xuezhao bao and richard a. secco [email protected]@uwo.ca;...
TRANSCRIPT
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Xuezhao Bao and Richard A. Secco
[email protected]; [email protected]
Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada N6A 5B7
Joel E. Gagnon and Brian J. Fryer
Department of Earth Sciences, University of Windsor, Windsor, Ontario, Canada N9B 3P4
U Solubility in Planetary Cores: Evidence From High Pressure and Temperature Experiments
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> Without U, Th, K40, inner core age is 1.5 Ga (should be >3.5Ga
see next) (Labrosse et al. 2001, Buffett, 2002, 2003; supported by Anderson, 2002, Nimmo, 2004, 2005, etc.).
Mantle
Current inner core size
Inner Core with U, Th, K40
Inner Core without U, Th, K40
13.5 TW
Introduction
Perovskite
postperovskite
Perovskite
Mantle
CMB
Outer core
13.5 TW
> Perovskite-postperovskite-perovskite forward and reverse transformations within CMB indicate a high heat flow of 13.5
TW (Lay et al., 2006)
T
P
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Tarduno et al., 2007
After Tarduno et al., 2006
McElhinny et al., 1980
The existence of a magnetic field of roughly present-day strength over 3.5 Gyr is more easily reconciled with an old inner core (Christensen & Tilgner, 2004).
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U in the outer core?
Two unusually long periods without any reversal
Courtillot, 1998
Computer simulations and field works (Coe and Glatzmaier, 2006)
N
S
S
Nreversal
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> upper limit of 65 TW radiogenic heat from U and Th;
central value of 16 TW?. Araki et al (2005)
> Problems: 1) Large range of 65 TW 2) U, Th: in mantle or core? 3) Geochemistry model dependent.
KamLAND
Mantle
Current inner core size
U,Th decay
Geo-neutrinos
Inner Core
Outer Core
Introduction
Information from geo-neutrino
studies
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0 % 0-10 % 14.5 % 0~0.2 %.
Sun Mercury Venus
Earth
Mars
Our starting materials: mixtures of the following powder
~ 65% ~33%? ~ 33% ~ 22% Righter, 2006
35% 67%? 67% 78%
Experiments and composition analytical methods
Silicate
Silicate ~ 54 wt% peridotite
U source ~3 wt% uraninite
simplified expression
Metal in the core
S in the core
metal phase - 40wt% Fe or
Fe-10% S
Fe-35wt%S
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500 ton Walker module multi-anvil press
Experiments and composition analytical methods
Pressure cell(inside)
An 3000 ton multi-anvil press will be coming in this Fall
Welcome to use this lab. You can email me at: [email protected], or [email protected]
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Pressure Cell
Graphite, MgO
Experiments and composition analytical methods
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Composition Analytical methods
> LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry)
University of Windsor, University of McGill
> EM (Electron Microprobe).
> SIMS( Secondary Ion Mass Spectrometer)
Experiments and composition analytical methods
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P, T range of experiments
Mg2SiO4
Davis & England (1964)
Ohtani & Kumazawa (1981)
Fe
Saxena & Dubrovinsky (2000)
0 2 4 6 8 10 12 14
1600
1800
2000
2200
2400
184
333
334
360332330
358357
260?
262?
207
198
201
Mg 2SiO 4
melting
Fe melting
Sample container BN
Graphite MgO 191
115
157
154
167
253?
202
212
214
199190
183
182
205
204
200
T (
OC
)
P (GPa)
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Run 182: 5GPa; 2050 ; metallic phase: Fe℃ Backscattered electron image
500 m
Results and discussion
Fe
Silicate
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Run 238: 5 GPa; 1972 ; metallic phase: Fe-10% S℃ Backscattered electron image
Fe-10% S
silicate
500 m
Results and discussion
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Spectra of U in the Fe phases at different pressures analyzed by LA-ICP-MS.
0 20 40 60 80 100
0
20
40
60
80
0 20 40 60 80 100 1200
50
100
150
200
250
300
350
400
Background Fe
Time (second, 10 µm/s)
Run 198, 0 GPa, 1850 Co
Run 207, 6.7 GPa, 2010 Co
Cou
nt
(nu
mb
er)
Results and discussion
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0 20 40 60 80 100
0
2000
4000
6000
8000
10000
0 20 40 60 80 100
0
200
400
600
800
1000
1200
1400
Background Fe
Time (second, 10 µm/s)
Run 199, 8 GPa, 2249 Co
Run 191, 8.5 GPa, 2287 CoCou
nt
(nu
mb
er)
Summary: U in the pure Fe: from 0 to 7.5 GPa, 50 to 10, 000 counts/s
Results and discussion
Oxides and silicates
Fe
Spectra of U in the Fe phases at different pressures analyzed by LA-ICP-MS.
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0 2 4 6 8 10 12 14
0.00
0.01
0.02
0.03
0.04
0.08
0.09
201
DU(U
in m
etal
/U in
sili
cate
)
Pressure (GPa)
207
262
198
260
ContainerT<T
silicate melt
BN Graphite
T>Tsilicate melt
BN Graphite
191
253
183
115
202
200182
214
205
204
199
167
190
154157
Run products with Fe in metal phase: DU= UFe / Usilicate
Silicate FeUP
Results and discussion
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Run Products With Fe-S: DU= UFe-10%S / Usilicate
LA-ICP-MS
D(U
/U)
U
metal o
r metal
-sul
fide
silic
ate
P (GPa )
0 2 4 6 8 10 12 14 16
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
123
224
237
207
190
198113
161
115
191
199
262
260
231
232
243
240
239
238
235
236
234
Silicate Fe10%SUP
T>Silicate melting line
T<Silicate melting line
Results and discussion
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-5
-4
-3
-2
-1
0
1
2
Fe20
FeS 80
0-28
wt%
S in
Fe
phas
e
Study
Whe
eler
Mur
rell+
Bur
nett
Mal
aver
gne
Fe-3
5wt%
S
Fe-1
0wt%
S
this
stu
dy
this
stu
dy
Fe-SFe+SiFeth
is s
tudy
log
DU
Results and discussion
Possible mechanism for U entry the cores
Low fo2 is the key for U entry metal phases. Uis very soluble in metal Fe in situations without O and other oxidative elements.
Fe0 + U0 + Si0 UFe10Si2
reducing
-2-1 ~ -6
-7~-3
log fo2(∆IW)
-1 ~ -6
Silicate Silicate Silicate
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-7 -6 -5 -4 -3 -2 -1 0
0
200
400
600
800
1000
333
201
157
212200154 204
182 253
183
205
207
190
191260
202
214
198
metal Fe with BN capsulemetal Fe with C capsulemetal Fe-10wt%S with MgOcapsulemetal Fe with MgO capsule
The others after Fig. 3
199(2303 ppm)262(8686 ppm)
167
Oxygen fugacity (∆IW)
Possible mechanism for U entry the cores
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0 2 4 6 8 10 12 14 16 18-2.0
-1.5
-1.0
-0.5
0.0
0.5
198
260
262
199
207
201
191
184
153
150 Container:BNGraphite
253212
157
154
167
115
214
202
190
183
182
205
204
200
P (GPa)
∆ = -2.0IW
∆ = -6.0IW
Run products with Fe: Si increases with P(consistent with previous studies of Ito et al., 1995, Gessmann et
al., 2001 and Malavergne et al., 2004).Based on log(wt%)Si – fo2 relation (Kilburn and Wood,1997). fo2 below the IW buffer decreases with P, coinciding with U increase with P in Fe
Results and discussion
log D(S)
log D(Si)
For geo-neutrino study: it may be impossible for significant K in the core.
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Possible mechanism for U entry the cores
0 2 4 6 8 10 12 14 16-9
-8
-7
-6
-5
-4
-3
-2
-1
0
metal Fe withBNcapsulemetal Fe withC capsulemetal Fe-10wt%S with MgO capsulemetal Fe withMgOcapsule
Smaller signs identify samples with very small metal phases. Their Uconcentrations inmetalphases are difficult toobtain with LA-ICP-MS analysis.
115330
200
357
204
182 183
358
212
190
360
371
333
157137
207
191
199
201
202205
214
154
153
253
262
226
260
198 150
184
167
P (GPa)
Oxides and silicates
Fe
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Possible mechanism for U entry the cores
According to Arora (2000) increase P, reactions proceed to left, fo2 increase.
Possible mechanism for U entry the cores
Fe can be oxidized even at room T (Kubaschewski et al., 1962)
Fe-O O, Fe
0 2 4 6 8 10 12 14 16-9
-8
-7
-6
-5
-4
-3
-2
-1
0
metal Fe with BNcapsulemetal Fe with C capsulemetal Fe-10wt%S with MgO capsulemetal Fe with MgOcapsule
Smaller signs identify samples with very small metal phases. Their Uconcentrations in metalphases are difficult to obtain with LA-ICP-MS analysis.
115330
200
357
204
182 183
358
212
190
360
371
333
157137
207
191
199
201
202205
214
154
153
253
262
226
260
198 150
184
167
P (GPa)
But in fact, increase P, fo2 decrease.
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Si 4+, Al 3+ Si 0, Al 0A)impact Al-O
Si-O
Possible mechanism for U entry the cores
Al
Si
O
O
Nb can’t be oxidized by O below 200oC (Kubaschewski et al., 1962)
O-Nb-O
B) Serghiou et al., (1992) found at room T, high P (19.2 GPa) can decrease the O content of Tetragonal Nb2O5, and it was amorphized. O-Nb,O
Yakovlev (1993) discovered metallic Si and Al in the impacted products.
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Possible mechanism for U entry the cores
P
Confirmed by Drickamer et al., 1969 and Burns, 1993, etc. in a wide variety of compounds.
P
P
Burns (1993). Similar elements: Mn, Cu, Ti, Cr and Ni
Core-mantle boundary
If Burns (1993) is right, then it is natural
P
Fe--OP
Fe O
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Possible mechanism for U entry the cores
McCammon & Kopylova, 2004 Woodland & Koch, 2003
Xenoliths from the mantle under Lesotho and South African (Mossbauer data)
Xenoliths from the mantle under northern Canada (Mossbauer data)
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0 2 4 6 8 10 12 14 16-9
-8
-7
-6
-5
-4
-3
-2
-1
0
metal Fe withBNcapsulemetal Fe withC capsulemetal Fe-10wt%S with MgO capsulemetal Fe withMgOcapsule
Smaller signs identify samples with very small metal phases. Their Uconcentrations inmetalphases are difficult toobtain with LA-ICP-MS analysis.
115330
200
357
204
182 183
358
212
190
360
371
333
157137
207
191
199
201
202205
214
154
153
253
262
226
260
198 150
184
167
P (GPa)
Possible mechanism for U entry the cores
Average ∆IW +4, basalts on the surface (Karner et al., 2006) ~0.6/GPa (Ballhaus,
1995)
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~ IW -6.8 (Average, ⊿Grossman et al., 2008) ~ IW -13~8.9 (enstatite chondrite ⊿
(EC), Grossman et al., 2008)
EC model: based on composition (Javoy, 1995,1998), and O(Clayton,1993),Cr(Lugmair, 1998), and Mo(Dauphas, 2002) isotope compositions.
Possible mechanism for U entry the cores
Proto-planetary disk or nebula
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Condensation T of Fe is higher than that of silicates at the relevent pressures of the solar nebula (~10-4 atm, Grossman, 1972)
~ IW -6.8 (Average, ⊿Grossman et al., 2008)
~ IW -13~8.9 (enstatite chondrite ⊿(EC), Grossman et al., 2008)
Earth
Metal core
Fe-rich region
Inner planet cores may have formed completely (Hwaung, 2000) or at least partially (Grossman, 1972), from the Fe-rich materials in the Fe-rich region.
Possible mechanism for U entry the cores
Proto-planetary disk or nebula
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Core has formed entirely under highly reducing conditions (Wänke, 1981; Javoy,1995; Allègre et al., 1995)
Core has formed mainly under an early highly reducing conditions (O’Nell, 1991; Javoy,1995; Wade & Wood, 2005; Wood et al.,2008 )
Core formation during Earth’s accretion Possible mechanism for U entry the cores
Wood et al.,2008
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-7 -6 -5 -4 -3 -2 -1 0
0
200
400
600
800
1000
333
201
157
212200154 204
182 253
183
205
207
190
191260
202
214
198
metal Fe with BN capsulemetal Fe with C capsulemetal Fe-10wt%S with MgOcapsulemetal Fe with MgO capsule
The others after Fig. 3
199(2303 ppm)262(8686 ppm)
167
Oxygen fugacity (∆IW)
Possible mechanism for U entry the cores
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O2 and water contents (McCammon et al., 2004 ) are the control factors of fo2.Other oxidative volatiles are also important.
Element Density Melting point
Compound Density Melting point Compound Density Melting point
Fe 7.86 1535Ni 8.90 1453U 19.05 1132Th 17.70 1700 K4ThOX4.4H2O soluble in water
ThO2 10.0 3220
ThN 10.6 2500ThS 1905ThCl4 600
Th(NO3)4.5H2O soluble in water
UO2 10.95 2800
UCl 4.86 567UN 14.32 2800UF6 4.68 ~0
UO3 7.29 soluble in water
The melting point (oC) and density (g/cm3) of U, Th, their compounds and complexes and core related elements (after Bao and Zhang, 1998).
U
O or other volatiles
Oxides and silicates
Metal
High density(19 g/cm3)
Small density (0.00143g/cm3)
Possible mechanism for U entry the cores
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Pacific ocean
U,Th rich zone Volatiles or their ionsU - Th -
nene
-
-
→ U → Th
n+
n+
The distribution of U and Th in the outer core and its influence on the formation of deep mantle plumes and subducted lithosphere plates in the Pacific Ocean.A-lithosphere; B- U,Th richer sphere or asthenosphere; C- mantle; D''- core-mantle boundary; D- outer core; E- inner core; O- center of Earth; M- center of geomagnetic field and thermal convection; the black points represent the relative concentration of U and Th (after Bao, 1999).
Continent
Only planet with life; only planet with plate tectonics; only planet with asthenosphere; only planet with granitoid continental crust; with magnetic field; with ocean…
Earth
Circum-pacific volcanic belt magmatism
Implication for planetary dynamics
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Cumulative volume of continental crust extracted through time from different researcher groups (After Abbott et al., 2000).
Th
U
5.6
1.4
0.1 0.2U
Th
Rudnick & Fountain, 1995
Taylor & McLennan, 1985
Oceanic crust
Continental crust
Average U, Th concentration (ppm)
EarthImplication for planetary dynamics
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0 2 4 6 8 10 12 14 16 18 20 22 24 26
0.0
0.1
0.2
T < silicate melting
T > silicate melting
DU= 0.047
DU= 0.17
DU(U
in m
etal
/U in
sili
cate
)
Pressure (GPa)
ContainerT<T
silicate melt
BN Graphite
T>Tsilicate melt
BN Graphite
0.17 x 20 ppb U in Primitive Silicate Earth (PSE, based Cl chondrite…) (McDonough, 2003)
3.4 ppb U
0.047 x 20 ppb U in PSE
0.21 x 20 ppb U in PSE(to CMB?)
1 ppb U
4.2 ppb U
Magma Ocean
Percolation
Earth’s major, minor, and isotopic composition is unlike any known chondrite group or mixture of chondrite group (Righter, 2003, Caro et al., etc.)
Application to Earth’s core Run product with Fe
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Influence of S concentration in metal phases on the extrapolated DU values at 26 GPa, the pressure at the bottom of a proposed magma ocean.
Application to Earth’s cores
T > silicate melting line
T < silicate melting line
5 ppb U in the core
10.2 ppb U in the core?
1 ppb U in the core
3.4
ppb
U in
the
core
22 ppb U at CMB?
4.2 ppb at CMB?
Mars
Earth
Mercury & Venus
Run product with Fe-10% S
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O and volatile contents are controlled by two factors: planetary heliocentric distances (Lewis, 1974a, 1984) and size (Ahrens, 1993)
(after Fegley, 1999).
P, T distribution in the protoplanetary disk from different researchers
Implication for planetary dynamics
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Formation Model of the magnetic field in Mercury.
639 km
U, Th
1800 km
Fe-Ni
O-Si-Mg (FeO<0.3%, Inferred spectrometry)
coreInner core?
Mantle
Mercury
Structure of Mercury
Giant impact: lost most its mantle and almost all its Fe has entered its core (Benz et al., 1988; Palme et al., 2003)
(After Strom and Sprague, 2003) (After Stanley et al., 2005)
Heat loss
Liquid outer core
Implication for planetary dynamics
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Mars
Magnetic records implies that a dynamo was present in its early stage but must have be absent after 4.0 Ga (Nimmo &Tanata, 2005; Fei & Bertka, 2005; Solomon et al.,2005), but why?
Mars possesses no internal magnetic field, but has old (>4 Ga) Magnetic records (After Solomon et al.,
2005)
Martian core is at least partially liquid; confirmed by solar tide study (Yoder et al., 2003)
Implication for planetary dynamics
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(Solomon et al., 2005)
Hemispheric dichotomy structure on Martian crust
> 3.7 Ga, less dense highlands, andesite or basaltic andesite, high radioactive elements (Nimmo&Tanaka, 2005)
< 3.7 Ga, lowlands, basaltic composition
4.12 Ma (Nimmo & Tanata, 2005)
A relatively small impact can drive off Martian volatiles (1034-1036.5 ergs). For Earth 1038 ergs (Ahrens, 1993)
MarsImplication for planetary dynamics
Today: dry Martian mantle from Martian meteorites (Dreibus & Wanke, 1985, 1987; Carr & Wanke, 1992; Ghosal et al., 1998; Reese & Solomatov, 2002; Herd et al., 2002, Jones, 2004 ); Infrared mineralogical mapping (Mustand et al., 2005; Bibring et al., 2005). The content of interior volatiles are 2000 times less than that of Earth (Beaty et al., 2005 ).
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Volatiles or their ions
U-4e U4+ U rich zone
A-lithosphere; B-asthenosphere (U,Th rich sphere); C-mantle; D’’-core-mantle boundary; D-outer core; E-inner core (after Bao, 1999)
Similar internal circulation system and plate tectonics might have developed before 4.12 Ga? Giant impacts have drove off Martian volatiles and then these systems stopped to work and the dynamo are dead since then.
14.5% S in the core (Fei & Bertka,2005)
U, Th(?) has entered the core
andesite or basaltic andesite
MarsImplication for planetary dynamics
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Conclusions
For P<15 GPa, T< 2500 , ℃ DU in FeS (including Fe-35% S and Fe-10% S) and Fe melts increase with P,T.
DU is 3-5 times larger when silicate was molten than silicate was solid.
For a 800 km thick magma ocean, Earth’s core could contain about ~10 ppb U? from the extrapolated result of Fe-10% S samples or 3.4 ppb U from Fe samples.
For percolation core formation, Earth’s core could contain 5ppb -- 22 ppb (?) U from the extrapolated result of Fe-10% S samples or 1ppb - 4 ppb(?) U from Fe samples.
Inside Earth, U may be a radioactive heat source to slow down the cooling and crystallization of the inner core.
U is also a possible energy source for maintaining the magnetic fields of Earth and Mercury.
•
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Acknowledgments
NSERC, CFI Dr. G. Young for partial funding. Drs. C. Cermignani and M. Liu for EM analyses Drs. W. Minarik and J. Gagnon for LA-ICP-MS analysis. Dr. A. Pratt and Mr. G.Gord for SIMS analysis. Dr. D. Liu and Mr. R. Tucker for help in sample preparation Mr. G. Wood for help in sample cutting and polishing.
Thanks to:
Thank you!
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Calculated fraction of atmosphere blown off versus impactor energy for Earth, Venus, and Mars. Lower and higher energy curves for each planet correspond to assumed polytropic exponent of ideal gas of γ =1.1 and 1.3, respectively (After Ahrens, 1993).
Implication for planetary dynamics
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Water-rich atmosphere formed by the impact-induced dehydration of water-bearing minerals by planetesimal impacts (After Ahrens, 1993)
Implication for planetary dynamics
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0 2 4 6 8 10 12 14
1600
1800
2000
2200
2400
fo2(IW)(Wheeler et al., 2006)
-4.69
-3.83
-4.01
-2.48
-1.83
-1.80
-1.98-1.92
-2.02
-2.31-2.13
-2.31
-2.06
184
333
334
360332330
358357
260?
262?
207
198
201
Mg 2SiO 4
melting
Fe melting
Sample container BN
Graphite MgO
191
115
157
154
167
253?
202
212
214
199190
183
182
205
204
200
T (
OC
)
P (GPa)
-1.19 -2.48
-0.97
-1.59
-1.14
-1.83
Fe can be oxidized even at room T (Kubaschewski et al., 1962)
Fe-O O, Fe