xuezhao bao and richard a. secco xbao2@uwo.caxbao2@uwo.ca;...

Xuezhao Bao and Richard A. Secco

xbao2@uwo.ca; xuezhaobao@hotmail.com

Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada N6A 5B7

Joel E. Gagnon and Brian J. Fryer

Department of Earth Sciences, University of Windsor, Windsor, Ontario, Canada N9B 3P4

U Solubility in Planetary Cores: Evidence From High Pressure and Temperature Experiments

> Without U, Th, K40, inner core age is 1.5 Ga (should be >3.5Ga

see next) (Labrosse et al. 2001, Buffett, 2002, 2003; supported by Anderson, 2002, Nimmo, 2004, 2005, etc.).

Mantle

Current inner core size

Inner Core with U, Th, K40

Inner Core without U, Th, K40

13.5 TW

Introduction

Perovskite

postperovskite

Perovskite

Mantle

Outer core

13.5 TW

> Perovskite-postperovskite-perovskite forward and reverse transformations within CMB indicate a high heat flow of 13.5

TW (Lay et al., 2006)

Tarduno et al., 2007

After Tarduno et al., 2006

McElhinny et al., 1980

The existence of a magnetic field of roughly present-day strength over 3.5 Gyr is more easily reconciled with an old inner core (Christensen & Tilgner, 2004).

U in the outer core?

Two unusually long periods without any reversal

Courtillot, 1998

Computer simulations and field works (Coe and Glatzmaier, 2006)

Nreversal

> upper limit of 65 TW radiogenic heat from U and Th;

central value of 16 TW?. Araki et al (2005)

> Problems: 1) Large range of 65 TW 2) U, Th: in mantle or core? 3) Geochemistry model dependent.

KamLAND

Mantle

Current inner core size

U,Th decay

Geo-neutrinos

Inner Core

Outer Core

Introduction

Information from geo-neutrino

studies

0 % 0-10 % 14.5 % 0~0.2 %.

Sun Mercury Venus

Our starting materials: mixtures of the following powder

~ 65% ~33%? ~ 33% ~ 22% Righter, 2006

35% 67%? 67% 78%

Experiments and composition analytical methods

Silicate

Silicate ~ 54 wt% peridotite

U source ~3 wt% uraninite

simplified expression

Metal in the core

S in the core

metal phase - 40wt% Fe or

Fe-10% S

Fe-35wt%S

500 ton Walker module multi-anvil press

Pressure cell(inside)

An 3000 ton multi-anvil press will be coming in this Fall

Welcome to use this lab. You can email me at: xbao2@uwo.ca, or xuezhaobao@hotmail.com

Pressure Cell

Graphite, MgO

Composition Analytical methods

> LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry)

University of Windsor, University of McGill

> EM (Electron Microprobe).

> SIMS( Secondary Ion Mass Spectrometer)

P, T range of experiments

Mg2SiO4

Davis & England (1964)

Ohtani & Kumazawa (1981)

Saxena & Dubrovinsky (2000)

0 2 4 6 8 10 12 14

360332330

358357

Mg 2SiO 4

melting

Fe melting

Sample container BN

Graphite MgO 191

199190

P (GPa)

Run 182: 5GPa; 2050 ; metallic phase: Fe℃ Backscattered electron image

Results and discussion

Silicate

Run 238: 5 GPa; 1972 ; metallic phase: Fe-10% S℃ Backscattered electron image

Fe-10% S

silicate

Spectra of U in the Fe phases at different pressures analyzed by LA-ICP-MS.

0 20 40 60 80 100

0 20 40 60 80 100 1200

Background Fe

Time (second, 10 µm/s)

Run 198, 0 GPa, 1850 Co

Run 207, 6.7 GPa, 2010 Co

0 20 40 60 80 100

Background Fe

Time (second, 10 µm/s)

Run 199, 8 GPa, 2249 Co

Run 191, 8.5 GPa, 2287 CoCou

Summary: U in the pure Fe: from 0 to 7.5 GPa, 50 to 10, 000 counts/s

Oxides and silicates

Spectra of U in the Fe phases at different pressures analyzed by LA-ICP-MS.

0 2 4 6 8 10 12 14

Pressure (GPa)

ContainerT<T

silicate melt

BN Graphite

T>Tsilicate melt

BN Graphite

200182

154157

Run products with Fe in metal phase: DU= UFe / Usilicate

Silicate FeUP

Run Products With Fe-S: DU= UFe-10%S / Usilicate

LA-ICP-MS

metal o

r metal

P (GPa )

0 2 4 6 8 10 12 14 16

198113

Silicate Fe10%SUP

T>Silicate melting line

T<Silicate melting line

FeS 80

Fe-SFe+SiFeth

Possible mechanism for U entry the cores

Low fo2 is the key for U entry metal phases. Uis very soluble in metal Fe in situations without O and other oxidative elements.

Fe0 + U0 + Si0 UFe10Si2

reducing

-2-1 ~ -6

log fo2(∆IW)

-1 ~ -6

Silicate Silicate Silicate

-7 -6 -5 -4 -3 -2 -1 0

212200154 204

182 253

191260

metal Fe with BN capsulemetal Fe with C capsulemetal Fe-10wt%S with MgOcapsulemetal Fe with MgO capsule

The others after Fig. 3

199(2303 ppm)262(8686 ppm)

Oxygen fugacity (∆IW)

0 2 4 6 8 10 12 14 16 18-2.0

150 Container:BNGraphite

253212

P (GPa)

∆ = -2.0IW

∆ = -6.0IW

Run products with Fe: Si increases with P(consistent with previous studies of Ito et al., 1995, Gessmann et

al., 2001 and Malavergne et al., 2004).Based on log(wt%)Si – fo2 relation (Kilburn and Wood,1997). fo2 below the IW buffer decreases with P, coinciding with U increase with P in Fe

log D(S)

log D(Si)

For geo-neutrino study: it may be impossible for significant K in the core.

0 2 4 6 8 10 12 14 16-9

metal Fe withBNcapsulemetal Fe withC capsulemetal Fe-10wt%S with MgO capsulemetal Fe withMgOcapsule

Smaller signs identify samples with very small metal phases. Their Uconcentrations inmetalphases are difficult toobtain with LA-ICP-MS analysis.

115330

182 183

157137

202205

198 150

P (GPa)

According to Arora (2000) increase P, reactions proceed to left, fo2 increase.

Fe can be oxidized even at room T (Kubaschewski et al., 1962)

Fe-O O, Fe

0 2 4 6 8 10 12 14 16-9

metal Fe with BNcapsulemetal Fe with C capsulemetal Fe-10wt%S with MgO capsulemetal Fe with MgOcapsule

Smaller signs identify samples with very small metal phases. Their Uconcentrations in metalphases are difficult to obtain with LA-ICP-MS analysis.

115330

182 183

157137

202205

198 150

P (GPa)

But in fact, increase P, fo2 decrease.

Si 4+, Al 3+ Si 0, Al 0A)impact Al-O

Nb can’t be oxidized by O below 200oC (Kubaschewski et al., 1962)

O-Nb-O

B) Serghiou et al., (1992) found at room T, high P (19.2 GPa) can decrease the O content of Tetragonal Nb2O5, and it was amorphized. O-Nb,O

Yakovlev (1993) discovered metallic Si and Al in the impacted products.

Confirmed by Drickamer et al., 1969 and Burns, 1993, etc. in a wide variety of compounds.

Burns (1993). Similar elements: Mn, Cu, Ti, Cr and Ni

Core-mantle boundary

If Burns (1993) is right, then it is natural

Fe--OP

McCammon & Kopylova, 2004 Woodland & Koch, 2003

Xenoliths from the mantle under Lesotho and South African (Mossbauer data)

Xenoliths from the mantle under northern Canada (Mossbauer data)

0 2 4 6 8 10 12 14 16-9

metal Fe withBNcapsulemetal Fe withC capsulemetal Fe-10wt%S with MgO capsulemetal Fe withMgOcapsule

Smaller signs identify samples with very small metal phases. Their Uconcentrations inmetalphases are difficult toobtain with LA-ICP-MS analysis.

115330

182 183

157137

202205

198 150

P (GPa)

Average ∆IW +4, basalts on the surface (Karner et al., 2006) ~0.6/GPa (Ballhaus,

~ IW -6.8 (Average, ⊿Grossman et al., 2008) ~ IW -13~8.9 (enstatite chondrite ⊿

(EC), Grossman et al., 2008)

EC model: based on composition (Javoy, 1995,1998), and O(Clayton,1993),Cr(Lugmair, 1998), and Mo(Dauphas, 2002) isotope compositions.

Proto-planetary disk or nebula

Condensation T of Fe is higher than that of silicates at the relevent pressures of the solar nebula (~10-4 atm, Grossman, 1972)

~ IW -6.8 (Average, ⊿Grossman et al., 2008)

~ IW -13~8.9 (enstatite chondrite ⊿(EC), Grossman et al., 2008)

Metal core

Fe-rich region

Inner planet cores may have formed completely (Hwaung, 2000) or at least partially (Grossman, 1972), from the Fe-rich materials in the Fe-rich region.

Proto-planetary disk or nebula

Core has formed entirely under highly reducing conditions (Wänke, 1981; Javoy,1995; Allègre et al., 1995)

Core has formed mainly under an early highly reducing conditions (O’Nell, 1991; Javoy,1995; Wade & Wood, 2005; Wood et al.,2008 )

Core formation during Earth’s accretion Possible mechanism for U entry the cores

Wood et al.,2008

-7 -6 -5 -4 -3 -2 -1 0

212200154 204

182 253

191260

metal Fe with BN capsulemetal Fe with C capsulemetal Fe-10wt%S with MgOcapsulemetal Fe with MgO capsule

The others after Fig. 3

199(2303 ppm)262(8686 ppm)

Oxygen fugacity (∆IW)

O2 and water contents (McCammon et al., 2004 ) are the control factors of fo2.Other oxidative volatiles are also important.

Element Density Melting point

Compound Density Melting point Compound Density Melting point

Fe 7.86 1535Ni 8.90 1453U 19.05 1132Th 17.70 1700 K4ThOX4.4H2O soluble in water

ThO2 10.0 3220

ThN 10.6 2500ThS 1905ThCl4 600

Th(NO3)4.5H2O soluble in water

UO2 10.95 2800

UCl 4.86 567UN 14.32 2800UF6 4.68 ~0

UO3 7.29 soluble in water

The melting point (oC) and density (g/cm3) of U, Th, their compounds and complexes and core related elements (after Bao and Zhang, 1998).

O or other volatiles

High density(19 g/cm3)

Small density (0.00143g/cm3)

Pacific ocean

U,Th rich zone Volatiles or their ionsU - Th -

→ U → Th

The distribution of U and Th in the outer core and its influence on the formation of deep mantle plumes and subducted lithosphere plates in the Pacific Ocean.A-lithosphere; B- U,Th richer sphere or asthenosphere; C- mantle; D''- core-mantle boundary; D- outer core; E- inner core; O- center of Earth; M- center of geomagnetic field and thermal convection; the black points represent the relative concentration of U and Th (after Bao, 1999).

Continent

Only planet with life; only planet with plate tectonics; only planet with asthenosphere; only planet with granitoid continental crust; with magnetic field; with ocean…

Circum-pacific volcanic belt magmatism

Implication for planetary dynamics

Cumulative volume of continental crust extracted through time from different researcher groups (After Abbott et al., 2000).

0.1 0.2U

Rudnick & Fountain, 1995

Taylor & McLennan, 1985

Oceanic crust

Continental crust

Average U, Th concentration (ppm)

EarthImplication for planetary dynamics

0 2 4 6 8 10 12 14 16 18 20 22 24 26

T < silicate melting

T > silicate melting

DU= 0.047

DU= 0.17

Pressure (GPa)

ContainerT<T

silicate melt

BN Graphite

T>Tsilicate melt

BN Graphite

0.17 x 20 ppb U in Primitive Silicate Earth (PSE, based Cl chondrite…) (McDonough, 2003)

3.4 ppb U

0.047 x 20 ppb U in PSE

0.21 x 20 ppb U in PSE(to CMB?)

1 ppb U

4.2 ppb U

Magma Ocean

Percolation

Earth’s major, minor, and isotopic composition is unlike any known chondrite group or mixture of chondrite group (Righter, 2003, Caro et al., etc.)

Application to Earth’s core Run product with Fe

Influence of S concentration in metal phases on the extrapolated DU values at 26 GPa, the pressure at the bottom of a proposed magma ocean.

Application to Earth’s cores

T > silicate melting line

T < silicate melting line

5 ppb U in the core

10.2 ppb U in the core?

1 ppb U in the core

22 ppb U at CMB?

4.2 ppb at CMB?

Mercury & Venus

Run product with Fe-10% S

O and volatile contents are controlled by two factors: planetary heliocentric distances (Lewis, 1974a, 1984) and size (Ahrens, 1993)

(after Fegley, 1999).

P, T distribution in the protoplanetary disk from different researchers

Formation Model of the magnetic field in Mercury.

639 km

1800 km

O-Si-Mg (FeO<0.3%, Inferred spectrometry)

coreInner core?

Mantle

Mercury

Structure of Mercury

Giant impact: lost most its mantle and almost all its Fe has entered its core (Benz et al., 1988; Palme et al., 2003)

(After Strom and Sprague, 2003) (After Stanley et al., 2005)

Heat loss

Liquid outer core

Magnetic records implies that a dynamo was present in its early stage but must have be absent after 4.0 Ga (Nimmo &Tanata, 2005; Fei & Bertka, 2005; Solomon et al.,2005), but why?

Mars possesses no internal magnetic field, but has old (>4 Ga) Magnetic records (After Solomon et al.,

Martian core is at least partially liquid; confirmed by solar tide study (Yoder et al., 2003)

(Solomon et al., 2005)

Hemispheric dichotomy structure on Martian crust

> 3.7 Ga, less dense highlands, andesite or basaltic andesite, high radioactive elements (Nimmo&Tanaka, 2005)

< 3.7 Ga, lowlands, basaltic composition

4.12 Ma (Nimmo & Tanata, 2005)

A relatively small impact can drive off Martian volatiles (1034-1036.5 ergs). For Earth 1038 ergs (Ahrens, 1993)

MarsImplication for planetary dynamics

Today: dry Martian mantle from Martian meteorites (Dreibus & Wanke, 1985, 1987; Carr & Wanke, 1992; Ghosal et al., 1998; Reese & Solomatov, 2002; Herd et al., 2002, Jones, 2004 ); Infrared mineralogical mapping (Mustand et al., 2005; Bibring et al., 2005). The content of interior volatiles are 2000 times less than that of Earth (Beaty et al., 2005 ).

Volatiles or their ions

U-4e U4+ U rich zone

A-lithosphere; B-asthenosphere (U,Th rich sphere); C-mantle; D’’-core-mantle boundary; D-outer core; E-inner core (after Bao, 1999)

Similar internal circulation system and plate tectonics might have developed before 4.12 Ga? Giant impacts have drove off Martian volatiles and then these systems stopped to work and the dynamo are dead since then.

14.5% S in the core (Fei & Bertka,2005)

U, Th(?) has entered the core

andesite or basaltic andesite

MarsImplication for planetary dynamics

Conclusions

For P<15 GPa, T< 2500 , ℃ DU in FeS (including Fe-35% S and Fe-10% S) and Fe melts increase with P,T.

DU is 3-5 times larger when silicate was molten than silicate was solid.

For a 800 km thick magma ocean, Earth’s core could contain about ~10 ppb U? from the extrapolated result of Fe-10% S samples or 3.4 ppb U from Fe samples.

For percolation core formation, Earth’s core could contain 5ppb -- 22 ppb (?) U from the extrapolated result of Fe-10% S samples or 1ppb - 4 ppb(?) U from Fe samples.

Inside Earth, U may be a radioactive heat source to slow down the cooling and crystallization of the inner core.

U is also a possible energy source for maintaining the magnetic fields of Earth and Mercury.

Acknowledgments

NSERC, CFI Dr. G. Young for partial funding. Drs. C. Cermignani and M. Liu for EM analyses Drs. W. Minarik and J. Gagnon for LA-ICP-MS analysis. Dr. A. Pratt and Mr. G.Gord for SIMS analysis. Dr. D. Liu and Mr. R. Tucker for help in sample preparation Mr. G. Wood for help in sample cutting and polishing.

Thanks to:

Thank you!

Calculated fraction of atmosphere blown off versus impactor energy for Earth, Venus, and Mars. Lower and higher energy curves for each planet correspond to assumed polytropic exponent of ideal gas of γ =1.1 and 1.3, respectively (After Ahrens, 1993).

Water-rich atmosphere formed by the impact-induced dehydration of water-bearing minerals by planetesimal impacts (After Ahrens, 1993)

0 2 4 6 8 10 12 14

fo2(IW)(Wheeler et al., 2006)

-1.98-1.92

-2.31-2.13

360332330

358357

Mg 2SiO 4

melting

Fe melting

Sample container BN

Graphite MgO

199190

P (GPa)

-1.19 -2.48

Fe can be oxidized even at room T (Kubaschewski et al., 1962)

Fe-O O, Fe

xuezhao bao and richard a. secco xbao2@uwo.caxbao2@uwo.ca;...

discussionspectra of

u solubility

metal fe

fe phases

u entry metal phases

pure fe

inner core age

k40inner core

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