Grand Challenges in solid Earth Sciences

Bill McDonough

Geology, U Maryland1

Observations about the mantle:- 142Nd controversy no longer exists – Earth is chondritic- Mg/Si of Earth: accretion of olivine to pyroxene; Sun is heterogeneous- Collisional erosion models are unnecessary and unfounded- 182W isotopes record early core-mantle separation- Geoneutrino flux of Th & U: define building blocks & describes convective state

Challenges we face in understanding the mantle:- rate of heat loss is known to be between 50 and 150 K/Ga
- Defining mineralogy of the mantle remains a challenge- LLSVP origins is either primordial or subduction related – no resolution in sight- Stirring efficiency of the mantle remains to be understood- Core-mantle mass exchange might occur, but not geochemical evidence- Primordial domains may remain from magma ocean conditions2

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Early Solar System chronology- CAI formed in ≤105 yrs at 4567.80 ± 0.16 ± 0.5 Ma (tzero)
stat.

syst.

- Chondrules formed from 0.5 to ~5 Ma after tzero- Cores and mantles of small planets (10 to 1000 km)
formed between tzero + 0.5 Ma to tzero + ~5 Ma- Earth’s core & Moon formed: tzero +~30 to +~150 Ma- Accretion models, consider rapid planetary growth4

Sun and Chondrites are related

NobleGasses

K, Th & U

heat producing elements

5Compositional space where we “might” fit…
4 elements describe>90% of the Earth!

6Earth @ 1 AU

Mars @ 2.5 AU
Olivine-rich

EARTH
COCloser to sun?

CIHLL
ELEHLMARS

CMCV?
-thermal-compositional-redox

Pyroxene-rich

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Mg/Si variation in a nebula disk
Olivine-high temperature-early crystallization-high, Mg2/Si-fewer volatile elements
Pyroxene-lower temperature-later crystallization-low, Mg/Si-more volatile elements
8Inner nebular regions of dust to be highly crystallized,
Outer region of one star has- equal amounts of pyroxene and olivine- while the inner regions are dominated by olivine.
Boekel et al (2004; Nature)

Olivine-rich

Ol & Pyx

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Small planet growth in just a few Myr
IAB irons : non-magmatic
Magmatic irons

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Kruijer et al. 2014 Science

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IVB

5

IVA

4

IIIABIID

3

IIAB

2
10246

Age Relative to CAI (Ma)W isotope constrain
these ages 11
Emily Worsham’s unpublished data (UMd Ph.D. student)

from R. Walker 2015

Redoxstate ofthe Earth

Whichchondriteis theEarth?

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142µ
Nd
What does this Nd datamean for the Earth?

Earth

Enstatitechondrites• Solar syst. heterogeneous• Chondrites are a guide• Planets ≠ chondrites ?

Data from:
Gannoun et al (2011, PNAS)
Carlson et al (Science, 2007)
Andreasen & Sharma (Science, 2006)
Boyet and Carlson (2005, Science)
Jacobsen & Wasserburg (EPSL, 1984)

Ordinarychondrites

Carbonaceouschondrites

13Plate Tectonics,Convection,Geodynamo

Radioactive decay driving theEarth’s engine!

K, Th & U!
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Earth’s surface heat flow 46 ± 3 (47 ± 1) TW
total R*
20 ± 4 TW

Mantle cooling(18 TW)

Crust R*
(7 ± 1 TW)(Huang et al ‘13)Core(~9 TW)
Mantle R*(13 ± 4 TW)*R radiogenic heat
(after McDonough & Sun ’95)

after Jaupart et al 2008 Treatise of Geophysics-(4-15 TW)(0.4 TW) Tidal dissipation
Chemical differentiation15

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Oceanic Plate stagnation- 660 km depth- 1000 km depthUnderstanding mantle
viscosity structure

Fukao & Obayashi (JGR ‘13)

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Hawaiian plume:- extending from CMB- rooted in large ULVZ

1st time: continuous connection between ULVZ's and mantle plumes

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French & Romanowicz (Nature,2015)

Zhang et al (GRL, 2016)

Deep mantle products(?) of an early magma ocean

Labrosse et al (Nature, 2007)

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What’s hidden in the mantle?
Seismically slow “red” regions in the deep mantle

From Alan McNamara after
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Ritsema et al (Science, 1999)

What’s hidden in the mantle?
Seismically slow “red” regions in the deep mantle

Can we image itwith geonus?

From Alan McNamara after
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Ritsema et al (Science, 1999)

Testing Earth Models
Mantle geoneutrino flux (238U & 232Th)

Šrámek et al (2013) EPSL, 361: 356–366, 10.1016/j.epsl.2012.11.001

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Predicted geoneutrino flux

Total flux at surface
dominated byContinental crust

Yu Huang et al (2013) G-cubed arXiv:1301.0365
10.1002/ggge.20129

Mantle flux at theEarth’s surface
dominated bydeep mantle structures23Šrámek et al (2013) EPSL 10.1016/j.epsl.2012.11.001; arXiv:1207.0853

Today, looks
bleak, but..
In 8 years,2025…
see Sramek
talk tomorrow
Watanabe’s
talk today?...

Tomorrow?

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Carsten Rott’s talkElectron density and neutrino oscillation

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• Big Unknowns:– Composition of silicate Earth (Mg, Si, Fe, O)•Amount of recycled basalt in the mantle– Mineralogy of the Lower mantle– geothermal gradient Mantle and Core
– Composition of Core (Fe-Ni + H, C, O, Si, S, ..)– Radioactive elements in Mantle and Core
– Core – Mantle exchange (do we see it?)
– Timing & rate of Inner Core crystallization
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