Linking Microstructures and Reactions

Porphyroblasts, poikiloblasts, and pseudomorphing

Part 2Mechanism and microstructure

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)++

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)

Criteria for sequence of mineral growth (1)

Andalusite, sillimanite. Which came first?

Why is this ambiguous?

Criteria for sequence of mineral growth (2)

Staurolite and muscovite: which came first?

Why is this obvious?

Criteria for sequence of mineral growth (3)

“Safe” criteria mainly involve

• Pseudomorphing – product occupies recognisable shape of precursor

• Inclusion fabrics – inherited, or obliterated

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

Grain size and overlapping sequences (1)

Andalusite partly enveloping staurolite, enclosing biotite.

Grain size and overlapping sequences (2)

Staurolite overgrowing two types of smaller porphyroblast

Poikiloblasts and mineral replacements

Staurolite growing by mineral replacement: mica -> St easy; Qtz -> St difficult

Staurolite

Biotite

Porphyroblast growth in graphitic rock

Different mechanism:selective dissolution, growth without entrapment, passive displacement of matrix

The “Staurolite-out” reaction

• Yardley’s scheme (Connemara). Elsewhere staurolite replaced by muscovite

Damara Belt, Trough Zone

Staurolite partly replaced by muscovite

Sillimanite growing within outline of resorbed garnet

Damara Belt, Central Zone

• Similar reaction textures• Different matrix microstructure

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

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

Metastability (2), Bushveld aureole

• Pseudomorph, hexagonal outline, now mostly quartz

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

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?

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

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