Campatibility diagrams Flashcards
What are compatibility diagrams?
Chemografic digrams; a grafical representation of the chemistry of the mineral assemblages
What are tie-lines?
Lines between the co-existing minerals in a compatibility diagram
- makes sub-triangles
- must not cross
What does the sub-triangles represent in a compatibility diagram?
the assemblages
Where will the positon of the bulk composition of the protolith plot in a compatibility diagram+
With-in the sub-triangle made from the minerals defining the mineral assemblage
How many degrees of freedom do we have in compatibility diagram?
F = 2, no matter what
When is compatibility diagram valid?
Bc. use of Phase rule;
- valid at specific P and T conditions (!)
- valid at very limited range of P,T conditions
What is the difference between ternary phase diagrams and compatibility diagrams?
Ternary:
- P is constant
- T varible
Compatibility:
- P, T is constant
Can Compatibility diagrams explain why an index mineral does not nessecarily show up in its zone (barrovian isograd concept)
An assemblage (sub-triangle) will be in equilibrium at the same conditions as the rest (where the index mineral can be found), but may not contain the nesscesay minerals to form the index mineral.
(Therefore the isograds a defined by the first occurence over a large area)
What does compatibility diagrams showcase/explain?
Why different rocks, equilibrated at the same grade (P-T), can develop differentmineral assemblages
- thus the importance of bulk composition
What can Compatibility diagrams also show?
what minerals that cannot co-exist at the P-T conditions
What can change the compatibility diagrams?
A change in P-T
- change the tie-lines - needs a metamorphic reaction
What does it mean when there is limited solid solution in a compatibility diagram?
The mineral with solid solution only has limited substitution of composition.
(writes thing in parathesis and there are lots of lines on the diagram)
How many degrees of freedom can a field (assemblage) have in a compatibility diagram with solid sultion? (not all)
F = 4 (- not breaking laws actually)
the 4 varibles:
- P
- T
- The composition (any 2 of 3 components)
(the area will be compositionally divariant)
How can tie-lines in compatibility diagram with solid solution, have 3 degrees of freedom?
The freedoms are
- P
- T
- 1 composition - lies between an assemblage that is (fully/partly) described by a solid solution
Why is compatibility diagrams with solid solutions relevant?
They better reflect natural metamorphic systems - typically they contain minerals with limited solid solutions
What does ‘univariant mineral composition’ mean?
Solid solution minerals are restricted to lie at the edges of their field, at the end of a tie-line (going through Bulk composition)
What is the problem with creating compatibility diagrams?
Most rocks contains the major oxides (9 so C=9) - this is too many components
How can natural system be reduces to a lower number of components?
- Combine components
- Limit the rock types shown
- Ignore components
- Use projections from phases
How does combining components work for lowering component amount?
Components that substitue for eachother in a solid solution get combined, and is just one component.
ex. Fe+Mg - 1 instead of 2
How does limiting the rock types shown, work for lowering component amount?
You only look at rocks with the minerals you are interested in and for which simplified systems work
How does ignoring components work for lowering component amount?
Ignore thing like:
- trace elements (not important for the chemical formula)
- elements entering only a single phase (can drop without violating the phase rule)
- Perfectly mobile elements (they can leave the system (fluids))
- minerals stable over a large range of conditions (they are not useful indicators for grade)
How does projection work?
It’s like a 4 component system (tetrahedron) where there ex. 6 phases (4 endmembers and 2 as mixtures). 1 component is on top, and the mixtures floating in the middle. Tie-lines between everything.The projecting component is like light shining down on the triangle below, and the mixtures will cast shadows - this is their new palcements.
What happens mathematically when projecting?
the projected component is being subtracted from the mixed components
What does an projection imply?
That the projected mineral is stable with all plotted assemblages. (in the whole triangle!)
