CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
THE POSSIBLE COMPOSITION AND THERMAL STRUCTURE OF
THE EARTH'S LOWER MANTLE AND CORE
Zizheng Gong1, Xijun Li2, Fuqian Jing3,1
1
Laboratory of High Pressure Physics, Southwest Jiaotong University, Chengdu 610031, China
Institute of Electronic Engineering, China Academy of Engineering Physics, Maianyang, Sichuan 621900,
3
Laboratory for Shock Wave and Detonation Physics Research, Southwest Institute of Fluid physics,
Maianyang, Sichuan 621900, China
Abstract. In order to constraint on the possible mineral composition, thermal structure of the Earth's
interior, equation of state, sound velocity and shock temperature and melting measurements of
perovskite-enstatite (Mg, Fe)SiO3 and porous iron, were performed through shock wave experiments. A
fully new mineralogical model of the lower mantle is proposed The temperature of the core-mantle
boundary (core side) is 3750(±150) K, the inner-outer core boundary is 5350(±150) K, and the center of the
coreis5500(±150)K.
The melting point of iron at the pressure of the
outer (liquid) core-inner (solid) core (330GPa) was
suggested to provide a constraint on the temperature
estimation [5]. However, there was a great gap of it
between former shock wave compression (SWC)
data (around 7500K)[6,7,8] and diamond anvil cell
(DAC) results (around 4800K)[9]. This deviation
was testified and discussed through shock wave
experimental results of porous iron. The
temperature profiles of the Earth's core was
suggested in term of corrected melting curve of
iron.
INTRODUCTION
For more than 30 years shock wave has played an
important and effective role in the study of the
Earth's science. Up to now, two aspects of the
Earth's deep interior are still unknown: (1)
Mineralogical composition and (2) Temperature
profiles. Comparing the in situ determination of
physical properties (density, bulk and shear moduli
etc.) of candidate mineral assemblages with that of
PREM (Preliminary reference Earth Model [1]), is
the main way to determine the possible composition
of Earth's deep interior.
(Mg, Fe) SiO3 with perovskite (pv) structure has
been considered to be main phase of the lower
mantle. Equation of state (EOS) and elastic
properties are increasingly well constrained [2, 3, 4].
Few studies, however, have been able to achieve
sound velocities on (Mg, Fe) SiO3-pv Under the
actual pressure and temperature condition of the
lower mantle. In this paper, shock wave study of
enstatite (Mg0.92, Fe0.o8)SiO3 pv at pressure from 40
GPa to 140GPa were reported and its geophysical
implications are discussed.
NEW MINERALOGY MODEL OF THE
LOWER MANTLE
Shock Wave Experiments of Enstatite-pv.
The enstatite samples studied in this experiment
were made of the natural enstatite minerals, which
were collected from Mine, Changjiakou, Hebei
Province, China. After purification, grinding and
chemical treatment, the tiny powder of enstatite
(<0.05mm in diameter) was hot pressed into disk-
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like sample, at 56MPa and 1700K using graphite
cylindrical mould at vacuum condition. The average
density of initial samples is 3.08g/cm3(its crystal
density is 3.27g/cm3).The chemical composition are
SiO2(54.72%), MgO (31.09%),FeO (5.00%), A12O3
(4.02%), Fe2O3 (2.44%), CaO (1.70%), and d values
of x-ray Diffraction are 3.167(100), 2.876(64),
2.499(6) and 2.940(5). It can be represented
typically as (Mg0.92, Fe0.08)SiO3.
(1) Hugoniot Equation of State ofEnstatite -pv.
The Hugoniot EOS experiments were carried out
with 37-mm two-stage light gas gun using metal
flyer plate bearing projectiles to impact samples at
speeds of up to 6.5km/s. In all experiments, shock
wave velocity, D, in the sample was measured using
the electrical probe technique. The particle velocity
behind the shock front, u^ and pressure-density
states were calculated through the impedance-match
method [10]. 8 shots of impact experiments was
conducted, and together with the experimental data
of McQueen [4] and Watt [2], a linear relation
between D and u (both in units of km/s) of En-pv
D=5.13+1.2w
(1)
is drawn in Fig.l. This shows that no phase change
happens for enstatite-pv in the temperature and
pressure ranges in the lower mantle (40-140GPa).
(2) Sound Velocity of Enstatite-pv
The compressional sound velocity vp for the
enstitate specimens was measured by the optical
analyzer techniques under shock loading [11]. Three
samples with the same diameter (15mm) and about
2, 3, 4 mm thick (stepped-sample) are arranged on
the same plane. The initially transparent bromoform
(BF) is used as the optical analyzer medium. Three
thin optical fibers are used to monitor the light
radiation from BF. Each fiber records the radiation
history for only one step of the stepped-sample.
The dependence of compressional sound
velocity VP of enstatite-pv on Hugoniot pressure
fitted for five shots can be described by [12]:
Invp (km/s) =2.030 +0.1041nP(GPa)
(2)
When compared it with the calculated bulk velocity,
no evidence of shock-induced melting is found.
This means what we measured is really the
compressional wave of sample. At the experimental
pressure range, the compressional wave velocity of
enstatite varies linearly with density (seeing Fig.2),
satisfied Birch's Law [13]: VP = 6.213 + 1.212p,
where p is the corresponding density, which means
enstatite (pv) is stable in the lower mantle
conditions.
(3) Shock Temperature and Melting ofEn.-pv
The shock temperature TH of enstatite-pv were
measured using a six-channel high sensitivity tran-
12
14
• Present work-En92
D McQueen-En90
ExperimBntal data
A Watt & Ahrens-En86
I
10
Unecr fit line of 5 data points
13
Linearfitlineof4datapoints
5
o
D=5.13+1.2u
•o
3
vF6.213fl.212p
11
10
3.3
4.3
4.8
5.5
5.8
FIGURE 1 All present shock velocity-particle velocity data and
linear fits for enstitate-pv.
FIGURE 2 Relationship between compression sound velocity
and density for enstatite-pv
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sient optical pyrometer. Three samples with the
same diameter (15mm) and about 2, 3, 4 mm of
thickness are arranged on the same plane, and LiF
window with the same diameter and about 8, 7, 6
mm of thickness were backed on them, respectively.
The parallelism is about 5 jam and Ijnm, and the
plainness is less than 2|iim and 0.5|im, for samples
and windows, respectively. Therefore, the width of
gap between sample/window none-ideal interface is
about l|im. TH of samples are calculated according
to the three layer heat conduction model for
sample-gap-window [14]. Measured TH increases
continuously with increasing pressure from
3319±160K at 46 GPa to 8272±420K at 140 GPa.
The P-T Hugoniot lies under the melting line
measured by static high pressure, inferring that no
melting or decomposing happened in our
experimental pressure range. According to the
shock temperature results, melting point of
perovskite (Mg0.92, Fe0.o8)SiO3 at core-mantle
boundary (CMB) pressure (136 GPa) should be
higher than 7600 K at least, this is in very good
agreement with the extrapolation of 8000±500K of
Zerr & Boehler's experiments [15], and the
extrapolation of 450011OOK of Knittle & Jeanloz'
[16] was negated.
Geophysical Implications
850
£
o
450
1771km
250
1000
1500
2000
2500
3000
Depth (km)
FIGURE 3 KS(P) of enstatite-pv. compared with PREM
was lower than that of PREM at depth shallower
than 1771 km and the deviation decreases with
depth, the largest deviation is -9.71% and the
average the -4.13%. At 1771±100km depth it is
equal to that of PREM. Below 1771 km depth, it is
higher than that of PREM and the deviation increase
with depth, the largest deviation is 8.22% and the
average 5.46%.
(3) Based on the above mentioned, a fully new
mineralogy model of the lower mantle is proposed
(seeing Fig.4): © (Mg0.92, Fe0.o8>SiO3-Pv is the
main mineral phase (not less than 85%).© A
chemical boundary exists at the depth of about
(1) A Fully New Mineralogical Model of the Lower
Mantle [17]
Wt
wt.%
(1) Both the corrected Hugoniot density and
compressional wave velocity are compared with
PREM. Both the profiles of density and P wave
velocity of (Mg0.92, Fe0.o8)SiO3-pv are parallel with
those of PREM. The density is by 1.6% on average
higher and P wave velocity is by 1.81% on average
higher than those of PREM, respectively. Based on
density and compressional velocity constraints,
Reuss-Voigt-Hill average conclude that the lower
mantle is mainly composed of 89.69%-88.65%
(wt.% or mol.%) (Mgo.92, Fe0.08)SiO3-Pv and
10.31%-! 1.35% (Mgo.92, Fe0.08)O, and the lower
mantle is chondrite-rich. This is consistent with
most of the previous results.
(2) As shown in Fig. 3, the profile of bulk
modulus (K$) was obviously divided into two parts
by that of PREM at depth of about 1771±100 km: it
XMw<20%
Boundary
transition zone
100km width
650
1771
3000
Depth (km)
FIGURE 4 A fully new mineralogy model of the lower mantle
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1771±100km in the middle of lower mantle. (3)
Above the boundary stishovite SiC>2 is the second
main mineral phase. SiC>2 (st) has a largest amount
(<30%) at the top of lower mantle and its amount
decreases with depth to the smallest (the absence'is
not excluded) at the boundary. @ Below the
boundary (Mg0.92, Fe0.o8)O (Mw) is the second main
mineral phase. Mw has a smallest amount (the
absence is not excluded) at the boundary and its
amount increases with depth to the largest amount
(<20%) at the bottom of lower mantle. This new
model is supported by the latest seismological
detection data.
(2) Lateral Thermal Heterogeneity in the Lower
Mantle [18]
In terms of the definition of sound velocity and
thermodynamics, the temperature coefficients of
sound velocity of perovskite-enstatite under high
pressure were obtained [18] | (3v/>/3 7)P|<0.25m
/K»S~] when pressure is high than lOOGPa. On the
basis of our data, we conclude that the
compressional wave velocity anomaly of 0.1-0.2%
in the deep lower mantle [19] and 2% in the D"
region [20]
would imply lateral thermal
heterogeneity with an amplitude of 53-106K and
1066K in these regions respectively.
corrected?
For a kind of porous iron with average initial
density p0=6.904g /cm3, in the pressure range
JP//<122GPa, the measured sound speed (using
optical analysis method with A12O3 windows) is the
longitudinal wave speed in nature, which can be
empirically fitted as VP= 5.951+ 1.224hiPH0.0349(lnPH)2> where PH is in unite of GPa, VP of
km/s; in the pressure range of PH >157GPa, the
measured sound speed is its bulk sound speed CB in
nature [22]. This means the shocked porous iron is
in solid states when PH <122GPa and transforms
into liquid state in the pressure range
122-15 7GPa. Following the interface temperature
model under the case of shock-induced melting
[23], two melting points of iron were measured
that are (171.4GPa, 5550-5730K) and (98GPa,
4470-461 OK). The two melting points are within
the scope of the SWC's data extrapolation. Both
the high pressure sound speed and melting
temperature experiments support the confidence
of the SWC's data extrapolation, or, in other
words, the systematic deviation between the
SWC's data and the DAC's melting data is indeed
a phenomena of objective reality. According to
overheating homogeneously nucleation and
catastrophe model, overhot melting point TM°ver
in SWC is 1.212 time of real melting point TM for
iron [24], and according to surface melting model,
the observed results TMExp in DAC should be 0.9
POSSIBLE COMPOSITION AND
TEMPERATURE PROFILE OF THE CORE
Possible Composition of the Core
It was generally accepted that about 90%(wt.) of
the core is iron, a small amount of light element is
present in it. According to the last point of view,
(Fe-S-O) or (Fe-S-Si) three element system were
suggested to be the possible composition of the core.
But we don't know what mineral styles they exist. As
shock Hugoniot of Nandan Iron meteorite (Fe 92.5%,
Ni6.8%, CoO.47%, wt.) from 60 to 208GPa showed
that its pressure-density fits with that of PREM [21].
So the iron-nickel alloy (Fe93%, Ni7%) core is
possible.
Thermal Structure of the Core
(1) Melting Point of Iron at 330GPa
As it was mentioned above, there is a large
difference in experimental results between
extrapolation of SWC and DC A. Is this a systematic
deviation between the two methods? Can this be
50
100
150
200
250
300
FIGURE 5 systematic deviation of melting data of iron between
the SWC method and DAC method.
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[5] Brich, F., J. Geophys. Res., 57, 227-286 (1952).
[6] Williams, Q., Jeanloz, R., and Bass, J. D., et al.,
Science, 236, 181 (1987).
[7] Bass, J. D., Svendsen, B., and Ahrens, T.J., in High
Pressure Research in Mineral Physics, Manghnani, M.
H. And Y. Syono (eds.), Terra Secentific, Tokyo, 1987,
pp.393-402.
[8] Yoo, C.S., Holmes, N. C., and Ross, M., et al., Phys.
Rev. Lett., 70, 3931-3934 (1993).
[9] Boehler, R., Nature, 363, 534-536 (1993)
[10]Jing, F. Q., Introduction to Experimental Equation of
State, second editor,. Academic Press, Beijing, 1999,
Chap 4
[ll]McQueen, R. G, Hopson, J. W. et al., Rev. Sci.
Instrum, 53, 245(1982).
[12] Gong, Z.Z., Jing, F.Q.et al., Chin. Phys. Lett., 16 (9),
695-697(1999).
[13]Campell, A. J.and Heinz, D.L., Science, 257 66
(1992).
[14]Tan, H. and Dai, C.D., Chinese J. High pressure
Phys., 14, 81 (2000), (in Chinese).
[15]Zerr, A. and Boehler, R., Science, 262, 553 (1993).
[16]Knittle, E. and Jeanloz, R., Geophys. Res. Lett., 16,
421 (1989).
[17] Gong, Z.Z., Equation of State, Sound velocity and
melting ofEnstatite at High Pressure: Constraints on
the Possible Composition and thermal structure of the
earth's lower Mantle, Postdoctoral Research Report,
Institute of Geochemistry, Chinese Academy of
Sciences. 1999.
[18]Gong, Z. Z., Jing, F. Q., et al., Chin. Phys. Lett., 17(3)
218-220(2000).
[19]Dziewonski, A. M. and J. H. Wooshouse, Science,
236,37(1987).
[20] .Lay, T., Rev. Geophys., Suppl, 325 (1995).
[21]Fu, S. Q., Jin, X.Gand Wang D. D., Chinese J.
Geophys 36 (2), 158-163 (1993).
[22] Li, X.J., Fu, Q.J., et al., Chin. Phys. Lett., 18(9)
(2001).will be published.
[23] Tan, H. And Ahrens, T.J., High Press. Res. 2, 159-182
(1990).
[24] Lu Ke and Li Yi, Phys. Rev. Lett., 80, 4470 (1998).
[25]Williams, Q. and Knittle, E., J. Geophys. Res., 96,
2171-2184(1990).
[26] Anderson, O.L., Phys. Earth Planet.Interiors,
109,179-197(1998).
I
flj 4500
0)
a
E4000
170
220
270
320
370
Pressure (GPa)
FIGURE 6 Temperature gradient in the Earth's core
time of the real melting point TM [25]. It is
surprising that these two corrected data are
almost in coincidence with each other. The
melting curve of iron can be fitted form corrected
data as: TM= 2523(±l53) +11.679 (±2.204) P-0.00157
(±0.00609) P2. From this equation, TM of pure iron
at 330GPa is 6200K. This value agrees well with
that many others have found.
(2) Temperature Profile of the Core
It was reported that depression of melting point
by impurities is about 700-1000K [26], So
temperature at the inner-outer core boundary is
constrained to be 5350(±150) K. Based on the
assumption of adiabatic compression, the
temperature at the boundary of the core-mantle
(core side), and the center of the core are around
3750(±150) K, 5500(±150) K, respectively, with
7=1.3 and 1.23 [26], respectively. Temperature
gradient of the Earth's core is shown in Fig.6.
ACKNOWLEDGMENTS
This research was supported by the National
Natural Science Foundation of China under Grant
No. 10032040.
REFERENCES
[1] Dziewonski, A. M. and Anderson, D. L., Phys. Earth
Planet. Interiors, 25, 295 (1981).
[2] Piquet, G, Dewaele, A., and Andrault, D., et al.,
Geophys. Res. Lett., 27, 21 (2000).
[3] Watt, J. P. and Atrens, T. J., J. Geophys. Res., 91,
7495(1986).
[4] McQueen,R. G, Marsh,S. P., et al.: J. Geophys. Res.,
72,4999(1967).
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