Transcript What is a rock

Helpful Hints
If you are an experienced petrologist
you might want to miss out section 1.
I’ve tried to pop in some stimulating
questions and summarise the
information in a way especially
relevant to experimental petrology so
it might be worth looking at anyway.
2. is background to the important
considerations for experimentalists
and a little insight into the limits of the
usefulness of experiments in the
shallow parts of subduction zones. It
contains a few of my own
thoughts,further reading etc.
Hopefully these sections will help you
to get more out of the two papers
which I have summarised in Section 3.
I’ll be interested to hear your thoughts
on some of the questions I have posed
(but of course I am not trying to
wriggle out of answering questions
myself!)
Enjoy.
• Welcome!
• I’ve added three sections
(labelled (usually )in
bottom left-hand corner
1,2 and 3)
• 1. Background to rocks in
relation to experiments
• 2. Experimental
approaches
• 3. Key findings from the
papers
NB : some of the slides are ‘animated’
What is a rock?
I’d like you to start by taking 5
– 10 minutes to discuss your
experiences of rocks and then
to describe what features of
(igneous) rocks you think are
useful for understanding
magmatic processes!
1
Andesite (opx,
amph,plag, Fe-Ti
oxides and accessory
pyrrhotite).
Rodin’s ‘The Thinker’
eruption
Many subduction-related
systems are open, with
almost all of these
processes happening to
individual rocks
degassing
assimilation
crystallisation
mixing
original melt
1
So when might analysing bulk
composition be
useful/important ? What
would the bulk actually
represent?
Important properties of the solid components (crystals) I
Pressure
Possible
Magma Ascent path
Crystallisation starts here
(v. few crystals)
Some phases (e.g. Plagioclase) have strong
temperature dependence of the liquidus at
low PH2O
Liquidus I
More crystals here
(more undercooled from the liquidus)
1
This is for a system
containing water,
volatiles are important
here
Temperature
Consequences in the rock
Microphenocrysts – may reflect storage OR ascent conditions
Microlites – smallest crystals, may reflect
Degassing or ascent related process.
Larger crystals (phenocrysts)Grow in the storage region,
Reflect ‘magma chamber’ processes
1
Important properties of the solid components (crystals) II
Pressure
I
Some phases (e.g. cpx) have a less
marked T dependency, thus differing
phenocryst components can reflect
differing conditions of formation.
Finally, hydrous minerals
(e.g.amphibole, biotite)
have a positive slope at
low pressures. This
reverses at higher
pressures. Some of you
may be able to explain this
in terms of the
thermodynamic properties
of the phase and melt!
II
I +II
Liquidus I
Liquidus III
1
This is for a system
containing water,
volatiles are important
here
I
Temperature
Now think about typical storage, ascent
paths and how this might be reflected in
the mineral assemblage
Important properties of the solid components (crystals) III
Most phenocryst phases in
subduction-related rocks exhibit
considerable solid solution. This
is often a strong function of one
or a few of of P,T, volatile
content and fO2 . Differing
phases can be used for
different purposes.
This is an example of the influence of P(H2O) and T on plagioclase composition
1
(Soufriere Hills groundmass composition experiments from Couch e
plag
plag
plag
This is a BSE-SEM image of
(predominantly plagioclase,
I’ve labelled a few)
phenocrysts) from Soufriere
Hills (Montserrat). Brightness
is proportionate to mass so the
differing colours reflect differing
compositions. Take a minute to
look at this image and think
about what these phenocrysts
could be reflecting
Keep thinking about that bulk composition question!
Scale bar is 500 mm
1
Rocks are about more than just composition I
Most melts are 20% less dense than their parent rock
1
Rocks are about more than just composition II
m (Pa s)
1014
107
Based on Lejeune & Richet (1995)
40
60
Crystal content
Crystallisation, cooling and changing composition all have implications for the
response of the magma to stress (affecting mobility and movement of
magmas). Some magmas behave as brittle solids.
1
Thinking pause
Individual crystals tell us a great
deal about magmatic history
But when we are thinking about
volcanic behaviour it is the
macroscopic changes to the
system that are important
Never forget to look at the rocks in
the field and think about this!!! You
can probably come up with some
examples.
The fluid phase. The amount of volatiles dissolved in the magma
markedly decreases as a function of pressure (or depth)
1
Moore et al., 1998b – ref at end
Volatiles
Phase change from dissolved fluids to exsolved gases is the fundamental driving
forced behind volcanic eruptions
Also have profound influence on physical properties of the magma (in what way?)
Exerts strong control on crystallisation.....
Many of you will have experience of volatile emissions,
perhaps good here to reflect among yourselves on the
way in which volatiles drive volcanic eruptions
1
Recall
‘plagioclase’
Pressure
‘Hydrous phases’
1
This is for a system
containing water,
volatiles are important
here
Stability and
compositions of
such phases could
be excellent record
of volatile content
or degassing
processes
Temperature
Experiments !!!
Can determine volatile concentrations (see Moore et al.,
1998b example)
Using phenocryst compositions and abundance can reproduce
conditions of storage and ascent, and perturbations relevant to
eruption (our papers focus on this)
Dynamic experiments can reproduce degassing-induced
crystallisation (see Couch et al., 2003 as an example)
BUT.......
2
Need to carefully consider the nature of
the liquid + crystals to be reproduced
What is useful about an equilibrium
assemblage?
Most useful to use low crystallinity magma
equivalent to that under investigation (cf
Moore et al., 1998) or natural glass or
experimental reversal with bulk and glass
Can anything meaningful be derived from
working with a hybridised magma?
2
From Pichavant et al., 2007
Problems with equilibrium:
Need to consider whether
want partial or total
equilibrium and what this
represents in the real
magmatic system. We need
to think again about the
complete system AND local
equilibrium.
Example from Pichavant et al., 2007
Some more observations
There are comparatively few studies of basaltic andesites, although these are
often implicated in the recharge and triggering of eruptions (incl. solubility data
relevant to excess degassing)
There is something of an experimental ‘gap’ above 3 Kbars and below 10kbars.
This is a technical problem rather than a lack of scientific interest (see e.g.
conclusions from Moore et al., and Barclay and Carmichael for reasons to work in
this zone). See also point above! ...But rescue may be at hand (see Moore et al.,
2008).
2
1atm gas-mixing furnace zone
6km
15km
120km
Cold seals
200MPa
IHPV zone
500MPa
Transition
Piston cylinder zone
4 Gpa
Multi anvil
domain
15 Gpa
450km
Diamond anvils
2
The kit
Rapid-quench cold seal
Pressure vessel
Sample
At the end of the
experiment the
magnet is pulled
down and sample
rapidly removed
from the hot zone
2
One of UEA’s RQCS – in its shiny orange safety cage
Thermocouple
Furnace and
insulation
Magnet
To water pump
Locations of the papers
(adapted From Carmichael et al., 2006)
3
Moore and Carmichael
• i
Usual Colima
assemblage: plag, opx,
cpx and hbl (matched
as shown)
No significant role for
CO2
NB Colima assemblage
too crystalline to use as
starting bulk.
Ascent-related
plagioclase growth
identified
Mascota spessartite
much increased water
content. Interesting!
3
Basaltic andesite
Equilibration at
water contents of 35 wt%
Reproduced
assemblage
compositions and
volumes
3
Notable findings
• Crystals in both compositions result of
degassing and decompression of hydrous
parent magmas
• Can fractionate (hbl + plag) from similar
hydrous parent
• Volatile phase dominated by water
• Limited role for magma mixing
3
Cerro la Pilita Trachybasalt (near
Jorullo Volcano)
Very rare global occurrence of amphibole-bearing basalts
Using a lava with amphibole allows us to consider why its absent!
Location of Cerro La Pilita Trachybasalt
o
o
101 48'
101 43'
0
2km
19o 00'
La Puerta de
la Playa
Hwy
120
La Huac ana
Jorullo
Volcano
Ce rro La
Pilita
Jor46
El Nara njo del
Jorullo
18 o 55'
Phase diagram
Stability
field for the
Cerro la
Pilita cone
Effect of mixed-volatile conditions
(produces unlikely plagioclase compositions)
Influence of fO2 (shift of amphibole)
Astonishing crystallisation!
0
% Crystals
All crystals
olivine
hornbl ende
cpx
o
Temperature ( C)
0
(a)
10mm
py x
hbde
(b)
Barclay and Carmichae l, Figure 6
3
Recall: the ‘mechanical’ consequence of the large increase in crystallinity in the
experiments of LeJeune and Richet
Barclay and Carmichael, Figure 7
Will all hydrous basalts stall?
60
50
40
30
20
0
-100
-50
0
50
100
150
80
% Crystallisation
Crystallisation effect associated
with ‘hornblende in’ most marked
for basalts
a.
70
10
70
b.
60
50
40
30
20
10
0
-100
-50
0
50
100
150
80
% Crystallisation
This considers other
experiments on other
compositions (Sisson and Grove,
Moore et al., and Blatter et al.)
% Crystallisation
80
c.
70
60
50
40
30
20
10
0
-100
-50
0
50
100
150
Temperature relative to ‘hornblende in’, C
o
Is this what happens?
Amphibole
‘in’
Taking it further….
The residual melt defines a
shoshonitic trend
SiO2 (wt%)
TiO2 (wt%)
FeO (wt%)
I’m using bulk compostions and
experimental glass compositions
here – what do you think of that?
SiO2 (wt%)
Fe2O3 (wt%)
A reasonable fit to the trends of
erupted material (perhaps plag
removal in Aurora)– could we
sometimes have re-melting of
stalled material as a source of
water-rich magma ?
Al2O3 (wt%)
Here, the experimental
composition are compared with
the Aurora Volcanic Field in
California
SiO2 (wt%)
SiO2 (wt%)
3
Davidson et al., (2007)
Cooling
rather than
Decompressi
on (Annen et
al., 2006)
3
• Finds geochemical
evidence for
fractionation from
amphibole
• Does this apply to
Mexico??
Conclusions
• Used well experiments are exceptionally
useful at determing pre-eruptive volatile
contents, storage conditions and quantifying
the nature of the perturbation prior to
eruption
• The attainment of equilibrium is very
important and the use of a ‘bulk’ that
represents a meaningful magma vital.
• Lots of exciting work to be done particularly at
moderate pressures
References not on the website