Igneous activity in the CircumMediterranean area during the Cenozoic Michele Lustrino Dipartimento d

1
Magmatismo cenozoico dellarea mediterraneaLaure
a Specialistica in Geofisica, Geodinamica e
Vulcanologia Dipartimento di Scienze della
TerraUniversità degli Studi di Roma La Sapienza
Michele Lustrino(michele.lustrino_at_uniroma1.it.
Tel [phone]. Stanza 116)
2
Part Three.
The role of volatiles in mantle
meltingSupra-subduction mantle melting.
3
Why a mantle melts

• You know (or should know) how a solid mantle rock
  can partially melt.
• Pressure decrease (as seen for MORBs but a common
  process also in other settings where lithospheric
  plates diverge)
• Addition of volatiles (H2O and CO2 mainly mostly
  in subduction zones)
• Increase of temperature (alleged Mantle Plumes
  thermal anomalies mostly along craton borders)
• Change in chemical/mineralogical composition
  (chemical anomalies everywhere).

4
Why a mantle melts
We will discuss in more detail the various
variables presenting case studies of Cenozoic
orogenic and anorogenic igneous rocks of the
Circum-Mediterranean area. We will see how many
different hypotheses have been proposed in the
literature to explain magma genesis in complex
systems like the Mediterranean. Before
proceeding, just a few words on basic concepts
like Geotherm, Mantle Adiabat and Potential
Temperature. A few comments also on dry and
H2O-CO2-bearing mantle solidi.
5
Geotherm
The Geotherm illustrates how the temperature of
the Earth changes with increasing
pressure. There is not just one geotherm in the
Earth.
Geotherms are relatively hot under oceanic basins
and relatively cold under cratons.
Here you see a classical example of how geotherms
are proposed in the literature.
From McKenzie and Bickle (1988) J. Petrol., 29,
625-679
6
Geotherm
The Geotherm illustrates how the temperature of
the Earth changes with increasing
pressure. There is not just one geotherm in the
Earth.
Geotherms are relatively hot under oceanic basins
and relatively cold under cratons.
This case shows convective geotherms with a
potential temperature of 1280 C calculated for
100 km-thick oceanic lithosphere and 200 km-thick
cratonic lithosphere.
From McKenzie and Bickle (1988) J. Petrol., 29,
625-679
7
Geotherm
The Geotherm illustrates how the temperature of
the Earth changes with increasing
pressure. There is not just one geotherm in the
Earth.
In other words In the upper mantle the
temperature gradients are high, decreasing from a
high conductive gradient at the surface to a low
convective gradient in the deeper interior.
8
Mantle Adiabat
When ascending mountains the temperature
decreases. This is consequence of air
compressibility With decreasing pressure air
expands and the temperature decreases. This means
that there exists an adiabatic temperature
gradient responsible for the decrease of
temperature at high altitudes.
This is the working principle of air cooling
systems.
9
Mantle Adiabat
The same happens if solid mantle or partial melt
ascends from the Earths interior to the
surface. The adiabat is defined as the decrease
in temperature with decrease in pressure. The
adiabat for solid mantle is 0.6 C/km (1.8
C/kbar). The adiabat for melt is higher (1
C/km 3 C/kbar).
10
Mantle Adiabat
Therefore it is possible to say that the
upwelling of mantle material (either solid or
melt) is not isothermal. The upwelling is
adiabatic (i.e., without exchange of heat, but
with decreasing of temperature).
If solid mantle upwells from the base of the
lithosphere (e.g., 100 km) to the surface, its
temperature would decrease by 60 C. Partial
melt coming from depths of 100 km would suffer a
100 C temperature decrease by the time it
reached the surface.
11
Mantle Adiabat
1280
1380
1480
1580
Adiabatic decompression paths
Solidus
Liquidus
90
30
70
50
20
10
1280
1380
1480
1580
12
Potential Temperature
In order to determine the heat content of two air
masses it is necessary to measure their
temperatures at the same pressure.
We need to know the heat content of two solid or
melt masses. Also in this case we should compare
these two masses at the same pressure. For this
reason a new temperature has been defined, i.e.
the Potential Temperature. In this case the
temperature of the mantle is referred to pressure
1 atm (i.e. pressure at sea level).
13
Potential Temperature
How is the potential temperature calculated?
The potential temperature is the temperature the
mass would have (hence the term potential) if
it were compressed or expanded to some constant
reference pressure (1 atm). The potential
temperature is calculated by continuing the
mantle adiabat in the convective region to P 1
atm.
14
Potential Temperature
Let us assume two different types of lithosphere.
MBL
Type 1 Normal continental lithosphere
(thickness of mechanical boundary layer 60 km
thermal boundary layer 130 km).
TBL
15
Potential Temperature
The geotherm of Type 1 lithosphere shows a clear
kink at the base of the thermal boundary layer.
MBL
TBL
This is related to the transition from the
convective to the conductive regions.
16
Potential Temperature
In this case the potential temperature is
obtained by continuing the mantle adiabat in the
convective region up to surface pressure.
1280 C
The potential temperature in this example is 1280
C.
17
Potential Temperature
Let we assume a second type of lithosphere.
MBL
Type 2 Cratonic continental lithosphere
(thickness of mechanical boundary layer 150 km
thermal boundary layer 230 km).
TBL
18
Potential Temperature
Also in this case continue the mantle adiabat in
the convecting region up to surface pressure.
1280 C
Also in this case the potential temperature is
1280 C.
19
Potential Temperature
The concept of potential temperature is used to
have an idea of the thermal state of the mantle.
1280 C
It is not possible to say the temperature of the
mantle of a given area is 1200 or 1500 C. We
can only say that the temperature of a mantle in
a given region is 1200 or 1500 C if we refer to
a reference pressure.
In these cases the reference pressure is 1 atm.
20
Potential Temperature
1280 C is considered the average potential
temperature of a normal mantle.
1280 C
1480 C
In the presence of alleged mantle plumes a much
hotter potential temperature has been
hypothesized (up to 1480 C).
In these cases the reference pressure is 1 atm.
21
Let us introduce another fundamental argument in
igneous petrology
The shape of the mantle solidus As a function of
its composition and volatile contents.
22
Experimental determinations of the solidus and
liquidus of garnet peridotite
From McKenzie and Bickle (1988) J. Petrol., 29,
625-679
23
Experimental determinations of the solidus and
liquidus of garnet peridotite
Blue symbols pre-1988 data (from McKenzie and
Bickle, 1988). Red Symbols 1988-2000 data.
From Hirschmann (2000) Geochem. Geophys.
Geosyst., 2000GC000070
24
Experimental determinations of the solidus and
liquidus of garnet peridotite
Comparison of the recommended dry peridotite
solidus of Hirschmann (2000) relative to selected
other model solidi from the literature.
Note this and the previously shown mantle
solidus corresponds ONLY to DRY assemblage and
Enriched and Depleted compositions exluded.
From Hirschmann (2000) Geochem. Geophys.
Geosyst., 2000GC000070
25
Experimental determinations of the solidus and
liquidus of garnet peridotite
Another proposed solidus for dry mantle.
40 C difference
Hirschmann (2000)
Katz et al (2005)
130 C difference
McKenzie and Bickle (1988)
From Katz et al. (2005) Geochem. Geophys.
Geosyst. doi10.1029/2002GC000433.
26
Top 800 km of pyrolitic mantle hypothetical
mineral assemblage.
Temperature (C)
1400
1600
1800
2000
2200
2400
2600
0
100
Liquid
5
200
Ol Opx Cpx Gt
300
10
a
The 660-670 km seismic discontinuity is extremely
important for mantle dynamics. It separates upper
from lower mantle.
b
b Cpx Gt
15
Pressure (Gpa)
Depth (km)
b
b Gt
g
20
Gt g CaPv
600
670
Gt MgPv Mw CaPv
25
700
Mw MgPv CaPv
From Herzberg and Zhang (1996) J Geophys. Res.,
101, 8271-8295
800
30
27
Question
Do you know what a PYROLITE is? Pyrolite is a
theoretical rock considered to be the best
approximation to the composition of Earths upper
mantle. It is generally considered to be 1 part
tholeiitic basalt and 3 parts peridotite residuum
(harzburgite or dunite). There are at least two
types of pyrolites (Hawaiian P. and MORB P.) used
as starting material in experimental petrology
studies.
28
At the 660-670 km seismic discontinuity the
subducted slab (eclogite) has nearly the same
density as ambient mantle or is even denser. At
P gt670 km the mantle assemblage becomes much
denser and a density crossover verifies. Under
these conditions the slab cannot penetrate more
and probably it folds and stagnates.
670
29
Similarly, since the mantle assemblage deeper
than 670 km is denser than the shallower
assemblage, a full convecting system is
considered improbable. This means that probably
Earths mantle is characterized by two different
convecting systems whose boundary is the 660-670
km discontinuity.
670
30
Note that the folded and stagnating subducted
slab (eclogite) is characterized by a solidus
temperature several 10s of degrees (up to 200 C)
lower than ambient mantle. This means that this
region of the mantle is prone to easy and
abundant melting. Often this has been considered
the source region for several CiMACI rocks.
CiMACI Province Circum- Mediterranean Anorogenic
Cenozoic Igneous Province
670
31
Mineral volume fractions for the top 1000 km of a
pyrolite mantle.
Small orange and pink regions in the top
right-hand corner denote the stabilities of
feldspar and spinel, respectively.
From Frost (2008) Elements, 4, 171-176
32
Mineral volume fractions for the top 1000 km of a
pyrolite mantle.
Do you know the peculiarity of the Majoritic
garnet? Two atoms of Al3 are substituted by
(Si4 Mg2). i.e., pyrope increases the
enstatite content in its formula.
From Frost (2008) Elements, 4, 171-176
33
The role of volatiles
Volatiles such as hydrogen, carbon, sulfur, and
halogens stored at depths in various forms and
concentrations, are known to have significant
effect on the generation of partial
melts. Extraction or impregnation of partial
melts in turn can deplete or enrich the mantle in
volatiles and cause compositional changes in
major and trace elements. The presence of
volatiles and melts are also known to influence
the rheology and seismic properties of minerals,
and likely have considerable influence on
convective processes in the mantle.
From Dasgupta and Dixon (2009) Chem. Geol. (in
press)
34
The role of volatiles
Hydrogen can be dissolved in minimum amounts in
the main silicates (ol, cpx and opx) and in
relatively high amounts (2-5 wt) in other
silicates (phlogopite, pargasite) or even more
(up to 15 wt in serpentine, talc, chlorite and
other dense hydrous silicate minerals). On the
other hand, Carbon is insoluble (absolutely
incompatible KD 0) in silicates and can be
found in different states as a function of fO2
(e.g., graphite, diamond, magnesite, dolomite,
CO2, CH4). The solubility of CO2 in melts
increases strongly with pressure.
35
The role of volatiles
Recent results indicate that clinopyroxene can
host up to 1400 ppm (0.14 wt) of H2O, garnet up
to 260 ppm (0.03 wt) and olivine up to 29 ppm
(0.003 wt). The partition coefficient of
hydrogen between minerals and melts is very low
(below 0.01). This implies that, assuming a
mantle with bulk H2O content of 50-200 ppm,
near-solidus partial melts beneath oceanic ridges
can have H2O content ranging from 0.5 to 3.8
wt. CL C0/DF(1-P)
Bulk Partition coefficient of the phases entering
the melt
Concentration of a trace element in a liquid.
Bulk Partition coefficient in the source
Concentration of the same element in the source
Molar fraction of melt produced
36
The role of volatiles
Recent results indicate that clinopyroxene can
host up to 1400 ppm (0.14 wt) of H2O, garnet up
to 260 ppm (0.03 wt) and olivine up to 29 ppm
(0.003 wt). The partition coefficient of
hydrogen between minerals and melts is very low
(below 0.01). This implies that, assuming a
mantle with bulk H2O content of 50-200 ppm,
near-solidus partial melts beneath oceanic ridges
can have H2O content ranging from 0.5 to 3.8
wt. CL C0/DF(1-P)
0.005-0.01
Minerals entering the melt
0.005-0.02 wt
0.005-0.01
0.0001
50-200 ppm
f 0.01
Minerals in the starting assemblage
From Tenner et al. (2009) Chem. Geol. (in press)
37
The role of volatiles
Similarly, assuming a mantle with bulk H2O
content of 300-1000 ppm, near-solidus partial
melts of oceanic islands (OIB) can have H2O
content ranging from 3 to 20 wt.
0.005-0.01
CL C0/DF(1-P)
Minerals entering the melt
0.03-0.1 wt
0.005-0.01
0.0001
300-1000 ppm
f 0.01
Minerals in the starting assemblage
Of course, increasing the degree of melting will
result in dilution of the amount of hydrogen in
the melt (e.g., with f 1 CL 1.5-10 wt).
From Tenner et al. (2009) Chem. Geol. (in press)
38
The role of volatiles
Earth's mantle contains a very small amount of
CO2 in the range 50 to 500 ppm. CO2 or
carbonate may not be the dominant carbon-bearing
phases in the mantle. According to the mantle
oxidation state, methane, diamond, and even some
metal carbides could be preferentially stabilized
in the Earth's mantle at pressures above 6
GPa. Nevertheless, CO2 and carbonates (dolomite
and magnesite) can be important even in the deep
mantle, for example, in oxidized
subduction-related environments.
From Ghosh et al. (2009) Chem. Geol. (in press)
39
The role of volatiles
At high P (gt2 GPa) , CO2 (if present) reacts with
silicate minerals to give a new carbonate phase
according to the reaction Cpx Ol CO2
Dolomite Opx Carbonates have been observed
rarely among mineral inclusions in diamonds and
in mantle xenoliths. This rarity is probably due
to the same decarbonation reaction acting during
decompression in the opposite direction (i.e.,
consuming dolomite). This reaction is responsible
for the formation of wehrlite.
Wehrlites?
40
The role of volatiles
dolomite orthopyroxene clinopyroxene
olivine CO2
41
The role of volatiles
At higher P (?gt3 GPa?), the composition of
carbonate minerals changes according to the
reaction Dolomite Opx Cpx Magnesite
Magnesite is richer in Mg than Dolomite. This
means that carbonatitic partial melts at
carbonated peridotite solidus are richer in Mg
where magnesite is a solidus phase (i.e., at
greater depths).
42
The role of volatiles
Are we sure that CO2 is present in the Earth's
mantle? Yes CO2-rich magmas (e.g., carbonatites
and kimberlites) originate at pressures of 28
GPa. Due to their low density, low viscosity
and highly reactive nature, carbonatite melts and
fluids are the most important metasomatic agents
in the lithospheric mantle.
43
The role of volatiles
Metasomatism? Metasomatic Agents? Mmmh
Important and fundamental concepts in modern
igneous petrology but too often used as magic
words to explain what is difficult to explain
We will come back later to these arguments, no
matter.
44
The role of volatiles
Carbonatite? What is a Carbonatite? Carbonatite
is an igneous rock which contains more than 50
vol. of carbonated minerals (e.g. dolomite,
calcite, magnesite, etc.). On the Earths
surface carbonatites are volumetrically
insignificant (much less than 10-9
) Carbonatites generally contain low silica
(010 wt.) and high incompatible trace elements,
which make them unique among other igneous rocks.
45
The role of volatiles
If minerals are visible, carbonatites can be
classified as follows
Sovite (plutonic rock with calcite as the main
carbonate mineral) Alvikite (volcanic rock with
calcine as the main carbonate mineral) Beforsite
(rock with dolomite as the main carbonate
mineral) Ferrocarbonatite (rock with Fe-rich
carbonate minerals) Natrocarbonatite
(essentially made up of Na-, K- and Ca- carbonate
minerals)
46
The role of volatiles
If minerals are too small, carbonatites can be
classified as follows
47
The role of volatiles
Are carbonatites common in the circum-Mediterranea
n region? ABSOLUTELY NO. Very few and scarce
occurrences (Canaries, Morocco, France, Germany
and, possibly, Italy). So WHY DO WE SPEND
PRECIOUS TIME WITH THIS RARE STUFF? Because a
(wrong) axiom has been often proposed in
literature Carbonatites? Presence of mantle
plume!
48
Simplified Solidus for Lherzolite H2O system
Simplified solidus for different bulk water
contents of the system.
From Katz et al. (2005) Geochem. Geophys.
Geosyst. doi10.1029/2002GC000433.
49
Did you really understand the significance of
these curves?
This is the solidus for a given dry starting
assemblage (it varies depending the composition
used).
This is the H2O-satureted solidus (i.e., for H2O
content greater that can be contained in
pargasite).
These are H2O-undersatureted solidi (i.e., for
H2O content lower that can be contained in
pargasite). In this case the solidus is given by
the amphibole stability limit.
50
Simplified Solidus for Lherzolite CO2 H2O
system
The average dry mantle solidus is the red line.
Adding CO2 H2O to peridotite increases the
complexity of the system.
From Ghosh et al. (2009) Chem. Geol. (in press).
51
Simplified Solidus for Lherzolite CO2 H2O
system
Here are reported also several mantle solidi
obtained on different non-dry starting
assemblages (CMS-CO2 CMAS-CO2 CMAS-CO2H2O
Natural PeridotiteCO2 Natural
peridotiteAlkaliesCO2).
From Ghosh et al. (2009) Chem. Geol. (in press).
52
Simplified Solidus for Lherzolite CO2 H2O
system
Dry Solidus
Hypothetical Geotherm
Solidus (CO2H2O)
In this case the geotherm never intersects the
dry solidus. This means that partial melting is
not possible under completely dry conditions.
MgCO3
Next Figure
DHMS
From Wyllie and Riabchikov (2000) J. Petrol.,
41, 1195-1206.
53
Melting relationships for pyrolite (CHO) of
the shallow mantle.
Oceanic Intraplate Geotherm
C-H-free Solidus
Dehydration Solidus
Plagioclase
The petrological base of the lithosphere defined
by high pressure pargasite breakdown and a
silicate solidus at 95 km. This is because
pargasite breakdown is always accompained by
melting.
Major Melting
Shield Continental Intraplate Geotherm
Spinel
Garnet
LITHOSPHERE
Amphibole Carbonate or Carbonatite Melt fO2
IW 3.4
Pargasite
Incipient Melting
ASTHENOSPHERE
Silicate Melt Dissolved CO3 and OH fO2 IW 2
Partial Melt present
CARBONATITE MELT
Adiabatic Upwelling
SUB-ASTHENOSPHERE
Subsolidus fO2 IW 1
D.H. Green (http//www.mantleplumes.org/MantleTemp
.html)
54
The base of the asthenosphere (incipient melting
regime along the geotherm) is drawn arbitrarily
at 150 km as the intersection of a mantle
adiabat of potential temperature Tp1450C with
the pyrolite-(CHO) solidus for fO2
IW1. Below this depth, a fluid with CH4 H2O is
present but there is no silicate melt unless fO2
gt IW1, i.e. a more oxidised region of the
mantle.
D.H. Green (http//www.mantleplumes.org/MantleTemp
.html)
55
Do you know (or remember) the significate of
Oxygen buffers?
Averagemantle value
More Oxidizing
Magnetite Quartz Fayalite O2 2Fe3O4
3SiO2 3Fe2SiO4 In other words, when the oxygen
fugacity is high the assemblage Fayalite O2 is
stable with respect to Magnetite Quartz.
More Reducing
56
If the peridotite system is sufficiently oxidized
(i.e., both CO2 and H2O are present), the Oceanic
Geotherm crosses the solidus at shallower depths
up to deeper levels.
Oceanic Intraplate Geotherm
C-H-free Solidus
Dehydration Solidus
Plagioclase
Major Melting
Shield Continental Intraplate Geotherm
Spinel
Garnet
LITHOSPHERE
Under these conditions (e.g., oxidized and with
free CO2 and H2O), the asthenosphere would occupy
a larger volume of the mantle.
Incipient Melting
ASTHENOSPHERE
Silicate Melt Dissolved CO3 and OH fO2 IW 2
Partial Melt present
CARBONATITE MELT
Adiabatic Upwelling
D.H. Green (http//www.mantleplumes.org/MantleTemp
.html)
57
The Sky Blue Line indicates the melting curve for
the C-H-free system (Dry Peridotite).
TEMPERATURE (C)
800
1200
1600
Oceanic Intraplate Geotherm
C-H-free Solidus
Dehydration Solidus
30
1
Plagioclase
Shield Continental Intraplate Geotherm
Spinel
60
Garnet
2
LITHOSPHERE
Major Melting
Amphibole Carbonate or Carbonatite Melt fO2
IW 3.4
DEPTH (km)
PRESSURE (GPa)
90
Pargasite
Incipient Melting
3
ASTHENOSPHERE
120
Silicate Melt Dissolved CO3 and OH fO2 IW 2
4
CARBONATITE MELT
Adiabatic Upwelling
150
SUB-ASTHENOSPHERE
5
Subsolidus fO2 IW 1
D.H. Green (http//www.mantleplumes.org/MantleTemp
.html)
58
TEMPERATURE (C)
The Yellow Line indicates the melting curve for
the system Peridotite H2O CO2 (Oxidized
Conditions). In this case at P gt2 GPa the
carbonatation reaction occurs Ol Cpx CO2
Opx Dolomite. Dolomite becomes the solidus
phase and the solidus is strongly depressed with
formation of carbonatitic melts.
800
1200
1600
Oceanic Intraplate Geotherm
C-H-free Solidus
Dehydration Solidus
Depth at which carbonatation reaction occurs
30
1
Plagioclase
Shield Continental Intraplate Geotherm
Spinel
60
Garnet
2
LITHOSPHERE
Major Melting
Amphibole Carbonate or Carbonatite Melt fO2
IW 3.4
DEPTH (km)
PRESSURE (GPa)
90
Pargasite
Incipient Melting
3
ASTHENOSPHERE
120
Silicate Melt Dissolved CO3 and OH fO2 IW 2
4
CARBONATITE MELT
Adiabatic Upwelling
150
SUB-ASTHENOSPHERE
5
Subsolidus fO2 IW 1
D.H. Green (http//www.mantleplumes.org/MantleTemp
.html)
59
The Red Line indicates the melting curve for the
system Peridotite H2O CH4 (Reduced
Conditions). In this case no carbonate is stable
as solidus phase and therefore no carbonatitic
melt is produced. The shape of the solidus is
mainly influenced by amphibole breakdown.
TEMPERATURE (C)
800
1200
1600
Oceanic Intraplate Geotherm
C-H-free Solidus
Dehydration Solidus
30
1
Plagioclase
Shield Continental Intraplate Geotherm
Spinel
Depth at which amphibole is unstable.
60
Garnet
2
LITHOSPHERE
Major Melting
Amphibole Carbonate or Carbonatite Melt fO2
IW 3.4
DEPTH (km)
PRESSURE (GPa)
90
Pargasite
Incipient Melting
3
ASTHENOSPHERE
120
Silicate Melt Dissolved CO3 and OH fO2 IW 2
4
CARBONATITE MELT
Adiabatic Upwelling
150
SUB-ASTHENOSPHERE
5
Subsolidus fO2 IW 1
60
The Green Line indicates the melting curve for
the system Peridotite CO2 (without H2O). In
this case there is no role for pargasite
breakdown (being as this system is H2O-free). The
solidus is sensibly lower compared to CO2-free
peridotite but there is no important kink.
TEMPERATURE (C)
800
1200
1600
Oceanic Intraplate Geotherm
C-H-free Solidus
Dehydration Solidus
30
1
Plagioclase
Shield Continental Intraplate Geotherm
Spinel
60
Garnet
2
LITHOSPHERE
Major Melting
Amphibole Carbonate or Carbonatite Melt fO2
IW 3.4
DEPTH (km)
PRESSURE (GPa)
90
Pargasite
Incipient Melting
3
ASTHENOSPHERE
120
Silicate Melt Dissolved CO3 and OH fO2 IW 2
4
CARBONATITE MELT
Adiabatic Upwelling
150
SUB-ASTHENOSPHERE
5
Subsolidus fO2 IW 1
D.H. Green (http//www.mantleplumes.org/MantleTemp
.html)
61
TEMPERATURE (C)
It was known from 1973 that carbonates may be
stable at mantle depths. CO2 solubilities in
silicate melts strongly increases at about 2 GPa.
800
1200
1600
Oceanic Intraplate Geotherm
C-H-free Solidus
Dehydration Solidus
30
1
Plagioclase
Shield Continental Intraplate Geotherm
Spinel
60
Garnet
2
LITHOSPHERE
Major Melting
Amphibole Carbonate or Carbonatite Melt fO2
IW 3.4
Doping peridotite with small amounts of CO2 and
H2O, experimental petrology showed that
Na-bearing carbonatite melts were stable from 2
to more than 5 GPa.
DEPTH (km)
PRESSURE (GPa)
90
Pargasite
Incipient Melting
3
ASTHENOSPHERE
120
Silicate Melt Dissolved CO3 and OH fO2 IW 2
4
CARBONATITE MELT
Adiabatic Upwelling
150
SUB-ASTHENOSPHERE
5
Subsolidus fO2 IW 1
D.H. Green (http//www.mantleplumes.org/MantleTemp
.html)
62
TEMPERATURE (C)
800
1200
1600
CO2 (as well as H2O) promotes melting at low
temperatures in peridotites. The melting
temperature for peridotite with both CO2 and H2O
was found to be lower than with either CO2 or H2O
alone (below 1000C at 2-3 GPa).
Oceanic Intraplate Geotherm
C-H-free Solidus
Dehydration Solidus
30
1
Plagioclase
Shield Continental Intraplate Geotherm
Spinel
60
Garnet
2
LITHOSPHERE
Major Melting
Amphibole Carbonate or Carbonatite Melt fO2
IW 3.4
DEPTH (km)
PRESSURE (GPa)
90
Pargasite
Incipient Melting
3
ASTHENOSPHERE
120
Silicate Melt Dissolved CO3 and OH fO2 IW 2
4
CARBONATITE MELT
Adiabatic Upwelling
150
SUB-ASTHENOSPHERE
5
Subsolidus fO2 IW 1
D.H. Green (http//www.mantleplumes.org/MantleTemp
.html)
63
The SiO2 content of basaltic melt is lower in
presence of CO2. Why? CO2 significantly lowers
the solidus temperature of a mantle assemblage at
pressures where carbonate is a solidus phase. Did
you understand what a solidus phase means? In
this case it means that carbonate is the first
mineral that starts melting.
64
The SiO2 content of basaltic melt is lower in
presence of CO2. Why? Of course, if carbonate
melts what type of composition would the have
liquid produced? Carbonatitic, of
course Therefore, when carbonate is the solidus
phase, a carbonatitic rather than silicatic melt
forms during partial melting processes. With
progressive melting at higher temperatures
partial melts change to silicate melts, but they
are characteristically SiO2-poor.
65
The SiO2 content of basaltic melt is lower in
presence of CO2. Why? Remember the carbonatation
reaction at P gt2 GPa Ol Cpx CO2 Opx
Dolomite This means that Opx is stabilized in the
source and it does not participate in the melting
assemblage. This, in turn, implies that the
peritectic reaction Opx Ol
SiO2-rich melt
does not occur. The final effect is that the
silicate melt produced is SiO2-poor.
66
At P 3 GPa and T 1350 C the melt produced
from the mix KLB-1 5 Magnesite (2.5 wt CO2)
is carbonatitic with 3 wt SiO2.
From Hirose (1997) Geophys. Res. Lett., 24,
2837-2840.
67
At similar P (3 GPa) other studies found
carbonatitic melts at much lower T (1050 C) with
lt10 wt SiO2. (Dasgupta et al., 2007 J.
Petrol.)
68

• The presence of carbonatitic melts at relatively
  low temperatures causes significant lowering of
  the solidus temperature of the mantle.
• However, in case of CO2-bearing mantle sources,
  silicate melts and carbonated-silicate melts also
  appear at temperatures sensibly lower than under
  dry (volatile-free) conditions.
• At 3 GPa
• A carbonatitic melt appears at T 1050 C (in a
  CO2-doped natural peridotite KLB-1)
• A carbonated-silicate melt appears at T 1350
  C
• A silicate melt appears at T 1500 C.

69
TEMPERATURE (C)
800
1200
1600
The carbonatite melt at gt95 km and T lt1000C is
represented as present or absent dependent on
local variation in fO2.
Oceanic Intraplate Geotherm
C-H-free Solidus
Dehydration Solidus
30
1
Plagioclase
Shield Continental Intraplate Geotherm
Spinel
60
Garnet
If fO2 IW3 (Oxidized conditions) then
carbonatite melt H2O-fluid is present.
2
LITHOSPHERE
Major Melting
Amphibole Carbonate or Carbonatite Melt fO2
IW 3.4
DEPTH (km)
PRESSURE (GPa)
90
Pargasite
Incipient Melting
3
If fO2 IW3 (Reducing conditions) then
graphite (diamond) H2O-fluids would be present.
ASTHENOSPHERE
120
Silicate Melt Dissolved CO3 and OH fO2 IW 2
4
CARBONATITE MELT
Adiabatic Upwelling
150
SUB-ASTHENOSPHERE
5
Subsolidus fO2 IW 1
D.H. Green (http//www.mantleplumes.org/MantleTemp
.html)
70
The melting regime for mantle-derived basaltic
magmas from intraplate settings (kimberlites,
olivine melilitites, olivine nephelinites,
basanites, alkali-olivine basalts) requires the
presence of carbon and hydrogen (dissolved (CO3
2-) and (OH-) in the melt phase.
Alkali Ol Basalt
Hawaiian Ol Tholeiite
Mid Ocean Ridge Picrite
Ol Basanite
Ol Nephelinite
Ol Melilitite
Leucitite
D.H. Green (http//www.mantleplumes.org/MantleTemp
.html)
71
Magma genesis in intraplate settings can be
understood in terms of the lherzolite (C-H-O)
system. Magmas are derived from an incipient
melting regime which lies at temperatures below
the volatile-free lherzolite solidus, marking
entry to the major melting regime.
Alkali Ol Basalt
Hawaiian Ol Tholeiite
Mid Ocean Ridge Picrite
Ol Basanite
Ol Nephelinite
Ol Melilitite
Leucitite
D.H. Green (http//www.mantleplumes.org/MantleTemp
.html)
72
Where are volatiles stored in the solid mantle?
H2O is stored also in nominally anhydrous
minerals at mantle depths. Spinel-lherzolite
facies rocks probably contain 25 ppm H2O,
whereas garnet-lherzolites reach up to 175
ppm. Primitive mantle H2O content estimate is
1160 ppm. Primitive mantle CO2 content range
between 230 and 550 ppm. Due to these low
abundances, H2O and CO2 can be entirely stored in
minerals. However, with mantle upwelling these
volatiles may exolve.
73
Where volatiles are stored in the solid mantle?
Under relatively oxidized conditions (i.e.,
uppermost mantle) H2O and CO2 are the most likely
compounds. Under more reducing conditions (i.e.,
at deeper levels) H2O-C (C graphite or diamond,
with increasing depth), H2O-CH4 and CH4-H2
become
stable.
How can we be sure that the upper mantle is
relatively oxidized? CO2 and H2O micro-inclusions
in diamonds
74
Where volatiles are stored in the solid mantle?
CO2 is stored in subsolidus dolomite or magnesite
at depths of gt70 km and 100 km, respectively.
At depths of gt300 km, H2O is stored in DHMS
(Dense Hydrous Magnesium Silicates) like Brucite
Mg(OH)2 and Wadsleyite at P 410-520 km.
Remember Wadsleyite?
b-Olivine
75
Where are volatiles stored in the solid mantle?
The olivinewadsleyite transformation in the
Mg2SiO4Fe2SiO4 system calculated at 1400 C
under dry conditions and with 0.4 wt H2O, which
is the H2O content required to saturate olivine
at these conditions.
From Frost (2008) Elements, 4, 171-176.
76
Where are volatiles stored in the solid mantle?
The downward displacement of the two-phase region
in the hydrous system is shown by the red curves.
The vertical dashed line shows a typical mantle
Fe/(FeMg) ratio of 0.1. The dry transition is
approximately 7 km wide, while the addition of
H2O broadens the transition at these conditions
to a maximum of 11 km.
From Frost (2008) Elements, 4, 171-176.
77
The effects of water
Water 1. Has a large effect on the mantle
solidus 2. Is also responsible for the
broadening of the melting zone under mid-ocean
ridges 3. Is responsible for the initiation of
melting in subduction zones 4. Drastically
changes the liquid line of descent of basaltic
magmas (it is responsible for the calcalkaline
versus tholeiitic differentiation trend).
78
Focus on Dense Hydrous Silicate Minerals
Experimental studies of hydrous systems under
mantle pressures suggest the existence of several
dense hydrous phases. The major hydrous phases in
pyrolite compositions with increasing pressure
should be represented by serpentine, chlorite,
amphibole (pargasite) and so-called dense hydrous
magnesium silicate (DHMS) phases phase A, phase
E, superhydrous phase B (or phase C) and phase G
(or phases D/F). All these phases are stable at
temperatures below 1300 C. Most of the hydrous
phases were synthesized via a simple
silicatewater system or under watersaturated
conditions.
From Litasov and Ohtani (2003) Phys. Chem.
Mineral., 30, 147-156.
79
Principal hydrous phases in peridotites
The principal hydrous phases in H2O-saturated
peridotite to 8 GPa are serpentine, phase A,
chlorite, talc, and amphibole.
From Katz et al. (2005) Geochem. Geophys.
Geosyst. doi10.1029/2002GC000433.
A phase A, amph amphibole, chl chlorite,
cpx clinopyroxene, gar garnet, ol olivine,
opx orthopyroxene, serp serpentine, sp
spinel, tc talc.
80
Principal hydrous phases in peridotites
The principal hydrous phases in H2O-saturated
peridotite to 8 GPa are serpentine, phase A,
chlorite, talc, and amphibole.
Temperature (C)
400
500
600
700
800
900
1000
1100
tc ol H2O
amph ol H2O
opx H2O
tc ol
opx ol sp H2O
solidus
amph sp opx ol
chl
serp chl cpx
serp
amph chl opx ol
1
1
50
serp chl amph ol
sp
amph ol H2O
amph gar opx ol
2
2
gar
tc amph chl ol
amph ol
chl opx cpx
70
cpx gar opx H2O
chl cpx opx ol
serp chl cpx ol
3
3
100
chl opx
Absence of stable hydrous phases
serp
Depth (km)
opx ol H2O
ol gar H2O
4
4
solidus
Pressure (GPa)
Pressure (GPa)
cpx opx ol gar
Absence of stable hydrous phases
150
serp gar cpx ol
5
5
ol H2O
serp phase A
serp phase A cpx gar
180
Peridotite H2O
6
6
200
ol H2O
serp
A opx H2O
A opx
7
7
phase A opx cpx gar
From Fumagalli and Poli (2005) J. Petrol., 46,
555-578
240
8
From Schmidt and Poli (1998) Earth Planet. Sci.
Lett., 163, 361-379
81
Principal hydrous phases in peridotites
The principal hydrous phases in H2O-saturated
peridotite to 8 GPa are serpentine, phase A,
chlorite, talc, and amphibole.
Temperature (C)
400
500
600
700
800
900
1000
1100
tc ol H2O
amph ol H2O
opx H2O
tc ol
opx ol sp H2O
solidus
amph sp opx ol
chl
serp chl cpx
serp
amph chl opx ol
1
50
serp chl amph ol
sp
amph ol H2O
amph gar opx ol
2
gar
tc amph chl ol
amph ol
chl opx cpx
70
cpx gar opx H2O
chl cpx opx ol
serp chl cpx ol
3
100
chl opx
serp
Depth (km)
opx ol H2O
ol gar H2O
4
solidus
Pressure (GPa)
cpx opx ol gar
150
serp gar cpx ol
5
ol H2O
serp phase A
serp phase A cpx gar
180
6
Peridotite H2O
200
ol H2O
serp
A opx H2O
A opx
7
phase A opx cpx gar
From Niida and Green (1999) Contrib. Mineral.
Petrol., 135, 18-40
240
8
From Schmidt and Poli (1998) Earth Planet. Sci.
Lett., 163, 361-379
82
Principal hydrous phases in peridotites
The principal hydrous phases in H2O-saturated
peridotite to 8 GPa are serpentine, phase A,
chlorite, talc, and amphibole.
Temperature (C)
900
1000
1100
Temperature (C)
MORB pyrolite -40 olivine ( 0.6 H2O)
400
500
600
700
800
900
1000
1100
tc ol H2O
amph ol H2O
opx H2O
tc ol
opx ol sp H2O
solidus
amph sp opx ol
chl
1
serp chl cpx
Pl-out
serp
amph chl opx ol
1
Lherzolite melt
Amphibole Lherzolite
50
serp chl amph ol
sp
amph ol H2O
amph gar opx ol
2
Pressure (GPa)
gar
tc amph chl ol
amph ol
chl opx cpx
2
70
cpx gar opx H2O
Gar-in
chl cpx opx ol
serp chl cpx ol
3
100
chl opx
3
serp
Depth (km)
opx ol H2O
ol gar H2O
4
Amph-out
solidus
Pressure (GPa)
cpx opx ol gar
150
serp gar cpx ol
5
From Niida and Green (1999) Contrib. Mineral.
Petrol., 135, 18-40
ol H2O
serp phase A
serp phase A cpx gar
180
6
Peridotite H2O
Amph-out 3 GPa
200
ol H2O
serp
? Who is wrong?
A opx H2O
A opx
7
phase A opx cpx gar
240
8
Amph-out 2 GPa
From Schmidt and Poli (1998) Earth Planet. Sci.
Lett., 163, 361-379
83
Principal hydrous phases in peridotites
Stabilities of serpentine, phase A, and talc are
expected to be almost identical for depleted or
fertile peridotite because these phases are
almost Ca- and Na-free and contain only minor
amounts of Al2O3.
On the other hand, the stabilities of chlorite
and amphibole are expected to vary for
harzburgitic, lherzolitic, and pyrolitic bulk
compositions.
Temperature (C)
400
500
600
700
800
900
1000
1100
tc ol H2O
amph ol H2O
opx H2O
tc ol
opx ol sp H2O
solidus
amph sp opx ol
chl
serp chl cpx
serp
amph chl opx ol
1
50
serp chl amph ol
sp
amph ol H2O
amph gar opx ol
2
gar
tc amph chl ol
amph ol
chl opx cpx
70
cpx gar opx H2O
chl cpx opx ol
serp chl cpx ol
3
100
chl opx
serp
Depth (km)
opx ol H2O
ol gar H2O
4
solidus
Pressure (GPa)
cpx opx ol gar
150
serp gar cpx ol
5
ol H2O
serp phase A
serp phase A cpx gar
180
6
Peridotite H2O
200
ol H2O
serp
A opx H2O
A opx
7
phase A opx cpx gar
240
8
From Schmidt and Poli (1998) Earth Planet. Sci.
Lett., 163, 361-379
84
Principal hydrous phases in peridotites
Serpentine contains 12.3 wt H2O and dominates,
together with chlorite (13.0 wt H2O), the water
budget of hydrous peridotite down to ca. 150 km
depth.
Temperature (C)
In average mantle compositions, talc (4.7 wt
H2O) olivine have a rather limited stability
field less than 100 ºC wide, and they decompose
to enstatite H2O between 690 ºC (1 GPa) and 720
ºC (2 GPa).
400
500
600
700
800
900
1000
1100
tc ol H2O
amph ol H2O
opx H2O
tc ol
opx ol sp H2O
solidus
amph sp opx ol
chl
serp chl cpx
serp
amph chl opx ol
1
50
serp chl amph ol
sp
amph ol H2O
amph gar opx ol
2
gar
tc amph chl ol
amph ol
chl opx cpx
70
cpx gar opx H2O
chl cpx opx ol
serp chl cpx ol
3
100
chl opx
serp
Depth (km)
opx ol H2O
ol gar H2O
4
solidus
Pressure (GPa)
cpx opx ol gar
150
serp gar cpx ol
5
ol H2O
serp phase A
serp phase A cpx gar
180
6
Peridotite H2O
200
ol H2O
serp
A opx H2O
A opx
7
phase A opx cpx gar
240
8
From Schmidt and Poli (1998) Earth Planet. Sci.
Lett., 163, 361-379
85
Principal hydrous phases in peridotites
In natural peridotites, chlorites have
compositions close to clinochlor
(Mg5Al2Si3O10(OH)8). Synthetic clinochlor
decomposes between 3 and 21 kbar to orthopyroxene
olivine spinel H2O with a maximum
temperature stability of 870ºC
Temperature (C)
In subducted peridotite, phase A (Mg7Si2O8(OH)6,
11.8 wt H2O) replaces serpentine at pressures
between 6 and 7 GPa.
400
500
600
700
800
900
1000
1100
tc ol H2O
amph ol H2O
opx H2O
tc ol
opx ol sp H2O
solidus
amph sp opx ol
chl
serp chl cpx
serp
amph chl opx ol
1
50
serp chl amph ol
sp
amph ol H2O
amph gar opx ol
2
gar
tc amph chl ol
amph ol
chl opx cpx
70
cpx gar opx H2O
chl cpx opx ol
serp chl cpx ol
3
100
chl opx
serp
Depth (km)
opx ol H2O
ol gar H2O
4
solidus
Pressure (GPa)
cpx opx ol gar
150
serp gar cpx ol
5
ol H2O
serp phase A
serp phase A cpx gar
180
6
Peridotite H2O
200
ol H2O
serp
A opx H2O
A opx
7
phase A opx cpx gar
240
8
From Schmidt and Poli (1998) Earth Planet. Sci.
Lett., 163, 361-379
86
Principal hydrous phases in peridotites
Close to the water-saturated solidus
(approximately 1000 ºC, at 23 GPa)
amphibole is pargasitic
hornblende in composition and decomposes between
2.2 and 3 GPa.
Temperature (C)
In harzburgite, calcic amphibole decomposes at
2.2 GPa. In lherzolite at 2.52.8 GPa. In
enriched pyrolite at 2.83.0 GPa.
400
500
600
700
800
900
1000
1100
tc ol H2O
amph ol H2O
opx H2O
tc ol
opx ol sp H2O
solidus
amph sp opx ol
chl
serp chl cpx
serp
amph chl opx ol
1
50
serp chl amph ol
sp
amph ol H2O
amph gar opx ol
2
gar
tc amph chl ol
amph ol
chl opx cpx
70
cpx gar opx H2O
chl cpx opx ol
serp chl cpx ol
3
100
chl opx
serp
Depth (km)
opx ol H2O
ol gar H2O
4
solidus
Pressure (GPa)
cpx opx ol gar
150
serp gar cpx ol
5
ol H2O
serp phase A
serp phase A cpx gar
180
6
Peridotite H2O
200
ol H2O
serp
A opx H2O
A opx
7
phase A opx cpx gar
240
8
From Schmidt and Poli (1998) Earth Planet. Sci.
Lett., 163, 361-379
87
Principal hydrous phases in peridotites
When serpentine decomposes, chlorite becomes the
dominant hydrous phase. Although amphibole
contains ca. 40 of the H2O bound in the PT
domain of chlorite amphibole, water is not
liberated when amphibole decomposes.
Water is liberated only between 800850 ºC and
the wet solidus (1000ºC) from the breakdown of
amphibole.
Temperature (C)
400
500
600
700
800
900
1000
1100
tc ol H2O
amph ol H2O
opx H2O
tc ol
opx ol sp H2O
solidus
amph sp opx ol
chl
serp chl cpx
serp
amph chl opx ol
1
50
serp chl amph ol
sp
amph ol H2O
amph gar opx ol
2
gar
tc amph chl ol
amph ol
chl opx cpx
70
cpx gar opx H2O
chl cpx opx ol
The quantity of this fluid is small in
lherzolite, the maximum content of amphibole is
19 wt (harzburgite 9 wt) contributing 0.5
wt (harzburgite 0.2 wt) H2O.
serp chl cpx ol
3
100
chl opx
serp
Depth (km)
opx ol H2O
ol gar H2O
4
solidus
Pressure (GPa)
cpx opx ol gar
150
serp gar cpx ol
5
ol H2O
serp phase A
serp phase A cpx gar
180
6
Peridotite H2O
200
ol H2O
serp
A opx H2O
A opx
7
phase A opx cpx gar
240
8
From Schmidt and Poli (1998) Earth Planet. Sci.
Lett., 163, 361-379
88
Schematic stability of the hydrous phases in the
system CMAS pyrolite 2 wt H2O.
Ol olivine Cpx clinopyroxene Cen
clinoenstatite Gt garnet Rw
ringwoodite MgPv Mg-perovskite CaPv
Ca-perovskite Pc periclase A DHSM Phase
A E DHSM Phase E SuB Superhydrous Phase B.
Litasov and Ohtani (2003) Phys. Chem. Mineral.,
30, 147-156.
89
Schematic stability of the hydrous phases in the
system CMAS pyrolite 2 wt H2O.
Pyrolite 2 wt H2O
Serp
Hot
In simplified systems, the field of the Phase A
reaches 10 GPa. The stability of Phase E
partially overlaps that of wadsleyite. Wadsleyite
can accomodate up to 3 wt H2O. Where this phase
is stable, the field of stability of DHSM is
reduced.
Ol A
Subduction
Ol E
Ol
Ol
Wd E
Wd
0.5
2.0
0.2
1.5
0.8
Cold Subduction
Wd
0.8
Rw
Rw SuB
Rw
MgPv Pc
SuB out
Phase G/D/F
Litasov and Ohtani (2003) Phys. Chem. Mineral.,
30, 147-156.
90
Schematic stability of the hydrous phases in the
system CMAS pyrolite 2 wt H2O.
Pyrolite 2 wt H2O
Serp
Hot
DHSM (A, E, superB) cannot exist in pyrolite
along a typical mantle geotherm due to their
stability being restricted to lower temperatures
and can occur only under the PT conditions in
slabs descending into the mantle. Under normal
mantle conditions, wadsleyite or ringwoodite are
the major H2O-bearing phases.
Ol A
Subduction
Ol E
Ol
Ol
Wd E
Wd
0.5
2.0
0.2
1.5
0.8
Cold Subduction
Wd
0.8
Rw
Rw SuB
Rw
MgPv Pc
SuB out
Phase G/D/F
Litasov and Ohtani (2003) Phys. Chem. Mineral.,
30, 147-156.
91
Stability of volatile-bearing phases in
subduction zones.

• The variation of T is a fundamental parameter to
  estimate the stability field of H2O-CO2-bearing
  phases.
• The factors controlling the temperature
  distribution are
• Rate of subduction
• Age (and thus thickness) of the descending slab
• Frictional heating of the upper and lower
  surfaces of the slab
• Conduction of heat into the slab from the
  asthenosphere
• Adiabatic heating associated with compression of
  the slab
• Heat derived from radioactive decay. This is
  generally low due to the low content of
  radioactivie minerals in oceanic plates
• Latent heat associated with phase transitions.

Litasov and Ohtani (2003) Phys. Chem. Mineral.,
30, 147-156.
92
The fate of water released from the subducting
lithosphere
Water in subducting oceanic crust. Blueschists
of basaltic origin representing shallow portions
of subduction zones (1020 km) have typically 56
wt H2O bound in hydrous phases. With a 7 km
thick mafic oceanic crust, composed by equal
parts of basalt and gabbro, 0.50.6 109 g
H2O/m2 are bound in basaltic layer 2 at shallow
depth. Oceanic gabbros are only partially
hydrated with 2030 hydration by volume
0.110.18 109 g H2O/m2 are estimated for the
gabbroic layer. An additional amount of 0.010.02
109 g H2O/m2 will be subducted in the
sedimentary layer (200400 m thick).
From Schmidt and Poli (1998) Earth Planet. Sci.
Lett., 163, 361-379
93
The fate of water released from the subducting
lithosphere
Water in subducting oceanic crust. In a vertical
section of 1 m2, typical dehydration rates
between 20 and 70 km depth amount to 0.8 0.2
107 g H2O per kilometre of depth. When the
oceanic crust reaches depths of 7080 km the
resulting metamorphic assemblage retains 0.5-1
wt H2O, i.e. 0.1-0.2 109 g H2O/m2.
From Schmidt and Poli (1998) Earth Planet. Sci.
Lett., 163, 361-379
94
The fate of water released from the subducting
lithosphere
Only a portion of the H2O initially contained in
the subducting lithosphere will escape from the
slab in the depth range suitable for arc magma
formation. A large portion of H2O will be
dehydrated at relatively shallow levels and a
small portion will fade into the deeper mantle.
From Schmidt and Poli (1998) Earth Planet. Sci.
Lett., 163, 361-379
95
Model for the formation of the volcanic front
Dehydration from peridotite and oceanic crust
occurs at almost any depth to ca. 150200 km.
Thus water will be generally available above the
subducting lithosphere.
Rise of fluids
Rise of melts
From Schmidt and Poli (1998) Earth Planet. Sci.
Lett., 163, 361-379
96
Model for the formation of the volcanic front
The volcanic front forms where the amount of melt
is sufficient to mechanically extract and give
rise to arc magmatism.
Rise of fluids
Rise of melts
From Schmidt and Poli (1998) Earth Planet. Sci.
Lett., 163, 361-379
97
Model for the formation of the volcanic front

• Typical models involve a multistage process
• Dehydration of the slab at variable and
  relatively shallow depths
• Intermediate storage of H2O (and trace elements
  deriving from the subducted slab) in a single,
  key hydrous phase in the mantle wedge (typically
  amphibole)
• Subsequently the down-dragged mantle wedge
  dehydrates
• Melting occurs above this dehydration zone or
• Melting of the mantle wedge occurs on the wet
  solidus of peridotite.
• A number of hydrous phases have been called on to
  explain such transient storage of H2O and of
  other metasomatizing components within the
  mantle wedge.

From Kearey et al. (2008) Global Tectonics.
Wiley-Blackwell Publ. 482 pp.
98
Model for the formation of the volcanic front
Typical models involve a multistage
process Amphibole contributes moderately to the
water budget. This phase has a relatively small
contribution to the water budget of peridotite
(0.20.5 wt H2O of the bulk rock) with respect
to the other hydrous phases. Therefore it has
been supposed that the role of amphibole is not
outstanding with respect to other hydrous
phases. Amphibole cannot constitute the fluid
source situated below the volcanic front.
From Schmidt and Poli (1998) Earth Planet. Sci.
Lett., 163, 361-379
99
Model for the formation of the volcanic front
Typical models involve a multistage
process Fluids rise from the subducting slab
and hydrate a portion of the overlying mantle
wedge at almost any depth to 150200 km. At a
given depth, several hydrous phases will
decompose through either discontinuous or
continuous reactions. Dehydration at low
pressures is almost entirely due to
pota