Transcript Document

Mantle geochemistry: How
geochemists see the deep Earth
Don DePaolo/Stan Hart
CIDER - KITP Summer School
Lecture #1, July 2004
Geochemistry 50 years ago dealt with fewer
questions and parameters, e.g. Birch (1952)
• How does meteorite chemistry compare with
seismic properties of Earth’s interior
• Is it Olivine+Pyroxene or other phases ?
• How much Fe in the mantle ?
• How much Al,Ca,Na,K (“sialic components”) is in
the mantle ?
• 11 elements of interest:
O,Mg,Si,Fe,Ni,Al,Ca,S,Na,K,P
What can geochemistry do in 2004?
• The earth is made of 90 or so chemical elements,
about 30 w/isotopic variations
• Chemical/isotopic characteristics can be tied to
geological processes - mantle isotopic chemistry is
a tracer
• We can tell where a particular piece of mantle has
been in the past and/or what has happened to it
• Radiogenic isotopes provide clocks as well as
tracers
Questions for geochemistry
• How deeply does near surface material circulate into the
mantle? On what time scale?
• Does the mantle have large scale chemical structure
(layering?)
• Does the core exchange material with the mantle? (Do
plumes come from the CMB?)
• What are the characteristics of mantle convection in terms
of its ability to stir and homogenize heterogeneous
materials?
• What features of mantle seismic heterogeneity are thermal
and which are chemical?
• What aspects of mantle structure are congenital?, of recent
origin?; steady-state features?
Components of geochemistry
• Petrology of the mantle (proportions of minerals or rock
types - e.g. lherzolite, harzburgite, eclogite, pyroxenite)
• Melting of the mantle
• Trace element composition of the mantle (doesn’t affect
mineralogy, but can be indicative of history)
• Trace element composition II (water and CO2) - affects
melting behavior.
• Isotopic composition of the mantle (from radioactive
decay, input from surface reservoirs, input from core?)
• Sampling of the mantle (scale of sampling by magmatism;
sampling biases, invisible reservoirs)
• Material balance - the sum of the parts must equal the
whole Earth for every element and isotope
(1)
(2)
Upper mantle
(100)
Lower mantle
(300)
(200)
Mass in units of 1025g
Oceanic lithosphere ≈ 10
Continental lithosphere ≈ 5
(1)
(2)
Upper mantle
(100)
Lower mantle
(300)
(200)
Mass in units of 1025g
Consider this:
(1) “Heterogeneities” are introduced from the top and the bottom
(2) Magmatism samples only the top and the bottom
There are key elements of the system where chemistry is done.
(Most of what we infer about the mantle depends on how well
we understand the processes.)
Choose one:
There are...
(a) too few
(b) too many
(c) just the right
number
...of isotopic
tracers
There are
stable
isotopes
too !
Why the
crustal
reservoirs
matter...
Why the
crustal
reservoirs
matter...
Depleted mantle
103
.
MORB
 = 10-14 sec-1
Heterogeneity thickness (km)
102
101
Other
lava
sequences
Laminar
shear
100
10-1
T urbulent
shear
10-2
UM
rocks
10-3
Chemical Diffusion
-4
10
0
500
1000
Age (M a)
1500
2000
Thickness of geochemical anomalies
Geochemically Anomalous
Layer (Thickness = h
Isotopic contrast = ²R h
Concentration = Ch
Background mantle
concentration = Cb
Background mantle that must be averaged with
anomalous material (effective anomaly thickness):
b = h
Ch ²R h
Cb ²R a
Making heterogeneity at a mid-ocean ridge...
Mid-ocean ridge factory
Hydrothermally altered
sediment
basalt, gabbro
harzburgite
strongly
depleted
lherzolite
Incipiently
depleted
lherzolite
H2O-enhanced melting region
unmodified
lherzolite
-20
sediment
basalt, gabbro
harzburgite
strongly
depleted
lherzolite
Incipiently
depleted
lherzolite
unmodified
lherzolite
0.1 b.y. later
Nd
0
+20
-20
1.0 b.y. later
Nd
0
+20
Anything systematic about distribution of heterogeneities?
Anything systematic about distribution of heterogeneities?
Bulk Earth
Younger cont.
crust
Lower cont.
crust
Upper cont.
crust
Older cont.
crust
Anything systematic about distribution of heterogeneities?
Bulk Earth
Chondrites
Distribution of
isotopic ratios
among ocean
islands is not
entirely random
Al Hofmann’s analysis, 2003
120
Mid-Atlantic Ridge
(data from PETDB)
Material
balance for
Sm-Nd...
100
Average
Mantle
Count
80
60
40
Bulk
Earth
20
0
-8
-4
0
4

8
Nd
12
16
15
“DM”
10
Epsilon Nd
An example of
heterogeneity on
various scales Nd isotopes in
MAR basalts. 5
to 10 units of
variation can be
found over 10km
or 10,000km
along the ridge.
The entire range
of values
observed
worldwide (in all
types of oceanic
basalts) is found
along the ridge
5
AM
“PM”
0
-5
Mid-Atlantic Ridge
(data from PET DB)
-10
-60
-40
-20
0
20
LAT IT UDE
40
60
80
Recycled
MORB
Primitive
The
helium
problem
Melting region
Sampling issues: Pt. 1
edge
center
9.0
8.5
Present
8.0

200 Ka
400 Ka
Mahukona
Kohala
Nd
7.0
600 Ka
800 Ka
Melt Supply
max = 5 cm/yr
6.5
HSDP
Hualalai
0.1
0.05
0.001
N
400
500
600
25
20
3He/4He (R/Ra)
Loihi
300
Model Age (ka)
0.5
0.3
Mauna Loa
200
0.9
0.7
Kilauea
HSDP Mauna Kea
(Bryce & DePaolo, 2002)
6.0
100
1.0
Mauna Kea
Magma Capture
Area
7.5
HSDP Mauna Kea
(Kurz, 2002)
15
10
MORB Range
5
20 k m
0
100
200
300
400
Model Age (ka)
500
600
3
5
Width of melting region
10
4
He/ He
20 25 30
15
35
0
500
He-3 anomaly
( Pb anomaly?)
Depth (km)
Sr anomaly
1000
1500
2000
Sr
2500
Core or core-mantling dense layer
3000
He
0.7025
87
86
Sr/ Sr
0.7035
40
Things may get even
more interesting
when we model the
melting in the
context of the flow (M. Jull, unfinished,
2003)
Melting versus tracers...
(Modeling from Jull & Ribe, 2002)
Sampling issues, Pt. 2:
Over what vertical distance are isotopic ratios averaged?
Erupted Lava
Storage
Chamber ( V r)
Lithosphere
²z
Partial melt zone
(, w)
z=0
W
z
Plume

i
DePaolo, JGR, 1996
18.40
12
18.45
Pb/204Pb
14
10
206
8
6
4
200
400
600
800
Depth (meters)
1000
18.50
18.55
18.60
18.65
200
400
600
800
Depth (meters)
1000
104
Hydrodynamic Dispersivity (m)
3
He/4He (R/Ra)
Estimating dispersivity in the Hawaii melting region
He
Pb
Sr
Nd
103
1 km
102
Mauna Kea (380-1000m)
101
0
From DePaolo, JGR, 1996
5
10
w/W
15
20
For MOR’s the
channeling instability
may apply; makes for
very large vertical
dispersion - i.e. lots of
averaging. May not be
the case for plumes (?)
Spiegelman et. al, 2001 (JGR, 106,
2061-2077)
OK, so what do we think we know......?
Where we are going in the next 2 weeks....
Geochemistry Tutorials....