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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
on
cen
trati
on
/Pri
mit
ive m
an
tle
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…