PUBLICATIONS
Geophysical Research Letters
RESEARCH LETTER
10.1002/2014GL061954
Key Points:
• Melting curve of MgO determined
experimentally to 40 GPa
• Results consistent with recent ab initio
predictions and shock measurements
• MgO-FeO binary system is consistent
with ideal solid solution model up to
40 GPa
Supporting Information:
• Readme
• Table S1
• Table S2
• Table S3
• Text S1
• Figure S1
• Figure S2
• Figure S3
• Figure S4
• Figure S5
• Figure S6
Correspondence to:
Z. Du,
zhixue.du@yale.edu
Citation:
Du, Z., and K. K. M. Lee (2014), Highpressure melting of MgO from (Mg,Fe)O
solid solutions, Geophys. Res. Lett., 41,
doi:10.1002/2014GL061954.
Received 19 SEP 2014
Accepted 5 NOV 2014
Accepted article online 10 NOV 2014
High-pressure melting of MgO from (Mg,Fe)O
solid solutions
Zhixue Du1 and Kanani K. M. Lee1
1
Department of Geology and Geophysics, Yale University, New Haven, Connecticut, USA
Abstract Magnesium oxide (MgO) is a significant component of planetary interiors, particularly Earth’s mantle
and other rocky planets within and beyond our solar system; thus its high-pressure, high-temperature behavior is
important to understanding the thermochemical evolution of planets. Laser-heated diamond-anvil cell (DAC)
experiments on (Mg,Fe)O ferropericlase up to ~40 GPa show that previous DAC experiments on MgO melting are
too low, while previous multi-anvil experiments yield melting temperatures too high. Instead, our quasi-static
experimental results are consistent with recent ab initio predictions as well as dynamic shock measurements.
Extrapolated to the core-mantle boundary (CMB) of the Earth, MgO is expected to melt at ~8000 ± 500 K, much
greater than expected geotherm temperatures.
1. Introduction
The origin and subsequent evolution of the Earth are dictated by several factors including composition and
the melt behavior of the constituent phases. It is widely assumed that the early Earth was partially or fully
molten (e.g., [Labrosse et al., 2007; Ohtani, 2009; Walter and Tronnes, 2004]) following the Moon-forming giant
impact event (e.g., [Benz and Cameron, 1990; Canup, 2004; Melosh, 1990]). Even today, partial melt has been
invoked to explain the ultralow velocity zone (ULVZ) at the Earth’s CMB (e.g., [Lay et al., 2004; Williams and
Garnero, 1996]). However, estimates of the melting temperature of Earth’s mantle, which comprises more
than three-quarters of the Earth’s volume and nearly two thirds of its mass, remain controversial and elusive
(e.g., [Andrault et al., 2011; Boehler, 2000; Fiquet et al., 2010; Nomura et al., 2014]). The conditions at which
the mantle melts and solidifies is especially important for understanding heterogeneity, which constrains
deep structure and mixing and is crucial for inferring the thermo-chemical evolution of the Earth, from
planetary accretion and the magma ocean, to continental growth, and the current state of the core-mantle
boundary (e.g., [Hamano et al., 2013; Labrosse et al., 2007; Lay et al., 2004; Walter and Tronnes, 2004]).
As MgO is an important component of Earth’s mantle, its many physical properties, particularly its melting
temperature at high pressure, have been the focal point of several studies and remain controversial: The first
experimental study on MgO yields a very low melting curve [Zerr and Boehler, 1994]; while a recent set of
multi-anvil press experiments show a high and steep melting curve [Zhang and Fei, 2008]. The most recent
shockwave generated measurements show possible evidence for melting at temperatures of ~9000 K at
240 GPa along the pre-heated Hugoniot [Fat’yanov and Asimow, 2014]. Recent laser-driven shock measurements
on MgO find evidence for melting of the high-pressure B2 structure of MgO at 650 GPa and 14,000 K
[McWilliams et al., 2012]. Additionally, several theoretical computations (e.g., [Alfe, 2005; Belonoshko et al.,
2010; Boates and Bonev, 2013; Cohen and Weitz, 1998; de Koker and Stixrude, 2009; Strachan et al., 1999;
Vocadlo and Price, 1996; Yoshimoto, 2010]) have computed the melting curve of MgO, yielding consistently
higher melting temperatures than the DAC experiments [Zerr and Boehler, 1994], although there are still
significant discrepancies among the computations themselves. Thus, more reliable experimental results
are a necessary test for theoretical predictions of melting (e.g., [Gillan et al., 2006]).
2. Methods
In an attempt to constrain MgO melting, we performed melting experiments with a single-sided laser-heated
DAC equipped with 300 μm flat culet diamonds (Figures S1 and S2). Rhenium gaskets were preindented
to a thickness of 20 μm with a centered ~130 μm sample chamber. We used two different starting materials of
ferropericlase Mg1 xFexO (x = 0.09, 0.2, i.e., Mg# = 100 × (Mg/Mg + Fe), by mol, is 91 and 80, respectively),
prepared with MgO and Fe2O3 (Alfa Aesar, purity 99.99%) powders mixed and synthesized as described
DU AND LEE
©2014. American Geophysical Union. All Rights Reserved.
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Geophysical Research Letters
10.1002/2014GL061954
b
a
T (K)
4800
4600
4400
4200
I
4000
II
3800
III
3600
10µm
3400
c
1.0
d
0.8
0.4
Em/Emmax
0.6
I
II
0.2
Log(Em) (a.u.)
3600
3800
4000
4200
4400
4600
Temperature (K)
Figure 1. Melting of (Mg0.91Fe0.09O) at 27 GPa. (a) Optical image of quenched sample at 27 GPa with transmitted and
reflected light. Region I: central dark, iron-rich area, previously molten; Region II: outer, iron-depleted area, coexisting
solid; Region III: unmelted area, starting material. (b) 2-D temperature map. (c) 2-D emissivity map. (d) Temperature-emissivity
transects across the center.
elsewhere [Otsuka et al., 2010]. We used electron probe microanalysis (EPMA) to confirm their homogeneity
and determine their compositions. The starting material was loaded as a powder into the sample chamber
without pressure medium, thermal insulation, or laser absorber to minimize contamination and/or
chemical reactions (Figure S1, Top). We found that the samples are sufficiently insulating and absorbing to
heat between the thermally conducting diamonds. In a few experiments, we used Ar as pressure medium
to examine the possible chemical reaction between Ar and MgO at high temperatures in Zerr and Boehler
[1994] and found that Ar goes in to the melt; thus, we did not use those measurements in this study
(Figure S6 supporting information). We measured the pressure by the first-order Raman band [Akahama and
Kawamura, 2006] from the diamond culets both before and after heating and find that the pressures
measured are within 10% of each other; likewise, pressure gradients are minimal and found to be <10%
[Uts et al., 2013].
Each sample was the compressed to a pressure between 3 and 40 GPa and then annealed at ~1200 K
before subjected to a pre-defined ramped heating (Figures S1 and S3, supporting information). During
melting, the sample was kept at its peak temperature for 0.3–1 s to ensure chemical equilibrium between the
melt and coexisting solid (Figure S1, D–E). Two-dimensional temperature measurements were obtained
with four-color multispectral imaging radiometry [Du et al., 2013] for 20–40 ms just before the sample was
temperature quenched, synchronized with a mechanical shutter. An optical photomicrograph of the quenched
sample was taken while at high pressure with transmitted white light (Figure 1a). Several regions are clearly
identified: a central opaque region (I); a semi-transparent coronal region surrounding an opaque region (II); and a
transparent region just outside darker inner regions (III) (Figure 1a). Outlines of regions I and II are superimposed
DU AND LEE
©2014. American Geophysical Union. All Rights Reserved.
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Geophysical Research Letters
10.1002/2014GL061954
on to a 2-D temperature map (Figure 1b) and
emissivity map (Figure 1c), showing
geometrical consistencies between the maps
and the optical image. More importantly,
the outline of region I yields a temperature,
which we interpret as the temperature
when melt and solid are coexisting. This
measurement is in excellent agreement with
the discontinuity in the temperatureemissivity plot across the melted region
(Figure 1d), which corresponds to the
sudden change of optical properties
between region I and II at high temperature
and has been used as a signature of melting
[Fischer and Campbell, 2010].
To investigate the nature of the quenched
sample, we cut through the center of the
heated region using Electrical Discharge
Machining (EDM) and Focused Ion Beam
(FIB) techniques (Figure S4). The cross
section of the sample is shown in
Figure 2a. Similar to the optical image
(Figure 1a), the three regions are identified
and confirmed with electron microscopy:
region I: iron-enriched melt, showing
dendritic quenched melt texture
Figure 2. Cross section of recovered (Mg,Fe)O sample melted at
(Figure S5); region II: iron-depleted solid,
27 GPa. (a) Back Scattering Electron (BSE) images across the melted
showing smooth, uniform texture, and
sample. It shows clearly the following features: iron-enriched region I,
orientation; and region III: annealed
iron-depleted region II, unaltered region III. (b) Corresponding Fe
elemental mapping. Bright areas indicate higher iron concentration.
polycrystalline starting material. Overall Fe
(c) Compositional transect (Mg# = 100 × Mg/(Mg + Fe) by mol) across
distribution is shown in Figure 2b. Last,
the sample along the dashed line in Figure 2a.
chemical composition is determined by
EPMA across the sample. In addition, the
compositional transects across the sample are also examined in Figure 2c, confirming the compositional
homogeneity within each of the three regions (supporting information, Table S1). This is the key to
verifying that the coexisting regions I (melt) and II (solid) have reached chemical equilibrium (Table S2).
3. Results and Discussion
Our measurements of the temperatures, pressures, and compositions of region I and II are compiled in
Table S2 and plotted as solid circles in Figure 3. In order to construct the MgO-FeO binary, we take the
melting temperature of FeO (TFeO) from Fischer and Campbell [2010] as fixed points and assume an ideal
solution model. The enthalpy of melting for MgO (ΔHMgO) and FeO (ΔHFeO) is largely unconstrained at high
pressures (e.g., [Alfe, 2005; Vocadlo and Price, 1996]). At room pressures, no calorimetric data are available,
and estimates of ΔHMgO range from 34 to 125 kJ/mol [Yoshimoto, 2010]. Thus, we treat ΔHMgO and ΔHFeO
along with the melting temperature MgO (TMgO) as free parameters to fit our data, finding that ΔHMgO
remains roughly constant at high pressure up to 11 GPa. Unique best fits are obtained at 3 and 11 GPa.
However, at 27 and 40 GPa, there are strong trade-offs between ΔHMgO and TMgO. We assume a roughly
constant ΔHMgO up to 40 GPa, to compute TMgO at 27 GPa and 40 GPa. All the fitting parameters are listed in
Table S3, and TMgO are plotted in Figure 4. Our results are in sharp contrast with previous quasi-static
experimental results [Zerr and Boehler, 1994; Zhang and Fei, 2008], while consistent with many of the theoretical
predictions (e.g., [Alfe, 2005; Belonoshko et al., 2010; Boates and Bonev, 2013; Vocadlo and Price, 1996]) and
shockwave experiments [Fat’yanov and Asimow, 2014; McWilliams et al., 2012].
DU AND LEE
©2014. American Geophysical Union. All Rights Reserved.
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Geophysical Research Letters
10.1002/2014GL061954
Figure 3. MgO-FeO binary diagram at 3, 11, 27, and 40 GPa. The data (solid circles) are measured by EMPA in regions I and II
and temperature given by the boundary between regions I and II. FeO melting temperatures (solid diamonds) from Fischer
and Campbell [2010]. Inferred MgO melting temperature is shown with open diamonds.
Figure 4. Melting curves of MgO as determined by experiment and theory. This study’s results are shown in solid circles. Vertical
dashed line corresponds to CMB; gray shaded area represents an estimate of the lower mantle geotherm [Boehler, 2000; Brown
and Shankland, 1981]. Light gray band corresponds to a minimum estimate of melting as determined by shock measurements
along the pre-heated Hugoniot at ~240 GPa [Fat’yanov and Asimow, 2014], well above of [Zerr and Boehler, 1994] and consistent with an extrapolation of our measurements. Other studies’ symbols are shown in legend.
DU AND LEE
©2014. American Geophysical Union. All Rights Reserved.
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Geophysical Research Letters
10.1002/2014GL061954
It is speculated that MgO-FeO solid solution increasingly deviates from ideal solution behavior following
the insulator-metal phase transition of FeO at pressures between 30 and 90 GPa and at temperatures
between 1500 and 2500 K [Ohta et al., 2012]; or between 50 and 75 GPa at temperatures between 1300 and
2500 K [Fischer et al., 2011], respectively. Our data are consistent with an ideal solid solution model up to
pressures of 40 GPa. Non-ideal behavior may occur upon the transition of FeO to a metal, and indeed,
one may argue that a small level of non-ideality may be present given the slightly higher values our data
give as compared to the ideal solution fits at 27 and 40 GPa. Therefore, more studies are needed to
determine the insulator-metal phase boundary, as well as the MgO-FeO phase relations at pressure greater
than 40 GPa. Even so, while (Mg,Fe)O melting will likely be further complicated at high pressures due to
the spin transition in Fe (e.g., [Badro et al., 2003; Lin et al., 2013]), the melting temperature of MgO will not
be affected by electronic changes that Fe in (Mg,Fe)O undergoes.
One can infer the composition of ULVZs [de Koker and Stixrude, 2009; Labrosse et al., 2007] as the eutectic
composition in a simplified MgO-MgSiO3 binary system, if ULVZs are interpreted as partial melt or remnants of a
partial melt [Nomura et al., 2011; Williams and Garnero, 1996]. We determine the melting temperature of the end
member MgO to be fairly high. This implies the eutectic composition of MgO-MgSiO3 system to be Si rich,
consistent with de Koker et al. [2013], but in contrast with [Boehler, 2000]. Iron, as well as other minor elements,
additionally will likely have a large effect on the eutectic composition and temperature. Indeed our results
confirm iron preferably partitions into melt during partial melting [Andrault et al., 2012; Nomura et al., 2011]. This
preference of iron in the melt further densifies the melt, which will subsequently freeze as it cools thereby
giving another mechanism to explain ULVZs beyond the presence of partial melt today [Bower et al., 2011;
Nomura et al., 2011].
A high melting curve of MgO also results in low homologous temperature at high pressures, suggesting a
much higher viscosity of MgO than previous estimates [Yamazaki and Karato, 2001; Zerr and Boehler, 1994].
As the mantle is a multiphase assemblage which includes MgO, in the presence of another phase such
as Mg-silicate perovskite in the Earth’s lower mantle, the “stronger” MgO will also possibly influence the
overall texture of this assemblage [Wang et al., 2013; Yamazaki and Karato, 2001]. Likely high mantle viscosity
yields sluggish convection and thus has profound implications for the thermochemical evolution of Earth
and rocky exoplanets (e.g., [Karato, 2011; Stamenkovic et al., 2012; Tackley et al., 2013]).
4. Conclusions
Acknowledgments
Data supporting Figures 3 and 4 are
available as in supporting information
Tables S1–S3. We thank Z. Jing,
K. Otsuka, and L. Miyagi for extensive
discussions; M. Rooks and F. Camino for
FIB help at YINQE Yale University and
CFN at Brookhaven National lab,
respectively; Z. Jiang for SEM assistance;
J. Eckert for EPMA measurements; and
G. Amulele, W. Samela, and C. Fiederlein
for technical support. We thank T. Pier of
SPI Lasers and the support staff of
National Instruments for assistance on
LabVIEW programming. We also thank
A. Wallenta for artwork help. Facilities
use was supported by YINQE and NSF
MRSEC DMR 1119826. Research carried
out in part at the Center for Functional
Nanomaterials, Brookhaven National
Laboratory, which is supported by the
U.S. Department of Energy, Office of
Basic Energy Sciences, under contract
DE-AC02-98CH10886. This work was
funded in part by NSF (EAR-1321956,
EAR-0955824) and CDAC.
Michael Wysession thanks Paul Asimow
and one anonymous reviewer for their
assistance in evaluating this paper.
DU AND LEE
Our experiments confirm recent ab initio computations on the relatively high melting curve of MgO as well as
new shock measurements of MgO, thus reconciling decades of mismatch between static experiments
and theoretical predictions. An ideal solid solution model for MgO-FeO binary system is tested for the first
time up to 40 GPa and could further be examined for the effect of spin transition of iron or metallization of
FeO at higher pressures.
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