Chapter 8: Major Elements

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• Trace Element Behavior
• Size - Smaller ions are preferentially
  incorporated into the solid over the liquid.
• Charge - Ions with higher charge are
  preferentially incorporated into the solid over
  the liquid.

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Trace Element Fractionation

• The uneven distribution of an ions between two
  competing phases (e.g. solid and liquid) in
  equilibrium

• Partition Coefficient (KD or D) defines the
  magnitude of the fractionation
• KD
• CS the concentration of some element in the
  solid phase
• CL the concentration of some element in the
  liquid phase

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KD

• incompatible elements are concentrated in the
  melt
• (KD or D) 1
• compatible elements are concentrated in the solid
  
• KD or D 1

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Compatibility depends on minerals and melts
involved.
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• Incompatible Trace Elements
• High Field Strength Elements (HFSE) Zr, Hf, Ti,
  Nb, Ta - small, highly charged (4 or 5)
• Large Ion Lithophile Elements (LILE) K, Rb,
  Cs, Ba, Sr, - large size, low charge (1 or
  2) all fluid soluble
• Rare Earth Elements (REE) La to Lu -
  relatively large, 3 charge divided into LREE,
  MREE, and HREE show uniform decrease in size.

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• Compatible Trace Elements
• Most commonly transition metals Ni, Vr, Co, V -
  generally fit into specific mafic minerals
• Ni in olivine Cr in magnetite, Cpx
• Specific minerals
• e.g., HREE in garnet Eu2, Ba, Sr, in feldspars

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Crystallizing Minerals
Ni is compatible (KD gt 1) in olivine. Olivine
crystallization depletes residual magmas in Ni.
Zr is incompatible (KD lt 1). Residual magmas
are continuously enriched in Zr.
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Depth of melting from REE patterns

10.00
60 Ol 15 Opx 15 Cpx 10Plag
8.00
6.00
sample/chondrite
4.00
2.00
0.00
La Ce Nd Sm Eu Tb Er Yb
Lu
10.00
67 Ol 17 Opx 14 Cpx 3 Sp
8.00
6.00
sample/chondrite
4.00
2.00
0.00
56
58
60
62
64
66
68
70
72
La Ce Nd Sm Eu Tb Er Yb
Lu
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Multi-Element Diagrams
Tectonic Setting from distinctive trace element
patterns
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Rb-Sr Isotope System

• 87Rb ? 87Sr a beta particle (t1/2 70 Ga)
• Rb is more incompatible than Sr, thus Rb is
  partitioned into melt preferentially during
  partial melting
• Rb is concentrated in continental crust and
  depleted in the mantle due to partial melting of
  upper mantle through geologic time
• high Rb/Sr ? high 87Sr/86Sr (continental crust)
  low Rb/Sr ? low 87Sr/86Sr (depleted
  upper mantle)

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Sr isotope evolution curve
enriched mantle
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IAV Islands Arc Volcanics
OIB Oceanic Island Basalts
MORB Mid Ocean Ridge Basalts
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Sm-Nd Isotope System

• 147Sm ? 143Nd alpha particle (t1/2 106
  Ga)
• Sm is more compatible than Nd, thus Sm is
  partitioned into the solid preferentially during
  partial melting.
• Sm is concentrated in the mantle and depleted in
  the continental crust due to partial melting of
  upper mantle through geologic time
• high Sm/Nd ? high 143Nd/144Nd (depleted upper
  mantle) low Sm/Nd ? low 143Nd/144Nd (continental
  crust)

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Evolution curve is opposite to Rb - Sr
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?Nd is just another (easier) way to express
143Nd/144Nd ratios ?Nd gt 0 depleted
mantle ?Nd lt 0 enriched mantle or
continental crust
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IAV Islands Arc Volcanics
OIB Oceanic Island Basalts
MORB Mid Ocean Ridge Basalts
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Sr-Nd isotope plot
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Mid-Ocean Ridge Basalts (MORB)
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Mid-Ocean Ridge Basalts (MORB)
Figure 13-1. After Minster et al. (1974) Geophys.
J. Roy. Astr. Soc., 36, 541-576.
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MORB Chemistry

• MORBs are dominantly olivine tholeiites with
  distinctively low K2O (lt 0.2).
• All incompatible trace elements (LREE, K, Rb, Cs,
  Ba, Pb, Sr, Th, U, Ce, Zr, Hf, Nb, Ta, and Ti)
  are depleted.
• No depletion in HREE elements indicating
  relatively shallow melting within the spinel
  stability field (lt70 depth).
• Sr and Nd isotope ratios are depleted indicating
  a depleted upper mantle source region.

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MORB Chemistry

• No depletion in HREE indicating relatively
  shallow melting depths (lt70 km)

• Strong depletions in highly incompatible trace
  elements (REE, LILE) indicating depleted mantle
  source

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• MORBs 87Sr/86Sr lt 0.7035 and ?Nd gt 5,
  depleted mantle source

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MORB Petrogenesis

• Decompression partial melting associated with
  near-adiabatic rise of mantle due to plate
  separation.
• N-MORB melting initiated 60-80 km depth in
  upper depleted mantle where it inherits depleted
  trace element and isotope signatures.

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Oceanic Island Basalts (OIB)
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Oceanic Island Basalts (OIB)

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OIB Chemistry

• OIBs are range from olivine tholeiites to highly
  alkaline basalts with relatively high TiO2, Na2O,
  K2O, and P2O5.
• All incompatible trace elements (LREE, K, Rb, Cs,
  Ba, Pb, Sr, Th, U, Ce, Zr, Hf, Nb, Ta, and Ti)
  are enriched in OIB magmas with respect to MORBs.
• HREE elements are depleted indicating deep
  melting within the garnet stability field (gt70-80
  depth).
• Sr and Nd isotope ratios are relatively enriched
  with respect to MORBs indicating undepleted
  (primitive) and/or enriched mantle source
  components.

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Types of OIB Magmas

• 1. Tholeiitic series (dominant type)
• Shield-building stage - tremendous outpourings
  of tholeiitic basalts

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Types of OIB Magmas

• 2. Alkaline series (small volumes)

Post-shield stage - Waning activity. Lavas
are more diverse, with a larger proportion of
chemically differentiated magmas
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OIB Chemistry

• Depletion in HREE indicating relatively deep
  melting depths (gt80 km)

• Strong enrichment in highly incompatible trace
  elements (REE, LILE, HFSE) indicating undepleted
  (primitive) and/or enriched mantle sources

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• OIBs 87Sr/86Sr 0.7035 to 0.710 and ?Nd 5
  to -5, primitive and/or enriched mantle
  source(s)

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Figure 14-6. After Zindler and Hart (1986),
Staudigel et al. (1984), Hamelin et al. (1986)
and Wilson (1989).
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OIB Petrogenesis

• Adiabatic decompression melting associated with
  deep upwelling of anomalously hot (100-200 C)
  mantle.
• Partial melting initiated gt100 km depth well
  within garnet stability field (explains common
  HREE depletion).
• Relatively enriched trace element and isotope
  signatures indicate enriched mantle source
  components.

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OIB Petrogenesis
Continental Reservoirs
DM
OIB
EM and HIMU from crustal sources (subducted OC
CC seds)
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Continental Flood Basalts (CFB)

• Large Igneous Provinces (LIPs)
• Oceanic plateaus
• Some rifts
• Continental flood basalts (CFBs)

Figure 15-1. Columbia River Basalts at Hat Point,
Snake River area. Cover of Geol. Soc. Amer
Special Paper 239. Photo courtesy Steve Reidel.
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Tectonic Setting of CFBs

• Continental hot spots
• Columbia River Plateau Yellowstone
• Deccan Traps
• Continental rifting
• Parana-Entendeka
• CAMP (Central Atlantic Magmatic Province)

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Columbia River Basalts
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Columbia River Basalts
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CFB Chemistry

• CFBs are mostly tholeiitic and similar to OIB
• Incompatible trace elements and isotopes are
  enriched, like OIB BUT show much more variability
  toward more enriched compositions.
• Distinctive enrichments of the most highly
  incompatible elements (K, Ba, Rb, Th, Pb, and
  LREE) over typical OIB. Noticeable depletions in
  HFSE (Nb, Ta) compared to OIB.
• Sr and Nd isotope ratios overlap with OIB, but
  extend to more enriched compositions (higher
  87Sr/86Sr and lower ?Nd).

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CFB Chemistry

• No depletion in HREE indicating relatively
  shallow melting depths (lt70 km)

• Strong enrichments in highly incompatible trace
  elements (REE, LILE) indicating enriched mantle
  source(s)

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Distinctive ernirchments in Ba and Pb and
depletions in HFSE (Nb, Ta) compared to OIB.
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• CFBs 87Sr/86Sr 0.7035 to 0.713 and ?Nd 5
  to -10, enriched mantle source(s)

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CFB Petrogenesis

• Basically OIB petrogenesis added components
  from continental lithospheric mantle and crust

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Figure 15-14. Diagrammatic cross section
illustrating possible models for the development
of continental flood basalts. DM is the depleted
mantle (MORB source reservoir), and the area
below 660 km depth is the less depleted, or
enriched OIB source reservoir. Winter (20010 An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
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Mantle Sources and Structure

• Upper depleted mantle MORB source
• Lower undepleted enriched OIB source

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