FORMATION OF EARTH’S CRUSTContinental and oceanic crust

Melissa MaisonneuveOct 2011

MAJOR ELEMENTS & MINERALOGY

Oceanic (MORB) Continental SiO2 45.2 49.4 60.3

TiO2 0.7 1.4 1.0

Al2O3 3.5 15.4 15.6

MgO 37.5 7.6 3.9 FeO 8.5 10.1 7.2 CaO 3.1 12.5 5.8 Na2O 0.6 2.6 3.2

K2O 0.1 0.3 2.5

Total 99.2 99.3 99.5

Mantle

*in weight percent

Mineralogy (oxygen units, XFe3+ = 0.10)Quartz 0.0 0.0 13.0 Feldspar 13.2 57.3 64.3 C-pyroxene 6.7 25.7 5.9 O-pyroxene 18.3 4.1 14.7 Olivine 59.9 9.9 0.0 Oxides 1.8 3.0 2.0

MAJOR ELEMENTS & MINERALOGY

Oceanic (MORB) Continental SiO2 45.2 49.4 60.3

TiO2 0.7 1.4 1.0

Al2O3 3.5 15.4 15.6

MgO 37.5 7.6 3.9 FeO 8.5 10.1 7.2 CaO 3.1 12.5 5.8 Na2O 0.6 2.6 3.2

K2O 0.1 0.3 2.5

Total 99.2 99.3 99.5

Mantle

*in weight percent

Mineralogy (oxygen units, XFe3+ = 0.10)Quartz 0.0 0.0 13.0 Feldspar 13.2 57.3 64.3 C-pyroxene 6.7 25.7 5.9 O-pyroxene 18.3 4.1 14.7 Olivine 59.9 9.9 0.0 Oxides 1.8 3.0 2.0

TRACE ELEMENTSC

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CRUSTAL FORMATION WITH TIME

Early crust hot mantle melting (komatiites, TTGs)

Increased stability of cratons over time allowing build-up of crust

Taylor & McLennan 1995

PUNCTUATED CRUSTAL PRODUCTION?

McCulloch & Bennett (1994)

Sm-Nd model crustal ages from Aus, N.Am, Scan. cratons (ziron & whole rock)

TRACE ELEMENTS

TWO-STAGE MODEL (HOFMANN 1988)

Primitive mantle

Continental crust

1) extraction of continental crust (1.5% melting*)

2) formation of oceanic crust (8-10% melting*)

Oceanic crust (MORB)

*batch or partial melting- either are presented by the author

THERMAL PULSE MODEL (DAVIES 1995)

Thermal pulses

Mantle overturn events

THERMAL PULSE MODEL (DAVIES 1995)

Thermal pulses

Mantle overturn events Increased production @ ridges (thicker oceanic crust)

Crustal thickening (stabilize of cratons?)

Wait…. no subduction?!

HOTTER

NOT SO HOT

Flood basaltsSpreading centre

LOW-ANGLE SUBDUCTION (FLAT-SUBDUCTION)

Smithies et al (2003)

Many Archean low-Mg TTGs Dearth of calc-alkaline-type rocks in Archean Continental crust: p. melting right off the oceanic slab (without

wedge) Eclogite residue from TTG formation delaminates back to mantle

BACK TO …NOW

“GOD DIAGRAM” DON FRANCIS

“GOD DIAGRAM” DON FRANCIS

REFERENCESDavies, G.F. (1995) Punctuated tectonic evolution of the earth. Earth & Planetary Science Letters, 136: 363-379.

Francis, D. Course notes from EPSC 423 Igneous Petrology, McGill University (2009)

Hawkesworth, C.J. & Kemp, I.S. (2006) Evolution of the continental crust. Nature, 443: 811-817

Hofmann, A.W. (1988) Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust. Earth & Planetary Science Letters, 90: 297-314.

McCulloch, M.T. & Bennett, V.C. (1994) Progressive growth of the Earth’s continental crust and depleted mantle: Geochemical constraints. Geochimica et Cosmochimica Acta, 58(21): 4717-4738.

Smithies, R.H., Champion, D.C. & Cassidy, K.F. (2003) Formation of Earth’s early Archaean continental crust. Precambrian Research, 127: 89-101

Taylor, S.R. & McLennan, S.M. (1995) The geochemical evolution of the continental crust. Reviews of Geophysics, 33: 241-265.

MIDTERM QUESTION

Why is it difficult to model crustal production in the Archaen (e.g. any time before the Phanerozoic)? Tectonics, geochemistry…