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implications for the Earth’s core composition
R. Torchio, S. Boccato, F. Miozzi, A. Rosa, N. Ishimatsu, I. Kantor, N Sévelin-Radiguet, R. Briggs, C. Meneghini, T. Irifune, et al.
To cite this version:
R. Torchio, S. Boccato, F. Miozzi, A. Rosa, N. Ishimatsu, et al.. Melting curve and phase relations of
Fe-Ni alloys: implications for the Earth’s core composition. Geophysical Research Letters, American
Geophysical Union, 2020, 47 (14), �10.1029/2020GL088169�. �hal-03006347�
Melting curve and phase relations of Fe-Ni alloys:
1
implications for the Earth’s core composition
2
R. Torchio 1 , S. Boccato 1(1) , F. Miozzi 2 , A.D. Rosa 1 , N. Ishimatsu 3 , I. Kantor 4 ,
3
N. S´ evelin-Radiguet 1 , R. Briggs 5 , C. Meneghini 6 , T. Irifune 7 and G. Morard 2,8
4
1 European Synchrotron Radiation Facility, Grenoble, France
5
2 Institut de Min´ eralogie, de Physique des Mat´ eriaux, et de Cosmochimie (IMPMC), Sorbonne Universit´ e -
6
UPMC, UMR CNRS 7590, Mus´ eum National d’Histoire Naturelle, IRD UMR 206, F-75005, Paris, France
7
3 Graduate School of Advanced Science and Engineering, Hiroshima Univ., Higashi-Hiroshima, Japan
8
4 Institut for Fysik, Danmarks Tekniske Universitet, Kongens Lyngby, Denmark
9
5 Lawrence Livermore National Laboratory, Livermore, CA, USA
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6 Department of Sciences, Roma Tre University of Rome, Via della Vasca Navale 84, 00146, Rome, Italy
11
7 Geodynamics Research Center, Ehime University, Matsuyama, Japan
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8 Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, 38000 Grenoble,
13
France
14
Key Points:
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• Melting curve and phase diagram of Fe-20wt%Ni and Fe-36wt%Ni have been in-
16
vestigated by in situ x-ray absorption up to 120 GPa and 3500 K.
17
• Ni alloying shifts the hcp/fcc/liquid triple point to higher pressures and temper-
18
atures.
19
• The triple point for Fe-10wt%Ni is predicted to be around 135 GPa and 3800 K
20
fixing new benchmarks for the Earth’s core composition.
21
1 Present address: Sorbonne Universit´ e, Mus´ eum National d’Histoire Naturelle, UMR CNRS 7590, Institut de Min´ eralogie, de Physique des Mat´ eriaux, et de Cosmochimie (IMPMC), 75005 Paris, France
Corresponding author: R.Torchio, [email protected]
Abstract
22
Nickel is the second most abundant element in the Earth’s core. However, the prop-
23
erties of Fe-Ni alloys are still poorly constrained under planetary cores conditions, in par-
24
ticular concerning the effect of Ni on the melting curve of Fe. Here we show that Ni al-
25
loying up to 36wt% doesn’t affect the melting curve of Fe up to 100 GPa. However, Ni
26
strongly modifies the hcp/fcc phase boundary, pushing the hcp/fcc/liquid triple point
27
of Fe-20wt%Ni to higher pressures and temperatures. Our results allow constraining the
28
triple point for Fe-10wt%Ni, a composition relevant for the Earth interior, and point out
29
a decrease of the melting temperature at core-mantle boundary by 400 K with respect
30
to pure Fe. A lower amount of light elements than previously predicted is thus required
31
to reduce the crystallization temperature of core materials below that of a peridotitic
32
lower mantle, in better agreement with geochemical observations.
33
Plain Language Summary
34
The Earth’s core is believed to be composed of Fe alloyed with Ni and several lighter
35
elements. In this paper, we investigate the effect of Ni alloying on the Fe phase diagram.
36
The main effect of Ni addition is to enlarge the pressure/temperature stability domain
37
of the face-centred-cubic (fcc) phase with respect to the hexagonal-closed-packed (hcp)
38
phase and to shift the hcp/fcc/liquid triple point to higher pressures and temperatures.
39
This implies a depression of the melting curve of Fe-Ni alloys by around 400 K at man-
40
tle boundary conditions, at Ni concentrations pertinent for the Earth interior. This con-
41
sequently decreases the temperature of the liquidus for Fe alloys constituting the Earth’s
42
core, in turn implying a reduced amount of light elements than previously predicted.
43
1 Introduction
44
Constraining the thermal profile of planetary cores is mandatory to model their dy-
45
namics, from the generation of the magnetic field to the mechanism of differentiation.
46
Melting curves of materials constituting their interior provide a first constrain on their
47
thermal profile. For instance, in the Earth, the crystallization temperature of liquid iron
48
alloys, forming the solid inner core, represents an anchoring point for the geotherm through
49
the Earth’s core.
50
Nickel is the second most abundant element in planetary cores, after iron. How-
51
ever, as these two metals have a similar atomic number, the effects of nickel on the prop-
52
erties of iron are often assumed to be small in comparison to those of light elements (i.e.
53
C, O, S, Si, H) (Poirier, 1994). Based on cosmochemical models and studies on iron me-
54
teorites, the Earth’s core is believed to be composed by iron alloyed with 5-15 wt% of
55
Ni (and other light elements) (Birch, 1952; Hirose et al., 2013), while iron meteorites have
56
been found to contain up to 35 wt% of Ni (Buchwald, 1975).
57
While numerous investigations on pure Fe phase diagram under extreme conditions
58
of pressure and temperature have been conducted, only few have been dedicated to Fe-
59
Ni alloys. In particular, measurements of their melting curves are very scarce. To the
60
best of our knowledge, there is only one study dedicated to the melting curve of the Fe-
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Ni alloy with 10 wt% of nickel, using Synchrotron M¨ ossbauer Spectroscopy (SMS) in the
62
LH-DAC (Zhang et al., 2016). In this study, no obvious effect of Ni was observed on the
63
melting temperature of Fe up to 125 GPa.
64
Solid phase relations for Fe-Ni alloys at concentrations between 5 and 50 wt% of
65
Ni have been previously investigated (Huang et al., 1988, 1992; Lin et al., 2002; W. L. Mao
66
et al., 2006; Kuwayama et al., 2008; Komabayashi et al., 2012; Dubrovinsky et al., 2007)
67
up to 300 GPa and 3000 K showing that Ni alloying enlarges the fcc phase stability to
68
higher pressures (P) and lower temperatures (T) with increasing Ni content. Ni alloy-
69
ing has also proved to dramatically increase the strength of pure Fe (Reagan et al., 2018).
70
On the other hand, the Fe equation of state (H. Mao et al., 1990) and c/a ratio of the
71
hcp phase at high-P and high-T (Lin et al., 2002; Sakai et al., 2014; Komabayashi et al.,
72
2012) have shown to remain practically unaffected. These studies have contributed to
73
the formulation of hypotesis regarding the properties and structure of the Earth’s inner
74
core that include several extrapolations and assumption, one of those being the effect
75
of Ni addition on the melting curve of Fe.
76
The purpose of this study is to clarify how Ni alloying affects the Fe phase diagram
77
and in particular its melting curve. The melting temperature of this end-member is manda-
78
tory to constrain the liquidus shape.
79
2 Experimental methods
80
Fe-Ni samples with 20 wt% and 36 wt% of Ni were compressed and heated in Di-
81
amond Anvil Cells (DACs) equipped with nanocrystalline diamond anvils (Rosa et al.,
82
2019) in order to avoid diamond Bragg peaks. The cullet sizes range was from 150 to
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400 microns in diameter. The sample chamber was made of pre-indented rhenium gas-
84
kets with holes of 70-150 microns in diameter and the sample was placed between two
85
KCl disks, previously dried in vacuum oven, used both as thermal insulator and pres-
86
sure transmitting medium. The samples were polycrystalline foils by Goodfellow with
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initial thickness of 6µm (Fe-20wt%Ni) and 8µm (Fe-36wt%Ni).
88
Fe K-edge (7.112 KeV) and Ni K-edge (8.333 KeV) EXAFS (Extended X-ray Ab-
89
sorption Fine Structure) measurements were performed at the energy dispersive beam-
90
line ID24 at the European Synchrotron Radiation Facility (ESRF) (Pascarelli et al., 2016).
91
The X-ray spot size on the sample was 4x5 µ m 2 FWHM.
92
The laser heating facility at ID24 (Kantor et al., 2018) allows to measure EXAFS
93
spectra of a compressed sample while it is heated from both sides of the DAC. The tem-
94
perature is measured by spectral-radiometry also from both sides. The heating lasers are
95
two infrared (IR) CW Nd:YAG fiber lasers (IPG photonics) with λ = 1064 nm, focused
96
to a spot size of around 20 µ m diameter on the sample. Once the sample is compressed
97
to the desired pressure, a triggering system allows to launch the laser heating, the tem-
98
perature measurement and the X-ray acquisition simultaneously, according to the scheme
99
described in (Boccato et al., 2017). The error over the measured temperature is chosen
100
as the maximum between the error obtained from the sliding two-colors fit (Benedetti
101
& Loubeyre, 2004; Giampaoli et al., 2018) and the difference in the temperature mea-
102
surements from the two sides of the DAC. Temperature uncertainties are typically within
103
10% of the measured temperature but can reach 14% at low temperature because of low
104
signal-to-noise ratio of the emission signal. The pressure was measured using the ruby
105
fluorescence (Dewaele et al., 2008) before and after the heating run, the pressure uncer-
106
tainty typically accounts for around 10 % of the measured pressure. Thermal pressure
107
was estimated as described in (Morard et al., 2018). Further details on the experimen-
108
tal procedure can be found in the supporting information.
109
3 Results and Discussion
110
Solid-solid and solid-liquid relations were investigated for Fe-Ni with Ni at 20 wt%
111
and 36 wt% in the 0-120 GPa and 300-3500 K range.
112
At ambient conditions, Fe-20wt%Ni is stable with bcc structure, like pure iron. Up
113
to 12 GPa all EXAFS oscillations shift to higher energies as a consequence of the inter-
114
atomic distance compression (see Fig.1 left). Above 12 GPa the bcc to hcp structural
115
transition starts and is well visible in the transformations of the XANES (X-ray Absorp-
116
tion Near Edge Structure) features and EXAFS oscillations (A-E features). The tran-
117
7100 7200 7300 7400 0
1 2 3
7100 7200 7300
-1 0 1
normalizedabsorption(a.u.)
50
20
17
16
15
14
12
6.5
0 GPa
bcc hcp Fe-20wt%Niat RT
A B
C D
E
Energy(eV) Fe-20wt%Ni at 52 GPa
ambient hcp
hcp 1550 K
hcp/fcc mix 1760 K
fcc 2030-3135 K
solid/liquid mix
3230-3320 K
liquid 3488 K
Energy(eV)
Figure 1. Left: selection of EXAFS spectra of Fe-20wt%Ni acquired under pressure and room temperature. The data are shifted in vertical for clarity. The bcc to hcp structural transition starts above 12 GPa (blue lines) and it is completed from 17 GPa (red lines). Right: example of heating run performed at a starting pressure of 52 GPa. A selection of EXAFS spectra have been chosen to show the subsequent transitions from hcp (black and blue line) to fcc (pink) to the liquid phase (red). The data are shifted in vertical for clarity (the temperature increases downwards).
sition is completed at 17 GPa and above this pressure only the energy shift due to com-
118
pression is observed. The transition pressure and width are similar to that of pure iron,
119
reported to occur around 13 GPa, with width variations depending on the transmitting
120
medium (Barge & Boehler, 1990).
121
Upon heating, hcp Fe-20wt%Ni transforms into the fcc structure. The hcp to fcc
122
transition is hard to observe in the EXAFS region because the oscillations are very sim-
123
ilar for the two phases (see Fig.1 right panel). This is not surprising since the hcp and
124
fcc structures differ only in their stacking sequence of the planes. However significant
125
modifications occur in the XANES region as it is shown in the left panel of Fig.2. Tem-
126
perature slightly modifies the XANES of the hcp structure. The subsequent transition
127
to the fcc structure shows up in the appearance of a two peaks structure (B) and flat-
128
tening of the post shoulder slope (A). The same transformation was observed at differ-
129
ent pressures from 15 to 100 GPa. However below 40 GPa it is not possible to measure
130
the transition temperature as the emission is too low for spectro-radiometry. At 70 GPa,
131
the example shown in Fig.2, the transition starts at 1990 K and is completed at 2210
132
K. A similar behaviour has been observed in pure iron for the T-induced hcp-fcc tran-
133
sition (Morard et al., 2018). Fe-36wt%Ni was found in the fcc structure at ambient con-
134
ditions and in all the explored P/T range before melting.
135
We report hcp/fcc co-existence data in Fig.3 in comparison with previous exper-
136
imental works. Our results extend the hcp/fcc boundary of Fe-20wt%Ni up to 3000 K
137
and 120 GPa. The Clapeyron slope (dT/dP) for the hcp/fcc boundary is found to be
138
steeper with respect to previous studies at similar Ni content (Kuwayama et al., 2008;
139
W. L. Mao et al., 2006) and closer to previously reported boundary for Fe-10wt%Ni (Dubrovinsky
140
et al., 2007). Komabayashi and co-authors (Komabayashi et al., 2012) claim that dif-
141
ferences in the phase boundaries reported for Fe-10wt%Ni can be explained in terms of
142
different evaluation or neglecting of the thermal pressure. This could also apply to the
143
Fe-20wt%Ni case even though it does not explain all the discrepancies. In the work of
144
(Kuwayama et al., 2008) the pressure at high T was determined using the PVT equa-
145
7100 7120 7140 7160 0
1
7130 7135 7140 7145 0.9
1.0
7100 7120 7140
0 1
7130 7135 7140
7120 7125
normalizedX-rayabsorption(a.u.)
Energy (eV) hcp 300 K
hcp 1870 K
fcc 2210 K
A
B
Energy (eV) hcp 1710-1870K
mixed 1990-2040K
fcc 2210-2350K
solid 3024K
liquid 3488K
Fe-20wt%Niat 50 GPa
A
B Fe-20wt%Niat 70 GPa
3024K
3134K
3230K
3321K
3488K
Figure 2. Left: selection of XANES spectra showing the temperature induced hcp-fcc phase transition in Fe-20wt%Ni at 70 GPa. The black, blue and pink solid lines represent the XANES signal from the hcp phase at room temperature, the hcp phase at high temperature and the fcc phase respectively. The inset shows spectra of mixed hcp-fcc phases (violet lines) along the tran- sition. Right: selection of XANES spectra showing the melting of Fe-20wt%Ni at 50 GPa. The two smaller panels on the right are zooms over the regions where the modifications occur.
tion of MgO and could be overestimated by 8 GPa at most because of axial tempera-
146
ture gradients. In the work of (W. L. Mao et al., 2006), the thermal pressure was neglected
147
therefore a steeper slope would be expected.
148
The previous results on the hcp/fcc boundary in Fe rich Fe-Ni alloys shown in Fig.3
149
have been used to formulate important hypothesis on the structure of the Earth’s inner
150
core (IC, 330-364 GPa and 4000–7000 K (Anzellini et al., 2013)), mostly proposing iron
151
rich Fe-Ni alloys to crystallize in the hcp phase (Lin et al., 2002; W. L. Mao et al., 2006;
152
Komabayashi et al., 2012; Sakai et al., 2014; Tateno et al., 2012). A simple extrapola-
153
tion of our hcp/fcc boundary supports the hypothesis of the hcp structure for Fe-Ni with
154
20 wt% of Ni at IC conditions, and the same would apply to lower Ni contents.
155
The temperature-induced transition to the liquid phase can also be clearly distin-
156
guished in the XANES region. As temperature is increased the two peaks (B in Fig.2,
157
right panel) get suppressed while the shoulder shape flattens (A). Similar changes are
158
observed at different pressures for the two alloys. The melting detection with XANES
159
has been recently applied to determine the melting curves of pure Ni (Boccato et al., 2017)
160
and pure Fe (Morard et al., 2018). This method has been validated by scanning electron
161
microscope (SEM) textural analyses, performed on exposed transverse sections of sam-
162
ples recovered from the in situ experiments and theoretical calculations in the case of
163
pure Co (Boccato, 2017).
164
In order to avoid any data misinterpretation due to modifications in the sample as-
165
sembly after melting, i.e. sample diffusion and/or chemical reactions (Boccato et al., 2017),
166
only the first appearance of mix or full liquid phase was considered for the melting curve
167
determination, and each heating run has been performed on a fresh portion of the sam-
168
ple. Chemical reactions with carbon and oxygen are clearly detectable through modi-
169
fications in the XANES (Aprilis et al., 2019) and have been monitored by acquiring a
170
quench (cold) spectrum in between each increase of laser power (see supporting infor-
171
mation).
172
this work
5%
10%
9.7%
9.8%
18%
25%
15-20%
Kuwayama 2008 ý
ý
Figure 3. Hcp/fcc boundary found in this study for Fe-20wt%Ni and comparison to other experimental works at close compositions (as indicated next to the corresponding shaded area).
T and P errors are around 10% of the measured values.
The Fe-20wt%Ni melting curve, obtained using a Simon-Glatzel fit (Simon & Glatzel,
173
1929) of the first liquid (or mixtures) data, reported in the phase diagram in Fig.4, al-
174
most superimposes on the pure Fe one up to 100 GPa. The Fe melting curve reported
175
by (Morard et al., 2018) combines results of XANES, XRD (Anzellini et al., 2013) and
176
results of synchrotron M¨ ossbauer spectroscopy (Jackson et al., 2013; Zhang et al., 2016).
177
The negligible effect of Ni is in agreement with a previous Fe-Ni study (Zhang et al., 2016),
178
the discrepancies of the absolute values, in fact, is given by a different estimate of the
179
thermal pressure (Morard et al., 2018). A kink of the melting line of Fe-20wt%Ni can
180
be also expected in correspondence to the triple point, as in pure Fe (Morard et al., 2018).
181
Therefore, an extrapolation of this melting curve to IC boundary conditions would not
182
be reliable and measurements above 120 GPa, where the melting occurs from the hcp
183
phase, are needed.
184
Three heating runs, at 30, 50 and 60 GPa were performed on the Fe-Ni alloy with
185
36 wt% of Ni. For sake of clarity, only the first melting signatures are reported on the
186
phase diagram for this alloy. These three melting points nicely overlap on the Fe-20wt%Ni
187
melting curve, indicating that even higher Ni content does not affect the melting curve
188
of Fe, differently from the effect of light elements alloying (Morard et al., 2017).
189
The hcp/fcc boundary is fitted linearly. The extrapolation of the melting curve and
190
of the hcp/fcc phase boundary from 120 GPa allows to locate the hcp/fcc/liquid triple
191
point of Fe-20wt%Ni at 170±20 GPa and 4000±400 K. For pure Fe, the triple point has
192
been recently found at 100 ±10 GPa and 3500±200 K (Morard et al., 2018). Therefore,
193
while the melting curve of fcc Fe is substantially unaffected by the addition of 20 wt%
194
of Ni, because of the shift of the hcp/fcc boundary, the triple point moves significantly
195
to higher pressures and temperatures.
196
The position of the triple point is important for thermodynamic modeling (Komabayashi
197
& Fei, 2010) or to constrain entropy change upon melting related to heat flow at the core-
198
mantle boundary (CMB) (Anderson, 1990). Our results allow constraining the position
199
of the triple point for a geophysical pertinent composition, Fe-10wt%Ni, by assuming melt-
200
fcc
liquid
fit
This study Fe-36%wtNi liquid
T(K)
P (GPa) hcp
fcc
triple point Fe Morard 2018
This study
Fe-20%wtNi Ni Boccato 2017
ý
Figure 4. Left: Phase diagram obtained from the experimental data. Black, orange and cyan rotated squares are bcc, hcp and fcc solid phases respectively. The bcc/hcp and hcp/fcc co-existence range is indicated as two colors squares. Pink circles indicate liquid phases, and two colours (pink and green) hexagons are solid-liquid mixtures. The black triangle represents the predicted position for the hcp/fcc/liquid triple point of Fe-20wt%Ni. Black hexagonal symbols represent melting points for the Fe-36wt%Ni alloy. Black crosses represent P/T errors for the melting points in different pressure regions of the phase diagram and have been shifted in vertical for clarity. The melting curves of pure Fe from (Morard et al., 2018) and pure Ni from (Boccato et al., 2017) are also shown. Right: phase boundaries incertitude evaluated as described in SI Text7.
ing curve similar to pure Fe and Fe-20wt%Ni in the fcc phase, and a linear variation of
201
the hcp/fcc boundary with Ni content, that is also in agreement with most recent mea-
202
surements of the hcp/fcc boundary by (Komabayashi et al., 2012) (Fig. 5 left panel). The
203
position of the triple point for Fe-10wt%Ni is then expected around 135 GPa and 3800
204
K, and indicates a reduction of the melting temperature at CMB conditions by around
205
400 K with respect to pure Fe. This impacts the liquidus for Fe alloys constituting the
206
Earth’s core, by shifting it toward lower temperature. As pure Fe has a melting temper-
207
ature comparable to mantle silicates at CMB pressure (∼4200 K), Ni addition will re-
208
duce this to ∼3800 K.
209
In a previous discussion, we have shown that the presence of a notable amount of
210
light elements is required to lower the crystallization temperature of core materials at
211
CMB below 4150 K (Fig7 in (Morard et al., 2017)), an upper bound which corresponds
212
to the mantle melting temperature at 136 GPa (Fiquet et al., 2010; Andrault et al., 2011)
213
(Fig. 5 right panel). The observation that the main constituting alloy (Fe-Ni) in the core
214
exhibits already a reduced melting temperature, suggests that a lower fraction of light
215
elements is required. Such a low fraction is in better agreement with the one estimated
216
from geochemical observations and the one required to reduce the density (and seismic
217
wave speed) of the core (Badro et al., 2014).
218
4 Conclusions
219
In this study we present the first experimental determination of melting curves of
220
Fe-Ni alloys at 20 and 36 wt% of Ni by means of x-ray absorption spectroscopy in the
221
laser heated DAC, as well as their high-pressure, high-temperature phase diagram in the
222
Fe
FeNi
10 wt%
C S
O solidus
of peridotite
