MORB source

Lecture 9:
Oceanic Basalts
So-called “spidergrams” – concentration normalised to estimate of “primitive
mantle”.
Mostly chondrite-normalised except for volatile elements
Incompatible
Compatible
Unusual
to plot
major
elements
Rb, Sr, Nd and Sm are all incompatible during mantle melting i.e. they
partition preferentially into the melt phase.
Nd is more incompatible than Sm and Rb is more incompatible than Sr
Source
Normalised
Concentration
1
Melt
La Nd Sm
Rb
Sr
Lu
Residue
1
1
La Nd Sm
Lu
Rb
Sr
Low Sm/Nd ► low 143Nd/144Nd
High Rb/Sr ► high 87Sr/86Sr
La Nd Sm
Lu
Rb
Sr
High Sm/Nd ► high 143Nd/144Nd
Low Rb/Sr ► low 87Sr/86Sr
0.5130
ε Nd = {
143Nd/144Nd
0.5120
143Nd/144Nd
143Nd/144Nd
s
}
- 1 x 104
CHUR
Residual mantle
high Sm/Nd - +ve ε Nd
0.5110
0.5100
Partial melts of
the mantle have
lower Sm/Nd than
their sources
because DNd <DSm
0.5090
0.5080
0.5070
Evolves –ve
ε Nd
0.5060
0.0
1.0
2.0
Time (Ga)
3.0
4.0
Concentrate solely on oceanic basalts:
No possibility of sampling continental material en
route to eruption – any isotopic variation observed
can be safely attributed to the mantle source
Two main classes of oceanic basalts:
Mid-Ocean Ridge Basalts – MORB
Ocean Island Basalts - OIB
Basalt & Source
La
Lu
Melts have high Rb/Sr and low
Sm/Nd, so evolve high 87Sr/86Sr
and low 143Nd/144Nd
MORB
Residues have low Rb/Sr and
high Sm/Nd, so evolve low
87Sr/86Sr and high 143Nd/144Nd
CHUR
143Nd/144Nd
or εNd
Depleted
Quadrant
Continental
Crust
Enriched
Quadrant
High 143Nd/144Nd – ancient
LREE depletion
La
87Sr/86Sr
or εSr
Lu
Low 143Nd/144Nd – ancient
LREE enrichment
MORB –
Relatively homogeneous source.
Depleted in incompatible elements and 87Sr/86Sr and
143Nd/144Nd indicate ancient “time-integrated”
depletion.
MORB sample upper mantle that was depleted in
incompatible elements by the extraction of the
continental crust.
Upper mantle represents the complementary reservoir
from which continental crust was extracted.
Source
La
MORB
OIB
Basalt
Lu La
Lu
However, OIB magmas tend
to be enriched in
incompatible trace elements
CHUR
143Nd/144Nd
or εNd
Depleted
Quadrant
Continental
Crust
Enriched
Quadrant
La
87Sr/86Sr
or εSr
Many OIB fall between CHUR
and MORB implying a source
that is depleted relative to
CHUR but less depleted than
the MORB source.
Lu
So, Rb/Sr and Sm/Nd
observed in OIB do not
reflect the “time-integrated”
Rb/Sr and Sm/Nd of the OIBsource mantle represented
by the isotope data.
OIB –
Source
La
Apparent paradox is mainly due to
differences in the degree of partial
melting.
Basalt
Lu
LREE depleted
La
Lu
LREE enriched
If OIB represent small melt fractions
than MORB, then even a relatively
depleted source can yield LREEenriched melts – especially so if
melting starts in the garnet field
because HREE are compatible in
garnet.
However, isotope data still indicate
that OIB sample (or preserve) a
greater variety of mantle
heterogeneity.
OIB
MORB
CHUR
143Nd/144Nd
Depleted
Quadrant
or εNd
Enriched
Quadrant
Continental
Crust
87Sr/86Sr
or εSr
OIB MORB
Continental
Crust
Depleted
Upper Mantle
650 km
Transition
Zone
Primordial
Lower Mantle
?
?
DMM
CHUR
HIMU
EM2
EM1
From: Hofmann, Nature 385, 219-229 (1997)
OIB
Andean Margin
MORB
IAB
Well Stirred
Depleted Upper
Mantle
Recycled
Ocean
Crust
Less Well
Stirred
Ancient “streaks”
And “blobs”
OIB
Eiler et al. (1996) Geochim Cosmochim Acta 61 2281-2293
143Nd/144Nd
OIB
Peridotite
Massifs
87Sr/86Sr
Zindler & Hart (1984) Annu. Rev. Earth Planet. Sci. 14, 493-571
Continental Crust
15.70
G eo
207Pb/204Pb
15.60
ch ro
n
8.5
8.4
8.3
re
he
p
is
m
e
H
rn
e
r th
o
N
Δ7/4
15.50
μ = 8.2
re
e
f
Re
ine
L
e
nc
15.40
15.30
17.0
17.5
18.0
18.5
19.0
206Pb/204Pb
19.5
20.0
20.5
The DUPAL Anomaly
Δ7/4 > 11
Δ7/4 > 7
Hart Nature 309, 753-757 (1984)
Δ7/4>3
Mantle Models
Fine scale
heterogeneity
Large scale
layering
Zindler et al (1984) Earth Planet. Sci. Lett. 70, 175-195
250
200
MORB
150
N
OIB
100
Primordial
He > 50 Ra
50
0
10
20
3He/4He
(R/Ra)
MORB = 8 ± 2Ra
Some OIB (though by no means all) > 8 Ra
30
The Geochemical “Standard Model”
Continental Crust
&
Lithospheric Mantle
Best estimates suggest only 1/3-1/2 of
mantle is depleted to form Continents.
Depleted Upper Mantle
Source of MORB and
apparently devoid of high
3He/4He
Primitive Lower Mantle
Somewhere there is a source of
primitive He which suggests
mantle that has never melted
Kárason & van der Hilst (2000)
AGU Monog. 121, 277-288
Kellogg et al. Science 283, 1881-1884 (1999)
A Numerical Model
(Van Keken and Ballentine 1998; 1999)
Formulation:
330,000 Tracers
U+Th+K production of Heat, Ar and He.
Degassing
Phase changes and P,T dependant rheology
Internal heating and U+Th concentration
Secular cooling of mantle and core
Benchmarks
Surface heat flow
Plate velocity
Viscosity profile
Caveat
Not spherical
Low effective temperature dependence
No continental crust formation
Mid Ocean
Ridges
What we think we know:
The depleted upper mantle is broadly complementary to the continents.
The uppermost mantle (MORB source) is fairly homogeneous and
devoid of primordial He.
The OIB source is more heterogeneous and includes both enriched and
primordial material.
Possible enriched components include subducted ocean crust,
terrigeneous and pelagic sediments, delaminated continental mantle
and intra-mantle differentiates.
The heterogeneities need to persist for Ga to evolve their isotopic
signatures. Until recently this was thought to require them to be large
but this is now less clear.
A source of high 3He/4He is required and this is most likely primordial
mantle that has never melted. Whether this is a discrete layer or
admixed into the mantle remains controversial

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