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Open Questions in Geosciences. Mapping Elements & Structure in the Earth. Geophysics tells us where we are at today Geochemistry tells us how we got there…. Collaborators : - Ricardo Arévalo, Mario Luong : UMD - Kevin Wheeler, Dave Walker : Columbia Univ

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slide1

Open Questions in Geosciences

Mapping Elements & Structure in the Earth

Geophysics tells us where we are at today

Geochemistry tells us how we got there…

Collaborators:

- Ricardo Arévalo, Mario Luong : UMD

- Kevin Wheeler, Dave Walker : Columbia Univ

- Corgne, Keshav & Fei : Geophysical Lab, CIW

- John Learned : University of Hawaii

- Steve Dye: Hawaii Pacific University

slide2

5 Big Questions:

  • What is the Planetary K/U ratio?
  • Radiogenic contribution to heat flow?
  • Distribution of reservoirs in mantle?
  • Radiogenic elements in the core??
  • Nature of the Core-Mantle Boundary?

planetary volatility curve

secular cooling

whole vs layered convection

Earth energy budget

hidden reservoirs

slide3

How much Th, U and K

is there in the Earth?

Heat flow measurements

Geochemical modeling

Neutrino Geophysics

slide4

Time Line

1st order Structure of Earth

Rock surrounding metal

1897

Emil Wiechert

1915

CORE-MANTLE

1925

UPPER-LOWER MANTLE

1935

CORE-MANTLE

PLATE TECTONICS

1970

1995

slide5

Earth’s Total

Surface Heat Flow

  • Conductive heat flow measured from bore-hole temperature gradient and conductivity

Data sources

Total heat flow

Conventional view

463 TW

Challenged recently

311 TW

urey ratio and mantle convection models
Urey Ratio and Mantle Convection Models
  • Mantle convection models typically assume:

mantleUrey ratio: 0.4 to 1.0, generally ~0.7

  • Geochemical models predict:

mantleUrey ratio 0.3 to 0.5

radioactive heat production

Urey ratio =

heat loss

discrepancy
Discrepancy?
  • Est. total heat flow, 46 or 31TW

est. radiogenic heat production 20TW or 31TW

give Urey ratio ~0.3 to ~1

  • Where are the problems?
    • Mantle convection models?
    • Total heat flow estimates?
    • Estimates of radiogenic heat production rate?
  • Geoneutrino measurements can constrain the planetary radiogenic heat production.
slide9

“Standard” Planetary Model

  • Chondrites, primitive meteorites, are key
  • So too, the composition of the solar photosphere
  • Refractory elements (RE) in chondritic proportions
  • Absolute abundances of RE – model dependent
  • Mg, Fe & Si are non-refractory elements
  • Chemical gradient in solar system
  • Non-refractory elements – model dependent
  • U & Th are RE, whereas K is moderately volatile
u and th and k distribution in the earth
U and Th (and K) Distribution in the Earth
  • U and Th (K?) are thought to be absent from the core and present in the mantle and crust.
    • Core: Fe-Ni metal alloy
    • Crust and mantle: silicates
  • U and Th (and K) concentrations are the highest in the continental crust.
    • Continents formed by melting of the mantle.
    • K, U and Th prefer to enter the melt phase
  • Continental crust: insignificant in terms of mass but major reservoir for U, Th, K.
slide11

Th & U

Volatility trend

@ 1AU from Sun

slide12

Silicate Earth

REFRACTORY ELEMENTS

VOLATILE ELEMENTS

Allegre et al (1995), McD & Sun (’95)

Palme & O’Neill (2003)

?

Lyubetskaya & Korenaga (2007)

Normalized concentration

Potassium

in the core

Half-mass Condensation Temperature

slide13

sub-title: What is the K/U Th/U ratios

for the Earth and modern mantle?

Implications and History

-- implications: K, Th and U are the radioactive elements that provide the sum of the internal radiogenic heat for the planet

-- history: Urey 1950s

Wasserburg 1960s

Jochum 1980s

slide15

SCIENCE

Accepted as the fundamental reference and set the bar at

K/U = 104

Th/U = 3.5 to 4.0

slide16

MORB (i.e., the Depleted Mantle ~ Upper Mantle)

K/U ~ 104 and

slightly sub-chondritic Th/U

DM & Continental Crust – complementary reservoirs

DM + Cc = BSE

ahh, but the assumptions and samples…

slide17

Two types of crust: Oceanic & Continental

Oceanic crust: single stage melting of the mantle

Continental crust: multi-stage melting processes

Compositionally distinct

slide18

Continental Crust

Not simple to interpret, but K/U ~104

Th/U ~ 4

slide19

Oeanic Crust

K/U ~ 104

Th/U ~ 4

Crust above = basalt

(melt, enriched in K, Th U)

Mantle beneath = Refractory peridotite

(residue, deplete in K, Th U)

slide22

MORBs:

a compositional spectrum

slide24

*BABB not included in above

diagram,nor in the MORB average

urey ratio and mantle convection models1
Urey Ratio and Mantle Convection Models
  • Mantle convection models typically assume:

mantleUrey ratio: 0.4 to 1.0, generally ~0.7

  • Geochemical models predict:

Urey ratio 0.3 to 0.5.

radioactive heat production

Urey ratio =

heat loss

slide26

Mantle is depleted in some elements (e.g., Th & U)

that are enriched in the continents.

-- models of mantle convection and element distribution

Th & U

poor

Th & U

rich

slide27

5 Big Questions:

  • What is the Planetary K/U ratio?
  • Radiogenic contribution to heat flow?
  • Distribution of reservoirs in mantle?
  • Radiogenic elements in the core??
  • Nature of the Core-Mantle Boundary?

planetary volatility curve

secular cooling

whole vs layered convection

Earth energy budget

hidden reservoirs

slide28

Long-lived chronometer:

147Sm 143Nd

(T1/2 = 106 Ga)

147Sm abundance decreased by only 3% in 4.56 Ga

Short-lived chronometer:

146Sm 142Nd

(T1/2= 103 Ma)

146Sm exists only in the first 500 Myr of

solar system history