Origin and Evolution of the Earth's Crust Two Kinds of Crust

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?

1 Oceanic Crust

 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.

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 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

Jack Hills

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.