Metamorphism - Lecture 5 Composition Diagrams
Review Phase rule, components, etc. - beginning of chapter 20.
V. M. Goldschmidt in 1912: first applied phase rule to rocks.
Minerals in metamorphic rocks closely approach equilibrium assemblages
Goldschmidt noted that common metamorphic assemblages found worldwide. Probably not exact same P&T, these assemblages found over a range of P & T.
Thus 2 degrees of freedom, they must be divariant assemblages
F = C - P + 2, thus Divariant assemblages
(review P-T diagram of andalusite, sillimanite, kyanite)
If divariant assemblages, then number of minerals is kept to a minimum.
Usually: # minerals = # components
(2 = C - P + 2)
(and never more than 2 more phases than components)
P, T diagram is OK, but need to consider composition.
Because number of minerals is small, can make graphical representations easily
Fix P and T (below solidus)
4 components, 3 dimensional plot
Hard to read, so project into triangular plot.
1. Start simple:
Project 2 dimensions to 1 (handout page 1).
Since all these minerals contain H2O, can project them according to Mg, Si proportions
2. Triangular composition diagrams
Project 3 dimensions to 2 (handout p. 2, Figure 2.1b)
This one is drawn for a particular T & P. At this T & P, equilibrium assemblages are indicated by lines connecting phases that coexist - TIE LINES
DIVARIANT ASSEMBLAGE - within triangle. Depends on bulk composition
Certain bulk compositions have particular mineral assemblages.
Example in Fig. 2.1b, at this P & T, can't have An + Wo
- Series of diagrams for successive P-T conditions (experimentally determined) illustrates changes in mineral assemblage and allows determination of reaction that took place
Graphical representation of mineral assemblages
A reaction separates diagrams 1 and 2. Univariant reaction
In pelites, have 6 components
K2O, MgO, FeO, Al2O3, SiO2, H2O
Would need very complicated diagram to graph this. Need simplification. Projection.
For pelites, always have quartz present (in all grade) and always have H2O present (most metamorphic reactions produce water)
So can depict 4 remaining components with tetrahedra:
Al2O3, K2O, FeO, MgO
But still difficult to visualize
Two ways to reduce:
I. AKF. Eskola in 1920s - since solid solution between FeO + MgO, combine them. Collapsed plane to form a line.
II. AFM. J. B. Thompson 1957 - Project from muscovite onto AFM plane. But biotite and kspar are above the musc-FeO-MgO plane.
Most pelitic rocks contain muscovite rather than Kspar, so just ignore it (except when?).
Biotite - when biotite projected to AFM plane from muscovite, plots outside tetrahedron - at extension of AFM plane
At high grades, use Barker AFM
Metabasites (mafic rocks) & impure carbonates
III. ACF diagram - also Eskola - for metabasites and impure carbonates
IV. A'KF (instead of AKF)
Summary of diagrams:
Pelites: AKF, A'KF, AFM (Both Thompson & Barker)
Metabasite: ACF or CFM (CaO+Na2O+K2O-Al2O3 vs. FeO-Fe2O3 vs MgO)