Download Chapter 21: Metamorphism

For pelites, Fe-Mg
solid solution is
important for many
major phases, but
this is not visible in
the ACF diagram.
J.B. Thompson
developed the AFM
diagram to display
Fe-Mg solid
solution in
important pelitic
minerals.
http://minerva.union.edu/hollochk/c_petrology/old_drawings.htm
Note that Qtz + Muscovite must be part of the mineral assemblage to use
the AFM diagram. A modified version is used for rocks that have Kfeldspar instead of muscovite (see your text).
http://minerva.union.edu/hollochk/c_petrology/old_drawings.htm
Discontinuous Reaction: Tie-Line Switch
Consider a metamorphic system of bulk composition x. For this system, quartz
and muscovite are part of the mineral assemblage but not shown on the diagram.
What is the stable assemblage at the start of the reaction? At the end? Don’t
forget the phases that are present but not plotted on the diagram! What is the
(approximate) reaction?
This reaction occurs in meduim grade pelites, and is thought to be responsible for
the appearance of staurolite and the disappearance of chlorite in typical pelites.
Discontinuous Reaction: Terminal Disappearance
In this case, staurolite disappears.
What is the new assemblage for composition “z”?
What new index mineral will appear from this reaction?
In this case, reaction represents increasing P and T (higher grade).
Also involves dehydration.
Continuous or Divariant Reaction
Type 3: Involves the gradual change in the composition of
the minerals in a system and the ultimately, potential
disappearance of a mineral.
Because change is gradual and does not ALWAYS result in
disappearance of a mineral, called continuous or divariant.
Continuous or Divariant Reaction
+ quartz and muscovite
For the AFM diagram shown above, in addition to
muscovite and quartz, the stable three phase mineral
assemblage is
garnet + chlorite + biotite
Continuous or Divariant Reaction
+ quartz and muscovite
Note that all three minerals have solid solutions that
involve Fe and Mg.
How do we know this?
Continuous or Divariant Reaction
+ quartz and muscovite
So in this continuous reaction, the mineral
compositions GRADUALLY change, for
example, from more Fe-rich to more Mg-rich
compositions (which is toward higher
temperature). This is noted by a gradual
change in the position of the triangle as it shifts
from left to right on the diagram.
Continuous or Divariant Reaction
+ quartz and muscovite
If you consider the bulk composition x, then as the minerals become
more Mg-rich, you’ll note that it is possible that eventually, there will
be no chlorite in the system.
You can tell this because as the mineral compositions shift right,
eventually the bulk composition x (which does NOT move) will
intersect the tie line between garnet and biotite.
This means there is no chlorite in the system. All of the chlorite has
been reacted away.
Petrogenetic Grid
A petrogenetic grid is simply a collection of ternary
triangles (showing stability of a certain group of
three minerals) plus some reaction boundaries
(which show exactly where in P-T space the
reaction occurs).
So a petrogenetic grid tells us:
1. Potentially stable mineral assemblages (mineral
assemblages that are in equilibrium at particular P,
T, X)
2. Potential reactions that might occur with changing
conditions.
Petrogenetic Grid
Note: three phase triangles, which tell us which
minerals are stable.
“Reaction lines” which tell us what minerals are
reacting. These are also called phase boundaries.
Petrogenetic Grid
Let’s look at an example:
Consider a system
undergoing a change
in temperature from
~550 °C to ~750°C at
~12.5 kbars.
For each distinct divariant region (the regions between the
phase boundaries), what is the three phase mineral
assemblage and at each phase boundary, what is the
relevant reaction?
Petrogenetic Grid
Using a diagram like this, I could ask you to identify index
minerals as metamorphic grade increases.
Summary
1. We try to understand metamorphic regions because they
provide information about P-T-X conditions.
2. To understand changes in conditions, we apply
chemographic diagrams, which tell us about minerals that
may be stable in a particular part of a metamorphic terrane.
3. To understand metamorphic reactions, we also use these
diagrams.
4. We need to recognize and understand the three types of
reactions and what these might tell us about changes in
mineral assemblage within a particular metamorphic region.
5. A petrogenetic grid is a more complex way of understanding
changes in a particular metamorphic region.
Compare the H2O
content of the Cascadia
slab and the NE Japan
slab at various depths.
At what depth does the
most significant fluid
release occur in each
case?
Not modeled in white
areas
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