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Linking Microstructures and Reactions

This article explores the mechanism and microstructure of reactions in metamorphic rocks, focusing on the link between porphyroblasts, poikiloblasts, and pseudomorphing. It discusses the reaction mechanism at the sillimanite isograd and provides textual evidence for the proposed mechanism. The article also examines the criteria for determining the sequence of mineral growth and the chemistry of mineral replacements.

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Linking Microstructures and Reactions

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  1. Linking Microstructures and Reactions Porphyroblasts, poikiloblasts, and pseudomorphing Part 2 Mechanism and microstructure

  2. 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 K+ 2H+ Na+ + 4H+ K+ 3H2O H2O 3(Mg,Fe)++ biotite => albite

  3. 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)

  4. Andalusite, sillimanite. Which came first? Why is this ambiguous? Criteria for sequence of mineral growth (1)

  5. Staurolite and muscovite: which came first? Why is this obvious? Criteria for sequence of mineral growth (2)

  6. Criteria for sequence of mineral growth (3) “Safe” criteria mainly involve • Pseudomorphing – product occupies recognisable shape of precursor • Inclusion fabrics – inherited, or obliterated

  7. 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 Chemistry of mineral replacements

  8. Grain size and overlapping sequences (1) Andalusite partly enveloping staurolite, enclosing biotite.

  9. Grain size and overlapping sequences (2) Staurolite overgrowing two types of smaller porphyroblast

  10. Poikiloblasts and mineral replacements Staurolite growing by mineral replacement: mica -> St easy; Qtz -> St difficult Staurolite Biotite

  11. Porphyroblast growth in graphitic rock Different mechanism:selective dissolution, growth without entrapment, passive displacement of matrix

  12. The “Staurolite-out” reaction • Yardley’s scheme (Connemara). Elsewhere staurolite replaced by muscovite

  13. Staurolite partly replaced by muscovite Sillimanite growing within outline of resorbed garnet Damara Belt, Trough Zone

  14. Damara Belt, Central Zone • Similar reaction textures • Different matrix microstructure

  15. 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

  16. 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 Metastability (1), Damara belt

  17. Pseudomorph, hexagonal outline, now mostly quartz Tiny blebs of relict cordierite (bright) in quartz (dark)[backscattered electron image] Metastability (2), Bushveld aureole

  18. 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? Sequence of reactions: metastability?

  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

  20. 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

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