.7050 0.7055 0.7060

87Sr/86Sr

143Nd/144Nd

MORB, ridge segment aves.

OIB Averages

MORB data: PetDB (Su, Y.J. Dissertation Columbia Univ., 2002OIB data: Albarède, Introduction to Geochemical Modeling, 1995.

Tolstikhin 3He/4He frequency

4He/3He distributions in Plumes and MORB

MORB-source low in 3He: „relatively degassed“

OIB-source high in 3He:„relatively undegassed“

3He = primordial, solar4He = radiogenic, particles

Tolstikhin & Hofmann, (2005) PEPI, 148, 109-130.

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

Xenon isotope production from extinct radioactivity

Xenon isotopes produced by decay of iodineand by fission of uranium and plutonium

129I → 129Xe (T1/2 = 16 Ma)

238U → 136,134Xe (spontaneou sfissio n T1/2 = 4.47 Ga)

244Pu → 136,134Xe (spontaneou sfissio n T1/2 = 82 Ma)

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

The 129Xe excess requires very early mantle degassing

and preservation of a very ancient noble gas reservoir

Contradictory closure ages for 2-stage Xe degassing models

Closure age for 129Xe production ≤ 150 Ma

Closure age for 136Xe production ≥ 550 Ma

Closure age for 129Xe production ≤ 150 Ma

Closure age for 136Xe production ≥ 550 Ma

Assume

Complete Xe loss until T(clo)

Complete retention after T(clo)

Assume

Complete Xe loss until T(clo)

Complete retention after T(clo)

This discrepancy requires existence of two Xe reservoirs

This discrepancy requires existence of two Xe reservoirs

Classical Geochemical Model:

Earth Consists of Three Reservoirs Only

1. Continental Crust = 0.5%

2. Depleted Mantle = 30 – 70 %

3. Primitive Mantle = 70 – 30 %

(4. No enriched source, e.g. OIB source, subducted crust etc.)

Results appeared consistent with compositional

boundary at 660 km

Many authors using different degrees of sophistication, e.g.Jacobsen & Wasserburg, 1979;O’Nions et al., 1979;DePaolo, 1980;Davies, 1980;Allègre et al., 1983, 1986;Hofmann et al., 1986.

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.

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 , 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

Result of the two-layer degassing model: Mass transfer from the lower mantle to the upper mantle,described by the coefficient , 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

EVIDENCE FAVORING WHOLE-MANTLE CONVECTION

(1) SEISMIC (TOMOGRAPHY) EVIDENCE FOR DEEP SUBDUCTION AND DEEP PLUMES

(2) MINERAL PHYSICS: LOW CLAPEYRON SLOPE MEANS THAT PHASE BOUNDARY AT

660 KM M AY SLOW, BUT DOES NOT STOP VERTICAL MASS MOVEMENT.

(3) MINERAL PHYSICS AND SEISMIC DATA CONSISTENT WITH SINGLE (“PYROLIT E”)

BULK COMPOSITION ABOVE AND BELOW THE 660 KM DISCONTINUITY

(4) VOLUME AND DIAMETE R OF LARGE IGNEOUS PROVINCES, INTERPRETED AS

PLUME HEADS, PROBABLY REQUIRE DEEP MANTLE ORIGIN.

Seismic Tomography: Deep subduction of Farallon Slab

Grand & van der Hilst (1997)

Seismic tomography:Strong evidence for whole-mantleCirculation.

Conventional two-layer convectionis no longer tenable

Figure 1. Cross sections of mantle P-wave (A) and S-wave (B) velocity variations along a sectionthrough 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 thesurface to the core-mantle boundary. Blues indicate faster than average and reds slower than averageseismic velocity. The large tabular blue anomaly that crosses the entire lower mantle is probably thedescending Farallon plate that subducted over the past ~100 m.y. Differences in structure between thetwo models in the transition zone (400 to 660 km depth) and at the base of the mantle are probablydue to different data sampling in the two studies.

Finite Frequency Tomographic Images of Deep-Mantle Plumes

Montelli et al., 2004, Science 303, 338-343

How can we reconcile the contradictory evidence for

whole-mantle and

layered-mantle convection?

Kellogg‘s Stealth Layer

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

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%.

BUT WE NEED A VERY

ANCIENT RESERVOIR THAT

PRESERVES PRIMORDIAL

(≥4.5 Ga) COMPOSITION

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

Older two-layer degassing models require a huge, almost undegassed lower

mantle reservoir with enormous amounts of helium,

because the initial amount of helium (constrained by the known

terrestrial U-Th abundances and measured 3He/4He ratios) is very large

and the measured fluxes are very small.

This model is inconsistent with results of seismic tomography.

But, a much smaller primordial noble-gas reservoir is sufficient.

The D’’ layer at the base of the mantle is quite large enough,

but it must be essentially primordial in nature and it cannot be

regenerated.

New Model: Noble gases stored in primitive D‘‘ layer

New Model: Noble gases stored in primitive D‘‘ layer

Creation of Ancient Chondritic Reservoir in D‘‘

Postulated Sequence of Events:

Giant impact causes

Extensive melting (“Magma ocean”)Moon formationCompletion of core formation

Solidification of magma ocean: primitive mantle and crust

Continued accretion adds smaller bodies of Fe-rich, dense meteoriticmaterial to crust

Subduction of this “densified” crust (Δρ ≥ 4 %)

Storag e in D’ ’ laye r a t bas eof mantle

Evolution of Nd Isotopes in the Mantle

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.

Non-chondritic 142Nd Isotopes

in Crust-Mantle

Science, in press

Requires ancient hidden reservoir (D‘‘ ?) with low Sm/Nd ratio.

142Nd produced by extinct 146Sm

Consequence for 143Nd isotope mantle evolution

Boyet & Carlson, Submitted to ScienceHidden reservoir (D‘‘ ?) with low Sm/Nd ratio must be ancient and small The rest of the mantle is depleted

(=high Sm/Nd) and MORB-like

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

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

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

Evolution of Mass Fluxes

D‘‘- Mantle - Crust - Atmosphere Mass Balance

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.

D‘‘ Eutectic (Boehler)

T Dense Melt

Magnesiowüstite(Mg,Fe)O

PerovskiteMgSiO3

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

Three Wise Men (Jacoby)

Wolfgang Jacoby