Origin and Evolution of the Earth’’s Crust EAS 302 Lecture 9
Two Kinds of Crust Oceanic Thin (~6 km) Fairly uniform Basaltic Dark, volcanic rock, rich in Mg, Fe Dense (~2.9 g/cc) impermanent Continental Thick (~35 km on average) Heterogeneous “Granitic” - more properly “granodioritic” (light colored igneous rock, rich in Al, Si) Less Dense (2.7 g/cc) Permanent?
Oceanic crust covers 60% of surface Continually created by sea-floor spreading Origin and fate closedly linked to plate tectonics - which will cover in coming weeks. Today, let’s focus on Continental Crust.
Origin and Evolution of the Continental Crust Questions When did the crust form? How did the crust form? Some possible hypotheses (1) Crust formed early from a late accretionary veneer (of more volatile elements) (2) Crust formed early by crystallization of an early magma ocean (Moon’s crust appears to have formed this way) (3) Crust formed by magmatism through time (Related to what geological process?)
2 Testing the hypotheses
Hypothesis (1) predicts crust should be old and rich in volatile elements Hypothesis (2) predicts crust should be old and rich in incompatible elements Hypothesis (3) predicts crust should be younger and rich in incompatible elements
Composition of the Crust
While the crust is rich in some (moderately) volatile elements such as the alkalis (Na, K, Rb), these elements are also incompatible On the whole, the crust is clearly enriched in incompatible elements (elements concentrated in melts). This is illustrated by the REE (rare earth elements). From its composition, we can conclude the crust was created by magmatism.
3 Age of the Continental Crust
“Conventional Ages” of Continental Crust are relatively young (e.g., North America) But do these ages represent the time the crust was created or simply the last time it was metamorphosed?
Ages in Ga
Sm-Nd decay system
147Sm decays to 143Nd with half life of 106 Ga “Isochron” equation H He Li Be B C N O F Ne for this system is: Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe ! $ Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rd 143 143 147 Nd # Nd& Sm! t $ Fr Ra Ac = + #e' -1& The Rare Earth Elements # & # & 144Nd # 144Nd& 144Nd " % La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu " %0 Ac Th Pa U
143 " 143 % 147 Nd $ Nd' Sm ! $ ' + (t 144Nd $ 144Nd' 144Nd # &0
a Sm-Nd decay system and crust- mantle evolution
When 143Nd/144Nd is plotted against time, slope is proportional to 147Sm/144Nd. Since Nd is more incompatible than Sm, it is more concentrated in the crust than Sm, hence crust has low 147Sm/144Nd, mantle has high 147Sm/144Nd. In time, this leads to low 143Nd/144Nd in crust and high 143Nd/144Nd in mantle.
We can simplify things a bit by comparing Nd isotope ratios to the chondritic (=bulk Earth) value. This is the “epsilon” notation: deviations in parts in 10,000 from chondritic:
143 144 143 144 ⎡( Nd / Nd )sam −( Nd / Nd )Chon ⎤ ε Nd =⎢ 143 144 ⎥×10, 000 ⎣ ( Nd / Nd )Chon ⎦
In this notation, crust has negative values and mantle positive ones.
a Nd isotopic compositions of Southwestern US
Bennett & DePaolo studied the Nd isotopic composition of intrusive igneous rocks in the Western US - both young and old. Young rocks had negative εNd - indicating they are simply remelted crustal material. Older rocks had positive “initial” εNd - indicating they were mantle derived and new additions to crust.
Crustal Residence Times or Sm-Nd Model Ages Once crust is created, there is very little further change in its Sm/Nd ratio. We can therefore extrapolate the 143Nd/144Nd growth back to its intersection with either the chondritic growth curve, or the depleted mantle growth curve. Time at the point of intersection is the “Sm-Nd model age” or “crustal residence time”.
6 Growth of Western North America
In Western North America, Sm-Nd model ages indicate the crust is older than “conventional” ages (it has been internally reprocessed), but younger than the age of the Earth. Conclusion: Crust has grown through time.
Possible Mechanisms of Crustal Growth
Rifting-related Magmatism This is clearly the process creating oceanic crust Subduction-related Magmatism Most important mechanism at present But was it true in the past? Mantle-plumes Responsible for hot-spot volcanism such as Hawaii and Yellowstone Three mechanisms Volcanism, particularly flood basalts Accretion of oceanic plateaus Crustal underplating
7 Subduction-Related Volcanism
Volcanism almost always occurs above subducting lithospheric plates. Mostly likely due to dehydration of subducting oceanic crust When subduction occurs along a continental margin, the magmas add to the volume of continental crust e.g., Andes
The Chemical Fingerprint of Subduction-Related Volcanism
“Subduction-related” or “Island Arc” magmas have distinctive trace element composition Nb, Ta depletion Pb enrichment These characteristics are shared by the continental crust Conclusion: Subduction-related magmatism seems to be the dominant way in the the crust formed.
8 When did the continental crust begin to form?
Oldest known rocks are from the Great Slave Province in Canada and are approximately 4 Ga old. Oldest known mineral is a zircon is from Australian sediments whose metamorphic age is 3.5 Ga. The crystallization ages of these zircons are as old as 4.4 Ga.
Acasta Gneisses -World’’s Oldest Rocks
9 Jack Hills,, Western Australia
Just because an old zircon exists,, how do we know there was continental crust?
(1) zircon does not crystallize from basalt (too soluble). (2) REE pattern of this zircon suggests it formed from a “continental” type magma such as granite.
10 Calculating the REE content of Hadean crustal magmas
From the REE in the zircons, we can calculate the REE concentrations in the melt from which they crystallized We make use of “partition” or “distribution” coefficients These are simply the ratio of the concentration in the melt to the concentration in the mineral Can be determined empirically, experimentally, or theoretically. Calculated “melts” from the oldest zircon have REE patterns characteristic of granites.