Transcript Terrestrial budgets and evolution modeling

Mantle evolution models
• Some broad generalizations about isotopes in the mantle
• Isotopic evolution and mass balance - the Sm-Nd system
and 670km coincidence; eNd notation
• Box models for average isotopic composition of mantle
and crust - limits on continental recycling and upper-lower
mantle exchange
• Heterogeneity and sampling incorporated into a box model
(Kellogg et al. approach)
• (Brief) comments on rare gases
What is wrong with this picture?
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(1)
(2)
Upper mantle
(100)
Lower mantle
(300)
(200)
Mass in units of 1025g
Modern
distribution of
isotope ratios in
oceanic basalts
To trace back through time we need older rocks
Not many MORB analogues; must use island arc analogues
Ancient depleted mantle representatives
MORB
Prim.
Arcs
MORB
Prim.
Arcs
Whole
Mantle
200
Mass of MROB mantle / Mass of Continents
Nd isotope
Mass balance
Assumes no
other
depleted
reservoirs
+2
150
100
+8
e
MORB
= +10
+12
670 km
50
0
0
1
2
Mean Age of Cont (Gy)
3
4
Bulk
Mantle
A major problem
is that most
everything
appears to be too
depleted
For Sm-Nd, Q =25.1, fm ≈ 0.25 for present day Earth
So fmQ ≈ 6 Gy-1 whereas dem/dt ≈ 2 Gy-1
Kellogg et al., 2002
Stirring effect:
(turbulent)
(laminar)
Effect of
sampling
scale
Effect of
stirring time
scale
The MORB
distribution is
recovered,
but there
needs to be
some
cheating to
get an OIB
distribution
Time evolution: f((U+Th)/He)
From VanKeken, Hauri, Ballentine, 2002
What is wrong with this picture?
Bulk
Mantle
MOIBCOIBeOIB  MMORBCMORBeMORB  MCCCCCeCC