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Melting of forsterite Mg2SiO4 up to 15 GPa
32
Physics of the Earth and Planetary Interiors, 27 (1981) 32—38 Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands
Melting of forsterite Mg2 Si04 up to 15 GPa Eiji Ohtani * and Mineo Kumazawa Department of Earth Sciences, Nagoya Uninersity, Nagoya 464 (Japan) (Received March 3, 1981; revision accepted April 15, 1981)
Ohtani, E. and Kumazawa, M., 1981. Melting of forsterite Mg2SiO4 up to 15 GPa. Phys. Earth Planet. Inter., 27: 32—38. The melting curve of forsterite has been studied by static experiment up to a pressure of IS GPa. Forsterite melts congruently at least up to 12.7 GPa. The congruent melting temperature is expressed by the Kraut— Kennedy equation in the following form: Tm(K) 2163 (1 + 3.0( V0 — V)/ V0), where the volume change with pressure was calculated by the Birch—Managhan equation of state with the isothermal bulk modulus K0 = 125.4 GPa and its pressure derivative K’ = 5.33. The triple point of forsterite—fl-Mg2SiO4 —liquid will be located at about 2600°Cand 20 GPa, assuming that congruent melting persists up to the limit of the stability field of forsterite. The extrapolation of the previous melting data on enstatite and periclase indicates that the eutectic composition of the forsterite—enstatite system should shift toward the forsterite component with increasing pressure, and there is a possibility of incongruent melting of forsterite into periclase and liquid at higher pressure, although no evidence on incongruent melting has been obtained in the present experiment.
1. Introduction Knowledge of the melting relations of silicate systems at high pressures is vital to understanding magma generation and chemical differentiation in the Earth’s mantle. Various experiments on the melting of silicates have been performed previously in order to determine the origin of various types of magma and the nature of low-V and low-Q regions in the upper mantle. Most of these experiments, however, are restricted to those with pressures corresponding to the crust and uppermost mantle, and we cannot discuss the melting phenomena in the deep mantie. The authors have developed the experimental technique of generating higher temperatures combined with higher pressures above 10 GPa to study *
Present address: Department of Earth Sciences, Ehime University, Matsuyama 790, Japan.
the melting relationships of silicate systems in the deep mantle conditions. We have applied the melting experiment on Fe2 Si04 with this technique (Ohtani, 1979). The next approach which we performed along these lines was to clarify the melting relationship of Mg2Si04, the major constituent of the Earth’s mantle. The previous data on Mg2SiO4 were limited to pressures below 5 GPa (Davis and England, 1964). The purpose of this paper is to present the results of the melting experiment on Mg2SiO4 made in the range of 5—15 GPa.
2. Experimental technique 2.1. Apparatus and pressure generation The apparatus used in the present experiment was an MA8-type high pressure apparatus (Ohtani, 1979) composed of eight truncated cube anvils. It was driven by a pair of RH3-type guide blocks
0031-9201/81 /0000-0000/$02.50 © 1981 Elsevier Scientific Publishing Company
33
(Kumazawa et al., 1972) in a uniaxial press. For this experiment the truncated edge of the anvil
Sem-sintered TiC electrode Semi—sintered MgO pressure medium
-
corner was 8.0 mm long at pressures below 10 GPa, and 3.5 mm long at higher pressures. Pyrophyllite gaskets were used for the lateral support of the eight anvils. Semi-sintered magnesia was selected for the octahedral pressure medium because magnesia has a high melting point and no phase transition at high pressure. The edge-length of octahedral pressure medium was 14 mm for anvils with a truncated-edge length of 8.0 mm, and 9.5 nim for anvils with a truncated-edge length of 3.5 mm. The pressure values below 1.0 GPa were corrected on the basis of the phase boundary curve of olivine— spinel transition in Fe2 Si04 P (kbar) = 34.6 + 0.025T (°C) and that of coesite—stishovite transition in Si02 J). (kbar) = 80 + 0.011T (°C) (Akimoto et al,, 1977) which were determined by in situ X-ray diffraction experiments on the basis of the NaCI scale at high temperature. The pressure values above 10 GPa generated by the use of anvils with a truncated edge 3.5 mm long were calibrated at room temperature using the transition pressures on Bi, I—Il, 2.48 GPa and III—V, 7.34 GPa (Jeffery et a!., 1966), Pb, 13.0 GPa (Takahashi et aL, 1969), and ZnS, 15.0 GPa (Piermarini and Block, 1975), and GaAs, 18.0 GPa (G.J. Piermarini and S. Block, personal communication, 1975) as the calibration points. The hightemperature corrections were made by the use of the phase boundary curve of pyroxene—ilmenite transition in ZnSiO3 determined by the lTighpressure X-ray diffraction method at high temperature (Akimoto et al., 1977). The calibration curves of the generated pressure versus press load on the present apparatus were shown in a previous paper (Ohtam, 1979). 2.2. Temperature generation and measurement Two furnace assemblies, illustrated in Fig. 1, were used for the present experiments. These furnaces are basically the same as those used for the melting experiment of Fe2SiO4 (Ohtani, 1979). The furnace shown in Fig. Ia was used for the runs at pressures below 10 GPa. The sample was directly packed on the cylindrical graphite heater placed at
LaCrO
3 heater
sample Thermocouple Ceramic
sleeve Graphite heater Capsule for sealing of Graphite electrode a b Fig. I. Furnace assemblies used in the present experiment: (a) furnace A for pressures below 10 GPa; and (b) furnace B for pressures above 10 GPa.
the centre of the pressure medium. A W.-W 26% Re thermocouple with a diameter of 0.1 mm was placed at the centre of the furnace through two holes drilled radially on the cylindrical heater. The thermocouple was separated from the furnace assembly by a ceramic (alumina) tube in order to avoid contamination from the furnace materials. Figure lb shows the furnace assembly used for the runs at pressures exceeding 10 GPa. In this pressure range, graphite is not used for the heating material and electric leads because the phase transformation of graphite to diamond hinders stable generation of temperatures above 2000°C Then we used semi-sintered lanthanum chromite, LaCrO3, as heating material and semi-sintered titanium carbide, TiC, as electric leads. A powdered sample was placed in a graphite or boron nitride capsule to avoid reaction of the sample and heater. Powdered diamond was also used for sealing material in runs made at the stability field of diamond. In this case, a pellet of the sintered forsterite, at 1500°C in a vacuum of iO—~Torr, was enclosed in the diamond powder to separate the sample from the tube heater. The sealing of the thermocouple by a ceramic tube was not made for this furnace because the ceramic tube reacts with the lanthanum chromite heater at high temperatures. The temperature value was based on the e.m.f.—~ temperature relation of a W—W 26% Re thermo-
34
couple reported in ASTM (1974) up to 2320°C and its linear extrapolation to higher temperatures. The run temperatures were corrected for the increase of the surface temperature of the anvils to which the thermocouple leads were connected. The correction of the effect of pressure on e.m.f. was not performed because the preliminary study on the pressure effect on the e.m.f. of the W—W/Re thermocouple (Ohtani, 1979) indicates that it is very small at pressures around 7 GPa. 2.3. Sample preparation, experimental procedure, and examination of the run products Forsterite used for the starting material was prepared by heating the stoichiometric mixture of reagents MgO and Si02 at 6 GPa to 1300°Cfor a few hours. The sample so prepared was ascertained, by means of X-ray diffraction and microscopic observation, to be a pure single-phase material, The furnace assembly was dried in an oven at about 150°Cfor a few hours immediately before each run in order to avoid the effect of absorbed water on the melting temperature. The quenching method was used to determine the melting curve. Pressure was applied to the sample and then the temperature was brought to the desired value and held constant. Heating duration at the desired temperature was limited to 0.1—5.0 mm in order to avoid contamination of the sample by the furnace material and the thermocouple. After that the sample was quenched isobarically. The run products close to or around the thermocouple were picked out carefully and the thin sections were prepared for examination by optical microscopy. The X-ray diffraction method was also used to identify the reaction products and to insure absence of contamination,
3. Experimental results and discussions The results of the present experiment on the melting of forsterite are summarized in Table I. Evidence of melting is recognized by microscopic observation of the textures of the run products.
Forsterite melt cannot be quenched to a glass but forms quench crystals on rapid cooling. The quench crystals show a fibrous texture, whereas the crystals grown slowly below the melting point have a granular texture. The difference between the textures is so noticeable that it is easy to detect whether the sample has melted or not. In several runs, both quench crystals and the recrystallized crystals coexisted in the same run products (see Fig. 2a, b). This is caused by the presence of the temperature gradient in the furnace. Then a part of the run product that is close to or around the thermocouple is picked out carefully for observation of the texture. Figure 2a— d shows the photomicrographs of the textures of the run products at various pressures. X-ray observation of these products indicated that these quench crystals are forsterite. We have confirmed the melting of forsterite up to 15 GPa, and could not detect evidence of incongruent melting of forsterite in the present experiment. However, we cannot rule out the possibility that forsterite melts incongruently and that crystal and melt coexist in the narrow temperature range around 15 GPa, as is observed in MgSiO3 below 0.6 GPa (Boyd et a!., 1964), because the run temperatures made at 15 GPa were about 200°Chigher than the expected melting temperature of forsterite. At the present stage of our experiment we can safely conclude that forsterite melts congruently at least up to 12.7 GPa. For a run made at 5 GPa and two runs at 10 GPa, the melting point was evaluated by utilizing the temperature gradient in the furnace. The ternperature gradient in furnace A is illustrated in Fig. 3. The temperature difference between the central and end parts of the furnace amounted to about 100°C. The temperature of the textural boundary between the crystals recrystallized without melting and those quenched from melt is 2090°C for a run made at 5 GPa (Run 80506A), and 2308°C(Run 1102) and 2320°C(Run 1105) for runs at 10 GPa. Temperature fluctuation during a run performed below 10.3 GPa was within ±30°C in most runs. However, the uncertainty of the melting temperature determined in this pressure range will amount to ±50°Cbecause of the ambiguity
35 TABLE I Experimental results on the melting of forsterite Run No.
Pressure (GPa)
Temperature (°C)
Time
Furnace
Results
(mm)
506B 301 80601B 8060lA 120 80506A
4.7 4.7 4.7 4.7 4.7 4.7
1950±10 2030±5 2075±15 2100±10 2140±20 2155±10 2090
1.0 A S-fo 1.5 A S-fo 1.0 A S-fo 1.0 A Q-fo 5.0 A Q-fo 1.0 A Q-fo Temperature at the boundary between S-fo and Q-fo
122 608 607B 607A 80502 507A 1005 1004 1007 1009 1104B 1011 1104A 1105
4.7 6.2 7.4 7.4 7.7 7.7 8.4 8.4 8.4 9.8 10.0 9.8 10.0 10.0
2200±50 2185±10 2145±30 2510±60 2185±30 2360±90 2130±30 2230±10 2485 ±65 1965±30 2010±10 2140±20 2245±10 2340±20 2320
2.5 A Q-fo 2.0 A Q-fo 2.0 A S-fo 2.0 A Q-fo 1.0 A S-to 1.0 A Q-fo 2.0 B, g S-fo 0.5 B, g S-fo 0.3 B, g Q-fo 2.5 B,g S-fo 0.5 A S-to 1.0 B,g S-to 0.5 A S-to 0.1 A Q-fo Temperature at the boundaiy between S-to and Q-fo
1102
9.9
2355±10 2308
0.1 A Q-fo Temperature at the boundary between S-fo and Q.fo
1103 80101 80102 80207 80404 80409 80204 80104 80105
9.8 12.7 12.7 12.7 15.0 15.0 15.3 15.3 15.3
2440±50 2200±50 2360±30 2400±90 2600±90 2700±150 2360±120 2680±200 2680±90
0.5 1.0 0.5 1.5 0.5 1.0 0.3 0.7 1.0
A B,g B,g B, b B, d B, d B,b B,d B,d
Q-fo S-fo S-fo Q-fo Q-fo Q-fo S-to Q-fo Q-fo
S-to; recrystallized forsterite in solid state. Q.fo; forsterite crystals formed from melt during quenching (quench crystal). A; furnace assembly.in Fig. Ia. B, g; furnace assembly in Fig. lb with a graphite capsule. B, b; furnace assembly in Fig. lb with a boron nitride capsule. B, d; furnace assembly in Fig. lb with diamond seaiing.
of the temperature scale at high pressure. For runs above 10.3 GPa using furnace B, fluctuation of the temperature reading amounted to ±100°C because of the failure to control the electric power and deterioration of the thermocouple owing perhaps to the absence of its sealing in furnace B. The ambiguity of the pressures at high temperature
amounted to ±0.3 GPa at 7 GPa and ±0.6 GPa at 14 GPa, because of errors of determination of the phase boundary curves used for the calibration points. Figure 4 shows the experimental results plotted against pressure together with the results of Davis’ and England (1964). When the melting curve de-
Fig. 2. Photomicrographs of the run products quenched at various pressures. Forsterite crystals quenched at 5 (iPa (Run 80506A) observed under the optical microscope (a) with open nicols and (b) with cross nicols. Recrystallized crystals without melting are observed on the right-hand side of the photographs and the quench crystals formed from melt are on the left-hand side. The difference in texture is clearly observed. (c) Forsterite quench crystal formed from melt at 10 GPa (Run 1105) under the optical microscope with cross nicols. (d) That quenched at 15 GPa (Run 80409) under the optical microscope with cross nicols.
termined in the present experiment is approximated to a straight line, we have the slope dT/dP = 41°C/GPa, which is slightly smaller than
and England (1964) to the Kraut—Kennedy equation, we have the parameter C = 3.0 with the use of the melting point Tm0 = 2163 K (1890°C)
47.7°C/GPa determined in the range below 4.6 GPa by Davis and England (1964). The slope of the melting curve usually decreases with pressure. Such behaviour is well expressed by some empirical equations such as the Simon and Kraut—Kennedy equations. The Kraut—Kennedy equation can be written as follows
(Bowen and Anderson, 1914) at atmospheric pressure, isothermal bulk modulus K0 = 125.4 GPa and its pressure derivative K’ 5.33 (Kumazawa and Anderson, 1969). The derived melting curve is shown in Fig. 4. Assuming that the congruent melting persists up to the limit of the stability field of forsterite,
T
Tm0~ (i +
c(v0
—
v’~/ v0, ‘~‘
the triple point among forsterite—$-Mg2SiO4 liquid will be located at about 2600°Cand 20 GPa
-~
—
where Tm and V are the melting point and volume at pressure F, and Tm0 and V0 are those at atmospheric pressure. Using the Birch— Managhan equation of state, we can evaluate (V0 V)/V0. Fitting the present results and the results of Davis —
by extrapolating the experimental data of the ct—$ transition of Mg2SiO4 (Suito, 1977). However, the possibility of the incongruent melting of forsterite cannot be ruled out. The melting data of enstatite up to 4.6 GPa
37
2000
--
--
_—~-Periclase ----+—-—-.-------.-~
21600
3000
----~----~L~
-
_,,cr.r~C7’
~tte~
~
0.
E
_..—‘—Enstotite
~2000
—----~———------4-~J
1200
2.0 mm:
,
I,
/
0—q3transition-_.J / ri MQ2SiOu,,/
ion
•
0 distance
/
-
‘S
2.0
1’
/
0
5
Pressure, 10 GPo
15
20
Fig. 5. Melting curves of periclase, forsterite and enstatite at
________________________________
______________________________
high pressures. The a—/~transition in Mg
2SiO4 reported by
II
Suito (1977) is also shown.
/~ ~~~‘:‘:
0
~“\EIectrode _______
\\_Heat
by the Birch—Managhan equation of state with
0
K0 1972) = 98.8 and GPa a fixed andpoint K’ =1650°Cat 9.47 (Frisillo 0.8 and GPa,Barsch, which
er
Fig. 3. Temperature gradient in furnace A at 10 UPa.
(Boyd et Kennedy pressure. using the
al., 1964) were fitted by the Kraut— equation and extrapolated to higher The fitting to the data was made by volume change (V0 V)/V0, calculated —
should
is a triple point among rhombic enstatite, protoenstatite and liquid. We obtained the parameters C = 5.5 and Tm0 = 1843 K for enstatite and the extrapolated melting curve is illustrated in Fig. 5. From the present extrapolation, we can say that the melting temperature of forsterite and enstatite are almost the.sâme above 10 GPa, and the eutectic composition of the forsterite— enstatite system shift toward the forsterite component with
2800
Tn(K) = 2163(1 + 3.0(
v,-v)/ V~) D
2600
~
Liquid
increasing pressure. The experimental data on the pressure dependence of the melting temperature of periclase has
0 0
o
not yet been reported its high temperature, 2800°C because (Kracek of and Clark,melting 1966) even at atmospheric pressure. The initial slope
-~
0
02200
• Forsterite
1800
• •
clase using dTm/dP = the I 10°C/GPa data of thewas fluoride estimated analogue for (Jackperison, 1976). The parameter in the Kraut—Kennedy
1
3073 0
2
4
6
8 Pressure.
10
12
14
o~ Fig. 4. Melting curve of forsterite up to IS GPa. Large squares are the results of the present experiment, and small circles are the results by Davis and England (1964). Open symbols mdicate melting, and solid symbols indicate recrystallization without melting. The symbol, x, is the melting point determined by the temperature gradient in the furnace. The ambiguities of the pressure and temperature are also shown in this figure.
equation, C= 5.75, can be calculated by T~= K (2800°C),dTm/dP = 1 lO°C/GPa,and K0 = 160.5 GPa (Spetzler, 1970). The melting curve of periclase shown in Fig. 5 was estimated by the Kraut— Kennedy equation with the volume change (V0 V)/ V0, evaluated by the Birch—Managhan —
equation of state with K0 = 160.5 GPa and K’ = 3.89 (Spetzler, 1970). The slope of the melting. curve of periclase dT/dP is expected to be twice
38
as large as that of forsterite and enstatite even at pressures above 10 GPa. The estimated melting curve of periclase is also shown in Fig. ~ This estimate of the melting curves of enstatite and periclase suggests that the incongruent melting of forsterite into periclase and liquid may occur at higher pressure, although there exists no evidence of the incongruent melting of ,forsterite in the present experiment. Therefore, it might be possible that periclase occurs as a liquidus phase and plays an important role in the fractionation by melting in the deep upper mantle.
Acknowledgements The authors thank H. Mizutarn, Y. Fukao and H. Sawamoto of Nagoya University for their useful discussions and advice during this work. They also thank S. Yogo and I. Hiraiwa of Nagoya University for their preparation of the petrOgraphic thin sections of the run products. This work was supported by a grant-in-aid from the Ministry of Education, Japan.
References Akimoto, S., Yagi, T. and Inoue, K., 1977. High temperature pressure phase boundaries in silicate systems using in situ X-ray diffraction. In: M.H. Manghnani and S. Akimoto (Editors), High-Pressure Research—Application to Geophysics. Academic Press, New York, NY, pp. 586—602. ASTM, 1974. Temperature—electromotive force (EMF) tables for tungsten—rhenium thermocouple systems. Annual Book of ASTM Standards, part 44, pp. 730—737. Bowen, N.L. and Anderson, 0., 1914. The binary system MgO—Si02. Am. J. Sci., 37: 487—500.
Boyd, F.R., England, J.L. and Davis, B.T.C., 1964. Effects of pressure on the melting and polymorphism of enstatite, MgSiO 3. J. Geophys. Res., 69: 2101—2109. Davis, B.T.C. and England, J.L., 1964. The melting of forsterote up to 50 kbar. J. Geophys. Res., 69: 1113—1116. Frisillo, A.L. and Barsch, G.R., 1972. Measurement of singlecrystal elastic constants of bronzite as a function of pressure and temperature. J. Geophys. Res., 77: 6360—6384. Jackson, I., 1976. Phase relations in the system LiF—MgF2 at elevated pressures: implications for the proposed mixedoxide zone of the Earth’s mantle. Phys. Earth Planet. Inter., 14: 86-94. Jeffery, R.N., Barnett, J.D., Vanfleet, H.B. and Hall, H.T., 1966. Pressure calibration to 100 kbar based on the compression of NaCl. J. Appl. Phys., 37: 3172—3180. Kracek, F.C. and Clark, S.P., Jr., 1966. Melting and transformation points in oxide and silicate systems at low pressure. In: S.P. Clark, Jr. (Editor), Handbook of Physical Constants, Geol. Soc. Am. Inc., New York, NY: pp. 301—344. Kumazawa, M. and Anderson, O.L., 1969. Elastic moduli, pressure derivatives, and temperature derivatives of singlecrystal olivine and single-crystal forsterite. J. Geophys. Res., 74: 5961—5972. Kumazawa, M., Masaki, K., Sawamoto, H. and Kato, M., 1972. Guide blocks and compressible pads for the practical operation of multiple anvil sliding system for the production of high pressure. High Temp. High Press., 4: 293-310. Ohtani, E., 1979. Melting relation of Fe2 Si04 up to about 200 kbar. J. Phys. Earth, 27: 189—208. Piermarini, G.J. and Block, S., 1975. Ultrahigh pressure diamond anvil cell and several semi-conductor phase transition pressures in relation to the fixed point pressure scale. Rev. Sci. Instrum., 46: 973—979. Spetzler, H., 1970. Equation of state of polycrystalilne and single crystal MgO to 8 kbar and 800 K. J. Geophys. Res., 75: 2073—2087. Suito, K., 1977. Phase relation of pure Mg2 SiC)4 up to 200 kbar. In: M.H. Manghnani and S. Akimoto (Editors), High Pressure Research—Applications to Geophysics. Academic Press, New York, NY, pp. 255—266. Takahashi, T., Mao, H.K. and Bassett, W.A., 1969. Lead: X-ray diffraction study of a high pressure polymorph. Science, 165: 1352—1353.
Physics of the Earth and Planetary Interiors, 27 (1981) 32—38 Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands
Melting of forsterite Mg2 Si04 up to 15 GPa Eiji Ohtani * and Mineo Kumazawa Department of Earth Sciences, Nagoya Uninersity, Nagoya 464 (Japan) (Received March 3, 1981; revision accepted April 15, 1981)
Ohtani, E. and Kumazawa, M., 1981. Melting of forsterite Mg2SiO4 up to 15 GPa. Phys. Earth Planet. Inter., 27: 32—38. The melting curve of forsterite has been studied by static experiment up to a pressure of IS GPa. Forsterite melts congruently at least up to 12.7 GPa. The congruent melting temperature is expressed by the Kraut— Kennedy equation in the following form: Tm(K) 2163 (1 + 3.0( V0 — V)/ V0), where the volume change with pressure was calculated by the Birch—Managhan equation of state with the isothermal bulk modulus K0 = 125.4 GPa and its pressure derivative K’ = 5.33. The triple point of forsterite—fl-Mg2SiO4 —liquid will be located at about 2600°Cand 20 GPa, assuming that congruent melting persists up to the limit of the stability field of forsterite. The extrapolation of the previous melting data on enstatite and periclase indicates that the eutectic composition of the forsterite—enstatite system should shift toward the forsterite component with increasing pressure, and there is a possibility of incongruent melting of forsterite into periclase and liquid at higher pressure, although no evidence on incongruent melting has been obtained in the present experiment.
1. Introduction Knowledge of the melting relations of silicate systems at high pressures is vital to understanding magma generation and chemical differentiation in the Earth’s mantle. Various experiments on the melting of silicates have been performed previously in order to determine the origin of various types of magma and the nature of low-V and low-Q regions in the upper mantle. Most of these experiments, however, are restricted to those with pressures corresponding to the crust and uppermost mantle, and we cannot discuss the melting phenomena in the deep mantie. The authors have developed the experimental technique of generating higher temperatures combined with higher pressures above 10 GPa to study *
Present address: Department of Earth Sciences, Ehime University, Matsuyama 790, Japan.
the melting relationships of silicate systems in the deep mantle conditions. We have applied the melting experiment on Fe2 Si04 with this technique (Ohtani, 1979). The next approach which we performed along these lines was to clarify the melting relationship of Mg2Si04, the major constituent of the Earth’s mantle. The previous data on Mg2SiO4 were limited to pressures below 5 GPa (Davis and England, 1964). The purpose of this paper is to present the results of the melting experiment on Mg2SiO4 made in the range of 5—15 GPa.
2. Experimental technique 2.1. Apparatus and pressure generation The apparatus used in the present experiment was an MA8-type high pressure apparatus (Ohtani, 1979) composed of eight truncated cube anvils. It was driven by a pair of RH3-type guide blocks
0031-9201/81 /0000-0000/$02.50 © 1981 Elsevier Scientific Publishing Company
33
(Kumazawa et al., 1972) in a uniaxial press. For this experiment the truncated edge of the anvil
Sem-sintered TiC electrode Semi—sintered MgO pressure medium
-
corner was 8.0 mm long at pressures below 10 GPa, and 3.5 mm long at higher pressures. Pyrophyllite gaskets were used for the lateral support of the eight anvils. Semi-sintered magnesia was selected for the octahedral pressure medium because magnesia has a high melting point and no phase transition at high pressure. The edge-length of octahedral pressure medium was 14 mm for anvils with a truncated-edge length of 8.0 mm, and 9.5 nim for anvils with a truncated-edge length of 3.5 mm. The pressure values below 1.0 GPa were corrected on the basis of the phase boundary curve of olivine— spinel transition in Fe2 Si04 P (kbar) = 34.6 + 0.025T (°C) and that of coesite—stishovite transition in Si02 J). (kbar) = 80 + 0.011T (°C) (Akimoto et al,, 1977) which were determined by in situ X-ray diffraction experiments on the basis of the NaCI scale at high temperature. The pressure values above 10 GPa generated by the use of anvils with a truncated edge 3.5 mm long were calibrated at room temperature using the transition pressures on Bi, I—Il, 2.48 GPa and III—V, 7.34 GPa (Jeffery et a!., 1966), Pb, 13.0 GPa (Takahashi et aL, 1969), and ZnS, 15.0 GPa (Piermarini and Block, 1975), and GaAs, 18.0 GPa (G.J. Piermarini and S. Block, personal communication, 1975) as the calibration points. The hightemperature corrections were made by the use of the phase boundary curve of pyroxene—ilmenite transition in ZnSiO3 determined by the lTighpressure X-ray diffraction method at high temperature (Akimoto et al., 1977). The calibration curves of the generated pressure versus press load on the present apparatus were shown in a previous paper (Ohtam, 1979). 2.2. Temperature generation and measurement Two furnace assemblies, illustrated in Fig. 1, were used for the present experiments. These furnaces are basically the same as those used for the melting experiment of Fe2SiO4 (Ohtani, 1979). The furnace shown in Fig. Ia was used for the runs at pressures below 10 GPa. The sample was directly packed on the cylindrical graphite heater placed at
LaCrO
3 heater
sample Thermocouple Ceramic
sleeve Graphite heater Capsule for sealing of Graphite electrode a b Fig. I. Furnace assemblies used in the present experiment: (a) furnace A for pressures below 10 GPa; and (b) furnace B for pressures above 10 GPa.
the centre of the pressure medium. A W.-W 26% Re thermocouple with a diameter of 0.1 mm was placed at the centre of the furnace through two holes drilled radially on the cylindrical heater. The thermocouple was separated from the furnace assembly by a ceramic (alumina) tube in order to avoid contamination from the furnace materials. Figure lb shows the furnace assembly used for the runs at pressures exceeding 10 GPa. In this pressure range, graphite is not used for the heating material and electric leads because the phase transformation of graphite to diamond hinders stable generation of temperatures above 2000°C Then we used semi-sintered lanthanum chromite, LaCrO3, as heating material and semi-sintered titanium carbide, TiC, as electric leads. A powdered sample was placed in a graphite or boron nitride capsule to avoid reaction of the sample and heater. Powdered diamond was also used for sealing material in runs made at the stability field of diamond. In this case, a pellet of the sintered forsterite, at 1500°C in a vacuum of iO—~Torr, was enclosed in the diamond powder to separate the sample from the tube heater. The sealing of the thermocouple by a ceramic tube was not made for this furnace because the ceramic tube reacts with the lanthanum chromite heater at high temperatures. The temperature value was based on the e.m.f.—~ temperature relation of a W—W 26% Re thermo-
34
couple reported in ASTM (1974) up to 2320°C and its linear extrapolation to higher temperatures. The run temperatures were corrected for the increase of the surface temperature of the anvils to which the thermocouple leads were connected. The correction of the effect of pressure on e.m.f. was not performed because the preliminary study on the pressure effect on the e.m.f. of the W—W/Re thermocouple (Ohtani, 1979) indicates that it is very small at pressures around 7 GPa. 2.3. Sample preparation, experimental procedure, and examination of the run products Forsterite used for the starting material was prepared by heating the stoichiometric mixture of reagents MgO and Si02 at 6 GPa to 1300°Cfor a few hours. The sample so prepared was ascertained, by means of X-ray diffraction and microscopic observation, to be a pure single-phase material, The furnace assembly was dried in an oven at about 150°Cfor a few hours immediately before each run in order to avoid the effect of absorbed water on the melting temperature. The quenching method was used to determine the melting curve. Pressure was applied to the sample and then the temperature was brought to the desired value and held constant. Heating duration at the desired temperature was limited to 0.1—5.0 mm in order to avoid contamination of the sample by the furnace material and the thermocouple. After that the sample was quenched isobarically. The run products close to or around the thermocouple were picked out carefully and the thin sections were prepared for examination by optical microscopy. The X-ray diffraction method was also used to identify the reaction products and to insure absence of contamination,
3. Experimental results and discussions The results of the present experiment on the melting of forsterite are summarized in Table I. Evidence of melting is recognized by microscopic observation of the textures of the run products.
Forsterite melt cannot be quenched to a glass but forms quench crystals on rapid cooling. The quench crystals show a fibrous texture, whereas the crystals grown slowly below the melting point have a granular texture. The difference between the textures is so noticeable that it is easy to detect whether the sample has melted or not. In several runs, both quench crystals and the recrystallized crystals coexisted in the same run products (see Fig. 2a, b). This is caused by the presence of the temperature gradient in the furnace. Then a part of the run product that is close to or around the thermocouple is picked out carefully for observation of the texture. Figure 2a— d shows the photomicrographs of the textures of the run products at various pressures. X-ray observation of these products indicated that these quench crystals are forsterite. We have confirmed the melting of forsterite up to 15 GPa, and could not detect evidence of incongruent melting of forsterite in the present experiment. However, we cannot rule out the possibility that forsterite melts incongruently and that crystal and melt coexist in the narrow temperature range around 15 GPa, as is observed in MgSiO3 below 0.6 GPa (Boyd et a!., 1964), because the run temperatures made at 15 GPa were about 200°Chigher than the expected melting temperature of forsterite. At the present stage of our experiment we can safely conclude that forsterite melts congruently at least up to 12.7 GPa. For a run made at 5 GPa and two runs at 10 GPa, the melting point was evaluated by utilizing the temperature gradient in the furnace. The ternperature gradient in furnace A is illustrated in Fig. 3. The temperature difference between the central and end parts of the furnace amounted to about 100°C. The temperature of the textural boundary between the crystals recrystallized without melting and those quenched from melt is 2090°C for a run made at 5 GPa (Run 80506A), and 2308°C(Run 1102) and 2320°C(Run 1105) for runs at 10 GPa. Temperature fluctuation during a run performed below 10.3 GPa was within ±30°C in most runs. However, the uncertainty of the melting temperature determined in this pressure range will amount to ±50°Cbecause of the ambiguity
35 TABLE I Experimental results on the melting of forsterite Run No.
Pressure (GPa)
Temperature (°C)
Time
Furnace
Results
(mm)
506B 301 80601B 8060lA 120 80506A
4.7 4.7 4.7 4.7 4.7 4.7
1950±10 2030±5 2075±15 2100±10 2140±20 2155±10 2090
1.0 A S-fo 1.5 A S-fo 1.0 A S-fo 1.0 A Q-fo 5.0 A Q-fo 1.0 A Q-fo Temperature at the boundary between S-fo and Q-fo
122 608 607B 607A 80502 507A 1005 1004 1007 1009 1104B 1011 1104A 1105
4.7 6.2 7.4 7.4 7.7 7.7 8.4 8.4 8.4 9.8 10.0 9.8 10.0 10.0
2200±50 2185±10 2145±30 2510±60 2185±30 2360±90 2130±30 2230±10 2485 ±65 1965±30 2010±10 2140±20 2245±10 2340±20 2320
2.5 A Q-fo 2.0 A Q-fo 2.0 A S-fo 2.0 A Q-fo 1.0 A S-to 1.0 A Q-fo 2.0 B, g S-fo 0.5 B, g S-fo 0.3 B, g Q-fo 2.5 B,g S-fo 0.5 A S-to 1.0 B,g S-to 0.5 A S-to 0.1 A Q-fo Temperature at the boundaiy between S-to and Q-fo
1102
9.9
2355±10 2308
0.1 A Q-fo Temperature at the boundary between S-fo and Q.fo
1103 80101 80102 80207 80404 80409 80204 80104 80105
9.8 12.7 12.7 12.7 15.0 15.0 15.3 15.3 15.3
2440±50 2200±50 2360±30 2400±90 2600±90 2700±150 2360±120 2680±200 2680±90
0.5 1.0 0.5 1.5 0.5 1.0 0.3 0.7 1.0
A B,g B,g B, b B, d B, d B,b B,d B,d
Q-fo S-fo S-fo Q-fo Q-fo Q-fo S-to Q-fo Q-fo
S-to; recrystallized forsterite in solid state. Q.fo; forsterite crystals formed from melt during quenching (quench crystal). A; furnace assembly.in Fig. Ia. B, g; furnace assembly in Fig. lb with a graphite capsule. B, b; furnace assembly in Fig. lb with a boron nitride capsule. B, d; furnace assembly in Fig. lb with diamond seaiing.
of the temperature scale at high pressure. For runs above 10.3 GPa using furnace B, fluctuation of the temperature reading amounted to ±100°C because of the failure to control the electric power and deterioration of the thermocouple owing perhaps to the absence of its sealing in furnace B. The ambiguity of the pressures at high temperature
amounted to ±0.3 GPa at 7 GPa and ±0.6 GPa at 14 GPa, because of errors of determination of the phase boundary curves used for the calibration points. Figure 4 shows the experimental results plotted against pressure together with the results of Davis’ and England (1964). When the melting curve de-
Fig. 2. Photomicrographs of the run products quenched at various pressures. Forsterite crystals quenched at 5 (iPa (Run 80506A) observed under the optical microscope (a) with open nicols and (b) with cross nicols. Recrystallized crystals without melting are observed on the right-hand side of the photographs and the quench crystals formed from melt are on the left-hand side. The difference in texture is clearly observed. (c) Forsterite quench crystal formed from melt at 10 GPa (Run 1105) under the optical microscope with cross nicols. (d) That quenched at 15 GPa (Run 80409) under the optical microscope with cross nicols.
termined in the present experiment is approximated to a straight line, we have the slope dT/dP = 41°C/GPa, which is slightly smaller than
and England (1964) to the Kraut—Kennedy equation, we have the parameter C = 3.0 with the use of the melting point Tm0 = 2163 K (1890°C)
47.7°C/GPa determined in the range below 4.6 GPa by Davis and England (1964). The slope of the melting curve usually decreases with pressure. Such behaviour is well expressed by some empirical equations such as the Simon and Kraut—Kennedy equations. The Kraut—Kennedy equation can be written as follows
(Bowen and Anderson, 1914) at atmospheric pressure, isothermal bulk modulus K0 = 125.4 GPa and its pressure derivative K’ 5.33 (Kumazawa and Anderson, 1969). The derived melting curve is shown in Fig. 4. Assuming that the congruent melting persists up to the limit of the stability field of forsterite,
T
Tm0~ (i +
c(v0
—
v’~/ v0, ‘~‘
the triple point among forsterite—$-Mg2SiO4 liquid will be located at about 2600°Cand 20 GPa
-~
—
where Tm and V are the melting point and volume at pressure F, and Tm0 and V0 are those at atmospheric pressure. Using the Birch— Managhan equation of state, we can evaluate (V0 V)/V0. Fitting the present results and the results of Davis —
by extrapolating the experimental data of the ct—$ transition of Mg2SiO4 (Suito, 1977). However, the possibility of the incongruent melting of forsterite cannot be ruled out. The melting data of enstatite up to 4.6 GPa
37
2000
--
--
_—~-Periclase ----+—-—-.-------.-~
21600
3000
----~----~L~
-
_,,cr.r~C7’
~tte~
~
0.
E
_..—‘—Enstotite
~2000
—----~———------4-~J
1200
2.0 mm:
,
I,
/
0—q3transition-_.J / ri MQ2SiOu,,/
ion
•
0 distance
/
-
‘S
2.0
1’
/
0
5
Pressure, 10 GPo
15
20
Fig. 5. Melting curves of periclase, forsterite and enstatite at
________________________________
______________________________
high pressures. The a—/~transition in Mg
2SiO4 reported by
II
Suito (1977) is also shown.
/~ ~~~‘:‘:
0
~“\EIectrode _______
\\_Heat
by the Birch—Managhan equation of state with
0
K0 1972) = 98.8 and GPa a fixed andpoint K’ =1650°Cat 9.47 (Frisillo 0.8 and GPa,Barsch, which
er
Fig. 3. Temperature gradient in furnace A at 10 UPa.
(Boyd et Kennedy pressure. using the
al., 1964) were fitted by the Kraut— equation and extrapolated to higher The fitting to the data was made by volume change (V0 V)/V0, calculated —
should
is a triple point among rhombic enstatite, protoenstatite and liquid. We obtained the parameters C = 5.5 and Tm0 = 1843 K for enstatite and the extrapolated melting curve is illustrated in Fig. 5. From the present extrapolation, we can say that the melting temperature of forsterite and enstatite are almost the.sâme above 10 GPa, and the eutectic composition of the forsterite— enstatite system shift toward the forsterite component with
2800
Tn(K) = 2163(1 + 3.0(
v,-v)/ V~) D
2600
~
Liquid
increasing pressure. The experimental data on the pressure dependence of the melting temperature of periclase has
0 0
o
not yet been reported its high temperature, 2800°C because (Kracek of and Clark,melting 1966) even at atmospheric pressure. The initial slope
-~
0
02200
• Forsterite
1800
• •
clase using dTm/dP = the I 10°C/GPa data of thewas fluoride estimated analogue for (Jackperison, 1976). The parameter in the Kraut—Kennedy
1
3073 0
2
4
6
8 Pressure.
10
12
14
o~ Fig. 4. Melting curve of forsterite up to IS GPa. Large squares are the results of the present experiment, and small circles are the results by Davis and England (1964). Open symbols mdicate melting, and solid symbols indicate recrystallization without melting. The symbol, x, is the melting point determined by the temperature gradient in the furnace. The ambiguities of the pressure and temperature are also shown in this figure.
equation, C= 5.75, can be calculated by T~= K (2800°C),dTm/dP = 1 lO°C/GPa,and K0 = 160.5 GPa (Spetzler, 1970). The melting curve of periclase shown in Fig. 5 was estimated by the Kraut— Kennedy equation with the volume change (V0 V)/ V0, evaluated by the Birch—Managhan —
equation of state with K0 = 160.5 GPa and K’ = 3.89 (Spetzler, 1970). The slope of the melting. curve of periclase dT/dP is expected to be twice
38
as large as that of forsterite and enstatite even at pressures above 10 GPa. The estimated melting curve of periclase is also shown in Fig. ~ This estimate of the melting curves of enstatite and periclase suggests that the incongruent melting of forsterite into periclase and liquid may occur at higher pressure, although there exists no evidence of the incongruent melting of ,forsterite in the present experiment. Therefore, it might be possible that periclase occurs as a liquidus phase and plays an important role in the fractionation by melting in the deep upper mantle.
Acknowledgements The authors thank H. Mizutarn, Y. Fukao and H. Sawamoto of Nagoya University for their useful discussions and advice during this work. They also thank S. Yogo and I. Hiraiwa of Nagoya University for their preparation of the petrOgraphic thin sections of the run products. This work was supported by a grant-in-aid from the Ministry of Education, Japan.
References Akimoto, S., Yagi, T. and Inoue, K., 1977. High temperature pressure phase boundaries in silicate systems using in situ X-ray diffraction. In: M.H. Manghnani and S. Akimoto (Editors), High-Pressure Research—Application to Geophysics. Academic Press, New York, NY, pp. 586—602. ASTM, 1974. Temperature—electromotive force (EMF) tables for tungsten—rhenium thermocouple systems. Annual Book of ASTM Standards, part 44, pp. 730—737. Bowen, N.L. and Anderson, 0., 1914. The binary system MgO—Si02. Am. J. Sci., 37: 487—500.
Boyd, F.R., England, J.L. and Davis, B.T.C., 1964. Effects of pressure on the melting and polymorphism of enstatite, MgSiO 3. J. Geophys. Res., 69: 2101—2109. Davis, B.T.C. and England, J.L., 1964. The melting of forsterote up to 50 kbar. J. Geophys. Res., 69: 1113—1116. Frisillo, A.L. and Barsch, G.R., 1972. Measurement of singlecrystal elastic constants of bronzite as a function of pressure and temperature. J. Geophys. Res., 77: 6360—6384. Jackson, I., 1976. Phase relations in the system LiF—MgF2 at elevated pressures: implications for the proposed mixedoxide zone of the Earth’s mantle. Phys. Earth Planet. Inter., 14: 86-94. Jeffery, R.N., Barnett, J.D., Vanfleet, H.B. and Hall, H.T., 1966. Pressure calibration to 100 kbar based on the compression of NaCl. J. Appl. Phys., 37: 3172—3180. Kracek, F.C. and Clark, S.P., Jr., 1966. Melting and transformation points in oxide and silicate systems at low pressure. In: S.P. Clark, Jr. (Editor), Handbook of Physical Constants, Geol. Soc. Am. Inc., New York, NY: pp. 301—344. Kumazawa, M. and Anderson, O.L., 1969. Elastic moduli, pressure derivatives, and temperature derivatives of singlecrystal olivine and single-crystal forsterite. J. Geophys. Res., 74: 5961—5972. Kumazawa, M., Masaki, K., Sawamoto, H. and Kato, M., 1972. Guide blocks and compressible pads for the practical operation of multiple anvil sliding system for the production of high pressure. High Temp. High Press., 4: 293-310. Ohtani, E., 1979. Melting relation of Fe2 Si04 up to about 200 kbar. J. Phys. Earth, 27: 189—208. Piermarini, G.J. and Block, S., 1975. Ultrahigh pressure diamond anvil cell and several semi-conductor phase transition pressures in relation to the fixed point pressure scale. Rev. Sci. Instrum., 46: 973—979. Spetzler, H., 1970. Equation of state of polycrystalilne and single crystal MgO to 8 kbar and 800 K. J. Geophys. Res., 75: 2073—2087. Suito, K., 1977. Phase relation of pure Mg2 SiC)4 up to 200 kbar. In: M.H. Manghnani and S. Akimoto (Editors), High Pressure Research—Applications to Geophysics. Academic Press, New York, NY, pp. 255—266. Takahashi, T., Mao, H.K. and Bassett, W.A., 1969. Lead: X-ray diffraction study of a high pressure polymorph. Science, 165: 1352—1353.