Intraplate magmatism

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Intraplate magmatism
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Intraplate magmatism

• Hotspots
• Rift zones (often associated with hotspots)
• Intra-oceanic plate Tholeitic to alkaline
  series mostly basalts (OIB Oceanic Islands
  Basalts), some differenciated alkaline terms
• Intra-continental plate
• either large tholeitic basaltic provinces (CFB
  Continental Flood Basalts), occasionally bimodal
  (ass. with rhyolites)
• or smaller, alkaline to hyper-alkaline,
  differenciated intrusions/volcanoes
  (syenites/phonolites carbonatites kimberlites
  and more)

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Ocean islands and seamounts Commonly associated
with hot spots

Figure 14-1. After Crough (1983) Ann. Rev. Earth
Planet. Sci., 11, 165-193.
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Oceanic islands
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Hotspots
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Mantle convection and mantle plumes
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Types of OIB Magmas

• Two principal magma series
• Tholeiitic series (dominant type)
• Parental ocean island tholeiitic basalt, or OIT
• Similar to MORB, but some distinct chemical and
  mineralogical differences
• Alkaline series (subordinate)
• Parental ocean island alkaline basalt, or OIA
• Two principal alkaline sub-series
• silica undersaturated
• slightly silica oversaturated (less common
  series)

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Hawaiian Scenario

• Cyclic, pattern to the eruptive history
• 1. Pre-shield-building stage somewhat alkaline
  and variable
• 2. Shield-building stage begins with tremendous
  outpourings of tholeiitic basalts

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Hawaiian Scenario
3. Waning activity more alkaline, episodic, and
violent (Mauna Kea, Hualalai, and Kohala). Lavas
are also more diverse, with a larger proportion
of differentiated liquids 4. A long period of
dormancy, followed by a late, post-erosional
stage. Characterized by highly alkaline and
silica-undersaturated magmas, including alkali
basalts, nephelinites, melilite basalts, and
basanites
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Evolution in the Series

• Tholeiitic, alkaline, and highly alkaline

Figure 14-2. After Wilson (1989) Igneous
Petrogenesis. Kluwer.
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Trace Elements

• The LIL trace elements (K, Rb, Cs, Ba, Pb2 and
  Sr) are incompatible and are all enriched in OIB
  magmas with respect to MORBs
• The ratios of incompatible elements have been
  employed to distinguish between source reservoirs
  
• N-MORB the K/Ba ratio is high (usually gt 100)
• E-MORB the K/Ba ratio is in the mid 30s
• OITs range from 25-40, and OIAs in the upper 20s
• Thus all appear to have distinctive sources

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Trace Elements

• HFS elements (Th, U, Ce, Zr, Hf, Nb, Ta, and Ti)
  are also incompatible, and are enriched in OIBs gt
  MORBs
• Ratios of these elements are also used to
  distinguish mantle sources
• The Zr/Nb ratio
• N-MORB generally quite high (gt30)
• OIBs are low (lt10)

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Trace Elements REEs
Figure 14-2. After Wilson (1989) Igneous
Petrogenesis. Kluwer.
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MORB-normalized Spider Diagrams
Figure 14-3. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
Data from Sun and McDonough (1989).
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Generation of tholeiitic and alkaline basalts
from a chemically uniform mantle
Figure 10-2 After Wyllie, P. J. (1981). Geol.
Rundsch. 70, 128-153.
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Pressure effects
Figure 10-8 After Kushiro (1968), J. Geophys.
Res., 73, 619-634.
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• Tholeiites favored by shallower melting
• 25 melting at lt30 km tholeiite
• 25 melting at 60 km olivine basalt
• Tholeiites favored by greater partial melting
• 20 melting at 60 km alkaline basalt
• incompatibles (alkalis) initial melts
• 30 melting at 60 km tholeiite

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Isotope Geochemistry

• Isotopes do not fractionate during partial
  melting of fractional melting processes, so will
  reflect the characteristics of the source
• OIBs, which sample a great expanse of oceanic
  mantle in places where crustal contamination is
  minimal, provide incomparable evidence as to the
  nature of the mantle

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Simple Mixing Models
Ternary All analyses fall within triangle
determined by three reservoirs

• Binary
• All analyses fall between two reservoirs as
  magmas mix

Figure 14-5. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
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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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Mantle Reservoirs

• 1. DM (Depleted Mantle) N-MORB source

Figure 14-6. After Zindler and Hart (1986),
Staudigel et al. (1984), Hamelin et al. (1986)
and Wilson (1989).
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2. BSE (Bulk Silicate Earth) or the Primary
Uniform Reservoir
Figure 14-6. After Zindler and Hart (1986),
Staudigel et al. (1984), Hamelin et al. (1986)
and Wilson (1989).
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• 3. EMI enriched mantle type I has lower
  87Sr/86Sr (near primordial)
• 4. EMII enriched mantle type II has higher
  87Sr/86Sr (gt 0.720, well above any reasonable
  mantle sources

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

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5. PREMA (PREvalent MAntle)
Figure 14-6. After Zindler and Hart (1986),
Staudigel et al. (1984), Hamelin et al. (1986)
and Wilson (1989).
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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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Pb Isotopes

• Pb produced by radioactive decay of U Th
• 238U ? 234U ? 206Pb
• 235U ? 207Pb
• 232Th ? 208Pb
• Pb isotopes also characterize the different
  reservoirs (see paper presentation Hart 1984)

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Figure 14-8. After Wilson (1989) Igneous
Petrogenesis. Kluwer. Data from Hamelin and
Allègre (1985), Hart (1984), Vidal et al. (1984).
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Kellogg et al. (1999)
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A Model for Oceanic Magmatism
Continental Reservoirs
DM
OIB
EM and HIMU from crustal sources (subducted OC
CC seds)
Figure 14-10. Nomenclature from Zindler and Hart
(1986). After Wilson (1989) and Rollinson (1993).
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Marble cake model for mantle convection mixing
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Continental Flood Basalts

• 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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Trapp volcanism
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LIPs (Large Igneous Provinces)
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CFBs

• Associated to major continental break-up
• or/and to plume head impact

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Figure 15-2. Flood basalt provinces of
Gondwanaland prior to break-up and separation.
After Cox (1978) Nature, 274, 47-49.
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Figure 15-3. Relationship of the Etendeka and
Paraná plateau provinces to the Tristan hot spot.
After Wilson (1989), Igneous Petrogenesis. Kluwer.
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Geochemistry

• Deccan traps basalts

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Bimodal magmas

• Basalts and rhyolites
• Secondary melting?
• Effect of the two eutectics?

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Figure 15-7. Condrite-normalized rare earth
element patterns of some typical CRBG samples.
Winter (2001). An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall. Data from
Hooper and Hawkesworth (1993) J. Petrol., 34,
1203-1246.
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Figure 15-4. Present setting of the Columbia
River Basalt Group in the Northwestern United
States. Winter (2001). An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall. Also
shown is the Snake River Plain (SRP)
basalt-rhyolite province and proposed trace of
the Snake River-Yellowstone hot spot by Geist and
Richards (1993) Geology, 21, 789-792.
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Figure 15-13. A model for the origin of the
Columbia River Basalt Group From Takahahshi et
al. (1998) Earth Planet. Sci. Lett., 162, 63-80.
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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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LIPs and mass extinctions