Mantle geochemistry: How geochemists see the deep Earth Don DePaolo/Stan Hart CIDER - KITP Summer...
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Transcript of Mantle geochemistry: How geochemists see the deep Earth Don DePaolo/Stan Hart CIDER - KITP Summer...
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
(200)
Lower mantle
Upper mantle
(300)
(100)
(2)
(1)
Mass in units of 1025g
(200)
Lower mantle
Upper mantle
(300)
(100)
(2)
(1)
Mass in units of 1025g
Oceanic lithosphere ≈ 10Continental lithosphere ≈ 5
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...
Depleted mantle
Why the crustal reservoirs matter...
10-4
10-3
10-2
10-1
100
101
102
103
0 500 1000 1500 2000
Age (Ma)
Laminarshear
Turbulentshear
Chemical Diffusion
ε = 10-14 sec-1. MORB
Otherlavasequences
UMrocks
Geochemically AnomalousLayer (Thickness = λh )
Background mantle that must be averaged withanomalous material (effective anomaly thickness):
Concentration = Ch
Isotopic contrast = ∆R h
Background mantleconcentration = Cb
Ch ∆Rh
Cb ∆Raλb = λh
Thickness of geochemical anomalies
Making heterogeneity at a mid-ocean ridge...
Incipientlydepletedlherzolite
stronglydepletedlherzolite
harzburgite
basalt, gabbrosediment
unmodifiedlherzolite
H2O-enhanced melting region
Mid-ocean ridge factoryHydrothermally altered
Incipientlydepletedlherzolite
stronglydepletedlherzolite
harzburgite
basalt, gabbrosediment
unmodifiedlherzolite
-20 0 +20εNd -20 0 +20
εNd
0.1 b.y. later 1.0 b.y. later
Anything systematic about distribution of heterogeneities?
Anything systematic about distribution of heterogeneities?
Bulk Earth
Younger cont.crust
Older cont.crust
Lower cont.crust
Upper 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
0
20
40
60
80
100
120
-8 -4 0 4 8 12 16
εNd
- Mid Atlantic Ridge( )data from PETDB
AverageMantle
Material balance for Sm-Nd...
BulkEarth
-10
-5
0
5
10
15
-60 -40 -20 0 20 40 60 80
LATITUDE
Mid-Atlantic Ridge(data from PETDB)
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
“PM”
“DM”
AM
MORBRecycled Primitive
The helium problem
0
5
10
15
20
25
100 200 300 400 500 600Model Age (ka)
HSDP Mauna Kea(Kurz, 2002)
MORB Range
6.0
6.5
7.0
7.5
8.0
8.5
9.0
100 200 300 400 500 600
εNd
( )Model Age ka
HSDP Mauna Kea( & , 2002)Bryce DePaolo
edge center
20 km
N
Melt Supplymax = 5 cm/yr
0.001
0.1
0.3
0.5
0.7
0.9
1.0
0.05
Present
200 Ka
400 Ka
600 Ka
800 Ka
Kohala
Hualalai
Mauna Loa
Loihi
Kilauea
Mauna Kea
Magma CaptureArea
HSDP
Mahukona
Melting region
Sampling issues: Pt. 1
5 10 15 20 25 30 35 40
0.7025 0.7035
0
500
1000
1500
2000
2500
3000
3He/
4He
87Sr/
86Sr
Sr
He
He-3 anomaly(Pb anomaly?)
Sr anomaly
Core or core-mantling dense layer
Width of melting region
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)
Partial melt zone(φ , )w
W
∆z
Plume
Lithosphere
Storage (Chamber Vr)
z
Erupted Lava
= 0z
εi
Sampling issues, Pt. 2:Over what vertical distance are isotopic ratios averaged?
DePaolo, JGR, 1996
101
102
103
104
0 5 10 15 20
HePbSrNd
w/W
Mauna Kea (380-1000m)
4
6
8
10
12
14
200 400 600 800 1000Depth (meters)
18.40
18.45
18.50
18.55
18.60
18.65200 400 600 800 1000
Depth (meters)
Estimating dispersivity in the Hawaii melting region
From DePaolo, JGR, 1996
1 km
Spiegelman et. al, 2001 (JGR, 106, 2061-2077)
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 (?)
OK, so what do we think we know......?
Where we are going in the next 2 weeks....
Geochemistry Tutorials....