Origin and Evolution of the Earth’s Crust
EAS 302 Lecture 9
• Oceanic Two Kinds of Crust – 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. • 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?
Sm-Nd decay system
• 147Sm decays to 143Nd with half life of 106 Ga H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar • “Isochron” equation 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 for this system is: Fr Ra Ac The Rare Earth Elements 143 143 143 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Nd ⎛ Nd ⎞ Nd λt Ac Th Pa U 144 = ⎜ 144 ⎟ + 147 (e −1) Nd ⎝ Nd ⎠0 Sm 143 Nd ⎛ 143 Nd ⎞ 143 Nd 144 ≈⎜ 144 ⎟ + 147 λt Nd ⎝ Nd ⎠0 Sm
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.
Epsilon Nd • 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 “initial” 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
• In Western North America 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.