Lecture 9: Oceanic Basalts

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
Normalised
Concentration
Source
1
Melt
LaNd Sm
Rb Sr
Lu
Residue
1
1
La Nd Sm Lu
Rb Sr
Low Sm/Nd ► low 143 Nd/ 144 Nd
High Rb/Sr ► high 87 Sr/ 86 Sr
La Nd Sm Lu
Rb Sr
High Sm/Nd ► high 143 Nd/ 144 Nd
Low Rb/Sr ► low 87 Sr/ 86 Sr

0.5130
0.5120
ε
143
Nd = {
Nd/ 144 Nd s -1} x 10 4
143
Nd/ 144 Nd CHUR
0.5110
Residual mantle
143 Nd/
144 Nd
0.5100
0.5090
0.5080
Partial melts of
the mantle have
lower Sm/Nd than
their sources
because D Nd

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
MORB
Melts have high Rb/Sr and low
Sm/Nd, so evolve high 87 Sr/ 86 Sr
and low 143 Nd/ 144 Nd
143
Nd/ 144 Nd
Depleted
Quadrant
CHUR
Residues have low Rb/Sr and
high Sm/Nd, so evolve low
87
Sr/ 86 Sr and high 143 Nd/ 144 Nd
or ε Nd
Continental
Crust
High 143 Nd/ 144 Nd – ancient
LREE depletion
Enriched
Quadrant
La
Lu
Low 143 Nd/ 144 Nd – ancient
LREE enrichment
87
Sr/ 86 Sr
or ε Sr

MORB –
Relatively homogeneous source.
Depleted in incompatible elements and 87 Sr/ 86 Sr and
143
Nd/ 144 Nd 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
Basalt
MORB
La
OIB
Lu
La
Lu
Many OIB fall between CHUR
and MORB implying a source
that is depleted relative to
CHUR but less depleted than
the MORB source.
143
Nd/ 144 Nd
or ε Nd
Depleted
Quadrant
CHUR
Continental
Crust
Enriched
Quadrant
La
Lu
However, OIB magmas tend
to be enriched in
incompatible trace elements
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.
87
Sr/ 86 Sr
or ε Sr

OIB –
Source
La Lu
LREE depleted
Basalt
La Lu
LREE enriched
Apparent paradox is mainly due to
differences in the degree of partial
melting.
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.

MORB
OIB
143
Nd/ 144 Nd
Depleted
Quadrant
CHUR
or ε Nd
Enriched
Quadrant
Continental
Crust
87
Sr/ 86 Sr
OIB MORB
Continental
Crust
or ε Sr
650 km
Transition
Zone
Depleted
Upper Mantle
Primordial
Lower Mantle

DMM
CHUR
HIMU
EM2
EM1
From: Hofmann, Nature 385, 219-229 (1997)

OIB Andean Margin MORB IAB OIB
Well Stirred
Depleted Upper
Mantle
Recycled
Ocean
Crust
Less Well
Stirred
Ancient “streaks”
And “blobs”

Eiler et al. (1996) Geochim Cosmochim Acta 61 2281-2293

143 Nd/
144 Nd
OIB
Peridotite
Massifs
87
Sr/ 86 Sr
Zindler & Hart (1984) Annu. Rev. Earth Planet. Sci. 14, 493-571

Continental Crust

15.70
15.60
15.50
15.40
15.30
8.5
Northern Hemisphere Reference Line
8.4
Geochron
8.3
Δ7/4
μ = 8.2
17.0 17.5 18.0 18.5 19.0 19.5 20.0 20.5
206 Pb/
204 Pb
207 Pb/
204 Pb

The DUPAL Anomaly
Δ7/4 > 11
Δ7/4>3
Δ7/4 > 7
Hart Nature 309, 753-757 (1984)

Mantle Models
Fine scale
heterogeneity
Large scale
layering
Zindler et al (1984) Earth Planet. Sci. Lett. 70, 175-195

250
N
200
150
MORB
100
50
OIB
Primordial
He > 50 R a
0
10 20 30
3
He/ 4 He (R/R a )
MORB = 8 ± 2Ra
Some OIB (though by no means all) > 8 Ra

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
3
He/ 4 He
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)
Mid Ocean
Ridges
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

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 3 He/ 4 He 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