linking microstructures and reactions
DESCRIPTION
Linking Microstructures and Reactions. Porphyroblasts, poikiloblasts, and pseudomorphing Part 2 Mechanism and microstructure. Reaction mechanism at sillimanite isograd. From Carmichael, 1969, CMP 20. Net reaction is 3Ky = 3Sil. 3 kyanite + 3 quartz => 2 muscovite. - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/1.jpg)
Linking Microstructures and Reactions
Porphyroblasts, poikiloblasts, and pseudomorphing
Part 2Mechanism and microstructure
![Page 2: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/2.jpg)
Reaction mechanism at sillimanite isograd
From Carmichael, 1969, CMP 20. Net reaction is 3Ky = 3Sil
3 kyanite + 3 quartz=>
2 muscovite
2 muscovite + albite=>
3 sillimanite + biotite + 3 quartz
biotite=>
albite
K+
2H+
K+
3H2O
Na+ + 4H+ H2O
3(Mg,Fe)++
![Page 3: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/3.jpg)
Textural evidence for reaction mechanism
Carmichael's key observations and inferences:
• Reactants and products of simple reactions (e.g. Ky => Sil) commonly not found in contact.
• Local reactions conserve immobile components, are linked by movement of mobile species on > 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)
![Page 4: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/4.jpg)
Criteria for sequence of mineral growth (1)
Andalusite, sillimanite. Which came first?
Why is this ambiguous?
![Page 5: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/5.jpg)
Criteria for sequence of mineral growth (2)
Staurolite and muscovite: which came first?
Why is this obvious?
![Page 6: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/6.jpg)
Criteria for sequence of mineral growth (3)
“Safe” criteria mainly involve
• Pseudomorphing – product occupies recognisable shape of precursor
• Inclusion fabrics – inherited, or obliterated
![Page 7: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/7.jpg)
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
![Page 8: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/8.jpg)
Grain size and overlapping sequences (1)
Andalusite partly enveloping staurolite, enclosing biotite.
![Page 9: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/9.jpg)
Grain size and overlapping sequences (2)
Staurolite overgrowing two types of smaller porphyroblast
![Page 10: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/10.jpg)
Poikiloblasts and mineral replacements
Staurolite growing by mineral replacement: mica -> St easy; Qtz -> St difficult
Staurolite
Biotite
![Page 11: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/11.jpg)
Porphyroblast growth in graphitic rock
Different mechanism:selective dissolution, growth without entrapment, passive displacement of matrix
![Page 12: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/12.jpg)
The “Staurolite-out” reaction
• Yardley’s scheme (Connemara). Elsewhere staurolite replaced by muscovite
![Page 13: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/13.jpg)
Damara Belt, Trough Zone
Staurolite partly replaced by muscovite
Sillimanite growing within outline of resorbed garnet
![Page 14: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/14.jpg)
Damara Belt, Central Zone
• Similar reaction textures• Different matrix microstructure
![Page 15: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/15.jpg)
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
![Page 16: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/16.jpg)
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 -> Sil not overstepped enough for polymorphic transition
![Page 17: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/17.jpg)
Metastability (2), Bushveld aureole
• Pseudomorph, hexagonal outline, now mostly quartz
• Tiny blebs of relict cordierite (bright) in quartz (dark)[backscattered electron image]
![Page 18: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/18.jpg)
Sequence of reactions: metastability?
Compare predicted mineral changes in And-St hornfels with the observed sequence of porphyroblast growth
• All grow over same interval• What’s cordierite doing there?
![Page 19: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/19.jpg)
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
![Page 20: Linking Microstructures and Reactions](https://reader036.vdocuments.us/reader036/viewer/2022062408/56813066550346895d963eb6/html5/thumbnails/20.jpg)
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