Olivine Equilibrium in Calculating Original Magma Composition
Potential Temperature
Potential Temperature
To calculate the original MgO
To calculate the original MgO
content of a melt, a certain
content of a melt, a certain
amount of olivine is added to the
amount of olivine is added to the
composition of the rock until a
composition of the rock until a
composition in equilibrium with
composition in equilibrium with
the Fo-richest olivine is obtained
the Fo-richest olivine is obtained
.
.
This approach may be VERY DANGEROUS…
This approach may be VERY DANGEROUS…
Potential Temperature
Potential Temperature
Follow this sequence:
Follow this sequence:
1)
Analyze with EMP olivine in basaltic rocks;
Analyze with EMP olivine in basaltic rocks;
2)
Select the olivine with the highest Fo;
Select the olivine with the highest Fo;
3)
Calculate the composition (the MgO content) of
Calculate the composition (the MgO content) of
the melt in equilibrium with this olivine assuming
the melt in equilibrium with this olivine assuming
the relation:
the relation:
K
K
D
D
= (FeO
= (FeO
ol
ol
)/(FeO
)/(FeO
melt
melt
) * (MgO
) * (MgO
melt
melt
)/(MgO
)/(MgO
ol
ol
)
)
4)
On the basis of this MgO content (higher than
On the basis of this MgO content (higher than
that really measured in the rock) calculate the Tp
that really measured in the rock) calculate the Tp
using the relation:
using the relation:
Tp
Tp
(
(
o
o
C)
C)
=
=
1463+12.74*MgO-2924/MgO
1463+12.74*MgO-2924/MgO
5)
Often it is calculated the hypothetical composition
Often it is calculated the hypothetical composition
of the original magma, before olivine
of the original magma, before olivine
crystallization. This is done adding olivine to the
crystallization. This is done adding olivine to the
melt till approaching the hypothetical composition.
melt till approaching the hypothetical composition.
Potential Temperature
Potential Temperature
The distribution coefficient:
The distribution coefficient:
K
K
D
D
= (FeO
= (FeO
ol
ol
)/(FeO
)/(FeO
melt
melt
)*(MgO
)*(MgO
melt
melt
)/(MgO
)/(MgO
ol
ol
)
)
relates the partitioning of Fe and Mg between olivine
relates the partitioning of Fe and Mg between olivine
and liquid.
and liquid.
It has been calculated independent of temperature
It has been calculated independent of temperature
and equal to
and equal to
0.30
0.30
in 1970 (Roeder and Emslie, Contrib.
in 1970 (Roeder and Emslie, Contrib.
Mineral. Petrol.).
Mineral. Petrol.).
Following studies have slightly modified the value of
Following studies have slightly modified the value of
this K
this K
D
D
to 0.31-0.35, no more.
to 0.31-0.35, no more.
Potential Temperature
Potential Temperature
The distribution coefficient:
The distribution coefficient:
K
K
D
D
= (FeO
= (FeO
ol
ol
)/(FeO
)/(FeO
melt
melt
)*(MgO
)*(MgO
melt
melt
)/(MgO
)/(MgO
ol
ol
)
)
Falloon et al. (2007) Chem.
Geol., 241, 207-233
Potential Temperature
Potential Temperature
The distribution coefficient:
The distribution coefficient:
K
K
D
D
= (FeO
= (FeO
ol
ol
)/(FeO
)/(FeO
melt
melt
)*(MgO
)*(MgO
melt
melt
)/(MgO
)/(MgO
ol
ol
)
)
Potential Temperature
Potential Temperature
What does a K
What does a K
D
D
= 0.30 mean?
= 0.30 mean?
The distribution coefficient:
The distribution coefficient:
K
K
D
D
= (FeO
= (FeO
ol
ol
)/(FeO
)/(FeO
melt
melt
)*(MgO
)*(MgO
melt
melt
)/(MgO
)/(MgO
ol
ol
)
)
We can write the KD also in a different way:
We can write the KD also in a different way:
K
K
D
D
= (FeO/MgO)
= (FeO/MgO)
ol
ol
/(FeO/MgO)
/(FeO/MgO)
melt
melt
Let us assume an olivine that crystallizes
Let us assume an olivine that crystallizes
from a basaltic melt. Does this olivine have
from a basaltic melt. Does this olivine have
higher Fe/Mg ratio than the liquid?
higher Fe/Mg ratio than the liquid?
This means that, starting from a given Fe/Mg in
This means that, starting from a given Fe/Mg in
the melt, what is the element preferentially
the melt, what is the element preferentially
allocated into olivine?
allocated into olivine?
Mg
Mg
Potential Temperature
Potential Temperature
The distribution coefficient:
The distribution coefficient:
K
K
D
D
= (FeO
= (FeO
ol
ol
)/(FeO
)/(FeO
melt
melt
)*(MgO
)*(MgO
melt
melt
)/(MgO
)/(MgO
ol
ol
)
)
Do you remember
Do you remember
this diagram?
this diagram?
The olivine in
The olivine in
equilibrium with a
equilibrium with a
melt is always
melt is always
richer in MgO.
richer in MgO.
This means that
This means that
olivine has lower
olivine has lower
FeO/MgO than
FeO/MgO than
the melt.
the melt.
Potential Temperature
Potential Temperature
Let us make an example:
Let us make an example:
FeO
FeO
ol
ol
= 20.90 wt%; MgO
= 20.90 wt%; MgO
ol
ol
= 37.70 wt%.
= 37.70 wt%.
FeO
FeO
melt
melt
= 9.20 wt%; MgO
= 9.20 wt%; MgO
melt
melt
= 7.50 wt%.
= 7.50 wt%.
The distribution coefficient:
The distribution coefficient:
K
K
D
D
= (FeO
= (FeO
ol
ol
)/(FeO
)/(FeO
melt
melt
)*(MgO
)*(MgO
melt
melt
)/(MgO
)/(MgO
ol
ol
)
)
K
K
D
D
= ?
= ?
First divide the wt% per molecular
First divide the wt% per molecular
weights
weights
FeO = 71.85; MgO = 40.31
FeO = 71.85; MgO = 40.31
K
K
D
D
=
=
(0.291/0.128)*(0.187/0.935)
(0.291/0.128)*(0.187/0.935)
= 0.46
= 0.46
?
?
K
K
D
D
should be 0.30, why we obtained 0.46?
should be 0.30, why we obtained 0.46?
K
K
D
D
= (FeO/MgO)
= (FeO/MgO)
ol
ol
/(FeO/MgO)
/(FeO/MgO)
melt
melt
Potential Temperature
Potential Temperature
K
D
0.8
K
D
0.6
K
D
0.4
K
D
0.3
K
D
0.2
K
D
0.1
K
D
= 1 means identical distribution of Fe/Mg
(or Mg#) between melt and olivine. This is
the base (wrong) assumption of CIPW norm.
Mg# =
Mg/(Mg+Fe)
Potential Temperature
Potential Temperature
K
D
0.8
K
D
0.6
K
D
0.4
K
D
0.1
Next slide
K
D
0.3
K
D
0.2
Potential Temperature
Potential Temperature
In theory all the olivines should
plot along the red line, but...
Each color represents one sample.
In each sample coexist olivines with
different compositions (Fo)
Potential Temperature
Potential Temperature
Nearly all the analyzed olivines are
not in equilibrium with the melt
Olivines above the red line have too
much Mg to have been crystallized
from that melt.
K
D
= 0.30
Potential Temperature
Potential Temperature
Olivines below the red line have too
low Mg to have been crystallized from
that melt.
K
D
= 0.30
Nearly all the analyzed olivines are
not in equilibrium with the melt
Potential Temperature
Potential Temperature
How is it possible to have olivine Fo-
richer (Mg-richer) than equilibrium
olivine?
K
D
= 0.30
Xenocrysts
How is it possible to have olivine Fo-poorer
(Fe-richer) than equilibrium olivine?
Recycling of late crystallization phases
Potential Temperature
Potential Temperature
Is it correct to say that the TRUE
Mg# of the melt is that calculated
considering the olivine with the highest
Fo?
K
D
= 0.30
Potential Temperature
Potential Temperature
1)
1)
There are no olivine-controlled trends for
There are no olivine-controlled trends for
MORB glasses
MORB glasses
.
.
Except for the Puna Ridge trend, all of the
Except for the Puna Ridge trend, all of the
olivine-controlled trends are
olivine-controlled trends are
artificially
artificially
produced by adding olivine back into
produced by adding olivine back into
observed glass compositions.
observed glass compositions.
There are several problems in
There are several problems in
transforming the Fo content in olivine
transforming the Fo content in olivine
into temperature estimates:
into temperature estimates:
Potential Temperature
Potential Temperature
Presnall and Gudfinnsson (2011) J. Petrol., 52, 1533-1546
Potential Temperature
Potential Temperature
2)
2)
The Fo-richest olivine can be fragments
The Fo-richest olivine can be fragments
of the mantle, not liquidus olivine
of the mantle, not liquidus olivine
.
.
A detailed chemical
A detailed chemical
and petrographic
and petrographic
investigation is fundamental to distinguish
investigation is fundamental to distinguish
the two types of olivines.
the two types of olivines.
There are several problems in
There are several problems in
transforming the Fo content in olivine
transforming the Fo content in olivine
into temperature estimates:
into temperature estimates:
Potential Temperature
Potential Temperature
3)
3)
The high Fo content of olivine can be
The high Fo content of olivine can be
related to Fe-poor mantle
related to Fe-poor mantle
.
.
A depleted mantle is Fe-poor and Mg-rich.
A depleted mantle is Fe-poor and Mg-rich.
Partial melts of this source will be Fe-poor
Partial melts of this source will be Fe-poor
and Mg-rich and have high Mg#
and Mg-rich and have high Mg#
.
.
Olivine crystallizing from this kind of melts
Olivine crystallizing from this kind of melts
will be Fe-poor and, therefore, Fo-rich.
will be Fe-poor and, therefore, Fo-rich.
There are several problems in
There are several problems in
transforming the Fo content in olivine
transforming the Fo content in olivine
into temperature estimates:
into temperature estimates:
Potential Temperature
Potential Temperature
Potential Temperature
Potential Temperature
4)
4)
Melts generated after high degrees of
Melts generated after high degrees of
melting are characterized by high Mg# and,
melting are characterized by high Mg# and,
consequentially,
consequentially,
will
will
crystallize
crystallize
Fo-rich
Fo-rich
olivine
olivine
.
.
It is not necessary to have high T to
It is not necessary to have high T to
generate high amount of melting
generate high amount of melting
. Melting can
. Melting can
be generated with volatile flushing (H and C,
be generated with volatile flushing (H and C,
above all) and with the presence of low-
above all) and with the presence of low-
solidus olivine-poor lithologies (eclogites).
solidus olivine-poor lithologies (eclogites).
There are several problems in
There are several problems in
transforming the Fo content in olivine
transforming the Fo content in olivine
into temperature estimates:
into temperature estimates:
Potential Temperature
Potential Temperature
4)
4)
Melts generated after high degrees of
Melts generated after high degrees of
melting are characterized by high Mg# and,
melting are characterized by high Mg# and,
consequentially,
consequentially,
will
will
crystallize
crystallize
Fo-rich
Fo-rich
olivine
olivine
.
.
If an upwelling mantle melts at high depths,
If an upwelling mantle melts at high depths,
it will experience more and more melting
it will experience more and more melting
moving to the surface.
moving to the surface.
There are several problems in
There are several problems in
transforming the Fo content in olivine
transforming the Fo content in olivine
into temperature estimates:
into temperature estimates:
Potential Temperature
Potential Temperature
5)
5)
Fo-rich olivines can crystallize in a highly
Fo-rich olivines can crystallize in a highly
oxidized melt
oxidized melt
.
.
If Fe
If Fe
3+
3+
become particularly abundant in
become particularly abundant in
specific conditions magnetite crystallization
specific conditions magnetite crystallization
is favoured. This depletes the residual
is favoured. This depletes the residual
magma in Fe
magma in Fe
2+
2+
.
.
Late o
Late o
livine can crystallize
livine can crystallize
with MgO-rich (up to Fo
with MgO-rich (up to Fo
99
99
) compositions
) compositions
.
.
There are several problems in
There are several problems in
transforming the Fo content in olivine
transforming the Fo content in olivine
into temperature estimates:
into temperature estimates:
Potential Temperature
Potential Temperature
6)
6)
Fo-rich olivines can be generated after
Fo-rich olivines can be generated after
subsolidus substitution with spinel
subsolidus substitution with spinel
.
.
Cr-rich spinel prefers Fe (forming chromite
Cr-rich spinel prefers Fe (forming chromite
Fe
Fe
2+
2+
Cr
Cr
2
2
O
O
4
4
) rather than Mg (Mg-chromite
) rather than Mg (Mg-chromite
MgCr
MgCr
2
2
O
O
4
4
). If olivine is present, it will
). If olivine is present, it will
exchange Fe with chromite, accepting Mg
exchange Fe with chromite, accepting Mg
from the spinel. The result will be Fo-rich
from the spinel. The result will be Fo-rich
olivine.
olivine.
There are several problems in
There are several problems in
transforming the Fo content in olivine
transforming the Fo content in olivine
into temperature estimates:
into temperature estimates:
Potential Temperature
Potential Temperature
7)
7)
If present, phlogopite in the mantle melts
If present, phlogopite in the mantle melts
incongruently giving K-OH-rich melt + Fo-
incongruently giving K-OH-rich melt + Fo-
rich (up to Fo
rich (up to Fo
95
95
) peritectic olivine
) peritectic olivine
.
.
This SiO
This SiO
2
2
-undersaturated K-OH-rich melt
-undersaturated K-OH-rich melt
reacts with mantle enstatite dissolving it. In
reacts with mantle enstatite dissolving it. In
this way the melt becomes MgO-rich (18-30
this way the melt becomes MgO-rich (18-30
wt% MgO) and with high Mg#.
wt% MgO) and with high Mg#.
Olivine
Olivine
crystallizing from this melt will be Fo-rich.
crystallizing from this melt will be Fo-rich.
There are several problems in
There are several problems in
transforming the Fo content in olivine
transforming the Fo content in olivine
into temperature estimates:
into temperature estimates:
Prelevic (2015, written communication)
Prelevic (2015, written communication)
Potential Temperature
Potential Temperature
8)
8)
If present, phlogopite in the mantle melts
If present, phlogopite in the mantle melts
incongruently giving K-OH-rich melt + Fo-
incongruently giving K-OH-rich melt + Fo-
rich (up to Fo
rich (up to Fo
95
95
) peritectic olivine.
) peritectic olivine.
Possible subsequent melting of this
Possible subsequent melting of this
phlogopite-free mantle, rich in Fo-rich
phlogopite-free mantle, rich in Fo-rich
peritectic olivine, can produce Mg# rich
peritectic olivine, can produce Mg# rich
melts that can crystallize Fo-rich olivines.
melts that can crystallize Fo-rich olivines.
There are several problems in
There are several problems in
transforming the Fo content in olivine
transforming the Fo content in olivine
into temperature estimates:
into temperature estimates:
Prelevic (2015, written communication)
Prelevic (2015, written communication)
Potential Temperature
Potential Temperature
9)
9)
Fo-rich (
Fo-rich (
up to Fo
up to Fo
93
93
)
)
olivine forms
olivine forms
as
as
liquidus phase of a MARID melt
liquidus phase of a MARID melt
.
.
MARID
MARID
rocks are lithologies composed
rocks are lithologies composed
essentially of
essentially of
M
M
ica +
ica +
A
A
mphibole,
mphibole,
R
R
utile,
utile,
I
I
lmenite and
lmenite and
D
D
iopside
iopside
.
.
MARID partial melts are Mg-rich and
MARID partial melts are Mg-rich and
crystallize Fo-rich olivine, but at “normal” to
crystallize Fo-rich olivine, but at “normal” to
“relatively low” temperatures.
“relatively low” temperatures.
There are several problems in
There are several problems in
transforming the Fo content in olivine
transforming the Fo content in olivine
into temperature estimates:
into temperature estimates:
Prelevic (2015, written communication)
Prelevic (2015, written communication)
Potential Temperature
Potential Temperature
10)
10)
Fo-rich (
Fo-rich (
up to Fo
up to Fo
94
94
)
)
olivine forms
olivine forms
in a
in a
melt derived from dolomite-bearing mantle
melt derived from dolomite-bearing mantle
.
.
Dolomite is the stable carbonate in the ~2-3
Dolomite is the stable carbonate in the ~2-3
GPa depth range. A mixed carbonatitic-
GPa depth range. A mixed carbonatitic-
silicatic melt with cooling can split the MgO
silicatic melt with cooling can split the MgO
content of dolomite forming Fo-rich olivine,
content of dolomite forming Fo-rich olivine,
leaving CaO-rich carbonatite, which can
leaving CaO-rich carbonatite, which can
eventually collapse at P <2GPa.
eventually collapse at P <2GPa.
There are several problems in
There are several problems in
transforming the Fo content in olivine
transforming the Fo content in olivine
into temperature estimates:
into temperature estimates:
Prelevic (2015, written communication)
Prelevic (2015, written communication)
1)
1)
Picritic magmas (Olivine- and MgO-rich tholeiites)
Picritic magmas (Olivine- and MgO-rich tholeiites)
from Hawaii show high-Fo olivines (Fo
from Hawaii show high-Fo olivines (Fo
88-91
88-91
).
).
2)
2)
The magma in equilibrium with the highest Fo
The magma in equilibrium with the highest Fo
olivines must have ~
olivines must have ~
15-16 wt.% MgO
15-16 wt.% MgO
.
.
3)
3)
The calculated MgO composition is typically higher
The calculated MgO composition is typically higher
than the measured whole-rock MgO composition.
than the measured whole-rock MgO composition.
4)
4)
It is possible to calculate the temperature of
It is possible to calculate the temperature of
formation of a magma using geothermometers
formation of a magma using geothermometers
(olivine-melt Fe/Mg exchange).
(olivine-melt Fe/Mg exchange).
5)
5)
The geothermometric results for the Hawaiian
The geothermometric results for the Hawaiian
picrites indicate liquidus temperature
picrites indicate liquidus temperature
~1315 °C
~1315 °C
.
.
Potential Temperature
Potential Temperature
From Green et al. (2001) Eur. J. Mineral., 13, 437-451
From Green et al. (2001) Eur. J. Mineral., 13, 437-451
1)
1)
The most MgO-rich MORB have 13-14 wt.% MgO.
The most MgO-rich MORB have 13-14 wt.% MgO.
2)
2)
The most magnesian olivines of these picritic
The most magnesian olivines of these picritic
MORB reach Fo
MORB reach Fo
91-92
91-92
.
.
3)
3)
As observed for the Hawaiian picrites, the
As observed for the Hawaiian picrites, the
calculated MgO composition is typically higher than
calculated MgO composition is typically higher than
the measured whole-rock MgO composition.
the measured whole-rock MgO composition.
4)
4)
It is possible to calculate the temperature of
It is possible to calculate the temperature of
formation of a magma using geothermometers
formation of a magma using geothermometers
(olivine-melt Fe/Mg exchange).
(olivine-melt Fe/Mg exchange).
5)
5)
The geothermometric results for the MORB
The geothermometric results for the MORB
picrites indicate liquidus temperature
picrites indicate liquidus temperature
~1325 °C
~1325 °C
.
.
Potential Temperature
Potential Temperature
From Green et al. (2001) Eur. J. Mineral., 13, 437-451
From Green et al. (2001) Eur. J. Mineral., 13, 437-451
The process involves analyzing olivine in basaltic rocks, selecting olivine with the highest Fo content, calculating the melt composition in equilibrium with this olivine using the distribution coefficient (KD), and determining the potential temperature (Tp) based on the MgO content. The distribution coefficient KD relates the partitioning of Fe and Mg between olivine and liquid, with a typical value of 0.30. This method helps estimate the hypothetical composition of the original magma before olivine crystallization.
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Presentation Transcript
Potential Temperature To calculate the original MgO content of a melt, a certain amount of olivine is added to the composition of the rock until a composition in equilibrium with the Fo-richest olivine is obtained. This approach may be VERY DANGEROUS Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature Follow this sequence: 1) Analyze with EMP olivine in basaltic rocks; 2) Select the olivine with the highest Fo; 3) Calculate the composition (the MgO content) of the melt in equilibrium with this olivine assuming the relation: KD= (FeOol)/(FeOmelt) * (MgOmelt)/(MgOol) 4) On the basis of this MgO content (higher than that really measured in the rock) calculate the Tp using the relation:Tp(oC)=1463+12.74*MgO-2924/MgO 5) Often it is calculated the hypothetical composition of the original magma, before olivine crystallization. This is done adding olivine to the melt till approaching the hypothetical composition. Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature The distribution coefficient: KD= (FeOol)/(FeOmelt)*(MgOmelt)/(MgOol) relates the partitioning of Fe and Mg between olivine and liquid. It has been calculated independent of temperature and equal to 0.30 in 1970 (Roeder and Emslie, Contrib. Mineral. Petrol.). Following studies have slightly modified the value of this KDto 0.31-0.35, no more. Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature The distribution coefficient: KD= (FeOol)/(FeOmelt)*(MgOmelt)/(MgOol) Falloon et al. (2007) Chem. Geol., 241, 207-233 KD(FeOmelt/FeOol)*(MgOol/MgOmelt) Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature The distribution coefficient: KD= (FeOol)/(FeOmelt)*(MgOmelt)/(MgOol) Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature The distribution coefficient: KD= (FeOol)/(FeOmelt)*(MgOmelt)/(MgOol) What does a KD= 0.30 mean? We can write the KD also in a different way: KD= (FeO/MgO)ol/(FeO/MgO)melt Let us assume an olivine that crystallizes from a basaltic melt. Does this olivine have higher Fe/Mg ratio than the liquid? This means that, starting from a given Fe/Mg in the melt, what is the element preferentially allocated into olivine? Mg Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature The distribution coefficient: KD= (FeOol)/(FeOmelt)*(MgOmelt)/(MgOol) Do you remember this diagram? The olivine in equilibrium with a melt is always richer in MgO. This means that olivine has lower FeO/MgO than the melt. Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature The distribution coefficient: KD= (FeOol)/(FeOmelt)*(MgOmelt)/(MgOol) Let us make an example: FeOol= 20.90 wt%; MgOol= 37.70 wt%. FeOmelt= 9.20 wt%; MgOmelt= 7.50 wt%. KD= ? KD= (0.291/0.128)*(0.187/0.935) = 0.46 ? KDshould be 0.30, why we obtained 0.46? KD= (FeO/MgO)ol/(FeO/MgO)melt First divide the wt% per molecular weights FeO = 71.85; MgO = 40.31 Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature 1.0 KD 0.2 0.9 0.8 KD 0.1 0.7 Fo (Mg# ol) 0.6 KD 0.8 0.5 KD 0.6 Mg# = Mg/(Mg+Fe) 0.4 KD 0.4 0.3 0.2 KD = 1 means identical distribution of Fe/Mg (or Mg#) between melt and olivine. This is the base (wrong) assumption of CIPW norm. 0.1 0.0 0.0 0.1 0.2 0.3 0.4 Mg# melt 0.5 0.6 0.7 0.8 0.9 1.0 Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature 1.0 KD 0.2 0.9 0.8 KD 0.1 0.7 Fo (Mg# ol) 0.6 KD 0.8 0.5 KD 0.6 0.4 KD 0.4 0.3 0.2 Next slide 0.1 0.0 0.0 0.1 0.2 0.3 0.4 Mg# melt 0.5 0.6 0.7 0.8 0.9 1.0 Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature 1.0 Mg# in olivine (Fo content) 0.8 0.6 Each color represents one sample. In each sample coexist olivines with different compositions (Fo) 0.4 0.2 In theory all the olivines should plot along the red line, but... 0 0.3 0.4 0.5 0.6 0.7 Mg# in melt Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature 1.0 Mg# in olivine (Fo content) 0.8 0.6 Olivines above the red line have too much Mg to have been crystallized from that melt. 0.4 0.2 Nearly all the analyzed olivines are not in equilibrium with the melt 0 0.3 0.4 0.5 0.6 0.7 Mg# in melt Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature 1.0 Mg# in olivine (Fo content) 0.8 0.6 Olivines below the red line have too low Mg to have been crystallized from that melt. Nearly all the analyzed olivines are not in equilibrium with the melt 0.4 0.2 0 0.3 0.4 0.5 0.6 0.7 Mg# in melt Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature 1.0 Mg# in olivine (Fo content) 0.8 0.6 How is it possible to have olivine Fo- richer (Mg-richer) than equilibrium olivine? How is it possible to have olivine Fo-poorer (Fe-richer) than equilibrium olivine? Recycling of late crystallization phases 0.4 Xenocrysts 0.2 0 0.3 0.4 0.5 0.6 0.7 Mg# in melt Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature 1.0 Mg# in olivine (Fo content) 0.8 0.6 Is it correct to say that the TRUE Mg# of the melt is that calculated considering the olivine with the highest Fo? 0.4 0.2 0 0.3 0.4 0.5 0.6 0.7 Mg# in melt Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature There are several problems in transforming the Fo content in olivine into temperature estimates: 1) There are no olivine-controlled trends for MORB glasses. Except for the Puna Ridge trend, all of the olivine-controlled trends are artificially produced by adding olivine back into observed glass compositions. Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature Di MORB glasses MORB glasses (Mg#>68) X Iceland glasses Puna Ridge glasses from Kilauea Experimental glasses Pl Ol Presnall and Gudfinnsson (2011) J. Petrol., 52, 1533-1546 Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature There are several problems in transforming the Fo content in olivine into temperature estimates: 2) The Fo-richest olivine can be fragments of the mantle, not liquidus olivine. A detailed chemical and petrographic investigation is fundamental to distinguish the two types of olivines. Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature There are several problems in transforming the Fo content in olivine into temperature estimates: 3) The high Fo content of olivine can be related to Fe-poor mantle. A depleted mantle is Fe-poor and Mg-rich. Partial melts of this source will be Fe-poor and Mg-rich and have high Mg#. Olivine crystallizing from this kind of melts will be Fe-poor and, therefore, Fo-rich. Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature There are several problems in transforming the Fo content in olivine into temperature estimates: 4) Melts generated after high degrees of melting are characterized by high Mg# and, consequentially,willcrystallizeFo-rich olivine. It is not necessary to have high T to generate high amount of melting. Melting can be generated with volatile flushing (H and C, above all) and with the presence of low- solidus olivine-poor lithologies (eclogites). Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature There are several problems in transforming the Fo content in olivine into temperature estimates: 4) Melts generated after high degrees of melting are characterized by high Mg# and, consequentially,willcrystallizeFo-rich olivine. If an upwelling mantle melts at high depths, it will experience more and more melting moving to the surface. Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature There are several problems in transforming the Fo content in olivine into temperature estimates: 5) Fo-rich olivines can crystallize in a highly oxidized melt. If Fe3+become particularly abundant in specific conditions magnetite crystallization is favoured. This depletes the residual magma in Fe2+. Late olivine can crystallize with MgO-rich (up to Fo99) compositions. Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature There are several problems in transforming the Fo content in olivine into temperature estimates: 6) Fo-rich olivines can be generated after subsolidus substitution with spinel. Cr-rich spinel prefers Fe (forming chromite Fe2+Cr2O4) rather than Mg (Mg-chromite MgCr2O4). If olivine is present, it will exchange Fe with chromite, accepting Mg from the spinel. The result will be Fo-rich olivine. Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature There are several problems in transforming the Fo content in olivine into temperature estimates: 7) If present, phlogopite in the mantle melts incongruently giving K-OH-rich melt + Fo- rich (up to Fo95) peritectic olivine. This SiO2-undersaturated K-OH-rich melt reacts with mantle enstatite dissolving it. In this way the melt becomes MgO-rich (18-30 wt% MgO) and with high Mg#. Olivine crystallizing from this melt will be Fo-rich. Prelevic (2015, written communication) Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature There are several problems in transforming the Fo content in olivine into temperature estimates: 8) If present, phlogopite in the mantle melts incongruently giving K-OH-rich melt + Fo- rich (up to Fo95) peritectic olivine. Possible subsequent melting of this phlogopite-free mantle, rich in Fo-rich peritectic olivine, can produce Mg# rich melts that can crystallize Fo-rich olivines. Prelevic (2015, written communication) Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature There are several problems in transforming the Fo content in olivine into temperature estimates: 9) Fo-rich (up to Fo93) olivine forms as liquidus phase of a MARID melt. MARID rocks are lithologies composed essentially of Mica + Amphibole, Rutile, Ilmenite and Diopside. MARID partial melts are Mg-rich and crystallize Fo-rich olivine, but at normal to relatively low temperatures. Prelevic (2015, written communication) Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature There are several problems in transforming the Fo content in olivine into temperature estimates: 10) Fo-rich (up to Fo94) olivine forms in a melt derived from dolomite-bearing mantle. Dolomite is the stable carbonate in the ~2-3 GPa depth range. A mixed carbonatitic- silicatic melt with cooling can split the MgO content of dolomite forming Fo-rich olivine, leaving CaO-rich carbonatite, which can eventually collapse at P <2GPa. Prelevic (2015, written communication) Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature 1) Picritic magmas (Olivine- and MgO-rich tholeiites) from Hawaii show high-Fo olivines (Fo88-91). 2) The magma in equilibrium with the highest Fo olivines must have ~ 15-16 wt.% MgO. 3) The calculated MgO composition is typically higher than the measured whole-rock MgO composition. 4) It is possible to calculate the temperature of formation of a magma using geothermometers (olivine-melt Fe/Mg exchange). 5) The geothermometric results for the Hawaiian picrites indicate liquidus temperature ~1315 C. From Green et al. (2001) Eur. J. Mineral., 13, 437-451 Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
Potential Temperature 1) The most MgO-rich MORB have 13-14 wt.% MgO. 2) The most magnesian olivines of these picritic MORB reach Fo91-92. 3) As observed for the Hawaiian picrites, the calculated MgO composition is typically higher than the measured whole-rock MgO composition. 4) It is possible to calculate the temperature of formation of a magma using geothermometers (olivine-melt Fe/Mg exchange). 5) The geothermometric results for the MORB picrites indicate liquidus temperature ~1325 C. From Green et al. (2001) Eur. J. Mineral., 13, 437-451 Magmatismo cenozoico dell area mediterranea. Michele Lustrino. Univ. La Sapienza Roma A.A. 2015/2016
