FORMATION OF EARTHS CRUST Continental and oceanic crust

FORMATION OF EARTH’S CRUST Continental and oceanic crust Melissa Maisonneuve Oct 2011

MAJOR ELEMENTS & MINERALOGY Mantle Si. O 2 Ti. O 2 Al 2 O 3 Mg. O Fe. O Ca. O Na 2 O K 2 O Total 45. 2 0. 7 3. 5 37. 5 8. 5 3. 1 0. 6 0. 1 99. 2 Oceanic (MORB) 49. 4 15. 4 7. 6 10. 1 12. 5 2. 6 0. 3 99. 3 Mineralogy (oxygen units, XFe 3+ = 0. 10) Quartz 0. 0 Feldspar 13. 2 57. 3 C-pyroxene 6. 7 25. 7 O-pyroxene 18. 3 4. 1 Olivine 59. 9 Oxides 1. 8 3. 0 Continental 60. 3 1. 0 15. 6 3. 9 7. 2 5. 8 3. 2 2. 5 99. 5 13. 0 64. 3 5. 9 14. 7 0. 0 2. 0 *in weight percent

MAJOR ELEMENTS & MINERALOGY Mantle Si. O 2 Ti. O 2 Al 2 O 3 Mg. O Fe. O Ca. O Na 2 O K 2 O Total 45. 2 0. 7 3. 5 37. 5 8. 5 3. 1 0. 6 0. 1 99. 2 Oceanic (MORB) 49. 4 15. 4 7. 6 10. 1 12. 5 2. 6 0. 3 99. 3 Mineralogy (oxygen units, XFe 3+ = 0. 10) Quartz 0. 0 Feldspar 13. 2 57. 3 C-pyroxene 6. 7 25. 7 O-pyroxene 18. 3 4. 1 Olivine 59. 9 Oxides 1. 8 3. 0 Continental 60. 3 1. 0 15. 6 3. 9 7. 2 5. 8 3. 2 2. 5 99. 5 13. 0 64. 3 5. 9 14. 7 0. 0 2. 0 *in weight percent

Concentration/Primitive mantle TRACE ELEMENTS

CRUSTAL FORMATION WITH TIME Early crust hot mantle melting (komatiites, TTGs) Increased stability of cratons over time allowing build-up of crust Taylor & Mc. Lennan 1995

PUNCTUATED CRUSTAL PRODUCTION? Sm-Nd model crustal ages from Aus, N. Am, Scan. cratons (ziron & whole rock) Mc. Culloch & Bennett (1994)

TRACE ELEMENTS

TWO-STAGE MODEL (HOFMANN 1988) Continental crust Primitive mantle Oceanic crust (MORB) 1) extraction of continental crust (1. 5% melting*) 2) formation of oceanic crust (8 -10% melting*) *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) Spreading centre NOT SO HOTTER Flood basalts Thermal pulses Mantle overturn events Increased production @ ridges (thicker oceanic crust) Crustal thickening (stabilize of cratons? ) Wait…. no subduction? !

LOW-ANGLE SUBDUCTION (FLAT-SUBDUCTION) 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 Smithies et al (2003)

BACK TO …NOW

“GOD DIAGRAM” DON FRANCIS

“GOD DIAGRAM” DON FRANCIS

REFERENCES Davies, 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, Mc. Gill 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. Mc. Culloch, 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. & Mc. Lennan, 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…