ISSN 1028334X, Doklady Earth Sciences, 2013, Vol. 453, Part 2, pp. 1246–1249. © Pleiades Publishing, Ltd., 2013. Original Russian Text © I.D. Ryabchikov, F.V. Kaminsky, 2013, published in Doklady Akademii Nauk, 2013, Vol. 453, No. 5, pp. 540–543.
GEOCHEMISTRY
The Composition of the Lower Mantle: Evidence from Mineral Inclusions in Diamonds Academician I. D. Ryabchikova and F. V. Kaminskyb Received July 3, 2013
DOI: 10.1134/S1028334X13120155
The presence of diamonds with mineral inclusions, which, according to the data of experimental petrol ogy, may be stable only at pressures of the lower man tle, provides unambiguous evidence for vertical move ments resulting in transport of the material from deep geospheres (probably from the mantle/core boundary) to the Earth’s surface or upper horizons of the crust. Numerous data demonstrate that peridotites domi nate in the upper mantle, although eclogites are widely abundant and some other rock types are observed. The composition of the lower mantle zones may be studied from the composition of minerals included in sub lithospheric diamonds. Multiphase mineral inclusions are rarely observed in such diamonds, and therefore, we may judge on the rock types hosting these dia monds on the basis of the chemistry of individual min erals. Mineral inclusions in lower mantle diamonds are represented by the mineral association of MgSiper ovskite (MPv) + CaSiperovskite (CPv) + ferroperi clase (FP). These phases have structures of typical oxides (perovskite CaTiO3 and periclase MgO). There fore, we may conclude that the oxide, but not silicate set of mineral phases, is the fundamental difference of the lower mantle from the upper mantle. The upper horizons of the lower mantle should also contain a high aluminum mineral represented by garnet in prod ucts of experiments and the tetragonal phase of pyrope–almandine composition (TAPP) in inclusions in diamonds. With increasing depth and pressure, alu minum solubility in МРv increases and garnet disap pears. In addition, disproportionating of Fe2+ into Fe3+ strongly incorporated in МРv and Fe0 results in the appearance of a metal phase (indicated as Met below). FP is the most abundant lower mantle mineral, which is observed as an inclusion in sublithospheric
a Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Moscow email:
[email protected] b KM Diamond Exploration Ltd., West Vancouver, British Columbia, Canada
diamonds. There is an extensive database on the com positions of ferropericlase inclusions in diamonds [1]. Assuming that the bulk composition of lower man tle rocks is identical to pyrolite [2], we calculated Mg# of FP (atomic values of the Mg/(Mg + Fe) ratio) using the method suggested in [3, 4]. We applied equations of partition coefficients Ni/Fe and Mg/Fe between Met, FP, and МPv for these calculations: FP/MPv
(Ni/Fe) = (Ni/Fe)FP/(Ni/Fe)MPv,
(1)
(Mg/Fe) = (Mg/Fe)FP/(Mg/Fe)MPv,
(2)
Kd
FP/MPv
Kd
Met/FP
Kd
FP/MPv
Kd
(Ni/Fe) = (Ni/Fe)Met/(Ni/Fe)FP.
(3)
(Ni/Fe) = 5, the average value of the experi Met/FP
mental data [5], was accepted. The K d (Ni/Fe) val ues were previously calculated for various tempera tures and pressures [3, 4]. In addition, we applied the mass balance equations like
∑c ⋅ F i
i
= ci0,
(4)
i
where ci is the concentration of this component in the ith phase; ci0 is the concentration of the component in the whole system; Fi is the portion of the ith phase in the system. The assumption that aluminum is entirely incorporated in МРv and calcium in СРv allows us to reduce the number of equations (4) to two. The simul taneous equations were solved solved using the algo rithms available in the MATLAB software. At a fixed bulk composition of the system, Mg# of FР FP / MP v and МРv has been controlled by the K d (Mg/Fe) values. This parameter was measured experimentally in many studies ([6] and references therein). It was demonstrated that this value for pyrolite ranged within 0.4–0.85 depending on the pressure and, conse quently, on depth in the lower mantle, which is partly explained by variation of the aluminum concentration FP / MP v in MPv [6]. At such values of K d (Mg/Fe), the calculated variation of Mg# for the bulk pyrolite com
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THE COMPOSITION OF THE LOWER MANTLE Ni, wt % 1.8
Mg/(Mg + Fe) 0.90 F(Met) = 0 F(Met) = 0.01 PPv + FP
0.88
Brazil Canada Guinea South Australia South Australia Yakutia
1.4
0.86
1.0
0.84
0.6
0.82 20
1247
0.2 40
60
80
100
120 P, GPa
Fig. 1. Calculated Mg# of ferropericlase for the bulk com position of pyrolite depending on pressure in the lower mantle. The coefficients of Mg/Fe exchange between FP and MPv are taken from [6]. F(Met) is the weight fraction of the metal phase in the system. PPv is the postperovskite phase with CaIrO3type structure, for which the coeffi cient of exchange with FP is taken from [5].
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0.3
0.4
0.5
0.6
0.7
0.8 0.9 1.0 Mg/(Mg + Fe)
Fig. 2. Dependence of the nickel concentration on Mg# for ferropericlase in diamond inclusions from various regions worldwide. References are given in [1]; the data for Yakutia are taken from [7].
position provides a range of 0.82–0.88 (Fig. 1). The results presented in Fig. 2 demonstrate that, except for the data on Brazilian diamonds containing an unusu ally high proportion of ironrich ferropericlases or magnesiowustites, 92% of the values for Mg# of FP inclusions plot in this range. As a whole, this is consis tent with the assumption that the composition of the lower mantle is close to pyrolite in relation to major components. However, it is necessary to discuss the probable influence of other factors on compositional variations of lower mantle phases. The formation of the metal phase as a result of FeO disproportionation should result in an increase of Mg# of both FP and MPv. The content of the iron–nickel alloy is close to 1% under the lower mantle conditions [8]. As is evident from Fig. 1, the value of Mg/(Mg + Fe) increases in com parison with the system free of the metal phase by ~0.02 ranging from 0.87 to 0.9. However, the concen tration of nickel in ferropericlase should decrease remarkably due to its transition into the metal phase, as was demonstrated in [3, 4] (Fig. 3). In fact, the con centration of Ni in FР with a Mg# > 0.8 is close to 1% (Fig. 2), which is significantly higher than that esti mated for lherzolite with 1% of the metal alloy. Hence, FeO disproportionation does not have a significant influence on the composition of ferropericlases cap tured by lower mantle diamonds. Different bulk compositions of rocks are another factor that may control the compositional variations of sublithospheric FP. The abundance of the lower man tle minerals included in diamonds (ferropericlase, 56%; MgSiperovskite, 8%; CaSiperovskite, 12%; other minerals, 24%) differs significantly from the cal culated values for pyrolite (ferropericlase, 18%; MgSi DOKLADY EARTH SCIENCES
0.1 0.2
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perovskite, 77%; CaSiperovskite, 5%), which was considered [1] as proof of the difference between the compositions of the lower mantle and the upper man tle. In particular, the presence of more hightempera ture rocks, such as harzburgite, in addition to primitive lherzolite in the lower mantle may result in the appear ance of ferropericlases with slightly different composi tions in comparison with the minerals of lower mantle rocks of the pyrolite composition. On the one hand, Mg# of the bulk composition of mantle substrate increases, and, on the other hand, the constants of exchange reactions between components of FP and MPv change. Comparison of the data from [5, 6] demonstrates that at low Al2O3 concentrations typical of rocks of the Ni, wt % 1.2 1.0 F(Met) = 0 F(Met) = 0.01 PPv + FP
0.8 0.6 0.4 0.2 0 20
40
60
80
100
120 P, GPa
Fig. 3. Calculated values of nickel concentrations in ferro periclase in dependence on pressure for the pyrolitic bulk composition. Constants of Ni/Fe and Mg/Fe exchange are taken from [4, 6].
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lower mantle temperatures and pressures. Instead of this phase, highpressure silica polymorphs (stisho vite, and phases with CaCl2type and then αPbO2 type structures at higher pressures [13]) coexist with “perovskites” (MPv and CPv).
Mg/(Mg + Fe) 0.90 0.88 0.86 0.84 0.82 20
40
60
80
100
120 P, GPa
Fig. 4. Calculated Mg# of ferropericlase for the bulk com position of pyrolite (dashed line) and harzburgite (solid line) in dependence on pressure in the lower mantle. The coefficients of Mg/Fe exchange between FP and MPv for lherzolite are taken from [6]; for harzburgite, from [5]. The triangle (for lherzolite) and square (for harzburgite) are the results of calculations for the equilibrium of FP with post perovskite phase (CaIrO3type structure), for which the exchange coefficient is taken from [5].
FP/MPv
harzburgite composition, K d (Mg/Fe) values should decrease remarkably at pressures >70 GPa due to the transition of iron to the lowspin state in ferro periclase. The calculations performed for the typical composition of mantle harzburgite (Sample 125 FP/MPv 780C6R1,6162 [9]) using the K d (Mg/Fe) val ues given in [5] demonstrated that at pressures up to 100 GPa Mg# of ferropericlases from rocks of the harzburgite composition were consistent with the val ues estimated for the pyrolitic lower mantle (Fig. 4). At higher pressures corresponding to the D'' zone at the boundary between the mantle and the metallic core, when magnesium metasilicate is represented by the postperovskite phase with the CaIrO3type struc ture, the affinity of siderophile elements (Fe and Ni) to this phase is higher than that to MPv [5]. Conse quently, Mg# of ferropericlase increases up to 0.9, which is remarkably higher than for the pyrolitic bulk composition and becomes close to the maximal values observed in FP captured by diamonds. Diamonds from Brazilian deposits contains a sig nificant number of ferropericlase and magnesiowus tite inclusions with much higher Mg# than that of the minerals of the lower mantle parageneses of peridotite composition (Fig. 2) [10–12]. It is natural to assume that the presence of such material with a low Mg# may be related to subduction of the oceanic crust to the lower mantle. However, we should take into account that the average composition of MORB has significantly higher Mg# (Mg/(Mg + Fe) = 0.61 for NMORB) in compar ison with the least magnesium ferropericlases from inclusions in Brazilian diamonds (Mg/(Mg + Fe) < 0.5, Fig. 2). In addition, FP is absent in mineral associa tions of the bulk NMORB composition under the
We may assume that the formation of ironrich fer ropericlases occurred in rocks that are a mixture of peridotitic and basaltic compositions. Our calcula tions demonstrate that the mixture of 50% pyrolite + 50% NMORB provides ferropericlase with Mg/(Mg + Fe) = 0.66 under the lower mantle conditions at 70 GPa, which is still much higher than the values for the most ironrich magnesiowustites in diamonds from Brazilian deposits. In addition, we should men tion that a mixture of such composition will contain only ≈1% of FP; further increase of the NMORB content will result in disappearance of ferropericlase and the formation of silica phases. In principle, the source of lower mantle rocks with ironrich magnesiowustite may be represented by lay ered intrusions of ultrabasic–basic composition (like Skaergaard), including ironrich late differentiates, and subducted to the lower mantle. However, this sce nario seems unlikely. More likely we may assume that ironrich FP may be formed as a result of magma fractional crystallization directly in the lower mantle. Such a likelihood is sup ported by the experimental data [14, 15] demonstrating that constants of Mg/Fe exchange between the melt and lower mantle phases (Kd = (Mg/Fe)melt/(Mg/Fe)chrystals) are always
