Petrogenetic grid, AFM diagrams, bulk compositions, isograds and

Geology 533
Metamorphism and Lithosphere Evolution
Metapelites:
Petrogenetic grid, AFM diagrams, bulk compositions, isograds and grade!
using the Gibbs program of F.S. Spear
You are provided with a ‘map’ representing four different stratigraphic units going
upgrade towards a diorite intrusion. The horizontal axis on the ‘map’ is in T (°C), which
you can think of as roughly equivalent to distance away from the intrusive contact. The
four different stratigraphic units maintain constant bulk composition going upgrade.
The bulk compositions of the four units, in terms of coordinates in an AFM diagram
projected from Ms, Qtz and H2O, are:
Unit
Name
A
F
M
1
2
3
4
High-Al pelite
Low-Al intermediate Fe-Mg pelite
Low-Al Fe-rich pelite
Low-Al Mg-rich pelite
0.4
0.1
0.1
0.1
0.3
0.45
0.8
0.1
0.3
0.45
0.1
0.8
You are to use the AFM routine in Gibbs to record stable mineral assemblages and
isograds that are encountered in each lithological unit going upgrade towards the
intrusion. The metamorphic field gradient you are to examine ranges from 405 to 700 °C
at a uniform pressure of 3.8 kbar.
The mineral assemblages will contain two or three minerals plus the excess phases Ms,
Qtz and hydrous fluid. These will include variously biotite, chlorite, chloritoid, staurolite,
cordierite, andalusite, sillimanite and kyanite. Look at the attached AFM diagram to see
where these plot relative to one another.
Isograds are here defined as lines on a map across which there is a change in mineral
assemblage. The isograds may be of two types:
1. Continuous reactions, in which three phase fields on the AFM diagram sweep through
the bulk composition as P-T conditions change. These reactions involve changes between
divariant (3-phase) and trivariant (2-phase) assemblages. In these reactions, you either (a)
gain a phase, ie, go from two to three phases stable, as a triangle is entered, or (b) lose a
phase, ie go from three to two phases stable, as a triangle is exited. The first type is a
mineral-in isograd, the second type a mineral-out isograd.
Rarely you may encounter a 1-phase region (quadravariant), which may also be involved
in mineral-in or mineral-out isograds.
2. Discontinuous reactions, in which a tie line change occurs across a univariant reaction.
In these reactions, the number of phases on either side of the isograd is the same
(generally three = divariant, unless crossing a degenerate univariant reaction – see
below), but the mineral assemblage changes. These reactions involve both the loss of one
phase and gain of another, so they are simultaneously a mineral-in/mineral-out isograd.
There are three types of discontinuous reactions: (a) crossing tie-line reactions; (b)
‘internal’ phase disappearance or appearance reactions; and (c) degenerate reactions. An
example of a crossing tie-line reaction (in which the four phases lie at the points of a
quadrilateral) is St+Chl=Al2SiO5+Bt, whereas an example of an internal phase
disappearance reaction (in which one phase lies within a triangle defined by the other
three) is Chl=Al2SiO5+Crd+Bt (remember that the projected phases Ms, Qtz and H2O are
also involved). Look at an AFM diagram.
Some examples of degenerate univariant reactions are Ky=And, And=Sil and
Ms+Qtz=Sil+Kfs+H2O (they are degenerate because they proceed in a smaller chemical
subsystem than in the full KFMASH system, and univariant because within these smaller
chemical subsystems, they involve one more phase than component – eg, the 5-phase
Ms+Qtz=Sil+Kfs+H2O reaction is univariant in the Fe&Mg-absent 4-component KASH
system).
Exercise
Start the Gibbs program and follow the directions in the Instructions attached to this lab.
Choose the SPaC 2000 data base. Go to the ‘Plotting menu’ and choose the P-T diagram
that gives a P range of 0-10 kbar, 400-800 °C, and draw the (new) axes. Return to the
‘Main menu’ and select ‘MakeMyGrid’. Choose ‘KFMASH system (Spear, Pattison &
Cheney 2000)’. Choose ‘AFM diagram routines’. Choose ‘P&T dialogue box’. Input
Start T and P of 405 °C, 3800 bars; delta T and P of 5°C, 0 bars, and End T and P of 700
°C, 3800 bars. Press ‘Calculate AFM’. The AFM diagram for 405 °C, 3800 bars will be
displayed.
At this point, you need to add your bulk compositions to the AFM diagram. Hit ‘Done’.
Move the AFM window over to the right and expand it, if necessary, so that you can see
the whole thing (normally it will largely obscure the P-T grid but don’t worry – you have
a hard copy of the grid to look at). Using a felt pen, mark as accurately as you can on a
clear overlay the positions of your four bulk compositions on the AFM diagram on the
screen.
Go back to the Command window and choose ‘P&T dialogue box’ again. The same box
as before will appear. Hit ‘Calculate AFM’ and the diagram for 410, 3800 bars will
appear. Again make note of the mineral assemblage for each of the four bulk
compositions. Hit ‘Calculate AFM’ again and the one for 415, 3800 bars will appear.
Continue doing this, recording all mineral assemblages and isograds for each bulk
composition. Note whether the isograd is a continuous or discontinuous isograd, and the
mineral-in or mineral-out.
** For each P-T point, look at your P-T diagram so you can link what is going on in the
grid with what is happening with the AFM diagram. In particular, when you cross one of
the coloured red or blue lines on the grid (corresponding to end member KFASH or
KMASH reactions), you will notice that a skinny three phase triangle emanates from the
Fe-rich or Mg-rich side of the AFM diagram, respectively, and migrates and enlarges as
temperature is increased. These are continuous Fe-Mg reactions. When you cross one of
the black univariant lines, you will get a tie-line flip involving the four phases labelled on
the reaction.**
You may need to reduce the T-interval in places to figure out exactly what it happening
when things change markedly over small temperature intervals.
Complete the map and comment on the different isograds observed for each bulk
composition. Are all bulk compositions affected by all the grid reactions?
Lithological unit
Isograds for different bulk compositions
400
450
500
550
600
650
Increasing grade (T °C at 3.8 kbar)
700
750
SPaC (4/2000) KFMASH Grid
20
An Pr
d+ l
Q
tz
0
350
h
Bt l
Fe
An Chl
n
Cld +C
+C ld
Ky
And
St+
A
450
tz
+Q +Bt
s
M Kfs
+
Als
t
Alm
Ann+
St
St ls
A
nn+
ls
l+A
Ch gCrd
M
Als + Bt
MgCrd
il
l
Ch t+S
+B
d
Cr
St+ Cld
Grt
+B
Ann+Cld
Alm
hl
t+C t
Gr d+B
Cl
4
Al
m
St+C
Als+ hl
Bt
Cld
St+ +Als
Ch
l
tz
Q s
+
f
s
M il+K
S
St
+A
ls
+Al
St s
Cld
tz
Prl
Ky+Q
Ky
Sil
ls
6
t+A
t+B t
S
8
Gr
10
Grt Gr Cld
+
t+
St+ Chl St+ Chl
Bt
Cld
Alm+St
ls
A
l+
Ch
rt+ St
P kbar
G
Alm
Chl
12
KFMASH
Chl
Als+Bt
s
Al
d+
Cl t
rt+ S
G
14
2
Cld
Grt+Als
+Chl
16
KFASH
KMASH
Grt+Chl
Als+Bt
Cld
Alm+Als
Alm d
+Cl
Ann
18
l
Ch Bt
d+
Cr
Alm
Ann+Als
tz
+Q Bt
Ms Kfs+ Ann+Als
+
i
FeCrd
AlS
+Ann
FeCrd
Alm
550
T (C)
650
t
Als+B
d
r
Grt+C
Alm + Als
FeCrd
Sil
And
750
SPaC(4/2000) KFMASH
hl
C
+ Bt
St ls+
A
l
ls
Ch t+A
+B
rd
C
+G ld
rt+
B
t
Fe
An Chl
n+C
ld
St
3
ls
l+A
Ch Crd
Mg
Cl
d
St +Ch
+B l
t
P kbar
4
Ann+Cld
Alm
Cl
d+
St Als
Cl
St d+A
+C ls
hl
hl
t+C
Gr d+Bt
Cl
5
C
Alm ld
+S
t
Gr C l d
t+
Gr St+ C
t
St +Ch hl
+B l
t
6
Als
Bt+
St
C
+
Grt
St ls
+A
Alm
Alm
Alm
Ann+
St
Ann+Als
St
+Als
Ann
2
l
Ch Bt
d+
Cr
Qtz
Ms+ fs+Bt
K
i+
AlS
d
Cl Als
+
n
An
Als
Ann+ d
C
Fe r
1
0
500
520
540
560
T (C)
580
600