Growth of the
Continental Crust
Lecture 48
Age of the Crust
• The oceanic crust is ephemeral; its mean age is
60 Ma and, with the exception of possible Permian
age crust preserved in the Eastern Mediterranean, it
is nowhere older than about 167Ma.
• In contrast, the continental crust is much older. How
old?
o When did production of continental crust begin?
o Has its production been constant through time?
o Is continental crust destroyed?
The Acasta Gneisses
•
•
•
The oldest crustal
rocks, Acasta gneisses
in Canada’s
Northwest Territory
have U-Pb zircon ages
of 3.98 Ga.
These oldest rocks
define the end of the
Hadean and
beginning of the
Archean eons.
One zircon from the
Acasta gneisses has a
4.20±0.06 Ga core
surrounded by a rim
with an age of 3.8 to
3.9 Ga indicating
these gneisses formed
from even older,
Hadean crustal
protoliths.
Jack Hills
Zircons
• Even older zircon cores,
with ages of up to 4.4 Ga,
have been found in midto late-Archean quartzite
(metamorphosed
conglomerate) from the
Jack Hills of western
Australia.
• The metasedimentary
conglomerate itself is
thought to be about 3
billion years old.
Jack Hills
Jack Hills Zircons
•
•
The zircons have REE patterns indicating that they originally
crystallized in a granitic magma.
Furthermore, the zircons have δ18O that varies from +4.8 to +8,
indicating that magma interacted with water as it cooled.
Nuvvuagittuq Greenstone Belt
•
•
•
•
An apparent 146Sm-142Nd
isochron suggests an age of
4388 Ma for rocks mafic
amphibolites from the
Nuvvuagittuq Greenstone
Belt of Labrador.
However, 147Sm-143Nd and UPb zircon ages are only 3.8
Ga.
Whether this is the age of
these rocks or simply a mixing
line when an Eoarchean
magma assimilated Hadean
crust is highly controversial.
Either way, however, these
data provide evidence of
very early crust and early
differentiation of the Earth.
Hadean Crust
• It is clear now that the process of forming
continental crust began in the Hadean (the time
prior to 4 Ga from which no rocks survive).
• Essentially none of this Hadean crust survives.
Indeed, even very little early Archean crust survives.
• How much Hadean/Early Archean crust was
created is very much debated.
Growth through time
•
•
•
•
The rate of crustal growth
through time is still not known.
Armstrong argued the the mass
of continental crust has been
virtually constant since the
Hadean(curve E) because the
rate of creation has been
matched by the rate of
destruction. This idea cannot be
ruled out.
The idea of young crust (curve A
- Hurley & Rand) can be ruled
out.
Studies of detrital zircons show
that surviving crust has been
produced in pulses, perhaps
related to cycles of
supercontinent construction and
breakup.
Sm-Nd Model Ages and
Crustal Growth
• Because there is little
fractionation of Sm
from Nd in the crust,
Sm-Nd model ages
provide a means of
“seeing through”
metamorphic ages
and deducing the time
regions of the
continents were first
created.
Growth of the Western US
• Bennett and DePaolo
used Sm-Nd model
ages to define broad
regions of crust
creation in the western
US.
• Ages become
progressively younger
away from the
Wyoming craton,
suggesting the crust
was built progressively
outward.
Growth of the Western US
• εNd indicates the initial
continental blocks
themselves were
mixtures of mantlederived magmas and
older crustal material.
• Young granitic plutons
in the southwestern US
have been produced
by remelting of the
older continental
protoliths.
How were the continents
created?
• So far, we have talked about the when, but not the
how.
• Today, there are 3 main mechanisms by which new
crust is created:
o Rifting (e.g., African and Rio Grande Rifts) and divergent
plate boundaries (North Atlantic Tertiary Province is an
example).
o Flood basalt events and associated underplating by
basaltic magma associated with mantle plumes (and
accretion of oceanic plateaus).
o Subduction zones, for example the Andes, the Pacific
Northwest, the Alaska Peninsula and the Aleutians.
• Which is most important?
o Of course, geochemistry provides the essential clue.
The Clue
Subduction Zones and
Subduction Zone Processes
•
There are clear similarities between the continental crust and the island arc lava:
both exhibit incompatible element enrichment, negative Nb-Ta anomalies, and
positive Pb anomalies.
•
It is also true that at present, most new additions to crust occur in subduction zones.
•
Let’s consider the geochemistry of subduction zone volcanism in a bit more detail.
Major Elements in Arc Magmas
• Magmas found in island arcs & continental margins are
predominantly andesitic.
• It is unlikely that andesite is the principle magma produced in
arcs. The lower parts of arc volcanic edifices may be basaltic.
• Andesite cannot be produced by partial melting of the
mantle, except at shallow depth under high water pressure.
Most arcs sit about 100 km above the Benioff zone, and
magmas may be generated close to this depth.
• A safer bet is that the primary magma is actually basaltic, of
which andesites are fractional crystallization products.
• In major element composition, island arc volcanics (IAV) are
not much different from other volcanic rocks. Compared with
MORB, the major difference is perhaps simply that siliceous
compositions are much more common among the island arc
volcanics. Most IAV are silica-saturated or oversaturated; silica
undersaturated magmas (alkali basalts) are rare.
Two Magma Series
•
Two principal magma series are
recognized, one called tholeiitic, the
other called calc-alkaline. There are
two principal differences between
these rock series.
o
o
•
Fe
First, tholeiites differentiate initially toward higher Fe
and tend to maintain higher Fe/Mg than the calcalkaline lavas. This reflects the suppression of
plagioclase crystallization as a result of (1) higher
pressure and (2) higher water content of the magma.
The second difference is that calc-alkaline magmas
are richer in alkalis (K2O and Na2O) than tholeiites.
Indeed, the calc-alkaline magmas are defined as
those that have Na2O + K2O ≈ CaO, whereas tholeiites
have Na2O + K2O < CaO.
Kay et al. found that for the Aleutians
occurrence of these 2 series related to
tectonic environment. Tholeiites occur
in extensional environments within the
arc where magmas ascend relatively
rapidly and undergo fractional
crystallization at low pressure. Calcalkaline lavas tend to occur in
compressional environments where
they cannot so readily ascend, and
undergo crystallization at greater Alkalis: K O+Na O
2
2
depth.
Mg
Controls on Magma Composition
•
•
•
Plank and Langmuir argued that
crustal thickness determines the
height of the mantle column
available for melting.
Most island arc volcanoes are
located above the point where
the subducting lithosphere
reaches a depth of 100–120 km.
This suggests that melting begins
at a relatively constant depth in
all island arcs.
If this is so, then the distance over
which mantle can rise and
undergo decompressional
melting will be less if the arc crust
is thick, leading to smaller extents
of melting beneath arcs with
thick crust, and higher Na6.0 and
Ca6.0 in the parental magmas.
Rare Earth in IAV
•
•
•
Island arc volcanics (IAV) are
typically, but not uniformly,
somewhat LRE-enriched.
Some show relative middle
rare earth depletion,
probably a result of
amphibole fractionation.
Negative Ce-anomalies
occur in some lavas. Ce,
normally in the III valence
state, can be in the IV
valence state under oxidizing
conditions at the Earth’s
surface. The Ce-anomalies
suggest an inherited
sedimentary component in
these lavas.
Alkali, Alkaline Earth Enrichment
• Another feature of IAV
is enrichment in the
alkali and alkaline
earth trace elements,
Rb, Cs, Sr, and Ba.
• This is apparent in the
Ba/La vs La/Sm plot.
Sr & Nd Isotope Ratios
Sr and Nd isotope ratios span a large range, overlapping a bit with
MORB and extensively with OIB. There is, however, a tendency to
plot to higher 87Sr/86Sr for a given εNd than the ‘mantle array. This
suggests an inherited component of altered oceanic crust.
Pb Isotope Ratios
Pb isotope ratios in IAV tend to define arrays that overlap
MORB at one end and marine sediments at the other indicating an inherited sedimentary component.