Transcript GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY Lecture 26 Metamorphic Reactions (cont.) Chemographics and Petrogenetic Grids April 8, 2009

GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 26
Metamorphic Reactions (cont.)
Chemographics and Petrogenetic Grids
April 8, 2009
TYPES OF METAMORPHIC REACTIONS
• Phase Transformations
• Exsolution Reactions
•Solid-Solid Net Transfer Reactions
• Devolatilization Reactions
• Continuous Reactions (THIS LECTURE)
• Ion Exchange Reactions
• Oxidation/Reduction (redox) Reactions
•Reactions involving Dissolved Species
CONTINUOUS REACTIONS
INVOLVING SOLID SOLUTION PHASES
An idealized field area of steeply dipping meta-pelitic sediments that strike directly up metamorphic grade
The bulk chemistry
of each unit is
homogeneous, but
differs somewhat
from the other units
in the area
The garnet-in field
isograd varies from
unit to unit,
occurring at
different grades.
WHY?
Winter (2001) Figure 26-8.
CONTINUOUS REACTIONS
Two possible reasons:
1. Each unit has contrasting compositions such that the garnet
reaction is different
Example: garnet in some pelites may be created by the
(unbalanced) reaction:
Chl + Ms + Qtz  Grt + Bt + H2O
(26-11)
Whereas in more Fe-rich and K-poor pelites, garnet might be
generated by an (unbalanced) reaction involving chloritoid:
Chl + Cld + Qtz  Grt + H2O
(26-12)
2. The reaction on which the isograd is based is the same in each
unit, but it is a continuous reaction, and its location is sensitive to
the composition of the solutions (either solid or fluid) involved
The offsets this creates in an isograd are usually more subtle than
for reason #1, but in some cases they can be substantial
Let’s evaluate the second situation
CONTINUOUS REACTIONS
Recall the igneous situation with solid solution
“Melt-in”
isograd?
Temperature
is strongly
X-dependent
Winter (2001) Fig. 6-10.
Isobaric T-X phase
diagram at atmospheric
pressure After Bowen
and Shairer (1932),
Amer. J. Sci. 5th Ser.,
24, 177-213
CONTINUOUS VS. DISCONTINUOUS REACTIONS
Consider the reaction: Chl + Ms + Qtz  Grt + Bt + H2O
F = C – f + 2 = 5 – 4 + 2 = 1 (univariant)
But effectively F=0 since T
and P are controlled by
moving along a geothermal
or field gradient
Therefore, the reaction occurs
at a particular T for a given
gradient and would be considered
DISCONTINUOUS
CONTINUOUS VS. DISCONTINUOUS REACTIONS
However, when considering the solid solution of Mg and Fe in garnet,
biotite, and chlorite, the general reaction is
(Fe,Mg)Chl + Ms + Qtz  (Fe,Mg)Grt + (Fe,Mg)Bt + H2O
This is a continuous reaction
and we expect to find chlorite,
muscovite, quartz, biotite, and
garnet all together in the same
rock over an interval of
metamorphic grade above the
garnet-in isograd
The composition of solid
solution phases will vary
across the interval, and the
proportions of the minerals
will change until one of the
reactants disappears with
increasing grade
CONTINUOUS REACTIONS
Perhaps a more
realistic way to
portray this
continuous reaction
(minus the K
component) is this.
Thus, the offsets in
the idealized field
area may be due to
differences in the
Mg/Fe ratios of the
different rock
layers.
TYPES OF METAMORPHIC REACTIONS
DEVOLATILIZATION
Another example of a continuous reaction over a range in T
due the potential for the fluid composition to change
CLOSED (buffered)
Volatiles are trapped in
the rock with degassing
XCO2 must follow the
reaction equibrium curve
to the max T.
a – degassing of CO2 &
H2O drives system to b
and increased T
b – Trem, Cal or Qtz
must be consumed
before T can increase
c – degassing of CO2
causes system to move
along reaction curve
toward XCO2 = 1
Fluid composition is controlled (buffered)
by the progress of the reaction
REACTIONS AND CHEMOGRAPHICS
We can use chemographics to infer reactions.
Consider the 2-component system MgO-SiO2
P
e
r
F
o
E
n
M
g
O
Q
t
z
S
i
O
2
Any two phases in a binary system can react to from a phase
between them
Fo + Qtz = En
Mg2SiO4 + SiO2 = Mg2Si2O6
En + Per = Fo
Mg2Si2O6 + 2 MgO = 2 Mg2SiO4
Per + Qtz = Fo or En
If we know the chemographics we can determine that a reaction
is possible (and can dispense with balancing it)
However, thermodynamics determines whether such a reaction
is probable
REACTIONS AND CHEMOGRAPHICS
For a ternary system,
any phase that is
coplanar with three
other phases can be
related by a chemical
reaction
2A + B + C = X
Again, whether this
reaction is probable
under natural
conditions of P & T
depends on
Thermodynamics.
If the diagram represents a projection
from another phase or component, then
that phase is implied in the reaction.
For component D, the reaction might be:
2A + B + C + #D = X
However, the amount of D in the
reaction would have to be figured out by
balancing the reaction.
REACTIONS AND CHEMOGRAPHICS
2A + B + C = X
At P/T
Conditions B
At P/T
Conditions A
X-in
Isograd
REACTIONS AND CHEMOGRAPHICS
Another Possibility
If phase X can be
defined by two
different reactions:
2A+B+C = X
D+E = X
Then, the reaction:
2A+B+C = D+E
is also true
REACTIONS AND CHEMOGRAPHICS
A Tie-line Flip - results in new groupings in the next metamorphic zone
Because A+B and C+ D share a common tie-line, they can be related by
the reaction:
A+B=C+D
Below the Isograd
At the Isograd
Increasing Grade
Above the Isograd
PETROGENETIC GRIDS
GRAPHICALLY PORTRAYING SEVERAL REACTIONS IN
PRESSURE – TEMPERATURE SPACE
Simple One Component System
Metastable
Extensions
Univariant
Curves can
be labelled
by Absent
Phase
PETROGENETIC GRIDS
MULTISYSTEMS f > C +2
One-Component System
Four Phases (ABDE)
Four invariant points
(labelled by the absent phase)
Seven univariant lines
(labelled by the absent phases)
Four divariant fields
(labelled by stable phase)
PETROGENETIC GRIDS
MULTISYSTEMS
Theoretically Possible vs.
Geologically Probable
PETROGENETIC GRIDS
MULTISYSTEMS
Many Petrogenetic Grids will overlay grids of multiple
component systems that pertain to a specific protolith type
Petrogenetic Grid
for Mafic Rocks
Lawsonite
Actinolite
P-T Range of Rock
with Lawsonite +
Actinolite + Pyrophyllite
Pyrophyllite
Winter (2001) Fig. 26-19. Simplified petrogenetic grid for metamorphosed mafic rocks showing the location of
several determined univariant reactions in the CaO-MgO-Al2O3-SiO2-H2O-(Na2O) system (“C(N)MASH”).