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Constitution of the mantle. 3. Density, elastic properties and the mineralogy of the 400 km discontinuity
Physics of the Earth and Planetary Interiors, 69 (1991) 85-100 Elsevier Science Publishers B.V., Amsterdam
85
Constitution of the mantle. 3. Density, elastic properties and the mineralogy of the 400 km discontinuity O.L. Kuskov and A.B. Panferov V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry, USSR Academy of Sciences, Kosygin str. 19, 117334 Moscow, USSR (Received 7 March 1989; revised and accepted 20 November 1990)
ABSTRACT Kuskov, O.L. and Panferov, A.B., 1991. Constitution of the mantle. 3. Density, elastic properties and the mineralogy of the 400 km discontinuity. Phys. Earth. Planet. Inter., 69: 85-100. On the basis of phase diagrams of the FeO-MgO-SiO 2 system and equations of state for minerals we have calculated internally consistent thermodynamic profiles of adiabatic bulk modulus, seismic parameter and density for a series of petrological models. It is shown that the sequences of phase assemblage changes across the 400 km discontinuity and the thickness of divariant transitions depend strongly on the iron content, the Ol/Px ratio and the temperature at the transition depth. Calculations show that the iron-rich model is inadequate to describe the seismic properties in the uppermost mantle and through the 400 km discontinuity. The distribution of physical properties in the iron-rich model at the depths of 360-400 km is of a gradient type which does not agree with seismic evidence; an important constraint on the upper-mantle chemistry is an Fe/(Fe + Mg) ratio of less than 0.17. Comparisons of calculated thermodynamic properties for various temperature and bulk composition models indicate that thermodynamic profiles of the olivine and pyroxene models do not match seismic observations through the 400 km discontinuity. Hence, the SiO 2 c o n t e n t in the isochemical upper mantle should be in excess of 36.4 mol.% and less than 44.4 tool.%. From all the petrological models considered, only a pyrolite model (SiO 2 = 40 mol.%, Fe/(Fe + Mg) = 0.12, O1/Px = 1) may satisfy available seismological constraints. The results obtained indicate that seismic properties across the 400 km discontinuity might be consistent with an isochemical pyrolite composition and do not require chemical stratification. The pyrolite models considered show that the phase transitions occur over a range of 8-15 km at a depth near 400 km. It is proposed that in the different tectonic regions of the Earth a 400 km discontinuity might be considered as a combination of first- and second-order discontinuities with a phase transition sequence a + Px --, a + "t + Px --, a + fl + Px --, fl + Px within a thickness range of around 15 km (low-temperature pyrolite model) or as pseudo-first-order discontinuity through a narrow divariant loop a + fl + Px about 8 km wide (high-temperature pyrolite model).
1. Introduction P h a s e t r a n s f o r m a t i o n s in m i n e r a l s y s t e m s at h i g h p r e s s u r e p r o v i d e a n e x c e l l e n t q u a l i t a t i v e exp l a n a t i o n o f the p r i n c i p a l f e a t u r e s o f t h e d i s t r i b u t i o n of elastic p r o p e r t i e s a n d d e n s i t y in the m a n t l e ( R i n g w o o d , 1975; A k a o g i a n d A k i m o t o , 1979; Liu, 1979; I t o a n d Y a m a d a , 1982; J e a n l o z a n d T h o m p son, 1983; K u s k o v a n d G a l i m z y a n o v , 1986; B i n a a n d W o o d , 1987; I r i f u n e a n d R i n g w o o d , 1987; I t o a n d T a k a h a s h i , 1987; K u s k o v et al., 1989a). 0031-9201/91/$03.50 © 1991
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We now examine whether the inferred phase transformations are capable of providing a q u a n t i t a t i v e e x p l a n a t i o n o f t h e m a g n i t u d e s o f the c h a n g e s in p h y s i c a l p r o p e r t i e s a s s o c i a t e d w i t h seismic discontinuities. Mineralogically, the transformation occurring at t h e s e i s m i c b o u n d a r y c a n b e d e f i n e d o n t h e b a s i s o f t h r e e c r i t e r i a s p e c i f i e d as f o l l o w s : (1) t h e p r e s s u r e at t h e b o u n d a r y d e p t h c o i n c i d e s w i t h t h a t o f t h e p h a s e t r a n s i t i o n ; (2) t h e s y n t h e t i c p r o f i l e s of physical properties descriptive of the mineral
Elsevier Science Publishers B.V. All rights reserved
86
paragenesis and their jumps at phase transitions are consistent with seismic evidence; (3) the thickness of seismic boundaries at 400 km and 650 km depths agrees well with the width of the interphase zone of phase transformations. In the first two papers of this series (Fabrichnaya and Kuskov, 1991; Kuskov et al., 1991) P - T - x diagrams calculated over a wide range of pressures, temperatures and compositions for the FeO-MgO-SiO 2 (FMS) system, which is fundamental in the petrology of the mantle, served as a basis for constructing petrological models of the mantle with a fixed bulk composition x: (P-T)~,. diagrams. The purpose of the present paper is the construction of internally consistent profiles of the elastic properties and density at depths of 300-400 km and through the discontinuity for different petrological models derived from the FMS system, covering all the compositions. Given the bulk Fe/(Fe + Mg) ratio and SiO2 concentrations, the amount and composition of the phase assemblages in mantle P - T conditions are determined from the phase diagram. Thus for different initial compositions, the F e / M g partitioning between the various phases, their amounts and compositions, equations of state of mineral constituents and thermodynamic profiles of elastic properties and density for a chosen petrological model, as well as through phase transitions, are internally consistent. It appears that the problems of modelling mineral and chemical composition and retrieving profiles of the physical properties of the planet's substance are intimately related. Solving them simultaneously results in the construction of a qualitatively new model: a petrological-geophysical model of the mantle. By means of petrological models inferred from phase diagrams of the system and equations of state of minerals and their mixtures, we can compute thermodynamic ('synthetic') profiles of elastic properties and density pertinent to various depths of the mantle and compare them with the seismological models available. The comparison of thermodynamic profiles of elastic properties with seismic profiles allows a petrological interpretation of the latter, with the anomalies in their behaviour shown to be associ-
O.L. K U S K O V A N D A.B. P A N F E R O V
ated with concrete phase (without compositional changes) or chemical (with compositional changes) boundaries. From the petrological-geophysical point of view this appears to be the only possible source of data on the bulk chemical composition of the Earth's interior, as regards the most widespread rock-forming oxides. In attempts to constrain mantle composition, Lees et al. (1983), Bass and Anderson (1984), Anderson and Bass (1986), Weidner (1986), Akaogi et al. (1987), Irifune (1987) and Duffy and Anderson (1989) have compared calculated velocities and densities with seismological data, taking into account more complex, but internally inconsistent phase diagrams of mineral systems. As there is no complete (experimental or theoretical) information on the elements' partitioning and full topologies of phase diagrams of the CFMAS system at various bulk compositions we do not include majorite-garnet in our calculations in this paper. Addition of majorite-garnet to the self-consistent FMS system will mean that the approach is not strictly thermodynamic and the internal consistency of the results will be violated. For this reason, all conclusions about the constitution of the mantle should be referred only to the mineralogy of the FMS system. Nevertheless, comparison of the thermodynamic profiles with seismological data allows interpretation of the uppermost mantle structure in terms of petrological models and refinement of seismological models in the vicinity of the 400 km discontinuity. 2. Profiles of the elastic properties and density in the Earth's mantle on the basis of phase relations in the F e O - M g O - S i O 2 system
The profiles of the physical properties have been computed from phase relations in the FeOMgO-SiO 2 system, which is presented in Fig. 1 as ( P - T ) x sections calculated after the technique discussed by Kuskov (1987), Kuskov and Fabrichnaya (1990) and Fabrichnaya and Kuskov (1991). Thermodynamic profiles were calculated for petrological models of the mantle of four different bulk compositions: (1) pyrolite (Py)--Xsio2 = 0.4, Fe/(Fe + Mg) = 0.12 (molar ratio O1/Px = 1);
87
C O N S T I T U T I O N OF T H E MANTLE. 3.
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Fig. 1. (P-T)x sections of the phase diagram for the FeO-MgO-SiO 2 system at various bulk compositions with low-temperature (LT) and high-temperature (HT) geotherms. The heavy lines represent univariant reactions; the stippled regions designate zones of divariant mineral assemblages. Olivine model composition: Xsio2= 0.364, F e / ( F e + M g ) = 0.12. Pyrolite model composition: Xsio2 = 0.4, Fe/(Fe + Mg) = 0.12. Iron-rich model composition: Xsio: = 0.4, Fe/(Fe+ Mg) = 0.17.
(2) olivine (O1)--Xsio2 = 0.364, F e / ( F e + Mg) = 0.12 (as compared with pyrolite, it is enriched with an olivine component, O l / P x = 3); (3) iron-rich (Fe)--Xsio2 = 0.4, F e / ( F e + Mg) = 0.17 (with the molar ratio O1/Px = 1 identical to that of pyrolite composition, but higher in iron);
= 1.07 K k m - l ) , originating from To= 1600 K (low-temperature m o d e l - - L T ) and TO= 1800 K (high-temperature m o d e l - - H T ) at a depth of 360 km (about 12 GPa). The elucidation of the temperature profiles in the mantle and of the role of the univariant and divariant phase transformations as the sources of h e a t i n f o r m i n g the t e m p e r -
(4) p y r o x e n e m o d e l ( P x ) - - g s i o 2 = 0.444, F e / ( F e + M g ) = 0.12 ( e n r i c h e d i n silica, w i t h the m o l a r ratio O 1 / P x --- 1 / 3 ) . T h e t e m p e r a t u r e d i s t r i b u t i o n i n the m a n t l e was m o d e l l e d u s i n g two g e o t h e r m s w i t h the s a m e geot h e r m a l g r a d i e n t d T / d P = 30 K G P a -1 ( d T / d H
a t u r e d i s t r i b u t i o n i n the m a n t l e h a s b e e n c a r r i e d o u t i n d e t a i l b y T r u s k i n o v s k y et al. (1983, 1985). T h e c h o s e n g e o t h e r m a l g r a d i e n t agrees i n g e n e r a l w i t h the e s t i m a t e d o n e of Lees et al. (1983), I r i f u n e (1987) a n d I t o a n d K a t s u r a (1988). F o r l a c k of r e l i a b l e e x p e r i m e n t a l d a t a o n shear
TABLE 1 Parameters of equations of state for minerals at 0.1 MPa and 298.15 K Mineral a-Mg2SiO 4 fl-Mg2SiO4
y-MgESiO4 MgSiO 3 enstatite SiO2 stishovite a-Fe2SiO4 fl-Fe2SiO4 7-Fe2SiO4 FeSiO3 ferrosilite
p
Ks
(gcm -3)
(GPa)
3.213 (0.001) 3.474 (0.010) 3.559 (0.010) 3.198 (0.005) 4.289 (0.005) 4.393 (0.001) 4.715 (0.010) 4.848 (0.005) 3.980 (0.005)
128.8 (1) 174 (6) 184 (5) 108 (3) 316 (8) 138 (1) 178 (6) 193 (4) 101 (4)
dKs/dP
a x 106 (K -1)
Cp (J mo1-1 K -1)
0 (K -I )
5.1 (0.2) 4.3 (0.5) 4.0 (0.5) 5.0 (0.5) 4.0 (1.0) 5.0 (0.5) 4.4 (1.0) 4.0 (1.0) 5.0 (0.5)
26.0 (1) 20.6 (1) 19.0 (1) 28.0 (3) 16.5 (1) 26.0 (1) 22.0 (2) 21.3 (1) 20.4 (5)
118.11 (0.16) 110.12 (1.00) 106.94 (1.00) 82.22 (0.08) 42.97 (0.10) 131.96 (0.20) 130.50 (1.30) 130.54 (1.30) 90.58 (1.70)
763 (5) 893 (25) 902 (25) 734 (30) 1190 (25) 512 (10) 609 (25) 609 (15) 540 (25)
Parameters of the equations of state for stable and virtual minerals are taken from Kuskov et al. (1989a), Kuskov and Fabrichnaya (1990) and Fabrichnaya and Kuskov (1991).
88
O . L K U S K O V A N D A.B. P A N F E R O V
moduli at ultrahigh pressure and temperature for basic minerals integrating the mantle, the comparison with seismological data was effected using three thermodynamic parameters, namely, density (~), adiabatic bulk modulus (Ks) and seismic parameter ( 4 ) s = K s / # = Vr~-4/3Vs~), inferred from the equations of state of minerals. These equations, constructed by the potential method
(Kuskov and Galimzyanov, 1986), rely on the supposition that the mantle substance is a mechanical mixture of isotropic phases. For each P-T condition the coexisting phases were mixed to form a petrological model. The Hill average of the Voigt and Reuss bounds was used for bulk moduli (Watt et al., 1976). Kuskov and Galimzyanov (1986) have carried out a detailed error analysis of
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Fig. 2. Thermodynamic profiles of density and seismic parameter calculated on the basis of a phase diagram of the F e O - M g O - S i O 2 system (see Fig. 1 and text) along low-temperature (LT) and high-temperature (HT) geotherms for the olivine (O1), pyrolite (Py), pyroxene (Px) and iron-rich (Fe) models. Seismic profiles for the ICO, ACY, G C A + T N A and PREM models are shown for comparison.
CONSTITUTION OF THE MANTLE. 3.
89
the thermal equation of state and have shown that the typical uncertainties of p, K s and ~s values for the mantle minerals existing at pressures of 10-30 GPa and temperatures ranging from 1500 to 2000 K are 0.2-0.4%, 2-4% and 2-4.5% respectively. The parameters of the equations of state for minerals used in the calculation procedure are listed in Table 1 and were cited by Kuskov et al. (1989a) and Fabrichnaya and Kuskov (1991). In the comparison of thermodynamic profiles pertinent to the various petrological models, the following seismological models were used: PREM (Dziewonski and Anderson, 1981), constrained reference mantle models ICO and ACY (Montagner and Anderson, 1989), GCA (Walck, 1984) and TNA (Grand and Helmberger, 1984). The GCA and TNA velocity models are for tectonic regions in North and Central America and are similar in general in the mantle region deeper than 300 km. Both models were used as reference by Weidner (1986), Bina and Wood (1986) and Akaogi et al. (1987).
3. Uppermost mantle at 300-400 kin depth In all four petrological models of the FMS system, the uppermost mantle substance consists of an assemblage composed of olivine (a) and pyroxene (Px), with olivine being rather high in iron and pyroxene being low. With increasing temperature and depth this difference in iron content slightly decreases, although the Fe/(Fe + Mg) ratio in coexisting phases is practically independent of P and T in the range 7-13 GPa and 1450-1650 K. The results of calculations are summarized in Fig. 2 and Tables 2 and 3, which compare the calculated thermodynamic profiles with those from the seismological models.
3.1. Pyrolite model The values of density, adiabatic bulk modulus and seismic parameter calculated for a lay model at depths of 300-400 km are lower than analogous
TABLE 2 Physical property values for the various petrological and seismological models at depths of 305-430 km H
P
Ta
Models (SiO2, Fe/(Fe + Mg))
(km)
(GPa)
(K)
PREM
O1-LT (36.4, 0.12)
P (g cm -3)
Ks (GPa)
q~s (kin2 s -2)
P (gcm -3)
Ks (GPa)
tbs (km2 S-2)
P (gcm -3)
Ks" (GPa)
tbs (kin2 s -2)
162.4 168.2 169.3 171.1 198.1
46.59 47.83 48.08 48.46 52.66
3.46 3.49 3.50 3.51 3.73
159.4 167.1 168.6 171.1 206.2
46.08 47.87 48.17 48.75 55.28
3.50 3.54 3.55 3.59 3.77
154.1 162.1 163.5 169.6 195.4
44.03 45.78 46.07 47.23 51.83
159.8 166.3 167.6 169.6 200.9
45.53 46.86 47.13 47.58 53.74
3.45 3.48 3.49 3.50 3.68
44.70 46.47 46.77 47.35 52.40
3.44 3.48 3.48 3.50 3.62
305 355 365 380 430
10.0 11.8 12,1 12.7 14.5
1540 1594 1603 1621 1675
3.486 3.516 3.522 3.531 3.762
305 355 365 380 430
10.0 11.8 12.1 12.7 14.5
1540 1594 1603 1621 1675
3.510 3.550 3.557 3.565 3.738
ICO
Py-LT (40, 0.12)
ACY 305 355 365 380 430
10.0 11.8 12.1 12.7 14.5
1740 1794 1703 1821 1875
Fe-LT (40, 0.17)
3.510 3.550 3.558 3.566 3.738
154.1 161.9 163.2 165.8 192.9
Px-LT (44.4, 0.12)
Py-HT (40, 0.12) 155.0 158.0 160.7 162.1 197.9
44.17 44.50 45.17 45.47 52.93
a Temperature is referred to only the petrological models.
3.43 3.46 3.47 3.48 3.64
151.7 159.5 160.8 163.4 189.3
148.2 156.1 157.1 160.0 178.4
43.09 44.85 45.14 45.72 49.27
Px-HT (44.4, 0.12) 44.49 46.08 46.37 46.96 51.94
3.42 3.46 3.46 3.47 3.58
146.0 153.8 154.9 157.3 174.5
42.69 44.46 44.76 45.33 48.73
400 400 400 390
392-407 417-425 392-413 373-407 392-419 382-407 413-430 400 400
PREM ICO ACY GCA + TNA
Py-LT Py-HT OI-LT Fe-LT Fe-HT Px-LT Px-HT Piclogite Pyrolite
13.1-13.7 14.0-14.3 13.1-13.9 12.4-13.7 13.1-14.1 12.8-13.7 13.9-14.5 13.4 13.4
13.4 13.4 13.4 13.0 1645 1860 1648 1644 1845 1635 1860 -
-
Tar (K)
P
(GPa)
3.51 3.50 3.51 3.56 3.54 3.50 3.50 3.65 3.54
3.54 3.58 3.58 -
(g cm -3)
p
Ap
0.14 0.14 0.20 0.18 0.16 0.09 0.08 0.05 0.18
0.18 0.14 0.14 -
(g cm -3)
P, Ks, Cs--parameters at the upper boundary. Ap, AKs, Atbs--changes in physical properties through the discontinuity. Piclogite and pyrolite models are after Bass and Anderson (1984).
Depth (km)
Model
Ks 174.1 172.4 164.2 167.1 168.5 172.3 165.1 165.3 159.8 162.1 169.0 164.9
4.0 4.0 5.7 5.1 4.5 2.6 2.3 1.4 5.1
(GPa)
5.1 3.8 3.8
(%)
Ap/p
21.0 19.5 30.9 25.6 23.0 14.0 11.5 18.6 43.8
16.5 21.2 25.7
AKs (GPa)
12.5 11.6 17.9 15.5 13.9 8.8 7.1 11.0 26.6
9.5 12.3 15.6
(%)
AKs/Ks
Changes in physical properties for various petrological and seismological models across the 400 km discontinuity
TABLE 3
47.30 47.64 48.11 49.04 46.43 46.63 45.72 46.38 46.30 46.59
49.05 48.20 45.89
Os (km 2 s - 2 )
4.85 3.89 3.57 5.70 4.60 4.18 2.65 2.18 4.40 9.50
1.95 3.91 5.24
AOS (km 2 s - z )
10.3 8.2 7.4 11.6 9.8 9.0 5.8 4.7 9.5 20.4
4.1 8.1 11.4
AOS/$S (%)
O <
O < > 7,
F"
~D
91
C O N S T I T U T I O N O F T H E M A N T L E . 3.
values used in the PREM model, but with increasing depth this discrepancy gradually decreases. In contrast to the PREM model, the seismic parameter of the G C A + T N A regional model at the P - T values of the uppermost mantle is on average 0.9 km 2 s -2 (2.0%) lower than the thermodynamic values for the LT pyrolite model. A temperature increase of 200 ° C with respect to the LT pyrolite model, given the same bulk composition, results in reduced values of p, K s and ~s, which lead to a still greater difference, as compared with the LT model, of thermodynamic values O, Ks and Cs from analogous values of the PREM model. At the same time, a 200 ° C increase in temperature means closer thermodynamic and regional GCA + T N A values of the Os seismic parameter, although thermodynamic values are still 0.4 km 2 s -z (0.9%) higher than seismic ones (Fig. 2, Table 2).
3.2. Olivine model At a depth of 300-400 km the O1 model density is estimated to be as little as 0.008 g cm -3 (0.2%) in excess of the density of the basic LT pyrolite model, which is associated with the forsterite and enstatite densities being rather close. Profiles of the adiabatic b u l k modulus and the seismic parameter appear to be more sensitive to changes in the proportion of olivine and pyroxene. A change in the O1/Px molar proportion from one (a pyrolite model) to three (an olivine model) results in higher K s and ~s values in the uppermost mantle, the increase being of 6.0 GPa (4%) and 1.5 km 2 s -2 (3%), respectively. The thermodynamic profile of the seismic parameter for the O1-LT model shows a perfect agreement with the values of the seismic parameter of the P R E M model (Fig. 2).
3.3. Pyroxene model A phase diagram for the Px model is not shown in Fig. 1 because it is identical to the olivine and pyrohte compositions. The seismic parameter profile for the equilibrium assemblage of O1 + Px (0.25(Fe0.14Mg0.86)2SiO 4 + 0.75(Fe0nlMg0.89)SiO3) in the range of 300-400 km for both the L T and
H T pyroxene models compares reasonably well with the ACY and G C A + T N A models but is in strong contradiction to the PREM and ICO models. In contrast, the calculated density profiles along any plausible geotherm are not consistent with the geophysical data inferred from the PREM, ICO and ACY models.
3.4. Iron-rich model Variations in the F e / ( F e + Mg) ratio appear to have an appreciable bearing on the distribution of density, as compared with the K s profile, which is more sensitive to changes in the O1/Px proportion. In the Fe model ( F e / ( F e + M g ) = 0.17), for the P - T values of the uppermost mantle, an increase in density of 0.05-0.1 g cm -3 (1.5-3%) compared with the LT pyrolite model ( F e / ( F e + Mg) = 0.12) is observed, whereas the bulk modulus increases by only 0.1-0.2 GPa (0.1%). This insensitivity of K s to changes in the iron content is associated with the fact that bulk moduli of the end-members in both the ohvine and pyroxene series prove to be rather close (Kuskov and Galimzyanov, 1986). At depths of 300-400 km the density values for the iron-rich model are higher and its bulk modulus and seismic parameter are lower than the analogous PREM values. In contrast, the density and seismic parameter profiles for the equilibrium assemblage O1+ Px (0.5(Fe0.19 Mg0.81)2SiO 4 + 0.5(Fe0.14Mgo.86)SiO3) agree well with those from the ACY and GCA + TNA models at depths of 300-360 km (Table 2, Fig. 2).
3.5. Comparison of the models Reviewing the analyses of physical property distribution in the uppermost mantle, we come to the following conclusions (Fig. 2, Table 2): an increase in temperature results in a decrease in all three thermodynamic parameters, i.e. O, Ks and ~s, whereas higher concentrations of olivine, as they have almost no bearing on the density, result in a dramatic increase in the adiabatic bulk modulus, and, vice versa, higher iron concentrations are responsible for a notable increase in density, but have practically no bearing on the bulk modulus. Moreover, the distribution of physical prop-
92
erties in the iron-rich model at depths ranging from 360 to 400 km is of a gradient type which does not agree with seismic evidence. Anderson (1988), on the basis of new estimates of solar composition, has proposed that the upper mantle may be enriched in FeO. The factors discussed, taken as a whole, suggest that the iron-rich model is inadequate to describe the physical properties of the uppermost mantle. Coming from this is an important constraint on the mantle chemistry at depths of 300-400 km, i,e. F e / ( F e + Mg) < 0.17. Comparison of the thermodynamic profiles for LT and H T pyroxene models with seismological data shows that a pyroxene-rich assemblage cannot be ruled out as important constituent of the uppermost mantle. The olivine and LT pyrolite models yield almost coinciding density distribution patterns for 300-400 km depth, which are in reasonable agreement with the PREM model. The 'I,s distribution in the olivine model gives the best fit with the PREM model but completely disagrees with the GCA + TNA seismic combination and with the new mantle models ICO and ACY. Duffy and Anderson (1989) have noticed that in the uppermost mantle there is considerable variation among seismological models and a variety of mineral combinations can be used to match the seismic data. However, our calculations show that the iron-rich and pyroxene models do not fit the seismic constraints and can be rejected in the uppermost mantle. Consequently, we can come to the conclusion that, within the uncertainty of calculations and resolution of seismic data, two compositional models (olivine and pyrolite models) have the best fit to the uppermost mantle physical properties. The compositions and amounts of the equilibrium O1 + Px assemblage are 0.75(Fe0.1EMg0.ss)2SiO 4 (81 vol.% Ol)+0.25(Fe0.09Mg0.91)SiO 3 (19 vol.% Px) for the olivine model and 0.5(Fe0.13Mg0.87): SiO4 (59 vol.% O1) + 0.5(Fe0.lMgo.9)SiO3 (41 vol.% Px) for the pyrolite model. It should be noted, however, that an addition of garnet to the FMS system (for example, 14 wt.% in pyrolite, as reported by Bass and Anderson (1984)) with the bulk modulus and seismic parameter well in excess of the olivine values
O.L. K U S K O V A N D A.B. P A N F E R O V
results in further increase in thermodynamic K s and tI,s values in comparison with the PREM and other seismic data. This might be the reason for discarding the olivine-rich model for the uppermost mantle. On the other hand, it should be emphasized that the ,I,s profiles for both LT and H T pyrolite models fall within the range of seismic parameters from the ICO and ACY models, as well as between the ~s values pertinent to the PREM model and the regional GCA + TNA model. Thus our preferred solution has an SiO 2 content of about 40 mol.% and an F e / ( F e + Mg) ratio of about 0.12 ('pyrolite' model), (see also Kuskov et al., 1989b).
4. Uppermost mantle-transition zone discontinuity In all petrological-geophysical models there occur various changes in elastic properties and density across the boundary separating the uppermost mantle and the transition zone. These changes appear to be associated with solid-phase reactions in the olivine component of the mantle substance. With increasing depth, pyroxene, which is an indifferent phase (its molar amount in the matter persists unchanged), undergoes an intensive depletion in iron over zones of solid-phase reactions, whereas in the uppermost mantle down to a depth of about 400 km an inverse, although less intensive, process is observed, involving pyroxene enrichment in iron. This relative impoverishment of pyroxene in iron is shown to be most intensive in the iron-rich model (the F e / ( F e + Mg) ratio in pyroxene drops from 0.14 at 375 km depth to 0.066 at 425 km depth). In the pyrolite and olivine models describing this range of depths, the pyroxene impoverishment in iron proves to be almost the same (the F e / ( F e + Mg) ratio varies from 0.098 to 0.065). The calculated iron to magnesium ratios for the pyroxene are in general agreement with those in the garnet lherzolite system studied by Akaogi and Akimoto (1979) at 1473 K (0.096 at 7.5 GPa and 0.064 at 14.6 GPa). Within the olivine component the redistribution of Fe and Mg always tends towards iron enrichment of a denser phase; in other words, the phases in equilibrium satisfy the relation term
CONSTITUTION OF THE MANTLE. 3.
Xpx < X,, < Xa < X.t, where Xi = F e / ( F e + Mg) in
the /-phase (see also Akaogi and Akimoto, 1979). The calculated sequence of phase transitions in the FMS system (Fig. 1) agrees with the results of Akimoto (1987) but does not agree below 1800 K with recent experimental work of Katsura and Ito (1988), who found that phase assemblage changes for the olivine composition ( F e / ( F e + Mg) = 0.11) occur in the following sequences: a ---, a +/3 ~ / 3 ---,/3 + y ---, y in the range 1473-1873 K. This discrepancy might be attributed to different experimental determinations of the P - T parameters of phase equilibria in the MgO-SiO 2 and F e O - S i O 2 systems. The present calculations are based on a refined set of internally consistent thermodynamic and experimental data in the FMS system proposed by Kuskov et al. (1989a) and Fabrichnaya and Kuskov (1991). The a-/3 transition in Mg2SiO 4 at 1473 and 1873 K was calculated by Kuskov et al. (1989a) at pressures of 13.6 GPa and 14.8 GPa respectively. These pressure values are in excellent agreement with the recent determination of Katsura and Ito (1988). The P - T parameters of the fl-y transition in Mg2SiO 4 taken from Sawamoto (1986) are located at a pressure 2 GPa lower than that of Katsura and Ito (1988). Only in this case are the sequences of the stable phase equilibria in the MgO-SiO 2 system thermodynamically consistent (for a review, see Kuskov et al., 1989a). 4.1. Olivine model
The sequence of mineral assemblages through the 400 km discontinuity for the LT olivine model is as follows: 392 km 402 km a+Px )a+y+Px )a+fl+Px 413 kin) fl +
Px
With reference to the LT pyrolite model, at 400 km depth the olivine model shows a significant increase in density, adiabatic bulk modulus and seismic parameter (Table 3). This means that the density changes described by the O1 model and PREM match perfectly, but K s and (I)s jumps in the Ol model are 2 - 3 times in excess of analogous values in PREM (Table 3). The olivine model
93
jump A(I)/(I)s (11.6%) agrees well with A(I)/¢ s (GCA + TNA) (10.3%) and AtI)/¢s(ACY) (11.4%), but absolute values of the seismic parameter in the olivine model are 2.5-3.5 km: s -2 in excess of regional seismic evidence. Figure 2 shows that the increasing ratio of O1/Px results in significantly higher values of K s and (I)s at the 400 km discontinuity as compared with seismological models of Dziewonski and Anderson (1981), Walck (1984), Grand and Helmberger (1984) and Montagner and Anderson (1989). This is a strong reason for discarding the isochemical olivine-rich upper mantle model. Hence the SiO 2 content in the isochemical mantle should be in excess of 36.4 mol.% and the molar ratio of O1/Px should be less than three (volume ratio of O l / P x less than 4). This constraint on the olivine content in the upper mantle does not contradict the conclusions of Weidner (1986) and Bina and Wood (1987), as well as those of Bass and Anderson (1984), Duffy and Anderson (1989) and Montagner and Anderson (1989), despite the differences in approach for calculation of the elastic properties and slight differences in values of pressure derivatives (Kos) for the fl-phase and y-spinel (compare Table 1 in this study and table 3 of Duffy and Anderson (1989)). 4.2. Pyroxene model
An increase of the SiO 2 content up to 44.4 mol.% (O1/Px ratio of 1 / 3 ) will lead to the following sequences of mineral assemblages in the Px model: LT model: a+Px
382 km
H T model: 413 km a+Px ) a+y+ 430 km ) fl +
407 km
)a+y+Px
Px
420 km
)fl+y+Px
) a+fl+
Px
Px
An increase in the SiO 2 content will reduce the jumps of the physical properties across the 400 km discontinuity. The density jump through the univariant reaction and divariant fields in the thickness range of about 20-25 km is two times less than for PREM. The jump of seismic parameter
94
O . L K U S K O V A N D A.B. P A N F E R O V
( A ~ / ~ s = 4.7-5.8%) is compatible with PREM, but is significantly less than in the ACY and G C A + T N A models (Table 3). Taking into account the physical property distribution in the uppermost mantle and through the discontinuity, we suggest that neither the LT nor the H T pyroxene model satisfies the mantle properties. As an isochemical Px-rich model can be rejected for the uppermost mantle and transition region, hence the SiO 2 content in the upper mantle should be less than 44.4 mol.% (or O1/Px molar ratio of more than 0.33 and O l / P x volume ratio of more than 0.47). 4.3. Iron-rich model
The sequence of mineral assemblages for the F e - L T model at the uppermost mantle-transition zone boundary appears to be as follows:
a + P x 373km) a + y + P x
407km) y + / 3 + P x
In contrast to the results of Weidner and Ito (1987), our self-consistent calculations show that a high F e / ( F e + Mg) ratio has a significant effect on the high density and seismic parameter gradients at depths of 370-410 km (Fig. 2). The profiles of physical properties for the Fe model show an extensive gradient zone (a + y + Px) near the 400 km discontinuity which is not observed in seismological models. An increase in the iron content smears the 400 km discontinuity, and the p, K s and ~s jumps associated with the a + Px---, a+y+Px-,/3+y+ Px transformations are spread over a 35 km depth interval, whereas the observed jumps in all seismological models are rather abrupt. Figure 2 shows that the iron-rich model is inadequate to describe the seismic properties in the uppermost mantle and through the 400 km discontinuity and must be immediately rejected. 4.4. Pyrolite model
The sequence of phase assemblages in the LT pyrolite model deduced from the phase diagram of the F e O - M g O - S i O 2 system (Fig. 1) is as follows: ot+Px 392km) a + y + P x 405kin a + / 3 + P x 407km~ /3+ Px
This sequence may account for the fine mineral structure of the 400 km discontinuity separated into a univariant reaction a + y ~ / 3 with a physical property j u m p of the first type at 405 km depth at a temperature of 1640 K and two adjacent gradient zones, namely, an upper 13 km thick zone associated with the solid-phase reaction a --, y (the a + y + Px zone) and a lower one of 2 km thickness associated with the divariant zone a +/3 + Px (a reaction a ~ / 3 ). Thus the LT pyrolite model indicates that the transition occurs over a range of 15 km centred on an average depth of 405 km. In this case, besides a first-order discontinuity (reaction a + y + Px --, a +/3 + Px) there are also second-order discontinuities at 392 km depth ( a + Px ~ a + Y + Px) and at 407 km depth ( a +/3 + Px ~ / 3 + Px). In the preliminary shear velocity model of Fukao et al. (1982), the thickness of 15 km at the depth around 430 km is consistent with the isochemical LT pyrolite model of the upper mantle. It follows from the phase diagram of the F e O M g O - S i O 2 system (Fig. 1) that the sequence of phase assemblages for the H T pyrolite model ( To = 1800 K) pertinent to the region of the first seismic discontinuity differs from the LT model (but coincides with Katsura and Ito's (1988) phase assemblage changes for the olivine composition): a+Px
417 km
~ a + fl + Px
425 km
~fl+Px
In contrast to all other models, the dramatic changes in physical properties described by the given model are constrained by a narrow (8 km) divariant zone a +/3 + Px where the phase reaction a - ~ fl is known to occur. The thickness of about 6 km which was observed by Leven (1985) for velocity structure in northern Australia is consistent with the above sequence of phase transitions in an isochemical H T pyrolite model. In this model the seismic boundary runs 15-20 km lower than in the LT model, but the pertinent effective jumps of density, bulk modulus and seismic parameter are rather close in magnitude to the analogous values in the LT pyrolite model (Fig. 2, Table 3). Thus, if the actual geotherm is slightly shifted toward higher temperatures, the evolutionary sequence will take the form a + Px ~ a +
CONSTITUTION
O F T H E M A N T L E . 3.
fl + Px ~ fl + Px. As the a + fl + Px divariant loop is extremely narrow in the mantle scale (see also Bina and Wood, 1986, 1987), this transition in the mantle would be sharp and could produce the pseudo-first-order seismic discontinuity at 420 km depth. The results obtained for the two pyrolite models in the FMS system reveal that the consistency between thermodynamic and seismic data through the 400 km discontinuity depends on the mineralogy and thickness of the phase transformation, and varies as a function of the parameter considered. Within a thickness range of 15 km for the LT pyrolite model and of 8 km for the H T pyrolite model, the jumps of physical properties are A p / p = 4%, A K / K s = 12.5% and A ~ / ~ s = 8.2%, and AO/O = 4%, A K / K s = 11.6% and A ~ / ~ s = 7.4% respectively. The density jump for a Py model is in agreement with the seismic models (Table 3), although the seismic data do not constrain density with good resolution. The calculated jump of bulk modulus agrees with PREM within the limits of uncertainties in both thermodynamic and seismological data. The seismic parameter jump is in the range of that of the PREM, ACY and GCA + TNA models. On the other hand, the jumps obtained at the 400 km discontinuity for both LT and H T pyrolite models are in excellent agreement with Ap/p = 3.8%, A K / K s = 12.3% and A ¢ / O s = 8.1% from the ICO starting model of Montagner and Anderson (1989). A choice of starting model, to satisfy some a priori constraints, can have an influence on a final seismic model. The ICO model is shifted with respect to PREM, which leads to discrepancies between ACY and PREM. It seems to us that the self-consistent thermodynamic models considered may specify ICO as other starting models. The PREM model has, as would be expected from the globally averaged models, a simple velocity structure with a first-order discontinuity at 400 km depth. The seismically determined thickness of the 400 km discontinuity within the real mantle has a value from 5 km (Lees et al., 1983; Leven, 1985) to 15 km (Fukao et al., 1982). For simplicity, transitions at a depth of about 400 km are generally displayed as first-order discontinuities.
95
Leven (1985) also interpreted the 400 km discontinuity as a first-order discontinuity at a depth of 406 km with a thickness of 6 km, although he did not discard completely second-order type discontinuities at this depth. Comparison of thermodynamic and seismic profiles shows that the discontinuity may represent, dependent on the choice of a reference seismological model, a chemical boundary as well as an isochemical boundary. If the PREM model correctly describes the seismic properties of the mantle then phase transformations in the olivine + orthopyroxene components of a pyrolite assemblage cannot explain the nature of the 400 km discontinuity because the calculated A ~ / ~ s jump in the Py model is twice as large as that in the PREM model. In this case, the 400 km discontinuity may represent a combination of phase transformations in olivine and chemical stratification. This assumption agrees with the conclusion of Bass and Anderson (1984), which suggested that phase change in pyrolite cannot explain the 400 km discontinuity. However, Bass and Anderson's calculations for the pyrolite model result in implausibly high jumps of A ~ / ~ s (20.4%) and A K / K s (26.6%) across the 400 km discontinuity in comparison with our selfconsistent calculations (see Table 3). On the other hand, if we compare calculated p, K s and ~s profiles of the pyrolite model with those from the global Earth model PREM, new reference mantle models ICO and ACY and regional models GCA + TNA, then the Py model matches a totality of the seismic data above, below and through the 400 km discontinuity. This agrees with calculations of Weidner (1986) and Akaogi et al. (1987), which suggested that pyrolite is an acceptable model for the upper mantle. In this case, the upper-mantle pyrolite assemblage in the FMS system is 59 wt.% (58.5 vol.%) olivine (XFTM = 0.13) and 41 wt.% (41.5 vol.%) pyroxene (XFP~ =
0.1). If an addition of garnet to the petrological models does not change the computed thermodynamic profiles in principle, then the results obtained may indicate that seismic properties across the 400 km discontinuity might be consistent with an isochemical pyrolite composition and do not
H
(km)
305 392
395 403
405
411 433 458
463
466
468 482
484
P
(GPa)
10.0 13.1
13.2 13.5
13.6
13.8 14.6 15.5
15.7
15.8
15.9 16.4
16.5
1735
1714 1732
1713
1712
1654 1678 1705
1648
1636 1645
1540 1633
(K)
T
T 2.5 11.2 Px 50.0 ~, 1.8 18.3 44.5 Px 50.0 T 51.6
3' 42.4 73.8 St 25.0
a 47.5 38.8 /3 50.0 /3 48.2 31.7 5.5 T 50.0 Px 46.7
/3 32.6 1.2 T 75.0
Px 50.0 50.0
50.0 50.0
a
M o l a r fractions (%)
-
St 25.0 25.0
St 1.6
-
Px 50.0 50.0 50.0
-
Px 50.0 50.0
-
3' 88.1
/3 37.7 1.4
Px 37.7
y 59.6
/3 57.1 37.1 6.4
/3 59.3
a 55.9 45.3
59.0 59.0
a
St 11.9
y 50.4 86.7
Y 61.5
Px 40.4
2.2 22.3 53.2
y
Px 40.7
3.1 14.0
T
41.0 41.0
Px
W e i g h t fractions (%)
St 11.9 11.9
St 0.8
Px 40.7 40.5 40.4
Px 41.0 40.8
-
F e / ( F e + Mg) ratios a n d a m o u n t s of the coexisting p h a s e s for the pyrolite m o d e l c o m p o s i t i o n
TABLE 4
"r 89.2
/3 39.3 1.5
Px 40.6
y 56.7
/3 55.1 36.3 6.4
/3 57.1
~ 55.8 45.9
58.6 58.6
a
St 10.8
y 50.1 87.7
T 58.7
Px 43.3
2.0 20.7 50.4
y
Px 42.9
2.7 12.3
y
41.4 41.4
Px
V o l u m e fractions (%)
-
St 10.7 10.8
St 0.7
-
Px 42.9 43.0 43.3
Px 41.5 41.8
-
~, 12.0
/3 8.1 6.5
Px 4.6
y 15.6
/3 13.9 11.5 8.9
fl 14.2
a 12.3 10.3
13.2 12.9
a
St -
-/ 15.0 12.1
Y 15.3
Px 4.7
y 24.5 20.7 16.4
Px 7.6
y 30.6 26.5
9.6 10.1
Px
-
St -
St
-
Px 7.5 6.3 4.9
Px 9.6 8.1
-
F e / ( F e + Mg) ratio (%)
o
C
c
97
C O N S T I T U T I O N O F T H E M A N T L E . 3.
require chemical stratification. In the isochemical pyrolite model the nature of the 400 km discontinuity may be associated with the above univariant (with divariant gradient zones) or divariant transitions in olivine which could produce, in different tectonic regions, pseudo-first-order or second-order seismic boundaries at depths of 390420 km. In both cases, the boundary does not exceed 8-15 km in thickness. The temperature at the seismic discontinuity is about 1650-1850 K. From seismological observations (Hales et al., 1980; Fukao et al., 1982; Walck, 1984; Grand and Helmberger, 1984; Yegorkin et al., 1984; Nakanishi, 1989) the 400 km discontinuity was found to be located at a depth range of 390-430 km. A comparison of seismological data with the LT and HT pyrolite models shows that the depth of a '400' km discontinuity and its mineralogical constitution may be different in the different tectonic regions of the mantle.
5. Phase relations in the F M S system at pressures and temperatures of the transition zone Figure 1 shows phase relations in the FMS system for olivine, pyrolite and ferrnginous compositions at pressures up to 17 GPa (510 km). For example, the sequence of phase assemblage in the LT pyrolite model deduced from Fig. 1 below the 400 km discontinuity is as follows: fl + Px -~ fl + ' t + Px -, 3, + P x - , ~/+ Px + St --~ fl + . t + S t---* y + S t
Table 4 shows the Fe/(Fe + Mg) ratios and amounts of the coexisting phases and thermodynamic properties of mineral assemblages for the pyrolite model composition at pressures and temperatures of the transition zone. In zones of solidphase reactions, pyroxene becomes gradually impoverished in iron. By the moment of its decomposition (460-475 km), the Fe/(Fe + Mg) ratio in Px is 0.045 for the LT pyrolite and olivine models, 0.053 for the iron-rich model and 0.068 for the HT pyrolite models. The calculated ratios are in general agreement with the measured values reported by Akaogi and Akimoto (1979) for orthopyroxene
(0.04-0.064 at about 14.5 GPa and 1473 K) in a sample of lherzolite. The iron content in the t phase in an equilibrium assemblage is higher than in pyroxene and lower than in -/-spinel. The lower portion of the transition zone (below 500 km) for all the models is composed of a trivariant assemblage ~/+ St (Fig. 1). Figures 1 and 2 show that in all petrological models the mineral assemblage at about 470 km depth undergoes phase transformations associated with the univariant reaction ~, + Px --, fl + St and various divariant fields. However, in actual seismological models, no evidence of such well-pronounced changes in physical properties pertinent to these depths has been obtained. Hence, none of the petrological models we have so far considered is confirmed by seismic data in the mantle transition zone. This inconsistency might be related to the fact that the FMS system does not include majorite-garnet, as, according to the data of Liu (1977), Akaogi and Akimoto (1977), Akaogi et al. (1987) and Irifune and Ringwood (1987) the PxGr transition should lead to smearing of the sharp phase boundary at depths of about 470 km. One should note also that the incorporation of a small amount of A1203 in the system will replace the stishovite-bearing assemblages by the garnet phase and will eliminate the "/+ St assemblage. Consequently, in the transition zone, t-phase or "y-spinel and garnet become the dominant constituents instead of pyroxene and stishovite (Irifune and Ringwood, 1987; Takahashi and Ito, 1987). It is clear that the studied FMS system lacks petrological representativeness for the description of the transition zone's seismic properties.
6. Conclusions
In this paper we have computed internally consistent phase diagrams of the FMS system and thermodynamic density, bulk modulus and seismic parameter profiles for a series of proposed bulk composition models. The profiles of the physical properties have been derived entirely from a thermodynamic model, with only thermodynamic data and bulk composition as input. It is shown that in different petrological models there are different
98
phase assemblage changes across the 400 km discontinuity. The sequences of phase transitions and thickness of their interphase zone depend strongly on the iron content, the O I / P x ratio and the temperature at the transition depth. Our calculations show that the iron-rich model is inadequate to describe the seismic properties in the uppermost mantle and through the 400 km discontinuity; an important constraint on the upper-mantle chemistry is F e / ( F e + Mg) < 0.17. Comparisons of calculated thermodynamic properties for various temperature and bulk composition models indicate that thermodynamic profiles of the olivine and pyroxene models do not match the seismic observations through the 400 km discontinuity. Hence, the SiO: content in the isochemical upper mantle should be in excess of 36.4 mol.% and less than 44.4 mol.%. From all the petrological models considered, only the LT and H T pyrolite models (SiO 2 = 40 mol.%, F e / ( F e + Mg) = 0.12, O l / P x = 1) may satisfy available seismological constraints. The resuits of calculations indicate that the model thermodynamic properties in the uppermost mantle and across the 400 km discontinuity may or may not be consistent with a transition zone of isochemical pyrohte composition, depending on the accuracy of the reference seismological model. If the PREM model correctly describes the seismic properties of the upper mantle then it is likely that phase changes in pyrolite cannot explain the 400 km discontinuity. On the other hand, the calculated changes in dPs in the uppermost mantle and across the discontinuity for both LT and H T pyrolite models are between the seismic estimates from the PREM, ACY and G C A + T N A models, whereas the calculated jumps of p, K s and (I)s for a pyrolite model coincide perfectly with those from the ICO model. Thus, the results indicate that seismic properties across the 400 km discontinuity might be consistent with an isochemical pyrolite composition and do not require chemical stratification. In that case, divariant fields in the phase diagram of
O.L. K U S K O V A N D A.B. P A N F E R O V
a pyrolite composition will represent gradient zones of seismic profiles, whereas univariant transformations give rise to more pronounced peculiarities with noticeable jumps in the elastic properties and density. The isochemical 400 km discontinuity may represent the univariant or divariant transitions in the olivine component of pyrolite within a thickness range of around 8-15 km and may be considered as a combination of the first- and (or) second-order type discontinuities dependent on temperature distribution in the mantle and mineralogy. Hence, in the various tectonic regions of the mantle a 400 km discontinuity might be considered as a combination of first- and second-order discontinuities with the phase transition sequence a + Px --, a + 3' +Px~ct+fl+Px~fl+Px within a thickness range of around 15 km (LT pyrolite model) or as a pseudo-first-order discontinuity through the narrow divariant loop a + f l + P x of about 8 km width ( H T pyrolite model). These thermodynamic results are in general agreement with the seismic observations of Fukao et al. (1982), Leven (1985) and Kind and Vinnik (1988) which imply the relative sharpness of the 400 km discontinuity; it should be noted, however, that the last result is less sharp than the 650 km discontinuity. In spite of the available discrepancies, our calculations allow us to identify the composition of the uppermost mantle and the transition zone as an isochemical pyrolitic one. It should be emphasized, however, that this conclusion is preliminary because of the insufficient representativeness of the petrological models considered and the insufficient resolution and coverage of seismic data for the simultaneous determination of Vr and Vs in a given tectonic region.
Acknowledgements We thank Dr. O.B. Fabrichnaya, Dr. L.M. Truskinovsky and reviewers for stimulating discussions and critical readings of the manuscript.
CONSTITUTION OF THE MANTLE. 3.
References Akaogi, M. and Akimoto, S., 1977. Pyroxene-garnet solidsolution equilibria in the systems Mg4Si4012-Mg3Ai2Si03012 and Fe4Si4012-FeaA12Si3O12 at high pressures and temperatures. Phys. Earth Planet. Inter., 15: 90-106. Akaogi, M. and Akimoto, S., 1979. High-pressure phase equilibria in a garnet lherzolite, with special reference to Mg z+ partitioning among constituent minerals. Phys. Earth Planet. Inter., 19: 31-51. Akaogi, M., Navrotsky, A., Yagi, T. and Akimoto, S., 1987. Pyroxene-garnet transformation: thermochemistry and elasticity of garnet solid solutions and application to a pyrolite mantle. In: M.H. Manglmani and Y. Syono (Editors), High-Pressure Research in Mineral Physics. The Akimoto Volume. Geophysical Monograph 39. Terra, Tokyo, pp. 251-260. Akimoto, S., 1987. High-pressure research in geophysics: past, present and future. In: M.H. Manghnani and Y. Syono (Editors), High-Pressure Research in Mineral Physics. The Akimoto Volume. Geophysical Monograph 39. Terra, Tokyo, pp. 1-13. Anderson, D.L., 1988. Bulk chemistry and compositional stratification of the Earth. In: Conference on the Origin of the Earth, LPI Contribution No. 681, Berkeley, CA, pp. 4-5. Anderson, D.L. and Bass, J.D., 1986. Transition region of the Earth's upper mantle. Nature, 320: 321-328. Bass, J.D. and Anderson, D.L., 1984. Composition of upper mantle: geophysical tests of two petrologic models. Geophys. Res. Lett., 11: 237-240. Bina, C.R. and Wood, B.J., 1986. The 400-km seismic discontinuity and the proportion of olivine in the Earth's upper mantle. Nature, 324: 449-451. Bina, C.R. and Wood, B.J., 1987. Olivine-spinel transitions: experimental and thermodynamic constraints and implications for the nature of the 4(K)-km seismic discontinuity. J. Geophys. Res., 92: 4853-4866. Duffy, T.S. and Anderson, D.L., 1989. Seismic velocities in mantle minerals and the mineralogy of the upper mantle. J. Geophys. Res., 94 (B2): 1895-1912. Dziewonski, A. and Anderson, D.L., 1981. Preliminary reference Earth model. Phys. Earth Planet. Inter., 25: 297-356. Fabrichnaya, O.B. and Kuskov, O.L., 1991. Constitution of the mantle. 1. Phase relations in t~e FeO-MgO-SiO 2 system at 10-30 GPa. Phys. Earth Planet. Inter., 69: 56-71. Fukao, J., Nagahashi, T. and Moil, S., 1982. Shear velocity in the mantle transition zone. In: S. Akimoto and M.H. Manghnani (Editors), High-Pressure Research in Geophysics: Center for Academic Publications, Tokyo, pp. 285-300. Grand, S.P. and Helmberger, D.V., 1984. Upper mantle shear structure of North America. Geophys. J. R. Astron. Soc., 76: 399-438. Hales, A.L., Muirhead, K.J. and Rynn, J.M.W., 1980. A compressional velocity distribution for the upper mantle. Tectonophysics, 63: 309-348.
99 Irifune, T., 1987. An experimental investigation of the pyroxene-garnet transformation in pyrolite composition and its bearing on the constitution of the mantle. Phys. Earth Planet. Inter., 45; 324-336. Irifune, T. and Ringwood, A.E., 1987. Phase transformations in primitive MORB and pyrolite compositions to 25 GPa and some geophysical implications. In: M.H. Manghnani and Y. Syono (Editors), High-Pressure Research in Mineral Physics. The Akimoto Volume. Geophysical Monograph 39. Terra, Tokyo, pp. 231-242. Ito, E. and Katsura, T., 1988. A temperature profile of the mantle transition zone. Tech. Rep. ISEI, Okayama Univ. Misasa, Ser. A, No. 18, pp. 1-10. Ito, E. and Takahashi, E., 1987. Ultrahigh-pressure phase transformation and constitution of the deep mantle. In: M.H. Manghnani and Y. Syono (Editors), High-Pressure Research in Mineral Physics. The Akimoto Volume. Geophysical Monograph 39. Terra, Tokyo, pp. 221-230. Ito, E. and Yamada, H., 1982. Stability relations of silicate spinels, ilmenites and perovskites. In: S. Akimoto and M.H. Manghnani (Editors), High-Pressure Research in Geophysics. Center for Academic Pulications, Tokyo, pp. 405-419. Jeanloz, R. and Thompson, A.B., 1983. Phase transitions and mantle discontinuities. Rev. Geophys. Space Phys., 21: 51-74. Katsura, T. and Ito, E., 1988. The system Mg2SiO4-Fe2SiO 4 at high pressures and temperatures: precise determination of stabilities of olivine, modified spinel and spinel. Tech. Rep. ISEI, Okayama Univ. Misasa, Ser. A, No. 16, pp. 1-15. Kind, R. and Vinnik, L.P., 1988. The upper-mantle discontinuities underneath the GRF array from P-to-S converted phases. J. Geophys., 62: 138-147. Kuskov, O.L., 1987. Thermodynamics of minerals of the mantle transition zone. Pure Appl. Chem., 59: 73-78. Kuskov, O.L. and Fabrichnaya, O.B., 1990. Phase relations in the FeO-MgO-SiO 2 system at the boundary between transition zone and lower mantle. Geokhimiya, 2:266-278 (in Russian). Kuskov, O.L. and Galimzyanov, R.F., 1986. Thermodynamics of stable mineral assemblages of the mantle transition zone. In: S.K. Saxena (Editor), Chemistry and Physics of Terrestrial Planets. Advances in Physical Geochemistry, Vol. 6. Springer-Verlag, New York, pp. 310-361. Kuskov, O.L., Fabrichnaya, O.B., Galimzyanov, R.F. and Truskinovsky, L.M., 1989a. Computer simulation of the phase diagram for the MgO-SiO 2 system at P - T parameters of the mantle transition zone. Phys. Chem. Minerals, 16: 442-454. Kuskov, O.L., Pan ferov, A.B., Fabrichnaya, O.B., Galimzyanov, R.F. and Truskinovsky, L.M., 1989b. Petrologic-geophysical model of the mantle transition zone. In: V.A. Magnitsky (Editor), Planetary Cosmogeny and the Earth's Sciences. Nauka, Moscow, pp. 140-173. Kuskov, O.L., Fabrichnaya, O.B. and Truskinovsky, L.M., 1991. Constitution of the mantle. 2. Petrological models of
100
transition zone based on FMS phase diagram. Phys, Earth Planet. Inter., 69: 72-84. Lees, A.C., Bukowinski, M.S.T. and Jeanloz, R., 1983. Reflection properties of phase transition and compositional change models of the 670-km discontinuity. J. Geophys. Res., 88: 8145-8159. Leven, J.H., 1985. The application of synthetic seismograms to the interpretation of the upper mantle P-wave velocity structure in northern Australia. Phys. Earth Planet. Inter., 38: 9-27. Liu, L., 1977. The system enstatite-pyrope at high pressures and temperatures and the mineralogy of the earth's mantle. Earth Planet. Sci. Lett., 36: 237-245. Liu, L., 1979. Phase transformations and constitution of deep mantle. In: M.W. McElhinny (Editor), The Earth: Its Origin, Structure and Evolution. Academic Press, New York, pp. 177-202. Montagner, J.-P. and Anderson, D.L., 1989. Constrained reference mantle model. Phys. Earth Planet. Inter., 58: 205-227. Nakanishi, I., 1989. A search for topography of the mantle discontinuities from precursors to P'P'. J. Phys. Earth, 37: 297-301. Ringwood, A.E., 1975. Composition and Petrology of the Earth's Mantle. McGraw-Hill, New York, 618 pp. Sawamoto, H., 1986. Single crystal growth of the modified spinel (fl) and spinel (y) phases of (Mg, Fe)2SiO 4 and some geophysical implications. Phys. Chem. Minerals, 13: 1-10. Takahashi, E. and Ito, E., 1987. Mineralogy of the mantle peridotite along a model geotherm up to 700 km depth. In:
O.L. KUSKOV A N D A.B. PANFEROV
M.H. Manghnani and Y. Syono (Editors), High-Pressure Research in Mineral Physics. The Akimoto Volume. Geophysical Monograph 39. Terra, Tokyo, pp. 427-437. Truskinovsky, L.M., Kuskov, O.L. and Khitarov, N.I. 1983. Adiabatic gradient in the mantle transition zone. Geokhimiya, 9:1222-1238 (in Russian). Truskinovsky, L.M., Kuskov, O.L. and Khitarov, N.I., 1985. Adiabatic gradient in the mantle transition zone: divariant transitions. Geokhimiya, 5:579-593 (in Russian). Walck, M.C., 1984. The P-wave upper mantle structure beneath an active spreading center: the Gulf of California. Geophys. J. R. Astron. Soc,, 76: 697--723. Watt, J.P., Davies, G.F. and O'Connell, R.J., 1976. The elastic properties of composite materials. Rev. Geophys. Space Phys., 14: 541-563. Weidner, D.J., 1986. Mantle model based on measured physical properties of minerals. In: S.K. Saxena (Editor), Chemistry and Physics of Terrestrial Planets. Advances in Physical Geochemistry, Vol. 6. Springer-Verlag, New York, pp. 251-274. Weidner, D.J. and lto, E., 1987. Mineral constraints on a uniform mantle composition. In: M.H. Manghnani and Y. Syono (Editors), High-Pressure Research in Mineral Physics. The Akimoto Volume. Geophysical Monograph 39. Terra, Tokyo, pp. 251-260. Yegorkin, A.V., Zyuganov, S.K. and Chernyshov, N.M., 1984. The upper mantle of Siberia. In: XXVII Int. Geol. Congress. Geophysics, Vol. 8, Section C.08. Nauka, Moscow, pp. 27-42.
85
Constitution of the mantle. 3. Density, elastic properties and the mineralogy of the 400 km discontinuity O.L. Kuskov and A.B. Panferov V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry, USSR Academy of Sciences, Kosygin str. 19, 117334 Moscow, USSR (Received 7 March 1989; revised and accepted 20 November 1990)
ABSTRACT Kuskov, O.L. and Panferov, A.B., 1991. Constitution of the mantle. 3. Density, elastic properties and the mineralogy of the 400 km discontinuity. Phys. Earth. Planet. Inter., 69: 85-100. On the basis of phase diagrams of the FeO-MgO-SiO 2 system and equations of state for minerals we have calculated internally consistent thermodynamic profiles of adiabatic bulk modulus, seismic parameter and density for a series of petrological models. It is shown that the sequences of phase assemblage changes across the 400 km discontinuity and the thickness of divariant transitions depend strongly on the iron content, the Ol/Px ratio and the temperature at the transition depth. Calculations show that the iron-rich model is inadequate to describe the seismic properties in the uppermost mantle and through the 400 km discontinuity. The distribution of physical properties in the iron-rich model at the depths of 360-400 km is of a gradient type which does not agree with seismic evidence; an important constraint on the upper-mantle chemistry is an Fe/(Fe + Mg) ratio of less than 0.17. Comparisons of calculated thermodynamic properties for various temperature and bulk composition models indicate that thermodynamic profiles of the olivine and pyroxene models do not match seismic observations through the 400 km discontinuity. Hence, the SiO 2 c o n t e n t in the isochemical upper mantle should be in excess of 36.4 mol.% and less than 44.4 tool.%. From all the petrological models considered, only a pyrolite model (SiO 2 = 40 mol.%, Fe/(Fe + Mg) = 0.12, O1/Px = 1) may satisfy available seismological constraints. The results obtained indicate that seismic properties across the 400 km discontinuity might be consistent with an isochemical pyrolite composition and do not require chemical stratification. The pyrolite models considered show that the phase transitions occur over a range of 8-15 km at a depth near 400 km. It is proposed that in the different tectonic regions of the Earth a 400 km discontinuity might be considered as a combination of first- and second-order discontinuities with a phase transition sequence a + Px --, a + "t + Px --, a + fl + Px --, fl + Px within a thickness range of around 15 km (low-temperature pyrolite model) or as pseudo-first-order discontinuity through a narrow divariant loop a + fl + Px about 8 km wide (high-temperature pyrolite model).
1. Introduction P h a s e t r a n s f o r m a t i o n s in m i n e r a l s y s t e m s at h i g h p r e s s u r e p r o v i d e a n e x c e l l e n t q u a l i t a t i v e exp l a n a t i o n o f the p r i n c i p a l f e a t u r e s o f t h e d i s t r i b u t i o n of elastic p r o p e r t i e s a n d d e n s i t y in the m a n t l e ( R i n g w o o d , 1975; A k a o g i a n d A k i m o t o , 1979; Liu, 1979; I t o a n d Y a m a d a , 1982; J e a n l o z a n d T h o m p son, 1983; K u s k o v a n d G a l i m z y a n o v , 1986; B i n a a n d W o o d , 1987; I r i f u n e a n d R i n g w o o d , 1987; I t o a n d T a k a h a s h i , 1987; K u s k o v et al., 1989a). 0031-9201/91/$03.50 © 1991
-
We now examine whether the inferred phase transformations are capable of providing a q u a n t i t a t i v e e x p l a n a t i o n o f t h e m a g n i t u d e s o f the c h a n g e s in p h y s i c a l p r o p e r t i e s a s s o c i a t e d w i t h seismic discontinuities. Mineralogically, the transformation occurring at t h e s e i s m i c b o u n d a r y c a n b e d e f i n e d o n t h e b a s i s o f t h r e e c r i t e r i a s p e c i f i e d as f o l l o w s : (1) t h e p r e s s u r e at t h e b o u n d a r y d e p t h c o i n c i d e s w i t h t h a t o f t h e p h a s e t r a n s i t i o n ; (2) t h e s y n t h e t i c p r o f i l e s of physical properties descriptive of the mineral
Elsevier Science Publishers B.V. All rights reserved
86
paragenesis and their jumps at phase transitions are consistent with seismic evidence; (3) the thickness of seismic boundaries at 400 km and 650 km depths agrees well with the width of the interphase zone of phase transformations. In the first two papers of this series (Fabrichnaya and Kuskov, 1991; Kuskov et al., 1991) P - T - x diagrams calculated over a wide range of pressures, temperatures and compositions for the FeO-MgO-SiO 2 (FMS) system, which is fundamental in the petrology of the mantle, served as a basis for constructing petrological models of the mantle with a fixed bulk composition x: (P-T)~,. diagrams. The purpose of the present paper is the construction of internally consistent profiles of the elastic properties and density at depths of 300-400 km and through the discontinuity for different petrological models derived from the FMS system, covering all the compositions. Given the bulk Fe/(Fe + Mg) ratio and SiO2 concentrations, the amount and composition of the phase assemblages in mantle P - T conditions are determined from the phase diagram. Thus for different initial compositions, the F e / M g partitioning between the various phases, their amounts and compositions, equations of state of mineral constituents and thermodynamic profiles of elastic properties and density for a chosen petrological model, as well as through phase transitions, are internally consistent. It appears that the problems of modelling mineral and chemical composition and retrieving profiles of the physical properties of the planet's substance are intimately related. Solving them simultaneously results in the construction of a qualitatively new model: a petrological-geophysical model of the mantle. By means of petrological models inferred from phase diagrams of the system and equations of state of minerals and their mixtures, we can compute thermodynamic ('synthetic') profiles of elastic properties and density pertinent to various depths of the mantle and compare them with the seismological models available. The comparison of thermodynamic profiles of elastic properties with seismic profiles allows a petrological interpretation of the latter, with the anomalies in their behaviour shown to be associ-
O.L. K U S K O V A N D A.B. P A N F E R O V
ated with concrete phase (without compositional changes) or chemical (with compositional changes) boundaries. From the petrological-geophysical point of view this appears to be the only possible source of data on the bulk chemical composition of the Earth's interior, as regards the most widespread rock-forming oxides. In attempts to constrain mantle composition, Lees et al. (1983), Bass and Anderson (1984), Anderson and Bass (1986), Weidner (1986), Akaogi et al. (1987), Irifune (1987) and Duffy and Anderson (1989) have compared calculated velocities and densities with seismological data, taking into account more complex, but internally inconsistent phase diagrams of mineral systems. As there is no complete (experimental or theoretical) information on the elements' partitioning and full topologies of phase diagrams of the CFMAS system at various bulk compositions we do not include majorite-garnet in our calculations in this paper. Addition of majorite-garnet to the self-consistent FMS system will mean that the approach is not strictly thermodynamic and the internal consistency of the results will be violated. For this reason, all conclusions about the constitution of the mantle should be referred only to the mineralogy of the FMS system. Nevertheless, comparison of the thermodynamic profiles with seismological data allows interpretation of the uppermost mantle structure in terms of petrological models and refinement of seismological models in the vicinity of the 400 km discontinuity. 2. Profiles of the elastic properties and density in the Earth's mantle on the basis of phase relations in the F e O - M g O - S i O 2 system
The profiles of the physical properties have been computed from phase relations in the FeOMgO-SiO 2 system, which is presented in Fig. 1 as ( P - T ) x sections calculated after the technique discussed by Kuskov (1987), Kuskov and Fabrichnaya (1990) and Fabrichnaya and Kuskov (1991). Thermodynamic profiles were calculated for petrological models of the mantle of four different bulk compositions: (1) pyrolite (Py)--Xsio2 = 0.4, Fe/(Fe + Mg) = 0.12 (molar ratio O1/Px = 1);
87
C O N S T I T U T I O N OF T H E MANTLE. 3.
p
1600
2000
1600 w
,
,
LT
*~,p
i
i
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,
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i
I
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,
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,.:' .,..~':'
141~("-:~'!~i~~ 13
2000 i
450
~1P~ 15
1600
'2O00
'~
"
400
1600
2000
Fig. 1. (P-T)x sections of the phase diagram for the FeO-MgO-SiO 2 system at various bulk compositions with low-temperature (LT) and high-temperature (HT) geotherms. The heavy lines represent univariant reactions; the stippled regions designate zones of divariant mineral assemblages. Olivine model composition: Xsio2= 0.364, F e / ( F e + M g ) = 0.12. Pyrolite model composition: Xsio2 = 0.4, Fe/(Fe + Mg) = 0.12. Iron-rich model composition: Xsio: = 0.4, Fe/(Fe+ Mg) = 0.17.
(2) olivine (O1)--Xsio2 = 0.364, F e / ( F e + Mg) = 0.12 (as compared with pyrolite, it is enriched with an olivine component, O l / P x = 3); (3) iron-rich (Fe)--Xsio2 = 0.4, F e / ( F e + Mg) = 0.17 (with the molar ratio O1/Px = 1 identical to that of pyrolite composition, but higher in iron);
= 1.07 K k m - l ) , originating from To= 1600 K (low-temperature m o d e l - - L T ) and TO= 1800 K (high-temperature m o d e l - - H T ) at a depth of 360 km (about 12 GPa). The elucidation of the temperature profiles in the mantle and of the role of the univariant and divariant phase transformations as the sources of h e a t i n f o r m i n g the t e m p e r -
(4) p y r o x e n e m o d e l ( P x ) - - g s i o 2 = 0.444, F e / ( F e + M g ) = 0.12 ( e n r i c h e d i n silica, w i t h the m o l a r ratio O 1 / P x --- 1 / 3 ) . T h e t e m p e r a t u r e d i s t r i b u t i o n i n the m a n t l e was m o d e l l e d u s i n g two g e o t h e r m s w i t h the s a m e geot h e r m a l g r a d i e n t d T / d P = 30 K G P a -1 ( d T / d H
a t u r e d i s t r i b u t i o n i n the m a n t l e h a s b e e n c a r r i e d o u t i n d e t a i l b y T r u s k i n o v s k y et al. (1983, 1985). T h e c h o s e n g e o t h e r m a l g r a d i e n t agrees i n g e n e r a l w i t h the e s t i m a t e d o n e of Lees et al. (1983), I r i f u n e (1987) a n d I t o a n d K a t s u r a (1988). F o r l a c k of r e l i a b l e e x p e r i m e n t a l d a t a o n shear
TABLE 1 Parameters of equations of state for minerals at 0.1 MPa and 298.15 K Mineral a-Mg2SiO 4 fl-Mg2SiO4
y-MgESiO4 MgSiO 3 enstatite SiO2 stishovite a-Fe2SiO4 fl-Fe2SiO4 7-Fe2SiO4 FeSiO3 ferrosilite
p
Ks
(gcm -3)
(GPa)
3.213 (0.001) 3.474 (0.010) 3.559 (0.010) 3.198 (0.005) 4.289 (0.005) 4.393 (0.001) 4.715 (0.010) 4.848 (0.005) 3.980 (0.005)
128.8 (1) 174 (6) 184 (5) 108 (3) 316 (8) 138 (1) 178 (6) 193 (4) 101 (4)
dKs/dP
a x 106 (K -1)
Cp (J mo1-1 K -1)
0 (K -I )
5.1 (0.2) 4.3 (0.5) 4.0 (0.5) 5.0 (0.5) 4.0 (1.0) 5.0 (0.5) 4.4 (1.0) 4.0 (1.0) 5.0 (0.5)
26.0 (1) 20.6 (1) 19.0 (1) 28.0 (3) 16.5 (1) 26.0 (1) 22.0 (2) 21.3 (1) 20.4 (5)
118.11 (0.16) 110.12 (1.00) 106.94 (1.00) 82.22 (0.08) 42.97 (0.10) 131.96 (0.20) 130.50 (1.30) 130.54 (1.30) 90.58 (1.70)
763 (5) 893 (25) 902 (25) 734 (30) 1190 (25) 512 (10) 609 (25) 609 (15) 540 (25)
Parameters of the equations of state for stable and virtual minerals are taken from Kuskov et al. (1989a), Kuskov and Fabrichnaya (1990) and Fabrichnaya and Kuskov (1991).
88
O . L K U S K O V A N D A.B. P A N F E R O V
moduli at ultrahigh pressure and temperature for basic minerals integrating the mantle, the comparison with seismological data was effected using three thermodynamic parameters, namely, density (~), adiabatic bulk modulus (Ks) and seismic parameter ( 4 ) s = K s / # = Vr~-4/3Vs~), inferred from the equations of state of minerals. These equations, constructed by the potential method
(Kuskov and Galimzyanov, 1986), rely on the supposition that the mantle substance is a mechanical mixture of isotropic phases. For each P-T condition the coexisting phases were mixed to form a petrological model. The Hill average of the Voigt and Reuss bounds was used for bulk moduli (Watt et al., 1976). Kuskov and Galimzyanov (1986) have carried out a detailed error analysis of
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36 37 3,8 DENSITY, g c m -~
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40
Fig. 2. Thermodynamic profiles of density and seismic parameter calculated on the basis of a phase diagram of the F e O - M g O - S i O 2 system (see Fig. 1 and text) along low-temperature (LT) and high-temperature (HT) geotherms for the olivine (O1), pyrolite (Py), pyroxene (Px) and iron-rich (Fe) models. Seismic profiles for the ICO, ACY, G C A + T N A and PREM models are shown for comparison.
CONSTITUTION OF THE MANTLE. 3.
89
the thermal equation of state and have shown that the typical uncertainties of p, K s and ~s values for the mantle minerals existing at pressures of 10-30 GPa and temperatures ranging from 1500 to 2000 K are 0.2-0.4%, 2-4% and 2-4.5% respectively. The parameters of the equations of state for minerals used in the calculation procedure are listed in Table 1 and were cited by Kuskov et al. (1989a) and Fabrichnaya and Kuskov (1991). In the comparison of thermodynamic profiles pertinent to the various petrological models, the following seismological models were used: PREM (Dziewonski and Anderson, 1981), constrained reference mantle models ICO and ACY (Montagner and Anderson, 1989), GCA (Walck, 1984) and TNA (Grand and Helmberger, 1984). The GCA and TNA velocity models are for tectonic regions in North and Central America and are similar in general in the mantle region deeper than 300 km. Both models were used as reference by Weidner (1986), Bina and Wood (1986) and Akaogi et al. (1987).
3. Uppermost mantle at 300-400 kin depth In all four petrological models of the FMS system, the uppermost mantle substance consists of an assemblage composed of olivine (a) and pyroxene (Px), with olivine being rather high in iron and pyroxene being low. With increasing temperature and depth this difference in iron content slightly decreases, although the Fe/(Fe + Mg) ratio in coexisting phases is practically independent of P and T in the range 7-13 GPa and 1450-1650 K. The results of calculations are summarized in Fig. 2 and Tables 2 and 3, which compare the calculated thermodynamic profiles with those from the seismological models.
3.1. Pyrolite model The values of density, adiabatic bulk modulus and seismic parameter calculated for a lay model at depths of 300-400 km are lower than analogous
TABLE 2 Physical property values for the various petrological and seismological models at depths of 305-430 km H
P
Ta
Models (SiO2, Fe/(Fe + Mg))
(km)
(GPa)
(K)
PREM
O1-LT (36.4, 0.12)
P (g cm -3)
Ks (GPa)
q~s (kin2 s -2)
P (gcm -3)
Ks (GPa)
tbs (km2 S-2)
P (gcm -3)
Ks" (GPa)
tbs (kin2 s -2)
162.4 168.2 169.3 171.1 198.1
46.59 47.83 48.08 48.46 52.66
3.46 3.49 3.50 3.51 3.73
159.4 167.1 168.6 171.1 206.2
46.08 47.87 48.17 48.75 55.28
3.50 3.54 3.55 3.59 3.77
154.1 162.1 163.5 169.6 195.4
44.03 45.78 46.07 47.23 51.83
159.8 166.3 167.6 169.6 200.9
45.53 46.86 47.13 47.58 53.74
3.45 3.48 3.49 3.50 3.68
44.70 46.47 46.77 47.35 52.40
3.44 3.48 3.48 3.50 3.62
305 355 365 380 430
10.0 11.8 12,1 12.7 14.5
1540 1594 1603 1621 1675
3.486 3.516 3.522 3.531 3.762
305 355 365 380 430
10.0 11.8 12.1 12.7 14.5
1540 1594 1603 1621 1675
3.510 3.550 3.557 3.565 3.738
ICO
Py-LT (40, 0.12)
ACY 305 355 365 380 430
10.0 11.8 12.1 12.7 14.5
1740 1794 1703 1821 1875
Fe-LT (40, 0.17)
3.510 3.550 3.558 3.566 3.738
154.1 161.9 163.2 165.8 192.9
Px-LT (44.4, 0.12)
Py-HT (40, 0.12) 155.0 158.0 160.7 162.1 197.9
44.17 44.50 45.17 45.47 52.93
a Temperature is referred to only the petrological models.
3.43 3.46 3.47 3.48 3.64
151.7 159.5 160.8 163.4 189.3
148.2 156.1 157.1 160.0 178.4
43.09 44.85 45.14 45.72 49.27
Px-HT (44.4, 0.12) 44.49 46.08 46.37 46.96 51.94
3.42 3.46 3.46 3.47 3.58
146.0 153.8 154.9 157.3 174.5
42.69 44.46 44.76 45.33 48.73
400 400 400 390
392-407 417-425 392-413 373-407 392-419 382-407 413-430 400 400
PREM ICO ACY GCA + TNA
Py-LT Py-HT OI-LT Fe-LT Fe-HT Px-LT Px-HT Piclogite Pyrolite
13.1-13.7 14.0-14.3 13.1-13.9 12.4-13.7 13.1-14.1 12.8-13.7 13.9-14.5 13.4 13.4
13.4 13.4 13.4 13.0 1645 1860 1648 1644 1845 1635 1860 -
-
Tar (K)
P
(GPa)
3.51 3.50 3.51 3.56 3.54 3.50 3.50 3.65 3.54
3.54 3.58 3.58 -
(g cm -3)
p
Ap
0.14 0.14 0.20 0.18 0.16 0.09 0.08 0.05 0.18
0.18 0.14 0.14 -
(g cm -3)
P, Ks, Cs--parameters at the upper boundary. Ap, AKs, Atbs--changes in physical properties through the discontinuity. Piclogite and pyrolite models are after Bass and Anderson (1984).
Depth (km)
Model
Ks 174.1 172.4 164.2 167.1 168.5 172.3 165.1 165.3 159.8 162.1 169.0 164.9
4.0 4.0 5.7 5.1 4.5 2.6 2.3 1.4 5.1
(GPa)
5.1 3.8 3.8
(%)
Ap/p
21.0 19.5 30.9 25.6 23.0 14.0 11.5 18.6 43.8
16.5 21.2 25.7
AKs (GPa)
12.5 11.6 17.9 15.5 13.9 8.8 7.1 11.0 26.6
9.5 12.3 15.6
(%)
AKs/Ks
Changes in physical properties for various petrological and seismological models across the 400 km discontinuity
TABLE 3
47.30 47.64 48.11 49.04 46.43 46.63 45.72 46.38 46.30 46.59
49.05 48.20 45.89
Os (km 2 s - 2 )
4.85 3.89 3.57 5.70 4.60 4.18 2.65 2.18 4.40 9.50
1.95 3.91 5.24
AOS (km 2 s - z )
10.3 8.2 7.4 11.6 9.8 9.0 5.8 4.7 9.5 20.4
4.1 8.1 11.4
AOS/$S (%)
O <
O < > 7,
F"
~D
91
C O N S T I T U T I O N O F T H E M A N T L E . 3.
values used in the PREM model, but with increasing depth this discrepancy gradually decreases. In contrast to the PREM model, the seismic parameter of the G C A + T N A regional model at the P - T values of the uppermost mantle is on average 0.9 km 2 s -2 (2.0%) lower than the thermodynamic values for the LT pyrolite model. A temperature increase of 200 ° C with respect to the LT pyrolite model, given the same bulk composition, results in reduced values of p, K s and ~s, which lead to a still greater difference, as compared with the LT model, of thermodynamic values O, Ks and Cs from analogous values of the PREM model. At the same time, a 200 ° C increase in temperature means closer thermodynamic and regional GCA + T N A values of the Os seismic parameter, although thermodynamic values are still 0.4 km 2 s -z (0.9%) higher than seismic ones (Fig. 2, Table 2).
3.2. Olivine model At a depth of 300-400 km the O1 model density is estimated to be as little as 0.008 g cm -3 (0.2%) in excess of the density of the basic LT pyrolite model, which is associated with the forsterite and enstatite densities being rather close. Profiles of the adiabatic b u l k modulus and the seismic parameter appear to be more sensitive to changes in the proportion of olivine and pyroxene. A change in the O1/Px molar proportion from one (a pyrolite model) to three (an olivine model) results in higher K s and ~s values in the uppermost mantle, the increase being of 6.0 GPa (4%) and 1.5 km 2 s -2 (3%), respectively. The thermodynamic profile of the seismic parameter for the O1-LT model shows a perfect agreement with the values of the seismic parameter of the P R E M model (Fig. 2).
3.3. Pyroxene model A phase diagram for the Px model is not shown in Fig. 1 because it is identical to the olivine and pyrohte compositions. The seismic parameter profile for the equilibrium assemblage of O1 + Px (0.25(Fe0.14Mg0.86)2SiO 4 + 0.75(Fe0nlMg0.89)SiO3) in the range of 300-400 km for both the L T and
H T pyroxene models compares reasonably well with the ACY and G C A + T N A models but is in strong contradiction to the PREM and ICO models. In contrast, the calculated density profiles along any plausible geotherm are not consistent with the geophysical data inferred from the PREM, ICO and ACY models.
3.4. Iron-rich model Variations in the F e / ( F e + Mg) ratio appear to have an appreciable bearing on the distribution of density, as compared with the K s profile, which is more sensitive to changes in the O1/Px proportion. In the Fe model ( F e / ( F e + M g ) = 0.17), for the P - T values of the uppermost mantle, an increase in density of 0.05-0.1 g cm -3 (1.5-3%) compared with the LT pyrolite model ( F e / ( F e + Mg) = 0.12) is observed, whereas the bulk modulus increases by only 0.1-0.2 GPa (0.1%). This insensitivity of K s to changes in the iron content is associated with the fact that bulk moduli of the end-members in both the ohvine and pyroxene series prove to be rather close (Kuskov and Galimzyanov, 1986). At depths of 300-400 km the density values for the iron-rich model are higher and its bulk modulus and seismic parameter are lower than the analogous PREM values. In contrast, the density and seismic parameter profiles for the equilibrium assemblage O1+ Px (0.5(Fe0.19 Mg0.81)2SiO 4 + 0.5(Fe0.14Mgo.86)SiO3) agree well with those from the ACY and GCA + TNA models at depths of 300-360 km (Table 2, Fig. 2).
3.5. Comparison of the models Reviewing the analyses of physical property distribution in the uppermost mantle, we come to the following conclusions (Fig. 2, Table 2): an increase in temperature results in a decrease in all three thermodynamic parameters, i.e. O, Ks and ~s, whereas higher concentrations of olivine, as they have almost no bearing on the density, result in a dramatic increase in the adiabatic bulk modulus, and, vice versa, higher iron concentrations are responsible for a notable increase in density, but have practically no bearing on the bulk modulus. Moreover, the distribution of physical prop-
92
erties in the iron-rich model at depths ranging from 360 to 400 km is of a gradient type which does not agree with seismic evidence. Anderson (1988), on the basis of new estimates of solar composition, has proposed that the upper mantle may be enriched in FeO. The factors discussed, taken as a whole, suggest that the iron-rich model is inadequate to describe the physical properties of the uppermost mantle. Coming from this is an important constraint on the mantle chemistry at depths of 300-400 km, i,e. F e / ( F e + Mg) < 0.17. Comparison of the thermodynamic profiles for LT and H T pyroxene models with seismological data shows that a pyroxene-rich assemblage cannot be ruled out as important constituent of the uppermost mantle. The olivine and LT pyrolite models yield almost coinciding density distribution patterns for 300-400 km depth, which are in reasonable agreement with the PREM model. The 'I,s distribution in the olivine model gives the best fit with the PREM model but completely disagrees with the GCA + TNA seismic combination and with the new mantle models ICO and ACY. Duffy and Anderson (1989) have noticed that in the uppermost mantle there is considerable variation among seismological models and a variety of mineral combinations can be used to match the seismic data. However, our calculations show that the iron-rich and pyroxene models do not fit the seismic constraints and can be rejected in the uppermost mantle. Consequently, we can come to the conclusion that, within the uncertainty of calculations and resolution of seismic data, two compositional models (olivine and pyrolite models) have the best fit to the uppermost mantle physical properties. The compositions and amounts of the equilibrium O1 + Px assemblage are 0.75(Fe0.1EMg0.ss)2SiO 4 (81 vol.% Ol)+0.25(Fe0.09Mg0.91)SiO 3 (19 vol.% Px) for the olivine model and 0.5(Fe0.13Mg0.87): SiO4 (59 vol.% O1) + 0.5(Fe0.lMgo.9)SiO3 (41 vol.% Px) for the pyrolite model. It should be noted, however, that an addition of garnet to the FMS system (for example, 14 wt.% in pyrolite, as reported by Bass and Anderson (1984)) with the bulk modulus and seismic parameter well in excess of the olivine values
O.L. K U S K O V A N D A.B. P A N F E R O V
results in further increase in thermodynamic K s and tI,s values in comparison with the PREM and other seismic data. This might be the reason for discarding the olivine-rich model for the uppermost mantle. On the other hand, it should be emphasized that the ,I,s profiles for both LT and H T pyrolite models fall within the range of seismic parameters from the ICO and ACY models, as well as between the ~s values pertinent to the PREM model and the regional GCA + TNA model. Thus our preferred solution has an SiO 2 content of about 40 mol.% and an F e / ( F e + Mg) ratio of about 0.12 ('pyrolite' model), (see also Kuskov et al., 1989b).
4. Uppermost mantle-transition zone discontinuity In all petrological-geophysical models there occur various changes in elastic properties and density across the boundary separating the uppermost mantle and the transition zone. These changes appear to be associated with solid-phase reactions in the olivine component of the mantle substance. With increasing depth, pyroxene, which is an indifferent phase (its molar amount in the matter persists unchanged), undergoes an intensive depletion in iron over zones of solid-phase reactions, whereas in the uppermost mantle down to a depth of about 400 km an inverse, although less intensive, process is observed, involving pyroxene enrichment in iron. This relative impoverishment of pyroxene in iron is shown to be most intensive in the iron-rich model (the F e / ( F e + Mg) ratio in pyroxene drops from 0.14 at 375 km depth to 0.066 at 425 km depth). In the pyrolite and olivine models describing this range of depths, the pyroxene impoverishment in iron proves to be almost the same (the F e / ( F e + Mg) ratio varies from 0.098 to 0.065). The calculated iron to magnesium ratios for the pyroxene are in general agreement with those in the garnet lherzolite system studied by Akaogi and Akimoto (1979) at 1473 K (0.096 at 7.5 GPa and 0.064 at 14.6 GPa). Within the olivine component the redistribution of Fe and Mg always tends towards iron enrichment of a denser phase; in other words, the phases in equilibrium satisfy the relation term
CONSTITUTION OF THE MANTLE. 3.
Xpx < X,, < Xa < X.t, where Xi = F e / ( F e + Mg) in
the /-phase (see also Akaogi and Akimoto, 1979). The calculated sequence of phase transitions in the FMS system (Fig. 1) agrees with the results of Akimoto (1987) but does not agree below 1800 K with recent experimental work of Katsura and Ito (1988), who found that phase assemblage changes for the olivine composition ( F e / ( F e + Mg) = 0.11) occur in the following sequences: a ---, a +/3 ~ / 3 ---,/3 + y ---, y in the range 1473-1873 K. This discrepancy might be attributed to different experimental determinations of the P - T parameters of phase equilibria in the MgO-SiO 2 and F e O - S i O 2 systems. The present calculations are based on a refined set of internally consistent thermodynamic and experimental data in the FMS system proposed by Kuskov et al. (1989a) and Fabrichnaya and Kuskov (1991). The a-/3 transition in Mg2SiO 4 at 1473 and 1873 K was calculated by Kuskov et al. (1989a) at pressures of 13.6 GPa and 14.8 GPa respectively. These pressure values are in excellent agreement with the recent determination of Katsura and Ito (1988). The P - T parameters of the fl-y transition in Mg2SiO 4 taken from Sawamoto (1986) are located at a pressure 2 GPa lower than that of Katsura and Ito (1988). Only in this case are the sequences of the stable phase equilibria in the MgO-SiO 2 system thermodynamically consistent (for a review, see Kuskov et al., 1989a). 4.1. Olivine model
The sequence of mineral assemblages through the 400 km discontinuity for the LT olivine model is as follows: 392 km 402 km a+Px )a+y+Px )a+fl+Px 413 kin) fl +
Px
With reference to the LT pyrolite model, at 400 km depth the olivine model shows a significant increase in density, adiabatic bulk modulus and seismic parameter (Table 3). This means that the density changes described by the O1 model and PREM match perfectly, but K s and (I)s jumps in the Ol model are 2 - 3 times in excess of analogous values in PREM (Table 3). The olivine model
93
jump A(I)/(I)s (11.6%) agrees well with A(I)/¢ s (GCA + TNA) (10.3%) and AtI)/¢s(ACY) (11.4%), but absolute values of the seismic parameter in the olivine model are 2.5-3.5 km: s -2 in excess of regional seismic evidence. Figure 2 shows that the increasing ratio of O1/Px results in significantly higher values of K s and (I)s at the 400 km discontinuity as compared with seismological models of Dziewonski and Anderson (1981), Walck (1984), Grand and Helmberger (1984) and Montagner and Anderson (1989). This is a strong reason for discarding the isochemical olivine-rich upper mantle model. Hence the SiO 2 content in the isochemical mantle should be in excess of 36.4 mol.% and the molar ratio of O1/Px should be less than three (volume ratio of O l / P x less than 4). This constraint on the olivine content in the upper mantle does not contradict the conclusions of Weidner (1986) and Bina and Wood (1987), as well as those of Bass and Anderson (1984), Duffy and Anderson (1989) and Montagner and Anderson (1989), despite the differences in approach for calculation of the elastic properties and slight differences in values of pressure derivatives (Kos) for the fl-phase and y-spinel (compare Table 1 in this study and table 3 of Duffy and Anderson (1989)). 4.2. Pyroxene model
An increase of the SiO 2 content up to 44.4 mol.% (O1/Px ratio of 1 / 3 ) will lead to the following sequences of mineral assemblages in the Px model: LT model: a+Px
382 km
H T model: 413 km a+Px ) a+y+ 430 km ) fl +
407 km
)a+y+Px
Px
420 km
)fl+y+Px
) a+fl+
Px
Px
An increase in the SiO 2 content will reduce the jumps of the physical properties across the 400 km discontinuity. The density jump through the univariant reaction and divariant fields in the thickness range of about 20-25 km is two times less than for PREM. The jump of seismic parameter
94
O . L K U S K O V A N D A.B. P A N F E R O V
( A ~ / ~ s = 4.7-5.8%) is compatible with PREM, but is significantly less than in the ACY and G C A + T N A models (Table 3). Taking into account the physical property distribution in the uppermost mantle and through the discontinuity, we suggest that neither the LT nor the H T pyroxene model satisfies the mantle properties. As an isochemical Px-rich model can be rejected for the uppermost mantle and transition region, hence the SiO 2 content in the upper mantle should be less than 44.4 mol.% (or O1/Px molar ratio of more than 0.33 and O l / P x volume ratio of more than 0.47). 4.3. Iron-rich model
The sequence of mineral assemblages for the F e - L T model at the uppermost mantle-transition zone boundary appears to be as follows:
a + P x 373km) a + y + P x
407km) y + / 3 + P x
In contrast to the results of Weidner and Ito (1987), our self-consistent calculations show that a high F e / ( F e + Mg) ratio has a significant effect on the high density and seismic parameter gradients at depths of 370-410 km (Fig. 2). The profiles of physical properties for the Fe model show an extensive gradient zone (a + y + Px) near the 400 km discontinuity which is not observed in seismological models. An increase in the iron content smears the 400 km discontinuity, and the p, K s and ~s jumps associated with the a + Px---, a+y+Px-,/3+y+ Px transformations are spread over a 35 km depth interval, whereas the observed jumps in all seismological models are rather abrupt. Figure 2 shows that the iron-rich model is inadequate to describe the seismic properties in the uppermost mantle and through the 400 km discontinuity and must be immediately rejected. 4.4. Pyrolite model
The sequence of phase assemblages in the LT pyrolite model deduced from the phase diagram of the F e O - M g O - S i O 2 system (Fig. 1) is as follows: ot+Px 392km) a + y + P x 405kin a + / 3 + P x 407km~ /3+ Px
This sequence may account for the fine mineral structure of the 400 km discontinuity separated into a univariant reaction a + y ~ / 3 with a physical property j u m p of the first type at 405 km depth at a temperature of 1640 K and two adjacent gradient zones, namely, an upper 13 km thick zone associated with the solid-phase reaction a --, y (the a + y + Px zone) and a lower one of 2 km thickness associated with the divariant zone a +/3 + Px (a reaction a ~ / 3 ). Thus the LT pyrolite model indicates that the transition occurs over a range of 15 km centred on an average depth of 405 km. In this case, besides a first-order discontinuity (reaction a + y + Px --, a +/3 + Px) there are also second-order discontinuities at 392 km depth ( a + Px ~ a + Y + Px) and at 407 km depth ( a +/3 + Px ~ / 3 + Px). In the preliminary shear velocity model of Fukao et al. (1982), the thickness of 15 km at the depth around 430 km is consistent with the isochemical LT pyrolite model of the upper mantle. It follows from the phase diagram of the F e O M g O - S i O 2 system (Fig. 1) that the sequence of phase assemblages for the H T pyrolite model ( To = 1800 K) pertinent to the region of the first seismic discontinuity differs from the LT model (but coincides with Katsura and Ito's (1988) phase assemblage changes for the olivine composition): a+Px
417 km
~ a + fl + Px
425 km
~fl+Px
In contrast to all other models, the dramatic changes in physical properties described by the given model are constrained by a narrow (8 km) divariant zone a +/3 + Px where the phase reaction a - ~ fl is known to occur. The thickness of about 6 km which was observed by Leven (1985) for velocity structure in northern Australia is consistent with the above sequence of phase transitions in an isochemical H T pyrolite model. In this model the seismic boundary runs 15-20 km lower than in the LT model, but the pertinent effective jumps of density, bulk modulus and seismic parameter are rather close in magnitude to the analogous values in the LT pyrolite model (Fig. 2, Table 3). Thus, if the actual geotherm is slightly shifted toward higher temperatures, the evolutionary sequence will take the form a + Px ~ a +
CONSTITUTION
O F T H E M A N T L E . 3.
fl + Px ~ fl + Px. As the a + fl + Px divariant loop is extremely narrow in the mantle scale (see also Bina and Wood, 1986, 1987), this transition in the mantle would be sharp and could produce the pseudo-first-order seismic discontinuity at 420 km depth. The results obtained for the two pyrolite models in the FMS system reveal that the consistency between thermodynamic and seismic data through the 400 km discontinuity depends on the mineralogy and thickness of the phase transformation, and varies as a function of the parameter considered. Within a thickness range of 15 km for the LT pyrolite model and of 8 km for the H T pyrolite model, the jumps of physical properties are A p / p = 4%, A K / K s = 12.5% and A ~ / ~ s = 8.2%, and AO/O = 4%, A K / K s = 11.6% and A ~ / ~ s = 7.4% respectively. The density jump for a Py model is in agreement with the seismic models (Table 3), although the seismic data do not constrain density with good resolution. The calculated jump of bulk modulus agrees with PREM within the limits of uncertainties in both thermodynamic and seismological data. The seismic parameter jump is in the range of that of the PREM, ACY and GCA + TNA models. On the other hand, the jumps obtained at the 400 km discontinuity for both LT and H T pyrolite models are in excellent agreement with Ap/p = 3.8%, A K / K s = 12.3% and A ¢ / O s = 8.1% from the ICO starting model of Montagner and Anderson (1989). A choice of starting model, to satisfy some a priori constraints, can have an influence on a final seismic model. The ICO model is shifted with respect to PREM, which leads to discrepancies between ACY and PREM. It seems to us that the self-consistent thermodynamic models considered may specify ICO as other starting models. The PREM model has, as would be expected from the globally averaged models, a simple velocity structure with a first-order discontinuity at 400 km depth. The seismically determined thickness of the 400 km discontinuity within the real mantle has a value from 5 km (Lees et al., 1983; Leven, 1985) to 15 km (Fukao et al., 1982). For simplicity, transitions at a depth of about 400 km are generally displayed as first-order discontinuities.
95
Leven (1985) also interpreted the 400 km discontinuity as a first-order discontinuity at a depth of 406 km with a thickness of 6 km, although he did not discard completely second-order type discontinuities at this depth. Comparison of thermodynamic and seismic profiles shows that the discontinuity may represent, dependent on the choice of a reference seismological model, a chemical boundary as well as an isochemical boundary. If the PREM model correctly describes the seismic properties of the mantle then phase transformations in the olivine + orthopyroxene components of a pyrolite assemblage cannot explain the nature of the 400 km discontinuity because the calculated A ~ / ~ s jump in the Py model is twice as large as that in the PREM model. In this case, the 400 km discontinuity may represent a combination of phase transformations in olivine and chemical stratification. This assumption agrees with the conclusion of Bass and Anderson (1984), which suggested that phase change in pyrolite cannot explain the 400 km discontinuity. However, Bass and Anderson's calculations for the pyrolite model result in implausibly high jumps of A ~ / ~ s (20.4%) and A K / K s (26.6%) across the 400 km discontinuity in comparison with our selfconsistent calculations (see Table 3). On the other hand, if we compare calculated p, K s and ~s profiles of the pyrolite model with those from the global Earth model PREM, new reference mantle models ICO and ACY and regional models GCA + TNA, then the Py model matches a totality of the seismic data above, below and through the 400 km discontinuity. This agrees with calculations of Weidner (1986) and Akaogi et al. (1987), which suggested that pyrolite is an acceptable model for the upper mantle. In this case, the upper-mantle pyrolite assemblage in the FMS system is 59 wt.% (58.5 vol.%) olivine (XFTM = 0.13) and 41 wt.% (41.5 vol.%) pyroxene (XFP~ =
0.1). If an addition of garnet to the petrological models does not change the computed thermodynamic profiles in principle, then the results obtained may indicate that seismic properties across the 400 km discontinuity might be consistent with an isochemical pyrolite composition and do not
H
(km)
305 392
395 403
405
411 433 458
463
466
468 482
484
P
(GPa)
10.0 13.1
13.2 13.5
13.6
13.8 14.6 15.5
15.7
15.8
15.9 16.4
16.5
1735
1714 1732
1713
1712
1654 1678 1705
1648
1636 1645
1540 1633
(K)
T
T 2.5 11.2 Px 50.0 ~, 1.8 18.3 44.5 Px 50.0 T 51.6
3' 42.4 73.8 St 25.0
a 47.5 38.8 /3 50.0 /3 48.2 31.7 5.5 T 50.0 Px 46.7
/3 32.6 1.2 T 75.0
Px 50.0 50.0
50.0 50.0
a
M o l a r fractions (%)
-
St 25.0 25.0
St 1.6
-
Px 50.0 50.0 50.0
-
Px 50.0 50.0
-
3' 88.1
/3 37.7 1.4
Px 37.7
y 59.6
/3 57.1 37.1 6.4
/3 59.3
a 55.9 45.3
59.0 59.0
a
St 11.9
y 50.4 86.7
Y 61.5
Px 40.4
2.2 22.3 53.2
y
Px 40.7
3.1 14.0
T
41.0 41.0
Px
W e i g h t fractions (%)
St 11.9 11.9
St 0.8
Px 40.7 40.5 40.4
Px 41.0 40.8
-
F e / ( F e + Mg) ratios a n d a m o u n t s of the coexisting p h a s e s for the pyrolite m o d e l c o m p o s i t i o n
TABLE 4
"r 89.2
/3 39.3 1.5
Px 40.6
y 56.7
/3 55.1 36.3 6.4
/3 57.1
~ 55.8 45.9
58.6 58.6
a
St 10.8
y 50.1 87.7
T 58.7
Px 43.3
2.0 20.7 50.4
y
Px 42.9
2.7 12.3
y
41.4 41.4
Px
V o l u m e fractions (%)
-
St 10.7 10.8
St 0.7
-
Px 42.9 43.0 43.3
Px 41.5 41.8
-
~, 12.0
/3 8.1 6.5
Px 4.6
y 15.6
/3 13.9 11.5 8.9
fl 14.2
a 12.3 10.3
13.2 12.9
a
St -
-/ 15.0 12.1
Y 15.3
Px 4.7
y 24.5 20.7 16.4
Px 7.6
y 30.6 26.5
9.6 10.1
Px
-
St -
St
-
Px 7.5 6.3 4.9
Px 9.6 8.1
-
F e / ( F e + Mg) ratio (%)
o
C
c
97
C O N S T I T U T I O N O F T H E M A N T L E . 3.
require chemical stratification. In the isochemical pyrolite model the nature of the 400 km discontinuity may be associated with the above univariant (with divariant gradient zones) or divariant transitions in olivine which could produce, in different tectonic regions, pseudo-first-order or second-order seismic boundaries at depths of 390420 km. In both cases, the boundary does not exceed 8-15 km in thickness. The temperature at the seismic discontinuity is about 1650-1850 K. From seismological observations (Hales et al., 1980; Fukao et al., 1982; Walck, 1984; Grand and Helmberger, 1984; Yegorkin et al., 1984; Nakanishi, 1989) the 400 km discontinuity was found to be located at a depth range of 390-430 km. A comparison of seismological data with the LT and HT pyrolite models shows that the depth of a '400' km discontinuity and its mineralogical constitution may be different in the different tectonic regions of the mantle.
5. Phase relations in the F M S system at pressures and temperatures of the transition zone Figure 1 shows phase relations in the FMS system for olivine, pyrolite and ferrnginous compositions at pressures up to 17 GPa (510 km). For example, the sequence of phase assemblage in the LT pyrolite model deduced from Fig. 1 below the 400 km discontinuity is as follows: fl + Px -~ fl + ' t + Px -, 3, + P x - , ~/+ Px + St --~ fl + . t + S t---* y + S t
Table 4 shows the Fe/(Fe + Mg) ratios and amounts of the coexisting phases and thermodynamic properties of mineral assemblages for the pyrolite model composition at pressures and temperatures of the transition zone. In zones of solidphase reactions, pyroxene becomes gradually impoverished in iron. By the moment of its decomposition (460-475 km), the Fe/(Fe + Mg) ratio in Px is 0.045 for the LT pyrolite and olivine models, 0.053 for the iron-rich model and 0.068 for the HT pyrolite models. The calculated ratios are in general agreement with the measured values reported by Akaogi and Akimoto (1979) for orthopyroxene
(0.04-0.064 at about 14.5 GPa and 1473 K) in a sample of lherzolite. The iron content in the t phase in an equilibrium assemblage is higher than in pyroxene and lower than in -/-spinel. The lower portion of the transition zone (below 500 km) for all the models is composed of a trivariant assemblage ~/+ St (Fig. 1). Figures 1 and 2 show that in all petrological models the mineral assemblage at about 470 km depth undergoes phase transformations associated with the univariant reaction ~, + Px --, fl + St and various divariant fields. However, in actual seismological models, no evidence of such well-pronounced changes in physical properties pertinent to these depths has been obtained. Hence, none of the petrological models we have so far considered is confirmed by seismic data in the mantle transition zone. This inconsistency might be related to the fact that the FMS system does not include majorite-garnet, as, according to the data of Liu (1977), Akaogi and Akimoto (1977), Akaogi et al. (1987) and Irifune and Ringwood (1987) the PxGr transition should lead to smearing of the sharp phase boundary at depths of about 470 km. One should note also that the incorporation of a small amount of A1203 in the system will replace the stishovite-bearing assemblages by the garnet phase and will eliminate the "/+ St assemblage. Consequently, in the transition zone, t-phase or "y-spinel and garnet become the dominant constituents instead of pyroxene and stishovite (Irifune and Ringwood, 1987; Takahashi and Ito, 1987). It is clear that the studied FMS system lacks petrological representativeness for the description of the transition zone's seismic properties.
6. Conclusions
In this paper we have computed internally consistent phase diagrams of the FMS system and thermodynamic density, bulk modulus and seismic parameter profiles for a series of proposed bulk composition models. The profiles of the physical properties have been derived entirely from a thermodynamic model, with only thermodynamic data and bulk composition as input. It is shown that in different petrological models there are different
98
phase assemblage changes across the 400 km discontinuity. The sequences of phase transitions and thickness of their interphase zone depend strongly on the iron content, the O I / P x ratio and the temperature at the transition depth. Our calculations show that the iron-rich model is inadequate to describe the seismic properties in the uppermost mantle and through the 400 km discontinuity; an important constraint on the upper-mantle chemistry is F e / ( F e + Mg) < 0.17. Comparisons of calculated thermodynamic properties for various temperature and bulk composition models indicate that thermodynamic profiles of the olivine and pyroxene models do not match the seismic observations through the 400 km discontinuity. Hence, the SiO: content in the isochemical upper mantle should be in excess of 36.4 mol.% and less than 44.4 mol.%. From all the petrological models considered, only the LT and H T pyrolite models (SiO 2 = 40 mol.%, F e / ( F e + Mg) = 0.12, O l / P x = 1) may satisfy available seismological constraints. The resuits of calculations indicate that the model thermodynamic properties in the uppermost mantle and across the 400 km discontinuity may or may not be consistent with a transition zone of isochemical pyrohte composition, depending on the accuracy of the reference seismological model. If the PREM model correctly describes the seismic properties of the upper mantle then it is likely that phase changes in pyrolite cannot explain the 400 km discontinuity. On the other hand, the calculated changes in dPs in the uppermost mantle and across the discontinuity for both LT and H T pyrolite models are between the seismic estimates from the PREM, ACY and G C A + T N A models, whereas the calculated jumps of p, K s and (I)s for a pyrolite model coincide perfectly with those from the ICO model. Thus, the results indicate that seismic properties across the 400 km discontinuity might be consistent with an isochemical pyrolite composition and do not require chemical stratification. In that case, divariant fields in the phase diagram of
O.L. K U S K O V A N D A.B. P A N F E R O V
a pyrolite composition will represent gradient zones of seismic profiles, whereas univariant transformations give rise to more pronounced peculiarities with noticeable jumps in the elastic properties and density. The isochemical 400 km discontinuity may represent the univariant or divariant transitions in the olivine component of pyrolite within a thickness range of around 8-15 km and may be considered as a combination of the first- and (or) second-order type discontinuities dependent on temperature distribution in the mantle and mineralogy. Hence, in the various tectonic regions of the mantle a 400 km discontinuity might be considered as a combination of first- and second-order discontinuities with the phase transition sequence a + Px --, a + 3' +Px~ct+fl+Px~fl+Px within a thickness range of around 15 km (LT pyrolite model) or as a pseudo-first-order discontinuity through the narrow divariant loop a + f l + P x of about 8 km width ( H T pyrolite model). These thermodynamic results are in general agreement with the seismic observations of Fukao et al. (1982), Leven (1985) and Kind and Vinnik (1988) which imply the relative sharpness of the 400 km discontinuity; it should be noted, however, that the last result is less sharp than the 650 km discontinuity. In spite of the available discrepancies, our calculations allow us to identify the composition of the uppermost mantle and the transition zone as an isochemical pyrolitic one. It should be emphasized, however, that this conclusion is preliminary because of the insufficient representativeness of the petrological models considered and the insufficient resolution and coverage of seismic data for the simultaneous determination of Vr and Vs in a given tectonic region.
Acknowledgements We thank Dr. O.B. Fabrichnaya, Dr. L.M. Truskinovsky and reviewers for stimulating discussions and critical readings of the manuscript.
CONSTITUTION OF THE MANTLE. 3.
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