Linking Microstructures and Reactions

1
Linking Microstructures and Reactions

• Porphyroblasts, poikiloblasts, and pseudomorphing
• Part 2
• Mechanism and microstructure

2
Reaction mechanism at sillimanite isograd
3 kyanite 3 quartz gt 2 muscovite
2 muscovite albite gt 3 sillimanite biotite
3 quartz
K
2H
Na 4H
K 3H2O
H2O 3(Mg,Fe)
biotite gt albite

• From Carmichael, 1969, CMP 20. Net reaction is
  3Ky 3Sil

3
Textural evidence for reaction mechanism

• Carmichael's key observations and inferences
• Reactants and products of simple reactions (e.g.
  Ky gt Sil) commonly not found in contact.
• Local reactions conserve immobile components, are
  linked by movement of mobile species on gt mm
  scale.
• Mobilities imply intergranular fluid present
  (temporarily?!)
• Al is the least mobile major species in prograde
  metamorphism
• Complex mechanism involving several other phases
  favoured because energy barriers are all lower
  than that for direct transformation.
• Plus, (after Yardley, 1977, Am Min)
• Patterns of mineral association probably
  controlled by nucleation preferences. (e.g. Sil
  prefers to nucleate on mica rather than on Ky)

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Criteria for sequence of mineral growth (1)

• Andalusite, sillimanite. Which came first?

• Why is this ambiguous?

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Criteria for sequence of mineral growth (2)

• Staurolite and muscovite which came first?

• Why is this obvious?

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Criteria for sequence of mineral growth (3)

• Safe criteria mainly involve
• Pseudomorphing product occupies recognisable
  shape of precursor
• Inclusion fabrics inherited, or obliterated

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Chemistry of mineral replacements

• Not obviously related to element mobility, but to
  exact nature of mineral pair in contact.
• Conserve volume (shape)

• Contact metamorphism, aureole of Bushveld
  Complex, S Africa

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Grain size and overlapping sequences (1)

• Andalusite partly enveloping staurolite,
  enclosing biotite.

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Grain size and overlapping sequences (2)

• Staurolite overgrowing two types of smaller
  porphyroblast

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Poikiloblasts and mineral replacements
Staurolite
Biotite

• Staurolite growing by mineral replacement mica
  -gt St easy Qtz -gt St difficult

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Porphyroblast growth in graphitic rock

• Different mechanismselective dissolution,
  growth without entrapment, passive displacement
  of matrix

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The Staurolite-out reaction

• Yardleys scheme (Connemara). Elsewhere
  staurolite replaced by muscovite

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Damara Belt, Trough Zone

• Staurolite partly replaced by muscovite

• Sillimanite growing within outline of resorbed
  garnet

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Damara Belt, Central Zone

• Similar reaction textures
• Different matrix microstructure

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Damara belt, structural/metamorphic setting

• Deformation, during high-T reactions
• Fluids (axial-planar quartz stringers)
• Trough Zone
• Much-thickened pile of clastic sediments
• Central Zone
• Thin sequence on granitic basement

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Metastability (1), Damara belt

• Both the Damara rocks contain two Al-silicates,
  without evidence for polymorphic transition

• Stable Al-silicate at St breakdown is
  sillimanite,
• But Ky/And -gt Sil not overstepped enough for
  polymorphic transition

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Metastability (2), Bushveld aureole

• Pseudomorph, hexagonal outline, now mostly
  quartz

• Tiny blebs of relict cordierite (bright) in
  quartz (dark)backscattered electron image

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Sequence of reactions metastability?

• Compare predicted mineral changes in And-St
  hornfels with the observed sequence of
  porphyroblast growth

• All grow over same interval
• Whats cordierite doing there?

19
Overstepping and metastable behaviour

• If driving force required to start nucleation is
  large
• A metastable reaction, rather than a stable one,
  may begin the growth of a new phase
• New minerals could appear out of sequence
  compared to the equilibrium phase diagram

Bushveld Complex aureole, Waters Lovegrove
2002 Observation is that Crd and Bt are already
present when andalusite appears
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Linking microstructures and reactions - summary

• We have examined
• Safe criteria for determining growth sequence
• Controls exerted by the nucleation process
• Porphyroblastic texture
• Mineral associations
• Probability of metastable growth sequences
• Processes at grain contacts
• Mineral replacement reactions and their
  constraints (volume, mass transfer)
• Poikiloblastic texture
• Effect of graphite
• Preservation (or not) of growth mechanisms