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Partitioning of Iron Between Magnesian Silicate Perovskite and Magnesiowüstite at About 1 Mbar S.E

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Partitioning of Iron Between Magnesian Silicate Perovskite and Magnesiowüstite at About 1 Mbar S.E Physics of the Earth and Planetary Interiors 131 (2002) 295–310 Partitioning of iron between magnesian silicate perovskite and magnesiowüstite at about 1 Mbar S.E. Kesson∗, J.D. Fitz Gerald, H.St.C. O’Neill, J.M.G. Shelley Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Received 3 December 2001; accepted 3 June 2002 Abstract The disproportionation products of olivine Fo90 in diamond–anvil cell experiments at about 1 Mbar are magnesian silicate perovskite Mg# ∼94 and a series of magnesiowüstites Mg# 85–90, yielding a recommended value of 0.4 ± 0.1 for the 2+ distribution coefficient that defines the exchange of Mg and Fe between the two phases (KD). The distinctive compositional trends which would signify that ferric iron had been stabilised during experiments are lacking. Pressure does not exert any significant effect on distribution behaviour, despite thermodynamic predictions of major reallocation of Fe2+ from magnesiowüstite into and in favour of perovskite, towards the base of the lower mantle. Nor does the presence of Al3+ in magnesian silicate perovskite in amounts appropriate for the lower mantle substantially modify distribution behaviour, and hence phase chemistry, provided the oxygen content of the system remains essentially constant. Wood’s [Earth Planet. Sci. Lett. 174 (2000) 341] multi-anvil study has been used to link KD with temperature and to calculate that magnesian silicate perovskite Mg# 93.0±0.6 would coexist with magnesiowüstite Mg# 82.6±0.9 just beyond the 660 km discontinuity (1800 K), whilst at the core–mantle boundary (2650 K) the corresponding values would be 91.9 ± 0.4 and 84.9 ± 0.8. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Partioning of iron; Magnesian silicate perovskite; Magnesiowüstite / 1. Introduction The equilibrium constant for this exchange, Kmw pvsk , Mg–Fe2+ is defined as: The two major phases in the Earth’s lower man- pvsk 2+ amw a tle are magnesian silicate perovskite (Mg,Fe )SiO3 mw/pvsk MgO Fe2+SiO + K = 3 2 2+ mw pvsk and magnesiowüstite (ferropericlase) (Mg,Fe )O. Mg–Fe a + a Fe2 O Allocation of ferrous iron and magnesium between MgSiO3 them can conveniently be described by an exchange where a is the activity, related to the mole fractions reaction: X by means of activity coefficients γ : mw mw mw + 2+ = 2+ + a = X γ MgSiO3 Fe O Fe SiO3 MgO MgO MgO MgO magnesiowustite¨ perovskite perovskite magnesiowustite¨ amw = Xmw γ mw Fe2+O Fe2+O Fe2+O amw = Xpvsk γ pvsk ∗ Corresponding author. Tel.: +61-2-6125-4228; MgSiO3 MgSiO3 MgSiO3 + fax: 61-2-6125-5989. apvsk = Xpvsk γ pvsk E-mail address: [email protected] (S.E. Kesson). 2+ 2+ 2+ Fe SiO3 Fe SiO3 Fe SiO3 0031-9201/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S0031-9201(02)00063-8 296 S.E. Kesson et al. / Physics of the Earth and Planetary Interiors 131 (2002) 295–310 Hence: in binary ferromagnesian silicate and ternary oxide systems reveals that all such systems approach ide- Kmw/pvsk = (K )mw/pvsk Kγmw/pvsk 2+ D 2+ 2+ ality even more closely than does MgO–“FeO” (von Mg–Fe Mg–Fe Mg–Fe mw Xpvsk mw γ pvsk Seckendorff and O’Neill, 1993, Fig. 8 therein). In X 2+ γ 2+ = MgO Fe SiO3 MgO Fe SiO3 summary, the simplifying approximation that Kγ is mw pvsk mw pvsk X + X γ + γ effectively unity so that K is equivalent to K, appears Fe2 O MgSiO Fe2 O MgSiO D 3 3 to be robust. / where (K )mw pvsk (henceforth K for simplicity) is Of course, it would be equally valid to express D Mg–Fe2+ D + the exchange reaction between magnesian silicate the distribution coefficient for Mg and Fe2 between perovskite and magnesiowüstite in the reverse sense, magnesiowüstite and perovskite. Many authors refer such that: to this parameter as a “partition coefficient”. Strictly amw apvsk speaking, that is incorrect because “partitioning” / 2+ Kmw pvsk = Fe O MgSiO3 describes the allocation of a single cation between 2+ Fe –Mg amw apvsk MgO 2+ phases. Fe SiO3 It is important to note that while K is a constant at Unfortunately, both conventions are in widespread use. constant temperature and pressure, KD is not, unless The former typically results in perovskite–magnesio- all values of γ are unity, i.e. mixing of all four compo- wüstite distribution coefficients (KD) having values nents in both phases is ideal. Although this is rarely the below unity, whereas the latter means that distribution case, the available empirical evidence demonstrates coefficients tend to be greater than unity. Here, we that the assumption of constancy for KD is a fair ap- adopt the former convention for both our own work, proximation in the context of other uncertainties to be and that of others. Note that is not correct to equate discussed later. Let us expand on this matter. Srecec ratios such as Xmw /Xpvsk ,orMg#pv/Mg#mw with et al. (1987) and Wiser and Wood (1991) measured Fe2+O Fe2+O + “distribution coefficients” (Mg#, i.e. Mg-number, activity–composition relations for MgO–“Fe2 O” being 100MgO/(MgO + total iron) molar). oxide solid solutions in equilibrium with metallic Fe + Over the past two decades, substantial effort has at 1 bar (i.e. oxides containing minimal Fe3 ). Their been directed towards measurements of perovskite– results may be described by a regular solution model magnesiowüstite distribution coefficients, with the with W mw of about 10 kJ/mol. After the effect of Mg–Fe2+ objective of understanding the effects of pressure, + Fe3 is allowed for, molar volumes of mixing appear temperature and system chemistry. These goals are im- approximately ideal, i.e. the excess volume of mixing portant because that distribution relationship describes ¯ is small. Now d(lnγ)/dP = Vex/RT so the response the essential phase chemistry of the lower mantle, and of activity coefficients to a change in pressure should also provides an independent constraint on geophysi- likewise be small. However, activity coefficient terms cal and geochemical models of the Earth’s interior. We are also composition-dependent. Magnesiowüstites begin by reviewing the experimental literature for the mw mw 2+ are regular solutions, such that ln γ + /γ = simple ternary system MgO–Fe O–SiO2, confining Fe2 O MgO mw mw our survey to olivine starting materials with compo- 1 − 2X + W + /RT. So if we were to in- Fe2 O Mg–Fe2 sitions between Fo80 and Fo90 that are therefore suit- crease Xmw say, from 0.1 to 0.2 at an arbitrary Fe2+O able lower mantle analogs. Fig. 1 confirms that iron temperature of 2000 K, the ratio γ mw /γ mw and is preferentially concentrated in magnesiowüstite, but Fe2+O MgO hence KD, would fall by only about 10%. That is when it comes to attributing any particular value to modest, seen in context of the entire gamut of uncer- the distribution coefficient, the matter can hardly be tainties. Although the activity–composition relations described as one of consensus. 2+ of MgSiO3–Fe SiO3 perovskites have not been Some of the dispersion amongst distribution co- measured, both due to the limited range of compo- efficient values arises from the inadvertent—often sitions over which this binary is stable, and also to unavoidable—oxidation of some ferrous iron to the experimental complications associated with Fe3+ (see trivalent state during experiments. Unfortunately, in later), an appraisal of experimental data for mixing many cases this process has been unrecognised or S.E. Kesson et al. / Physics of the Earth and Planetary Interiors 131 (2002) 295–310 297 Fig. 1. Earlier determinations of distribution coefficients for perovskite and magnesiowüstite produced by high-pressure disproportionation of olivine Fo80–90 between 25 and 50 GPa show considerable dispersion. Data from Ito et al. (1984); Ito and Takahashi (1989); Katsura and Ito (1996); Guyot et al. (1988); Fei et al. (1991, 1996); Martinez et al. (1997); Kesson and Fitz Gerald (1991); Mao et al. (1997). unacknowledged. And this ferric iron, we now know, interval that could well exceed a thousand degrees. If is concentrated in magnesian silicate perovskite in Mg–Fe2+ distribution, and hence phase chemistry, are preference to magnesiowüstite (McCammon, 1997; temperature-dependent, the potential for uncertainty Gloter et al., 2000) because at high pressures—in is substantial. Similar reservations apply to extrapo- contrast to ambient conditions—magnesiowüstite is lating magnesiowüstite’s composition from its lattice reluctant to accept Fe3+ (McCammon et al., 1998). parameters. In many studies, the composition of perovskite has Direct chemical analysis of individual phases, been deduced from its lattice parameters, by assum- either by electron microprobe or by analytical trans- ing that all of the iron in that perovskite was in the mission electron microscopy (TEM) possesses obvi- ferrous state. But if both ferrous and ferric iron are ous advantages. However, in the characterisation of simultaneously present, derivation of a perovskite’s most earlier work, neither instrument was then able composition from its lattice parameters must be in er- to discriminate between ferrous and ferric iron, with ror, because no such calibration yet exists. Moreover, convention dictating that the iron content of perovskite there is the uncomfortable probability that iron might be reported as if it were entirely ferrous. If, in reality, not have been exclusively ferrous in the perovskites a significant fraction of perovskite’s iron inventory that were used to quantify the original relationship. were ferric, then the “distribution coefficient” would Indeed, both phases probably require small but not correspond to: insignificant amounts of ferric iron for stabilisation pvsk pvsk mw (O’Neill et al., 1993), thereby further complicating X + + X + X ∗ Fe2 SiO Fe3 O .
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