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University of Glasgow, charity number SC0044

The role of plagioclase, grain boundaries

and fluids in retrograde metamorphic

reactions Allan Hollinsworth

Supervisor: Tim Dempster

1. Background 2. Research Questions

3. Methods

4. Current Results

• Samples cut into thin strips

• Snapped 90o to reaction boundary

• Rock should fracture along grain

boundaries!

• SEM + optical microscopy + reaction

balancing + volume change

calculations

• Controls of reaction distribution?

• Are Albite surfaces reaction sites for epidote?

• How is plagioclase epidote reaction

facilitated?

• Grain boundaries: pathways and reaction sites?

• Retrograde porosity and permeability evolution?

Sample showing both vein and diffuse boundaries between albite

zone and epidote zone

Albite-epidote interface with no

Chlorite at grain boundary zone

Porous interior of albite with

fluid dissolution structures Inter-grain zone between albite

and amphibole

5a. Preliminary Conclusions 5b. Further questions

Albite

Epidote

Ab

Ab

Amph

Chl

Micropores

Dissolution Structures

Chl

Chlorite rich

unaltered zone

Partial albite

replacement

Amph – Ab

boundaries =

chlorite zones

Albite

adjacent to

vein not

always

altered

Proportionally

less chlorite?

Potential structural

control of chlorite

nucleation

Field photo of reaction texture in metabasic gneiss

from Iona

• Plagioclase most voluminous mineral in crust

• Alteration could have regional

crustal implications!

• Metabasic gneisses from

Iona have distinct “patchy”

texture

• Albite (Na plagioclase) in

some areas completely

replaced by epidote.

• Gneisses also contain large

proportions of retrogressive

chlorite

Porous albite, twins/

cleavage = reaction

propagation!

Chlorite may be

inhibiting

albiteepidote

taking place

Effect of volume

changes

with hydration reaction

of albite epidote?

Opening veins:

reaction enhancers or

inhibitors?

Controls of hydraulic

conductivity? (stress

changes, fracturing,

grain boundary

changes)

1mm