Mineral Zoning Flashcards
(16 cards)
What’s chemical zoning in minerals
Spatial variation in the composition of minerals
Why is mineral zoning important in metamorphic rocks
Zoned minerals can show records of changing metamorphic conditions
2 major causes of chemical zoning in garnet?
Growth zoning and diffusion zoning
What is growth zoning in garnet
Where new shells of different compositions are added onto a growth crystal reflecting changing external conditions like P,T or X
What causes the compositional differences in growth zoning?
Changes in external conditions like T, P or X
How does each element change from core to rim during growth zoning in garnet?
1.Fe
2. Ca
3. Mn
4. Mg
All of the elements decrease from core to rim while Mg increases from core to rim
What is a ‘bell shaped profile’ in garnet zoning?
A Mn profile in growth-zoned garnet from greenschist and amphibolite facies
What’s diffusion zoning in garnet?
Modification of pre-existing garnet composition by volume diffusion driven by changes in external conditions without mineral growth timing
How does temperature affect diffusion zoning?
Diffusion in thermally active and gets faster with increasing temperature making diffusion less important in the low-grade rocks
What happens to garnet zoning during peak metamorphism at high grades?
Diffusion rates are rapid enough for most minerals to homogenize to equilibrium composition at the garnet rim at the peak of metamorphism
How does diffusion during cooling affect garnet and biotite?
During cooling, Fe at the rim of garnet increases but diffusion slows relative to cooling rates creating a diffusion profile
How does garnet consumption during cooling affect zoning
Garnet is absorbed and biotite is produced. Both garnet and biotite shift their rims more towards the Fe-rich compatibility
Why is mineral zoning significant for garnet-biotite geothermobarometry?
The calculated temp depends on which part of the zoned garnet and biotite is analyzed
Why do chlorite and garnet have fast diffusion rates?
Because they contain Fe-Mg and these elements don’t go require a coupled substitution and breaking through strong bonds
Why does plagioclase have a slow diffusion rate?
Ca-Na have to exchange but they have different charges so they need a charge balance with Al which requires a coupled substitution
How will the reverse reaction of garnet and biotite happen?
With decreasing temperature, causing the Fe/Fe+Mg at the rim of garnet to increase and the rim of biotite to decrease
