Petrology and Ore Deposits Course 10179

1
Petrology and Ore DepositsCourse 10179

• Week 2
• Origin of Magmas by Melting of the Mantle and
  Crust
• Evolution of Magmas Fractional Crystallization
  and Contamination
• Reflected Light Microscopy

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Ch5 Origin of Magmas by Melting of the Mantle and
Crust

• Virtually all magmas have been generated within
  the outermost 250 km of the Earth by melting of
  solid mineral assemblages in the crust or mantle
• Melting occurs when an assemblage of minerals
  (/-fluid) reaches a temperature that produces a
  silicate melt eg may be as low as 650?C for
  crustal feldspathic sandstones water, or as
  high as gt1200?C to melt dry mantle peridotite
• Understanding the processes that generate magmas
  first requires an analysis of the phase relations
  involved in melting
• P becomes an important variable when considering
  melting, in contrast to crystallization, which
  with volcanic rocks generally takes place at or
  near the surface where P rarely exceeds 2kbar.
  Plutonic generally crystallize within the
  uppermost 15-20 km of the lithosphere 5-7kbar.
  However, melting occurs over a much greater range
  of depths 10- gt100km (3- gt20kbars)
• The composition and T of the magmas derived from
  melting of the same parental material can vary
  significantly at different P

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Melting in Binary Systems

• Equilibrium melting
• Fractional melting
• Batch melting

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Binary Systems with Peritectites - Equilibrium
Melting
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Binary Systems with Peritectites Fractional
Melting
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Binary Systems with Complete Solid Solution
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Melting in Ternary Systems
Eutectic point
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Batch Melting

• We have been looking at two idealised concepts
  1) Equilibrium melting, in which all of the
  liquid remains in contact and equilibrium with
  the solids, and 2) Fractional melting, in which
  the liquid is constantly removed and therefore
  cannot interact with the residuum
• Do these processes actually occur?
• Experimental work has shown that melt cannot be
  removed from a system until a critical minimum
  volume of melt has been produced ie 30 for
  basaltic liquids and much higher for granitic
  melts. This indicates that a blend of equilibrium
  and fractional melting may be typical in nature,
  with equilibrium melting dominating in the early
  stages and fractional melting in the latter.
• This intermediate behaviour is referred to as
  batch melting. Importantly, batch melting can
  lead to the formation of a wide variety of melt
  compositions as opposed to pure fractional
  melting which only produces specific eutectic or
  peritectic compositions.

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Ch6 - Evolution of Magmas Fractional
Crystallization and Contamination

• We have now covered the basics of how magmas are
  formed by partial melting, we now need to explore
  how the composition of magma can be modified
  after it has left the place where it originated
  and again it relies heavily on phase diagrams!
• The processes that govern magma evolution are
  fractional crystallization and assimilation-contam
  ination.

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Fractional Crystallization
Rhythmic layering
Colloform growth structures
Cross-bedding
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Binary Eutectic and Peritectic Systems
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Layered Gabbroic Intrusions

• Typically olivine tholeiite composition
• Show prominent structural layering, which forms a
  quasi-stratigraphy that subdivides layered
  intrusives into various olivine- or
  pyroxene-rich, ultramafic rocks at the base
  plagioclase-rich, mafic, plutonic rocks such as
  gabbros, norites anorthosites in the middle
  levels and finally highly fractionated felsic
  rocks, such as syenites and granophyres at the
  top.

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Crystallization Sequence in Layered Intrusions
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Crystallization Sequence in Layered Intrusions
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Ore Microscopy (reflected light)
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Ch 5-6 Revision Questions

• Chapter 5 Review Questions
• 1. Using Figure 5-1, predict what the temperature
  for beginning of melting would be for a rock
  consisting of 40 anorthite and 60 diopside.
  Predict the initial melting temperature for a
  rock with 80 anorthite and 20 diopside.
• 2. Construct a melting scenario in the system
  forsteritesilica (Figure 5-2) for both
  equilibrium and fractional melting for a rock
  consisting of 75 enstatite and 25 forsterite.
  Follow the melting process from inception of
  melting to disappearance of the last few
  crystals.
• Optional
• 3. Construct a melting scenario in the ternary
  system diopside-albite-anorthite (Figure 5-5) for
  a rock composition between the diopside-plagioclas
  e cotectic line and the diopside comer.
• 4. In Figure 5-6B, trace the crystallization of a
  variety of melts just on either side of the C-D
  binary join to prove to yourself that C-D is in
  fact a thermal divide that cannot be crossed by
  evolving melts.
• 5. Why is melting in the ternary system
  diopside-forsterite-silica (Figure 5-7) a good
  and relatively simple model for melting of the
  upper mantle?
• Chapter 6
• 1. In your own words, describe how fractional
  crystallization differs from equilibrium
  crystallization.
• 2. How would you use patterns of igneous rock
  compositions (specifically, clustering of
  compositions for specific rock types such as
  ocean floor basalt or intraplate alkali basalt)
  to determine whether most magmas undergo
  substantial fractionation during ascent?
• 3. Using the ternary systems diopside-forsterites
  ilica (Figure 6-8) and forsterite-anorthite-silica
  (Figure 6-9), try to work out fractional
  crystallization sequences that would give rise to
  the layering found in layered intrusion (Figure
  6-6). Experiment with each diagram to see what
  compositions of mixed magmas would result from
  the mixing of fractionated melts and batches of
  new magma. (Hint You might find some interesting
  "zigzag" liquid descent lines.)
• Optional
• 4. Is there any sensible way to track the
  continuous evolution in the bulk compositions of
  fractionated magmas in a layered intrusion when a
  "stratigraphic" layering develops as a result of
  such physical processes as gravitational
  settling? If not, how do we know that
  fractionation actually occurred?
• 5. Summarize the ways that trace elements and iso
  topes can be used to track the processes of magma
  contamination and magma mixing.