Download Th/U - APC

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
5 Big Questions:
- What is the Planetary K/U ratio?
planetary volatility curve
- Radiogenic contribution to heat flow?
secular cooling
- Distribution of reservoirs in mantle?
whole vs layered convection
- Radiogenic elements in the core??
Earth energy budget
- Nature of the Core-Mantle Boundary?
hidden reservoirs
How much Th, U and K
is there in the Earth?
Heat flow measurements
Geochemical modeling
Neutrino Geophysics
Time Line
1st order Structure of Earth
Rock surrounding metal
1897
Emil Wiechert
1915
1925
1935
1970
1995
PLATE TECTONICS
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
radioactive heat production
Urey ratio =
heat loss
• Mantle convection models typically assume:
mantle Urey ratio: 0.4 to 1.0, generally ~0.7
• Geochemical models predict:
mantle Urey ratio 0.3 to 0.5
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.
“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 (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.
Th & U
Volatility trend
@ 1AU from Sun
Silicate Earth
Normalized concentration
REFRACTORY ELEMENTS
VOLATILE ELEMENTS
Allegre et al (1995), McD & Sun (’95)
Palme & O’Neill (2003)
?
Lyubetskaya & Korenaga (2007)
Potassium
in the core
Half-mass Condensation Temperature
What is the K/U Th/U ratios
for the Earth and modern mantle?
sub-title:
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
First observations -- got it right at the 1-sigma level
SCIENCE
Accepted as the
fundamental reference
and set the bar at
K/U = 104
Th/U = 3.5 to 4.0
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…
Two types of crust: Oceanic & Continental
Oceanic crust: single stage melting of the mantle
Continental crust: multi-stage melting processes
Compositionally distinct
Continental Crust
Not simple to interpret,
but K/U ~104
10
Th/U
8
6
4
2
Granites
Loess and Shales
0
1000
10000
K/U
100000
Th/U ~ 4
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)
MORB Sample Locations
a compositional
spectrum
PM Normalized
MORBs:
10
Atlantic Ocean
1
Ba Th U Nb K La Ce Pb Pr Sr Nd Zr Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
100
1
Indian Ocean
0.1
PM Normalized
10
10
1
Pacific Ocean
0.1
Ba Th U Nb K La Ce Pb Pr Sr Nd Zr SmEuGd Tb Dy Ho Er TmYb Lu
Ba Th U Nb K La Ce Pb Pr Sr Nd Zr SmEu Gd Tb Dy Ho Er Tm Yb Lu
100000
N-MORB
E-MORB
K/U
10000
MORB Average
19,200 ± 4300
(2Mean ± 590)
1000
100
1000
10000
K (g/g)
Avg. K/U
2σ Mean
2σ
RSD %
n
All MORB N-MORB E-MORB OIB BABB
19030 19208
19759
18139 15055 20185
495
586
420
1558
1240
949
4168 4267
2518
6423
3037 3550
22
22
13
35
20
13
71
53
36
17
6
14
*BABB not included in above
diagram,nor in the MORB average
Urey Ratio and
Mantle Convection Models
radioactive heat production
Urey ratio =
heat loss
• Mantle convection models typically assume:
mantle Urey ratio: 0.4 to 1.0, generally ~0.7
• Geochemical models predict:
Urey ratio 0.3 to 0.5.
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
5 Big Questions:
- What is the Planetary K/U ratio?
planetary volatility curve
- Radiogenic contribution to heat flow?
secular cooling
- Distribution of reservoirs in mantle?
whole vs layered convection
- Radiogenic elements in the core??
Earth energy budget
- Nature of the Core-Mantle Boundary?
hidden reservoirs
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
100000
Published ID-MS & ICP-MS
Jochum et al. (1983)
Our LA-ICP-MS Data
K/U
10000
Basalts (MORB, OIB, BABB)
1000
100
1000
K (g/g)
19,000 ± 500 (2Mean)
10000