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Iron oxidation state in lower mantle mineral assemblages
Earth and Planetary Science Letters 222 (2004) 423 – 434 www.elsevier.com/locate/epsl
Iron oxidation state in lower mantle mineral assemblages II. Inclusions in diamonds from Kankan, Guinea C.A. McCammon a,*, T. Stachel b, J.W. Harris c b
a Bayerisches Geoinstitut, Universita¨t Bayreuth, D-95440 Bayreuth, Germany Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 c Division of Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK
Received 2 September 2003; received in revised form 8 January 2004; accepted 16 March 2004
Abstract Inclusions of ferropericlase and former (Mg,Fe)(Si,Al)O3 perovskite in diamonds from Kankan, Guinea believed to originate in the lower mantle were studied using Mo¨ssbauer spectroscopy to determine Fe3 +/AFe. Fe3 + concentration in the (Mg,Fe)(Si,Al)O3 inclusion is consistent with empirical relations relating Fe3 +/AFe to Al concentration, supporting the inference that it crystallised in the perovskite structure at lower mantle conditions. In ferropericlase there is a nearly linear variation of trivalent cation abundance with monovalent cation abundance, suggesting a substitution of the form Na0.5M0.53 +O (M = Fe3 +, Cr3 +, Al3 +). Excess positive charge is likely balanced by cation vacancies, where their abundance is observed to increase with increasing iron concentration, consistent with high-pressure experiments. The abundance of cation vacancies is related to oxygen fugacity, where ferropericlase inclusions from Kankan and Sa˜o Luiz (Brazil) are inferred to have formed at conditions more oxidising than Fe – (Mg,Fe)O equilibrium, but more reducing than Re – ReO2 equilibrium. Fe2 +/Mg partition coefficients between (Mg,Fe)(Si,Al)O3 and ferropericlase were calculated for inclusions co-existing in the same diamond using Mo¨ssbauer data and empirical relations based on high-pressure experimental work. Most values are consistent with highpressure experiments, suggesting that these inclusions equilibrated at lower mantle conditions. The measured ferropericlase Fe3 + concentrations are consistent with diamond formation in a region of redox gradients, possibly arising from the subduction of oxidised material into reduced lower mantle. Reduction of carbonate to form ferropericlase and diamond is consistent with a slight shift of Kankan d13C values to isotopically heavy compositions compared to the worldwide dataset, and could supply the oxygen necessary to satisfy the high Fe3 + concentration in (Mg,Fe)(Si,Al)O3 perovskite, as well as account for the high proportion of ferropericlase in the lower mantle paragenesis. The heterogeneity of lower mantle diamond sources indicates that the composition of lower mantle diamonds do not necessarily reflect those of the bulk mantle. D 2004 Elsevier B.V. All rights reserved. Keywords: Mo¨ssbauer spectroscopy; ferric iron; oxygen fugacity; cation partitioning; perovskite; ferropericlase
1. Introduction * Corresponding author. Tel.: +49-921-553709; fax: +49-921553769. E-mail address: [email protected] (C.A. McCammon). 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.03.019
Inclusions in diamonds are the only known source for natural samples from the deeper parts of the Earth. The lower mantle paragenesis, which includes former (Mg,Fe)(Si,Al)O3 perovskite, (Mg,Fe)O and former
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CaSiO3 perovskite, provides an opportunity to study deep material that is essentially unaltered from the chemical composition existing at the time of diamond formation. While the extent to which such inclusions represent the bulk lower mantle is still an open question, there is overwhelming evidence that points to their origin below the enigmatic 660 km discontinuity. Indeed, the characteristics of such inclusions can place constraints on dynamic mantle processes such as convection and subduction (reviewed in [1]). Inclusions from the lower mantle paragenesis have been studied by a wide variety of techniques to elucidate their chemical and structural characteristics. Very few methods are able to distinguish between the different valence states of iron, however, which has restricted the amount of information available on aspects such as oxygen fugacity. Currently, Mo¨ssbauer spectroscopy is the only method which has been applied to the study of such inclusions [2], but even this method is limited, since inclusions must be at least 100 Am in diameter to give a reasonable signal. In Part I of this paper [3], the system (Mg,Fe)(Si,Al) O3 perovskite –(Mg,Fe)O ferropericlase was studied at pressures and temperatures similar to those near the top of the lower mantle in order to derive empirical relations to determine Fe3 + concentrations from the major element chemistry alone. In this paper we focus on the natural occurrence of these phases as inclusions in diamonds, and investigate the variation of Fe3 + concentration with composition. We find that the empirical relations derived in Part I are also valid for lower mantle inclusion phases, which allows the determination of true Fe2 +/Mg partition coefficients for pairs of co-existing phases in the same diamond. The results contribute new information about conditions during diamond formation.
2. Kankan (Guinea) diamonds Diamonds from Kankan, Guinea and their inclusions have been previously described in detail [4 – 8]. They represent a range of depths encompassing the lithosphere and asthenosphere, with parageneses from the transition zone and lower mantle. The latter includes abundant ferropericlase and less abundant former (Mg,Fe)(Si,Al)O3 perovskite (for consistency with previous work, we will refer in this paper to the
lower mantle inclusion phases as ferropericlase and MgSiO3). Lower mantle MgSiO3 can be distinguished from parageneses at shallower depths by its low Ni concentration, because Ni is partitioned preferentially into co-existing ferropericlase in the lower mantle paragenesis [6]. Ni concentrations for lower mantle MgSiO3 inclusions ( < 0.03 wt.% NiO) are significantly lower than those found in upper mantle orthopyroxene inclusions (typically 0.09 – 0.2 wt.% NiO), and lower mantle MgSiO3 inclusions from Kankan also have lower CaO (0.06 – 0.11 wt.%) and higher Al2O3 (1.1 –1.7 wt.%) compared to upper mantle orthopyroxene inclusions [6]. Ferropericlase is the most abundant mineral phase in Kankan inclusions of lower mantle origin, where a total of 39 inclusions were recovered from 32 diamonds [6]. Ni concentrations in Kankan ferropericlase inclusions fall in the range 0.08 –1.46 wt.% NiO [6], which is similar to the range for ferropericlase inclusions from Sao Luiz [9] and the Juina area (which also includes Sa˜o Luiz) [10]. Ni concentrations in ferropericlase are not necessarily a diagnostic of lower mantle origin. High Ni concentrations have been found in an inclusion containing an olivine – ferropericlase pair that was inferred to have formed in the upper mantle (sample KK-84 ferropericlase with 1.37 wt.% NiO) [6,11], and low Ni concentrations have been found in ferropericlase inferred to come from the lower mantle [6]. Recent experiments have demonstrated that high Na concentrations cannot be used to distinguish lower mantle ferropericlase, since Na is equally well incorporated at upper mantle conditions [11]. Lower mantle ferropericlase has been most reliably identified based on its association with other phases in the same diamond, such as MgSiO3 and CaSiO3. Cation partition coefficients for co-existing phases observed in lower mantle diamonds are generally consistent with those predicted from high-pressure experiments [6,9,12]. Lower mantle Kankan diamonds have been inferred to derive from the uppermost part of the lower mantle. This is based on the generally low Al concentrations in MgSiO3 inclusions ( < 1.7 wt.% Al2O3), since highpressure experiments have shown garnet would be stable at these depths and would incorporate the major portion of mantle Al, e.g. [13 – 16]. Harte et al. [9] also suggested an origin near the top of the lower mantle for
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most deep inclusions from Sa˜o Luiz based on a comparison of inclusion mineralogy and chemistry with high-pressure phase relations in the MgO – FeO – SiO2 and MgSiO3 –Al2O3 systems. Trace element analyses of Kankan inclusions show generally less than chondritic compositions for MgSiO3, but highly enriched compositions for CaSiO3 [6]. This is similar to results for diamond inclusions from Sa˜o Luiz [9] and Juina [10], and is consistent with high-pressure partitioning experiments at lower mantle conditions that show a preference of rare earth elements for CaSiO3 perovskite, e.g. [17,18]. CaSiO3 inclusions from Kankan [6] and Sa˜o Luiz [9] show Eu anomalies, which were interpreted to reflect a source rock containing feldspar, and therefore of crustal
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origin. The combination of such anomalies with the observed trace element enrichment and origin near the top of the lower mantle has led to suggestions of a subduction-related origin for lower mantle diamonds [6,9,10,19,20].
3. Experimental methods Inclusions were released from the host diamonds and prepared in epoxy mounts as previously described [5]. Inclusions of ferropericlase and MgSiO3 with diameters greater than 100 Am were selected for examination using Mo¨ssbauer spectroscopy. Specifications for the inclusions are listed in Table 1, and
Table 1 Inclusion specifications and hyperfine parameters Kankan (this study) Phase Sample number Other inclusionsa Size (Am) Fe2 + d (mm/s)b DEQ (mm/s) C (mm/s) Area Fe3 + d (mm/s)b DEQ (mm/s) C (mm/s) Area (Fe3 +/AFe)
MgSiO3 16c fp + FeCO3 80 100
fp fp fp 16b 38 43 MgSiO3 + FeCO3 100 200 150 250 150 150 300 400
fp 83c cpx-mix + ol?-Fe 250 300
fp 103a MgSiO3
fp 104
fp 108d MgSiO3 + Mg-hbl 50 100 150 150 100 200
100 150
1.148(10) 2.162(20) 0.281(36) 0.911(106) 0.401(75) 0.0(5) 0.28 0.09(8)
1.012(3) 0.649(6) 0.512(11) 0.964(14) 0.15 0.3(1) 0.4 0.036(18)
1.004(9) 0.672(14) 0.561(24) 0.977(28) 0.15 0.0(5) 0.4 0.023(34)
1.011(9) 0.727(13) 0.596(20) 0.952(26) 0.183(90) 0.3(1) 0.4 0.048(34)
1.014(8) 0.612(11) 0.523(18) 0.940(24) 0.127(73) 0.5(1) 0.4 0.060(34)
1.019(9) 0.766(13) 0.628(20) 0.949(24) 0.153(56) 0.2(1) 0.4 0.051(28)
1.037(13) 0.613(19) 0.539(36) 0.940(49) 0.15 0.2(2) 0.4 0.060(62)
1.006(10) 0.633(14) 0.518(24) 0.978(34) 0.145(90) 0.5(4) 0.4 0.022(48)
1.010(7) 0.577(10) 0.481(19) 0.991(24) 0.15 0.3(0) 0.4 0.009(30)
fp 13b
Sao Luiz (refit from [2]) Phase Sample number
fp 66
fp 67
fp 73
fp 238b
fp 251a
Fe2 + d (mm/s)b DEQ (mm/s) C or r (mm/s)c Area Fe3 + d (mm/s)b DEQ (mm/s) C (mm/s) Area (Fe3 +/AFe)
1.063(7) 0.916(11) 0.597(18) 0.943(21) 0.146(60) 0.453(91) 0.4 0.057(26)
1.068(2) 0.812(3) 0.184(2)d 0.964(6) 0.089(46) 0.278(68) 0.4 0.036(15)
1.070(2) 0.803(3) 0.147(7)e 0.966(10) 0.114(35) 0.255(54) 0.4 0.034(12)
1.066(4) 0.698(7) 0.502(11) 0.931(15) 0.113(41) 0.564(65) 0.4 0.069(20)
1.058(5) 0.679(8) 0.524(13) 0.999(16)
a
0.001(28)
Other inclusions in the same diamond. Relative to a-Fe. c C for Lorentzian lineshape and r for Voigt lineshape. d Fitted using Voigt lineshape with natural Lorentzian linewidth (0.194 mm/s). e Fitted using Voigt lineshape with variable Lorentzian linewidth (0.35 mm/s). b
1.021(8) 0.710(13) 0.559(22) 0.956(26) 0.15 0.3(2) 0.4 0.044(32)
fp 109b fp-ol
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complete microprobe analyses are given in the supplementary table of reference [6]. The epoxy sample mounts were cut with a diamond saw to thicknesses of no greater than 1 mm in order to minimise electronic absorption. Each inclusion was centred behind a hole in 25 Am thick Ta foil (absorbs 99% of 14.4 keV gamma rays), where the hole diameter varied from 150 to 500 Am depending on the size of the grain. Absorber thicknesses were estimated to be 0.2 mg Fe/ cm2 for MgSiO3 and 1.5 –2.1 mg Fe/cm2 for ferropericlase. Mo¨ssbauer spectra were recorded at room temperature (293 K) in transmission mode on a constant acceleration Mo¨ssbauer spectrometer with a nominal 370 MBq 57Co high specific activity source in a 12 Am Rh matrix. Further details of the method are given in the literature [21,22]. The velocity scale was calibrated relative to 25 Am a-Fe foil using the positions certified for National Bureau of Standards standard reference material no. 1541; linewidths of 0.34 mm/s for the outer lines of a-Fe were obtained at room temperature. The spectra were fitted to Lorentzian lineshapes using the commercially available fitting program NORMOS written by R.A. Brand (distributed by Wissenschaftliche Elektronik, Germany). Spectra were collected over periods ranging from 1 to 41 days.
ment with the empirical relation, even though two of the MgSiO3 inclusion compositions fall outside the calibration range (Fig. 1). The strong influence of Al on Fe3 +/AFe in MgSiO3 perovskite contrasts with the moderate influence of Al on Fe3 +/AFe observed in MgSiO3 orthopyroxene [24], providing strong evidence that the MgSiO3 inclusions crystallised in the perovskite structure and hence at conditions within the lower mantle. Ferropericlase spectra are similar to those in the literature, and show a dominant Fe2 + doublet with a weak shoulder at low velocity due to Fe3 + [25,26] (Fig. 2). The relative area of Fe3 + absorption is essentially constrained by the asymmetry of the main doublet, which can be measured with a relatively high degree of precision [25 –27]. The spectra were fit to two doublets, where linewidths and centre shifts of Fe3 + absorption were constrained in some spectra to values from previous data in order to avoid unrealistic linewidths (>1 mm/s) and, in some cases, abnormal Fe3 + centre shifts (deviations of more than 0.2 mm/s from expected values). Fe3 +/AFe values were calcu-
4. Measured versus calculated Fe3+ The spectrum of MgSiO3 resembles those from Sa˜o Luiz inclusions [2], and is dominated by a Fe2 + doublet. We fixed the positions and linewidths of Fe3 + absorption to those observed in previous inclusion spectra [23] due to low iron concentration and small thickness of the absorber, and derived a value for Fe3 +/AFe based on the relative areas (Table 1). We compared Fe3 +/AFe values determined using Mo¨ssbauer spectroscopy with the empirical relation [3]: Fe3þ =RFe ¼ x3þ Fe =RxFe ¼ 0:15ð4Þ þ 4:16ð51ÞxAl ; 0:03 < xAl < 0:15
ð1Þ
where xi represents the concentration of the ith element in cations per formula unit, and the numbers in parentheses represent standard deviations of the final digits. All available data (including the two data points from the literature [2]) are in excellent agree-
Fig. 1. Relative Fe3 + concentration (Fe3 +/AFe = x3Fe+ / AxFe) versus xAl for MgSiO3 inclusions, expressed in cations per formula unit. The solid circle is Mo¨ssbauer data from Kankan inclusions (this study), while open circles represent Mo¨ssbauer data from Sa˜o Luiz inclusions [2]. The solid line indicates the empirical relation determined from experimental data [3], while dotted lines indicate 90% prediction limits.
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substitution mechanisms. Kesson et al. [28] proposed a substitution of the form Na0.5M0.53 +O (M = Fe3 +, Cr3 +, Al3 +): þ 3þ 4Mg2þ Mg þ Na2 O þ M2 O3 ¼ 2ðNaMg MMg Þ þ 4MgO;
ð2Þ where the subscripts denotes the lattice sites occupied by the cations. If coupled substitution were the only mechanism for the incorporation of trivalent cations, charge balance would require that AxM3 + = xFe3 + + xAl + xCr = xNa, where xn is the cation abundance of the nth cation (per formula unit). There is a positive correlation between univalent and trivalent cation abundance (Fig. 3), which confirms that trivalent cations are stabilised primarily by coupled cation substitution; however, the excess positive charge indicates the presence of an additional substitution mechanism. The incorporation of Fe3 + into (Mg,Fe)O has been studied extensively, and considered to occur primarily through substitution of Fe3 + on the octahedral cation
Fig. 2. Room temperature Mo¨ssbauer spectra of selected ferropericlase inclusions recorded using a point source (milliprobe method): (a) KK 104; (b) KK 38. The doublets corresponding to Fe3 + are shaded black, and hyperfine parameters are given in Table 1.
lated based on the relative areas and were not corrected for recoil-free fraction effects, since systematic errors due to the constraints were estimated to be larger than the corrections would be. We calculated errors based on statistics of the fitting process as well as uncertainties in the fitting model (which includes the effect of fixing parameters). There are distinct trends significantly larger than the error bars that provide information on the substitution of Fe3 + into ferropericlase diamond inclusions. Ferropericlase exists at a wide range of pressures and temperatures, with the ability to incorporate heterovalent cations through coupled substitution as well as through creation of point defects. The accurate determination of Fe3 + concentrations using Mo¨ssbauer spectroscopy allows the examination of these
Fig. 3. Trivalent cation abundance versus Na abundance for ferropericlase inclusions in diamond, expressed in cations per formula unit. Solid circles represent Mo¨ssbauer data from Kankan inclusions (this study), while open circles represent Mo¨ssbauer data from Sa˜o Luiz inclusions [2]. Charge is balanced along the 1:1 correlation line, so deviations above the line likely indicate the presence of cation vacancies.
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site, coupled with the creation of cation vacancies, e.g. [29,30]: 2þ 3þ 3þ Mg2þ Mg þ 2FeMg þ 1=2 O2 ¼ 2ðFeMg 5FeMg Þ þ MgO;
ð3Þ where 5 is a vacancy on the octahedral cation site. The charge balance condition becomes x5 ¼ 1=2 ðxFe3þ þ xAl þ xCr xNa Þ;
ð4Þ
where x5 is the number of cation vacancies. The concentration of x5 (and hence Fe3 +) is a function of oxygen fugacity according to Eq. (3), and the number of cation vacancies (and hence Fe 3 + ) increases with increasing total iron composition at constant oxygen fugacity, e.g. [31]. Therefore a plot of cation vacancy concentration (or alternatively Fe3 +) versus total iron concentration will contain a family of curves, each curve corresponding to a different oxygen fugacity. Experiments have also confirmed this at high pressure [32], where Fe3 + concentrations are
reduced at high pressure due to the transformation of (Mg,Fe)Fe2O4 to a denser phase [33]. The concentration of cation vacancies in ferropericlase inclusions from Kankan and Sa˜o Luiz [2] generally shows a positive correlation with iron composition (Fig. 4), consistent with the experimentally observed behaviour of ferropericlase. The data lie in between the experimentally determined curves corresponding to Fe equilibrium and Re –ReO2, suggesting that the ferropericlase inclusions formed at oxygen fugacities somewhere in between. The variation of Fe3 + concentration with iron composition can be described by a linear fit of data in Fig. 4 combined with Eq. (4) to obtain the results in terms of Fe3 +: x3þ Fe ¼ xNa xAl xCr þ CxFe ;
0 < xFe < 0:6 ð5Þ
where C = 0.04 F 0.01 for the inclusion data. This lies in between values determined experimentally for ferropericlase in equilibrium with Fe (C = 0.008 F 0.004) and at the Re/ReO2 buffer (C = 0.20 F 0.05) [3]. 5. Fe2+/Mg partitioning
Fig. 4. Cation vacancy concentration ( = 1/2 (x3Fe+ + xAl + xCr xNa)) versus xFe for ferropericlase inclusions in diamond, all expressed in cations per formula unit. Solid circles represent Mo¨ssbauer data from Kankan inclusions (this study), while open circles represent Mo¨ssbauer data from Sa˜o Luiz inclusions [2]. The solid lines indicate variations for Fe – (Mg,Fe)O equilibrium and Re – ReO2 equilibrium [3], while the dotted line indicates the best fit to inclusion data.
The occurrence of MgSiO3 and ferropericlase inclusions in the same diamond enables the calculation of cation partition coefficients in order to assess aspects such as the conditions under which the inclusions might have been in equilibrium, e.g. [12]. Correction of major element analyses for Fe3 + gives Fe2 +/Mg partition coefficients that are essentially constant with pressure, temperature and composition at conditions in the range P = 24– 30 GPa, T = 1600– 2200 jC and Mg#>0.75 [3]. These analyses included data for which no direct Fe3 + measurements were available, where Fe3 + was determined empirically using Eq. (1) for MgSiO3 and Eq. (5) for ferropericlase, and where the constant was chosen based on whether the experiment was performed in Fe or Re capsules. The number of diamonds with co-existing MgSiO3 and ferropericlase inclusions where Fe3 +/AFe has been measured for both phases is severely limited (currently only KK16 from the Kankan suite and BZ251 from the Sa˜o Luiz suite). We therefore esti-
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mated Fe3 +/AFe for all co-existing MgSiO3 –ferropericlase inclusion pairs given in the literature based on Eqs. (1) and (5) (Table 2). We calculated the Fe2 +/Mg pv fp fp pv partition coefficient, KD=(xFe xMg )/(xFe xMg ), where pv,fp the xFe,Mg terms are the normalised Fe2 + or Mg concentrations in MgSiO3 or ferropericlase, e.g. x*Fe2þ ¼ xFe2þ =ðxFe þ þ xMg Þ:
ð6Þ
All results from Kankan show similar values, and are in excellent agreement with the regression line determined from the experimental data (Fig. 5). The Koffiefontein diamond inclusion pair reported in [34] and two of the Sa˜o Luiz pairs (BZ207; BZ251) also give KD (Fe2 +/Mg) values that are consistent with the experimental data trend [3]. The two remaining Sa˜o Luiz pairs (BZ120; BZ210), however, give KD (Fe2 +/Mg) values that are somewhat low (Fig. 5), even allowing for the larger uncertainties. We note in particular that Fe3 +/AFe of the BZ210 MgSiO3 inclusion was determined using Mo¨ssbauer spectroscopy, so the low Fe2 + concentration in BZ210 MgSiO3 is relatively well determined. The discrepancy between observed KD (Fe2 +/Mg) values and experimental data is unlikely due to pressure or temperature effects, since pressure would increase the value of KD (Fe2 +/Mg), and temperature was not observed to have a large effect on KD (Fe2 +/Mg) up to at least 2200 jC [3]. The discrepancy is also unlikely due to errors in Fe3 + estimation, since approximately 50% of iron would have to be present
Fig. 5. Fe2 + – Mg partitioning between co-existing MgSiO3 (pv) – ferropericlase (fp) inclusions in diamond. Fe2 + concentrations are 2+ 2+ 2+ = xFe /(xFe + xMg). Fe2 + concennormalised for each phase to x*Fe trations for each phase were determined either from Mo¨ssbauer data, if available, or estimated using Eqs. (1) and (5). Solid circles represent data from Kankan inclusions, open circles represent data from Sa˜o Luiz inclusions [9], and the open square represents data from Koffiefontein inclusions [34]. The dashed line indicates the 1:1 correlation, while the dotted line indicates the best fit regression line determined from experimental data at pressures and temperatures equivalent to those near the top of the lower mantle [3].
as Fe3 + in ferropericlase in order to match experimental results. The simplest explanation for the discrepancy, of course, is a lack of equilibrium
Table 2 Fe/Mg partition coefficients for co-existing MgSiO3 (pv) and ferropericlase (fp) inclusions Source
Sample
Fe3 +/AFe, pv, obs
Fe3 +/AFe, pv, calc
Fe3 +/AFe, fp, obs
Fe3 +/AFe, fp, calc
2 +a x*Fe , pv
2 +a x*Fe , fp
KDb, Fetotal/Mg
KDc, Fe2 +/Mg
Kankan [6]
16b 25 27 44 103a 108d BZ251 BZ120 BZ207 BZ210 A262
0.09(25)
0.25(25) 0.52(25) 0.54(25) 0.32(25) 0.26(25) 0.35(25) 0.34(25) 0.33(25) 0.42(25) 0.77(25) 0.32(25)
0.02(2)
0.05(5) 0.01(1) 0.01(1) 0.01(1) 0.05(5) 0.07(7) 0.04(4) 0.05(5) 0.03(3) 0.06(6) 0.05(5)
0.037(12) 0.045(23) 0.052(28) 0.035(13) 0.038(13) 0.048(18) 0.045(5) 0.047(17) 0.084(34) 0.023(7) 0.033(12)
0.142(2) 0.162(3) 0.148(3) 0.127(3) 0.121(7) 0.134(4) 0.150(2) 0.292(12) 0.299(9) 0.171(10) 0.119(6)
0.31 0.51 0.69 0.36 0.36 0.48 0.33 0.17 0.36 0.43 0.36
0.25(4) 0.26(6) 0.30(9) 0.28(5) 0.31(5) 0.36(6) 0.27(3) 0.13(2) 0.23(4) 0.12(4) 0.28(5)
Sao Luiz [9]
Koffiefontein [34] a
0.20(6)
0.75(3)
0.06(6) 0.04(3) 0.00(1)
Normalised to Fe2 +/(Fe2 + + Mg). Based on major element analysis. c Corrected using measured Fe3 +/AFe for each phase (if available); otherwise using calculated Fe3 +/AFe. b
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between co-existing inclusions. Such a conclusion has already been suggested [9], but the results of the present study allow a more quantitative evaluation of possible equilibrium compositions. For both BZ120 and BZ210 MgSiO3 inclusions, if we assume that Fe2 +/Mg partitioning follows the same trend shown by other inclusions, equilibrium could have occurred in each case with ferropericlase containing approximately half the concentration of iron, or alternatively, ferropericlase inclusions could have equilibrated with MgSiO3 that contained twice as much total iron. The estimated iron concentrations in all cases are within estimated iron stability limits for (Mg,Fe)(Si,Al)O3 perovskite and (Mg,Fe)O at lower mantle pressures and temperatures.
6. Implications for diamond formation The concentration of trivalent cations in ferropericlase is a function of oxygen fugacity, which enables an assessment of conditions based on major element and Fe3 +/AFe data. Ferropericlase inclusions from Sa˜o Luiz show cation vacancy concentrations significantly elevated above the expected trend for equilibrium with Fe (Fig. 4), which is consistent with the observation of tiny inclusions (typically 1 –3 Am) of (Mg,Fe)Fe2O4 in a number of the ferropericlase inclusion grains [9]. Their appearance suggested formation after primary crystallisation of the ferropericlase, perhaps as a result of decreasing temperature during transport within the mantle. Since the amount of (Mg,Fe)Fe2O4 lies below the detection limit for Mo¨ssbauer spectroscopy, the Fe3 + concentration measured for ferropericlase represents a minimum. In contrast, nearly all of the Kankan inclusions lie close to the Fe equilibrium curve (Figs. 3 and 4), and no compositional variations were noted in Kankan ferropericlase that would suggest the presence of (Mg,Fe)Fe2O4 (although we cannot rule it out) [6]. One possibility is therefore that Kankan diamonds formed at conditions more reducing than those for Sa˜o Luiz. However, since ferropericlase inclusions from Kankan contain less iron than those from Sa˜o Luiz, assessment of oxygen fugacity is more equivocal. In any case, diamonds are stable over a relatively large range of oxygen fugacity [35]; hence, results for both Kankan
and Sa˜o Luiz diamonds are consistent with existing diamond stability data. The oxygen fugacity recorded by ferropericlase inclusions can be placed within the larger framework of mantle redox conditions. Efforts over the past 20 years have produced much data concerning upper mantle oxygen fugacity, and although agreement is not universal, a relatively consistent picture is emerging. The oxygen fugacity measured for lithospheric upper mantle is heterogeneous on a scale of at least four log units, where the lowest values have been recorded for suboceanic abyssal peridotites and xenoliths from zones of continental extension, while more oxidised values have been recorded for xenoliths from regions of recent or active subduction [36]. Oxygen fugacity decreases in the deep upper mantle due to the enhanced incorporation of Fe3 + into garnet due to pressure, approaching conditions near the iron – wu¨stite buffer near the transition zone, e.g. [37]. The increased affinity of all transition zone minerals for Fe3 + compared to those in the upper mantle leads to a lowering its chemical potential, thus reducing oxygen fugacity, probably close to conditions of metal saturation [38]. There is hence no compelling reason for a redox gradient across the 410 km discontinuity. The oxygen fugacity of the lower mantle is still an open question, and while a reduced lower mantle with conditions similar to the transition zone is consistent with the more primitive nature inferred for the lower mantle, there is also evidence for heterogeneities in volatile content that would imply that some regions are more oxidised [39]. Superimposed on this picture is evidence from redox measurements of the mantle wedge above subduction zones that indicate values significantly more oxidised than the surrounding mantle [36, 40,41], where the source of oxygen is suggested to be the subducted slab [41]. This contrast in oxygen fugacity between slabs and surrounding mantle may be preserved to greater depths in a manner similar to thermal and petrological contrasts, e.g. [42]. Minerals within subducting slabs have the capacity to incorporate significant quantities of Fe3 + [43], hence providing a mechanism to transport oxygen into the deeper mantle. A preliminary picture of redox conditions in the deep Earth, therefore, shows a gener-
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ally reduced bulk mantle with possibly more oxidised regions, at least some associated with subducting slabs. Diamond growth is generally favoured in regions of redox gradients [35], which may provide a clue to its origin in the lower mantle. The introduction of oxidised subducted material into reduced lower mantle could generate redox fronts that provide conditions favourable for diamond growth [19]. Another consideration is the stability of Fe3 + in (Mg,Fe)(Si,Al)O3 perovskite relative to g-(Mg,Fe)2SiO4 ringwoodite. Fe3 +/AFe is relatively low in g-(Mg,Fe)2SiO4 ringwoodite, even under oxidising conditions (ca. 1%) [32], while in the perovskite phase Fe3 +/AFe is significantly higher. A bulk pyrolite composition at the top of the lower mantle requires at least 10% Fe3 +/AFe in (Mg,Fe)SiO3 perovskite at reducing conditions [23], while the high concentration of Al in MORB requires at least 60% Fe3 +/ AFe in (Mg,Fe)(Si,Al)O3 perovskite at reducing conditions [3,13]. Charge balance can be satisfied during the transformation of g-(Mg,Fe)2SiO4 ringwoodite to (Mg,Fe)SiO3 perovskite+(Mg,Fe)O ferropericlase either through disproportionation (3Fe2 + = Fe0 + 2Fe3 +) or oxidation (2FeO + 1/2O2 = Fe2O3). The latter could be coupled to the reduction of volatile species to release oxygen. Phase changes associated with the 660 km discontinuity might enhance the mobility of volatile species. For example, results from recent experiments suggest that subduction of oceanic lithosphere into the lower mantle may release large quantities of water as a separate fluid or melt phase and contribute to a local depression of the melting temperature [44]. The transition zone phases h-(Mg,Fe) 2 SiO 4 and g(Mg,Fe)2SiO4 are known to incorporate large amounts of water (up to 3 wt.% H2O); however, the dominant lower mantle phases (Mg,Fe)SiO3 perovskite and (Mg,Fe)O incorporate significantly less (ca. 20 ppm wt. H2O) [44,45]. At greater depths where garnet transforms to the perovskite structure, hence increasing the Al concentration of (Mg,Fe)(Si,Al)O3 perovskite, larger amounts of water can be incorporated in pyrolite compositions (up to 0.2 wt.% H2O) [46]. In MORB compositions, however, (Mg,Fe)(Si,Al)O3 perovskite incorporates significantly less water (40 –110 ppm wt. H2O), suggested to be due to a different substitution mechanism for trivalent cations at higher Al concentrations [46,47].
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Carbon isotopic compositions (d13C) of lower mantle diamonds show a narrow range with a mode at 4.5x, which is close to typical mantle compositions ([48] and references therein). Also, all lower mantle diamonds identified so far are Type II, implying negligible nitrogen concentration, in comparison to upper mantle diamonds where ca. 98% have been identified as Type I [1]. It is not clear, however, the degree to which these characteristics reflect those of their source region. Trends between d13C and nitrogen concentration in the worldwide diamond database have been suggested to arise from the evolution of mantle melts (or fluids) during differentiation, where the uptake of nitrogen depends on the rate of diamond growth [49]. These patterns may be complicated by different diffusive relaxation times between carbon and nitrogen isotopes [50]. According to these models the relatively uniform carbon isotopic compositions in lower mantle diamonds can be explained by relaxation of heterogeneities at high temperatures over long residence times [50] and the low nitrogen concentrations can be explained by growth over long periods of time [49]. Kankan diamonds may provide a clue to the origin of carbon in lower mantle diamonds. Carbon isotopic compositions are slightly shifted to isotopically heavier compositions, which was suggested to indicate that diamond formation may have occurred through reduction of subducted carbonates [6]. Magnesite (MgCO3) is known to be stable at conditions throughout the transition zone and into the lower mantle, e.g. [51,52], oceanic carbonates have d13C values that are typically close to 0x [53], and reduction of carbonate could occur according to the reaction MgCO3 ¼ MgO þ C þ O2 :
ð7Þ
Diamond formation according to reaction (7) could account for the high proportion of ferropericlase in lower mantle diamond inclusions (more than half of worldwide occurrences), as well as supply the oxygen needed to oxidise iron during transformation to the perovskite structure. The source region for lower mantle diamonds may undergo significant changes during the crystallisation of individual diamonds. This is illustrated by Fe2 +/Mg partitioning data for diamonds BZ210 and BZ120 from Sa˜o Luiz that show disequilibrium between co-
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existing MgSiO3 and ferropericlase inclusions. We may envisage at least two possible scenarios: (a) ferropericlase was included first, then a partial melting event depleted the source in iron before a second growth stage when MgSiO3 perovskite was included; (b) MgSiO3 perovskite was included first, then melt infiltration enriched the source in iron before a second growth stage when ferropericlase was included. While further data is required to resolve the possible scenarios, these results emphasise the potential heterogeneity of lower mantle diamond sources, and that compositions of lower mantle diamonds do not necessarily reflect those of the bulk mantle.
7. Concluding remarks This study has examined Fe3 + concentrations in inclusions from lower mantle diamonds using Mo¨ssbauer spectroscopy. We calculated Fe2 +/Mg partition coefficients between MgSiO3 and ferropericlase based on these results and empirical relations derived from high-pressure experimental work, and found values for most of the inclusion pairs that were consistent with high-pressure experiments, suggesting that these inclusions equilibrated at lower mantle conditions. Such determinations of Fe2 +/Mg partition coefficients are possible despite the lack of Fe3 + data for ferropericlase, because Fe3 + concentrations are low and hence contribute a relatively small uncertainty to the overall calculation. On the other hand, this study has also shown that Fe3 + concentrations in ferropericlase can be used to qualitatively evaluate oxygen fugacity conditions during diamond formation. More data is clearly needed to understand the mechanism of diamond formation in the lower mantle, but unfortunately techniques such as Mo¨ssbauer spectroscopy are limited to samples with grain size greater than 100 Am, and methods with higher spatial resolution such as electron energy loss spectroscopy require destructive sample preparation. Other methods such as X-ray absorption spectroscopy using synchrotron sources and X-ray emission spectroscopy using the electron microprobe show promise, and
may provide a means for rapid and routine determination of Fe3 +/AFe in ferropericlase in order to construct multiparameter databases for lower mantle diamonds and their inclusions.
Acknowledgements DeBeers Consolidated Mines is thanked for their support. The manuscript was significantly improved through constructive reviews by D. Andrault, P. Cartigny and B. Harte. [BW]
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Iron oxidation state in lower mantle mineral assemblages II. Inclusions in diamonds from Kankan, Guinea C.A. McCammon a,*, T. Stachel b, J.W. Harris c b
a Bayerisches Geoinstitut, Universita¨t Bayreuth, D-95440 Bayreuth, Germany Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 c Division of Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK
Received 2 September 2003; received in revised form 8 January 2004; accepted 16 March 2004
Abstract Inclusions of ferropericlase and former (Mg,Fe)(Si,Al)O3 perovskite in diamonds from Kankan, Guinea believed to originate in the lower mantle were studied using Mo¨ssbauer spectroscopy to determine Fe3 +/AFe. Fe3 + concentration in the (Mg,Fe)(Si,Al)O3 inclusion is consistent with empirical relations relating Fe3 +/AFe to Al concentration, supporting the inference that it crystallised in the perovskite structure at lower mantle conditions. In ferropericlase there is a nearly linear variation of trivalent cation abundance with monovalent cation abundance, suggesting a substitution of the form Na0.5M0.53 +O (M = Fe3 +, Cr3 +, Al3 +). Excess positive charge is likely balanced by cation vacancies, where their abundance is observed to increase with increasing iron concentration, consistent with high-pressure experiments. The abundance of cation vacancies is related to oxygen fugacity, where ferropericlase inclusions from Kankan and Sa˜o Luiz (Brazil) are inferred to have formed at conditions more oxidising than Fe – (Mg,Fe)O equilibrium, but more reducing than Re – ReO2 equilibrium. Fe2 +/Mg partition coefficients between (Mg,Fe)(Si,Al)O3 and ferropericlase were calculated for inclusions co-existing in the same diamond using Mo¨ssbauer data and empirical relations based on high-pressure experimental work. Most values are consistent with highpressure experiments, suggesting that these inclusions equilibrated at lower mantle conditions. The measured ferropericlase Fe3 + concentrations are consistent with diamond formation in a region of redox gradients, possibly arising from the subduction of oxidised material into reduced lower mantle. Reduction of carbonate to form ferropericlase and diamond is consistent with a slight shift of Kankan d13C values to isotopically heavy compositions compared to the worldwide dataset, and could supply the oxygen necessary to satisfy the high Fe3 + concentration in (Mg,Fe)(Si,Al)O3 perovskite, as well as account for the high proportion of ferropericlase in the lower mantle paragenesis. The heterogeneity of lower mantle diamond sources indicates that the composition of lower mantle diamonds do not necessarily reflect those of the bulk mantle. D 2004 Elsevier B.V. All rights reserved. Keywords: Mo¨ssbauer spectroscopy; ferric iron; oxygen fugacity; cation partitioning; perovskite; ferropericlase
1. Introduction * Corresponding author. Tel.: +49-921-553709; fax: +49-921553769. E-mail address: [email protected] (C.A. McCammon). 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.03.019
Inclusions in diamonds are the only known source for natural samples from the deeper parts of the Earth. The lower mantle paragenesis, which includes former (Mg,Fe)(Si,Al)O3 perovskite, (Mg,Fe)O and former
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CaSiO3 perovskite, provides an opportunity to study deep material that is essentially unaltered from the chemical composition existing at the time of diamond formation. While the extent to which such inclusions represent the bulk lower mantle is still an open question, there is overwhelming evidence that points to their origin below the enigmatic 660 km discontinuity. Indeed, the characteristics of such inclusions can place constraints on dynamic mantle processes such as convection and subduction (reviewed in [1]). Inclusions from the lower mantle paragenesis have been studied by a wide variety of techniques to elucidate their chemical and structural characteristics. Very few methods are able to distinguish between the different valence states of iron, however, which has restricted the amount of information available on aspects such as oxygen fugacity. Currently, Mo¨ssbauer spectroscopy is the only method which has been applied to the study of such inclusions [2], but even this method is limited, since inclusions must be at least 100 Am in diameter to give a reasonable signal. In Part I of this paper [3], the system (Mg,Fe)(Si,Al) O3 perovskite –(Mg,Fe)O ferropericlase was studied at pressures and temperatures similar to those near the top of the lower mantle in order to derive empirical relations to determine Fe3 + concentrations from the major element chemistry alone. In this paper we focus on the natural occurrence of these phases as inclusions in diamonds, and investigate the variation of Fe3 + concentration with composition. We find that the empirical relations derived in Part I are also valid for lower mantle inclusion phases, which allows the determination of true Fe2 +/Mg partition coefficients for pairs of co-existing phases in the same diamond. The results contribute new information about conditions during diamond formation.
2. Kankan (Guinea) diamonds Diamonds from Kankan, Guinea and their inclusions have been previously described in detail [4 – 8]. They represent a range of depths encompassing the lithosphere and asthenosphere, with parageneses from the transition zone and lower mantle. The latter includes abundant ferropericlase and less abundant former (Mg,Fe)(Si,Al)O3 perovskite (for consistency with previous work, we will refer in this paper to the
lower mantle inclusion phases as ferropericlase and MgSiO3). Lower mantle MgSiO3 can be distinguished from parageneses at shallower depths by its low Ni concentration, because Ni is partitioned preferentially into co-existing ferropericlase in the lower mantle paragenesis [6]. Ni concentrations for lower mantle MgSiO3 inclusions ( < 0.03 wt.% NiO) are significantly lower than those found in upper mantle orthopyroxene inclusions (typically 0.09 – 0.2 wt.% NiO), and lower mantle MgSiO3 inclusions from Kankan also have lower CaO (0.06 – 0.11 wt.%) and higher Al2O3 (1.1 –1.7 wt.%) compared to upper mantle orthopyroxene inclusions [6]. Ferropericlase is the most abundant mineral phase in Kankan inclusions of lower mantle origin, where a total of 39 inclusions were recovered from 32 diamonds [6]. Ni concentrations in Kankan ferropericlase inclusions fall in the range 0.08 –1.46 wt.% NiO [6], which is similar to the range for ferropericlase inclusions from Sao Luiz [9] and the Juina area (which also includes Sa˜o Luiz) [10]. Ni concentrations in ferropericlase are not necessarily a diagnostic of lower mantle origin. High Ni concentrations have been found in an inclusion containing an olivine – ferropericlase pair that was inferred to have formed in the upper mantle (sample KK-84 ferropericlase with 1.37 wt.% NiO) [6,11], and low Ni concentrations have been found in ferropericlase inferred to come from the lower mantle [6]. Recent experiments have demonstrated that high Na concentrations cannot be used to distinguish lower mantle ferropericlase, since Na is equally well incorporated at upper mantle conditions [11]. Lower mantle ferropericlase has been most reliably identified based on its association with other phases in the same diamond, such as MgSiO3 and CaSiO3. Cation partition coefficients for co-existing phases observed in lower mantle diamonds are generally consistent with those predicted from high-pressure experiments [6,9,12]. Lower mantle Kankan diamonds have been inferred to derive from the uppermost part of the lower mantle. This is based on the generally low Al concentrations in MgSiO3 inclusions ( < 1.7 wt.% Al2O3), since highpressure experiments have shown garnet would be stable at these depths and would incorporate the major portion of mantle Al, e.g. [13 – 16]. Harte et al. [9] also suggested an origin near the top of the lower mantle for
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most deep inclusions from Sa˜o Luiz based on a comparison of inclusion mineralogy and chemistry with high-pressure phase relations in the MgO – FeO – SiO2 and MgSiO3 –Al2O3 systems. Trace element analyses of Kankan inclusions show generally less than chondritic compositions for MgSiO3, but highly enriched compositions for CaSiO3 [6]. This is similar to results for diamond inclusions from Sa˜o Luiz [9] and Juina [10], and is consistent with high-pressure partitioning experiments at lower mantle conditions that show a preference of rare earth elements for CaSiO3 perovskite, e.g. [17,18]. CaSiO3 inclusions from Kankan [6] and Sa˜o Luiz [9] show Eu anomalies, which were interpreted to reflect a source rock containing feldspar, and therefore of crustal
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origin. The combination of such anomalies with the observed trace element enrichment and origin near the top of the lower mantle has led to suggestions of a subduction-related origin for lower mantle diamonds [6,9,10,19,20].
3. Experimental methods Inclusions were released from the host diamonds and prepared in epoxy mounts as previously described [5]. Inclusions of ferropericlase and MgSiO3 with diameters greater than 100 Am were selected for examination using Mo¨ssbauer spectroscopy. Specifications for the inclusions are listed in Table 1, and
Table 1 Inclusion specifications and hyperfine parameters Kankan (this study) Phase Sample number Other inclusionsa Size (Am) Fe2 + d (mm/s)b DEQ (mm/s) C (mm/s) Area Fe3 + d (mm/s)b DEQ (mm/s) C (mm/s) Area (Fe3 +/AFe)
MgSiO3 16c fp + FeCO3 80 100
fp fp fp 16b 38 43 MgSiO3 + FeCO3 100 200 150 250 150 150 300 400
fp 83c cpx-mix + ol?-Fe 250 300
fp 103a MgSiO3
fp 104
fp 108d MgSiO3 + Mg-hbl 50 100 150 150 100 200
100 150
1.148(10) 2.162(20) 0.281(36) 0.911(106) 0.401(75) 0.0(5) 0.28 0.09(8)
1.012(3) 0.649(6) 0.512(11) 0.964(14) 0.15 0.3(1) 0.4 0.036(18)
1.004(9) 0.672(14) 0.561(24) 0.977(28) 0.15 0.0(5) 0.4 0.023(34)
1.011(9) 0.727(13) 0.596(20) 0.952(26) 0.183(90) 0.3(1) 0.4 0.048(34)
1.014(8) 0.612(11) 0.523(18) 0.940(24) 0.127(73) 0.5(1) 0.4 0.060(34)
1.019(9) 0.766(13) 0.628(20) 0.949(24) 0.153(56) 0.2(1) 0.4 0.051(28)
1.037(13) 0.613(19) 0.539(36) 0.940(49) 0.15 0.2(2) 0.4 0.060(62)
1.006(10) 0.633(14) 0.518(24) 0.978(34) 0.145(90) 0.5(4) 0.4 0.022(48)
1.010(7) 0.577(10) 0.481(19) 0.991(24) 0.15 0.3(0) 0.4 0.009(30)
fp 13b
Sao Luiz (refit from [2]) Phase Sample number
fp 66
fp 67
fp 73
fp 238b
fp 251a
Fe2 + d (mm/s)b DEQ (mm/s) C or r (mm/s)c Area Fe3 + d (mm/s)b DEQ (mm/s) C (mm/s) Area (Fe3 +/AFe)
1.063(7) 0.916(11) 0.597(18) 0.943(21) 0.146(60) 0.453(91) 0.4 0.057(26)
1.068(2) 0.812(3) 0.184(2)d 0.964(6) 0.089(46) 0.278(68) 0.4 0.036(15)
1.070(2) 0.803(3) 0.147(7)e 0.966(10) 0.114(35) 0.255(54) 0.4 0.034(12)
1.066(4) 0.698(7) 0.502(11) 0.931(15) 0.113(41) 0.564(65) 0.4 0.069(20)
1.058(5) 0.679(8) 0.524(13) 0.999(16)
a
0.001(28)
Other inclusions in the same diamond. Relative to a-Fe. c C for Lorentzian lineshape and r for Voigt lineshape. d Fitted using Voigt lineshape with natural Lorentzian linewidth (0.194 mm/s). e Fitted using Voigt lineshape with variable Lorentzian linewidth (0.35 mm/s). b
1.021(8) 0.710(13) 0.559(22) 0.956(26) 0.15 0.3(2) 0.4 0.044(32)
fp 109b fp-ol
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complete microprobe analyses are given in the supplementary table of reference [6]. The epoxy sample mounts were cut with a diamond saw to thicknesses of no greater than 1 mm in order to minimise electronic absorption. Each inclusion was centred behind a hole in 25 Am thick Ta foil (absorbs 99% of 14.4 keV gamma rays), where the hole diameter varied from 150 to 500 Am depending on the size of the grain. Absorber thicknesses were estimated to be 0.2 mg Fe/ cm2 for MgSiO3 and 1.5 –2.1 mg Fe/cm2 for ferropericlase. Mo¨ssbauer spectra were recorded at room temperature (293 K) in transmission mode on a constant acceleration Mo¨ssbauer spectrometer with a nominal 370 MBq 57Co high specific activity source in a 12 Am Rh matrix. Further details of the method are given in the literature [21,22]. The velocity scale was calibrated relative to 25 Am a-Fe foil using the positions certified for National Bureau of Standards standard reference material no. 1541; linewidths of 0.34 mm/s for the outer lines of a-Fe were obtained at room temperature. The spectra were fitted to Lorentzian lineshapes using the commercially available fitting program NORMOS written by R.A. Brand (distributed by Wissenschaftliche Elektronik, Germany). Spectra were collected over periods ranging from 1 to 41 days.
ment with the empirical relation, even though two of the MgSiO3 inclusion compositions fall outside the calibration range (Fig. 1). The strong influence of Al on Fe3 +/AFe in MgSiO3 perovskite contrasts with the moderate influence of Al on Fe3 +/AFe observed in MgSiO3 orthopyroxene [24], providing strong evidence that the MgSiO3 inclusions crystallised in the perovskite structure and hence at conditions within the lower mantle. Ferropericlase spectra are similar to those in the literature, and show a dominant Fe2 + doublet with a weak shoulder at low velocity due to Fe3 + [25,26] (Fig. 2). The relative area of Fe3 + absorption is essentially constrained by the asymmetry of the main doublet, which can be measured with a relatively high degree of precision [25 –27]. The spectra were fit to two doublets, where linewidths and centre shifts of Fe3 + absorption were constrained in some spectra to values from previous data in order to avoid unrealistic linewidths (>1 mm/s) and, in some cases, abnormal Fe3 + centre shifts (deviations of more than 0.2 mm/s from expected values). Fe3 +/AFe values were calcu-
4. Measured versus calculated Fe3+ The spectrum of MgSiO3 resembles those from Sa˜o Luiz inclusions [2], and is dominated by a Fe2 + doublet. We fixed the positions and linewidths of Fe3 + absorption to those observed in previous inclusion spectra [23] due to low iron concentration and small thickness of the absorber, and derived a value for Fe3 +/AFe based on the relative areas (Table 1). We compared Fe3 +/AFe values determined using Mo¨ssbauer spectroscopy with the empirical relation [3]: Fe3þ =RFe ¼ x3þ Fe =RxFe ¼ 0:15ð4Þ þ 4:16ð51ÞxAl ; 0:03 < xAl < 0:15
ð1Þ
where xi represents the concentration of the ith element in cations per formula unit, and the numbers in parentheses represent standard deviations of the final digits. All available data (including the two data points from the literature [2]) are in excellent agree-
Fig. 1. Relative Fe3 + concentration (Fe3 +/AFe = x3Fe+ / AxFe) versus xAl for MgSiO3 inclusions, expressed in cations per formula unit. The solid circle is Mo¨ssbauer data from Kankan inclusions (this study), while open circles represent Mo¨ssbauer data from Sa˜o Luiz inclusions [2]. The solid line indicates the empirical relation determined from experimental data [3], while dotted lines indicate 90% prediction limits.
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427
substitution mechanisms. Kesson et al. [28] proposed a substitution of the form Na0.5M0.53 +O (M = Fe3 +, Cr3 +, Al3 +): þ 3þ 4Mg2þ Mg þ Na2 O þ M2 O3 ¼ 2ðNaMg MMg Þ þ 4MgO;
ð2Þ where the subscripts denotes the lattice sites occupied by the cations. If coupled substitution were the only mechanism for the incorporation of trivalent cations, charge balance would require that AxM3 + = xFe3 + + xAl + xCr = xNa, where xn is the cation abundance of the nth cation (per formula unit). There is a positive correlation between univalent and trivalent cation abundance (Fig. 3), which confirms that trivalent cations are stabilised primarily by coupled cation substitution; however, the excess positive charge indicates the presence of an additional substitution mechanism. The incorporation of Fe3 + into (Mg,Fe)O has been studied extensively, and considered to occur primarily through substitution of Fe3 + on the octahedral cation
Fig. 2. Room temperature Mo¨ssbauer spectra of selected ferropericlase inclusions recorded using a point source (milliprobe method): (a) KK 104; (b) KK 38. The doublets corresponding to Fe3 + are shaded black, and hyperfine parameters are given in Table 1.
lated based on the relative areas and were not corrected for recoil-free fraction effects, since systematic errors due to the constraints were estimated to be larger than the corrections would be. We calculated errors based on statistics of the fitting process as well as uncertainties in the fitting model (which includes the effect of fixing parameters). There are distinct trends significantly larger than the error bars that provide information on the substitution of Fe3 + into ferropericlase diamond inclusions. Ferropericlase exists at a wide range of pressures and temperatures, with the ability to incorporate heterovalent cations through coupled substitution as well as through creation of point defects. The accurate determination of Fe3 + concentrations using Mo¨ssbauer spectroscopy allows the examination of these
Fig. 3. Trivalent cation abundance versus Na abundance for ferropericlase inclusions in diamond, expressed in cations per formula unit. Solid circles represent Mo¨ssbauer data from Kankan inclusions (this study), while open circles represent Mo¨ssbauer data from Sa˜o Luiz inclusions [2]. Charge is balanced along the 1:1 correlation line, so deviations above the line likely indicate the presence of cation vacancies.
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site, coupled with the creation of cation vacancies, e.g. [29,30]: 2þ 3þ 3þ Mg2þ Mg þ 2FeMg þ 1=2 O2 ¼ 2ðFeMg 5FeMg Þ þ MgO;
ð3Þ where 5 is a vacancy on the octahedral cation site. The charge balance condition becomes x5 ¼ 1=2 ðxFe3þ þ xAl þ xCr xNa Þ;
ð4Þ
where x5 is the number of cation vacancies. The concentration of x5 (and hence Fe3 +) is a function of oxygen fugacity according to Eq. (3), and the number of cation vacancies (and hence Fe 3 + ) increases with increasing total iron composition at constant oxygen fugacity, e.g. [31]. Therefore a plot of cation vacancy concentration (or alternatively Fe3 +) versus total iron concentration will contain a family of curves, each curve corresponding to a different oxygen fugacity. Experiments have also confirmed this at high pressure [32], where Fe3 + concentrations are
reduced at high pressure due to the transformation of (Mg,Fe)Fe2O4 to a denser phase [33]. The concentration of cation vacancies in ferropericlase inclusions from Kankan and Sa˜o Luiz [2] generally shows a positive correlation with iron composition (Fig. 4), consistent with the experimentally observed behaviour of ferropericlase. The data lie in between the experimentally determined curves corresponding to Fe equilibrium and Re –ReO2, suggesting that the ferropericlase inclusions formed at oxygen fugacities somewhere in between. The variation of Fe3 + concentration with iron composition can be described by a linear fit of data in Fig. 4 combined with Eq. (4) to obtain the results in terms of Fe3 +: x3þ Fe ¼ xNa xAl xCr þ CxFe ;
0 < xFe < 0:6 ð5Þ
where C = 0.04 F 0.01 for the inclusion data. This lies in between values determined experimentally for ferropericlase in equilibrium with Fe (C = 0.008 F 0.004) and at the Re/ReO2 buffer (C = 0.20 F 0.05) [3]. 5. Fe2+/Mg partitioning
Fig. 4. Cation vacancy concentration ( = 1/2 (x3Fe+ + xAl + xCr xNa)) versus xFe for ferropericlase inclusions in diamond, all expressed in cations per formula unit. Solid circles represent Mo¨ssbauer data from Kankan inclusions (this study), while open circles represent Mo¨ssbauer data from Sa˜o Luiz inclusions [2]. The solid lines indicate variations for Fe – (Mg,Fe)O equilibrium and Re – ReO2 equilibrium [3], while the dotted line indicates the best fit to inclusion data.
The occurrence of MgSiO3 and ferropericlase inclusions in the same diamond enables the calculation of cation partition coefficients in order to assess aspects such as the conditions under which the inclusions might have been in equilibrium, e.g. [12]. Correction of major element analyses for Fe3 + gives Fe2 +/Mg partition coefficients that are essentially constant with pressure, temperature and composition at conditions in the range P = 24– 30 GPa, T = 1600– 2200 jC and Mg#>0.75 [3]. These analyses included data for which no direct Fe3 + measurements were available, where Fe3 + was determined empirically using Eq. (1) for MgSiO3 and Eq. (5) for ferropericlase, and where the constant was chosen based on whether the experiment was performed in Fe or Re capsules. The number of diamonds with co-existing MgSiO3 and ferropericlase inclusions where Fe3 +/AFe has been measured for both phases is severely limited (currently only KK16 from the Kankan suite and BZ251 from the Sa˜o Luiz suite). We therefore esti-
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mated Fe3 +/AFe for all co-existing MgSiO3 –ferropericlase inclusion pairs given in the literature based on Eqs. (1) and (5) (Table 2). We calculated the Fe2 +/Mg pv fp fp pv partition coefficient, KD=(xFe xMg )/(xFe xMg ), where pv,fp the xFe,Mg terms are the normalised Fe2 + or Mg concentrations in MgSiO3 or ferropericlase, e.g. x*Fe2þ ¼ xFe2þ =ðxFe þ þ xMg Þ:
ð6Þ
All results from Kankan show similar values, and are in excellent agreement with the regression line determined from the experimental data (Fig. 5). The Koffiefontein diamond inclusion pair reported in [34] and two of the Sa˜o Luiz pairs (BZ207; BZ251) also give KD (Fe2 +/Mg) values that are consistent with the experimental data trend [3]. The two remaining Sa˜o Luiz pairs (BZ120; BZ210), however, give KD (Fe2 +/Mg) values that are somewhat low (Fig. 5), even allowing for the larger uncertainties. We note in particular that Fe3 +/AFe of the BZ210 MgSiO3 inclusion was determined using Mo¨ssbauer spectroscopy, so the low Fe2 + concentration in BZ210 MgSiO3 is relatively well determined. The discrepancy between observed KD (Fe2 +/Mg) values and experimental data is unlikely due to pressure or temperature effects, since pressure would increase the value of KD (Fe2 +/Mg), and temperature was not observed to have a large effect on KD (Fe2 +/Mg) up to at least 2200 jC [3]. The discrepancy is also unlikely due to errors in Fe3 + estimation, since approximately 50% of iron would have to be present
Fig. 5. Fe2 + – Mg partitioning between co-existing MgSiO3 (pv) – ferropericlase (fp) inclusions in diamond. Fe2 + concentrations are 2+ 2+ 2+ = xFe /(xFe + xMg). Fe2 + concennormalised for each phase to x*Fe trations for each phase were determined either from Mo¨ssbauer data, if available, or estimated using Eqs. (1) and (5). Solid circles represent data from Kankan inclusions, open circles represent data from Sa˜o Luiz inclusions [9], and the open square represents data from Koffiefontein inclusions [34]. The dashed line indicates the 1:1 correlation, while the dotted line indicates the best fit regression line determined from experimental data at pressures and temperatures equivalent to those near the top of the lower mantle [3].
as Fe3 + in ferropericlase in order to match experimental results. The simplest explanation for the discrepancy, of course, is a lack of equilibrium
Table 2 Fe/Mg partition coefficients for co-existing MgSiO3 (pv) and ferropericlase (fp) inclusions Source
Sample
Fe3 +/AFe, pv, obs
Fe3 +/AFe, pv, calc
Fe3 +/AFe, fp, obs
Fe3 +/AFe, fp, calc
2 +a x*Fe , pv
2 +a x*Fe , fp
KDb, Fetotal/Mg
KDc, Fe2 +/Mg
Kankan [6]
16b 25 27 44 103a 108d BZ251 BZ120 BZ207 BZ210 A262
0.09(25)
0.25(25) 0.52(25) 0.54(25) 0.32(25) 0.26(25) 0.35(25) 0.34(25) 0.33(25) 0.42(25) 0.77(25) 0.32(25)
0.02(2)
0.05(5) 0.01(1) 0.01(1) 0.01(1) 0.05(5) 0.07(7) 0.04(4) 0.05(5) 0.03(3) 0.06(6) 0.05(5)
0.037(12) 0.045(23) 0.052(28) 0.035(13) 0.038(13) 0.048(18) 0.045(5) 0.047(17) 0.084(34) 0.023(7) 0.033(12)
0.142(2) 0.162(3) 0.148(3) 0.127(3) 0.121(7) 0.134(4) 0.150(2) 0.292(12) 0.299(9) 0.171(10) 0.119(6)
0.31 0.51 0.69 0.36 0.36 0.48 0.33 0.17 0.36 0.43 0.36
0.25(4) 0.26(6) 0.30(9) 0.28(5) 0.31(5) 0.36(6) 0.27(3) 0.13(2) 0.23(4) 0.12(4) 0.28(5)
Sao Luiz [9]
Koffiefontein [34] a
0.20(6)
0.75(3)
0.06(6) 0.04(3) 0.00(1)
Normalised to Fe2 +/(Fe2 + + Mg). Based on major element analysis. c Corrected using measured Fe3 +/AFe for each phase (if available); otherwise using calculated Fe3 +/AFe. b
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between co-existing inclusions. Such a conclusion has already been suggested [9], but the results of the present study allow a more quantitative evaluation of possible equilibrium compositions. For both BZ120 and BZ210 MgSiO3 inclusions, if we assume that Fe2 +/Mg partitioning follows the same trend shown by other inclusions, equilibrium could have occurred in each case with ferropericlase containing approximately half the concentration of iron, or alternatively, ferropericlase inclusions could have equilibrated with MgSiO3 that contained twice as much total iron. The estimated iron concentrations in all cases are within estimated iron stability limits for (Mg,Fe)(Si,Al)O3 perovskite and (Mg,Fe)O at lower mantle pressures and temperatures.
6. Implications for diamond formation The concentration of trivalent cations in ferropericlase is a function of oxygen fugacity, which enables an assessment of conditions based on major element and Fe3 +/AFe data. Ferropericlase inclusions from Sa˜o Luiz show cation vacancy concentrations significantly elevated above the expected trend for equilibrium with Fe (Fig. 4), which is consistent with the observation of tiny inclusions (typically 1 –3 Am) of (Mg,Fe)Fe2O4 in a number of the ferropericlase inclusion grains [9]. Their appearance suggested formation after primary crystallisation of the ferropericlase, perhaps as a result of decreasing temperature during transport within the mantle. Since the amount of (Mg,Fe)Fe2O4 lies below the detection limit for Mo¨ssbauer spectroscopy, the Fe3 + concentration measured for ferropericlase represents a minimum. In contrast, nearly all of the Kankan inclusions lie close to the Fe equilibrium curve (Figs. 3 and 4), and no compositional variations were noted in Kankan ferropericlase that would suggest the presence of (Mg,Fe)Fe2O4 (although we cannot rule it out) [6]. One possibility is therefore that Kankan diamonds formed at conditions more reducing than those for Sa˜o Luiz. However, since ferropericlase inclusions from Kankan contain less iron than those from Sa˜o Luiz, assessment of oxygen fugacity is more equivocal. In any case, diamonds are stable over a relatively large range of oxygen fugacity [35]; hence, results for both Kankan
and Sa˜o Luiz diamonds are consistent with existing diamond stability data. The oxygen fugacity recorded by ferropericlase inclusions can be placed within the larger framework of mantle redox conditions. Efforts over the past 20 years have produced much data concerning upper mantle oxygen fugacity, and although agreement is not universal, a relatively consistent picture is emerging. The oxygen fugacity measured for lithospheric upper mantle is heterogeneous on a scale of at least four log units, where the lowest values have been recorded for suboceanic abyssal peridotites and xenoliths from zones of continental extension, while more oxidised values have been recorded for xenoliths from regions of recent or active subduction [36]. Oxygen fugacity decreases in the deep upper mantle due to the enhanced incorporation of Fe3 + into garnet due to pressure, approaching conditions near the iron – wu¨stite buffer near the transition zone, e.g. [37]. The increased affinity of all transition zone minerals for Fe3 + compared to those in the upper mantle leads to a lowering its chemical potential, thus reducing oxygen fugacity, probably close to conditions of metal saturation [38]. There is hence no compelling reason for a redox gradient across the 410 km discontinuity. The oxygen fugacity of the lower mantle is still an open question, and while a reduced lower mantle with conditions similar to the transition zone is consistent with the more primitive nature inferred for the lower mantle, there is also evidence for heterogeneities in volatile content that would imply that some regions are more oxidised [39]. Superimposed on this picture is evidence from redox measurements of the mantle wedge above subduction zones that indicate values significantly more oxidised than the surrounding mantle [36, 40,41], where the source of oxygen is suggested to be the subducted slab [41]. This contrast in oxygen fugacity between slabs and surrounding mantle may be preserved to greater depths in a manner similar to thermal and petrological contrasts, e.g. [42]. Minerals within subducting slabs have the capacity to incorporate significant quantities of Fe3 + [43], hence providing a mechanism to transport oxygen into the deeper mantle. A preliminary picture of redox conditions in the deep Earth, therefore, shows a gener-
C.A. McCammon et al. / Earth and Planetary Science Letters 222 (2004) 423–434
ally reduced bulk mantle with possibly more oxidised regions, at least some associated with subducting slabs. Diamond growth is generally favoured in regions of redox gradients [35], which may provide a clue to its origin in the lower mantle. The introduction of oxidised subducted material into reduced lower mantle could generate redox fronts that provide conditions favourable for diamond growth [19]. Another consideration is the stability of Fe3 + in (Mg,Fe)(Si,Al)O3 perovskite relative to g-(Mg,Fe)2SiO4 ringwoodite. Fe3 +/AFe is relatively low in g-(Mg,Fe)2SiO4 ringwoodite, even under oxidising conditions (ca. 1%) [32], while in the perovskite phase Fe3 +/AFe is significantly higher. A bulk pyrolite composition at the top of the lower mantle requires at least 10% Fe3 +/AFe in (Mg,Fe)SiO3 perovskite at reducing conditions [23], while the high concentration of Al in MORB requires at least 60% Fe3 +/ AFe in (Mg,Fe)(Si,Al)O3 perovskite at reducing conditions [3,13]. Charge balance can be satisfied during the transformation of g-(Mg,Fe)2SiO4 ringwoodite to (Mg,Fe)SiO3 perovskite+(Mg,Fe)O ferropericlase either through disproportionation (3Fe2 + = Fe0 + 2Fe3 +) or oxidation (2FeO + 1/2O2 = Fe2O3). The latter could be coupled to the reduction of volatile species to release oxygen. Phase changes associated with the 660 km discontinuity might enhance the mobility of volatile species. For example, results from recent experiments suggest that subduction of oceanic lithosphere into the lower mantle may release large quantities of water as a separate fluid or melt phase and contribute to a local depression of the melting temperature [44]. The transition zone phases h-(Mg,Fe) 2 SiO 4 and g(Mg,Fe)2SiO4 are known to incorporate large amounts of water (up to 3 wt.% H2O); however, the dominant lower mantle phases (Mg,Fe)SiO3 perovskite and (Mg,Fe)O incorporate significantly less (ca. 20 ppm wt. H2O) [44,45]. At greater depths where garnet transforms to the perovskite structure, hence increasing the Al concentration of (Mg,Fe)(Si,Al)O3 perovskite, larger amounts of water can be incorporated in pyrolite compositions (up to 0.2 wt.% H2O) [46]. In MORB compositions, however, (Mg,Fe)(Si,Al)O3 perovskite incorporates significantly less water (40 –110 ppm wt. H2O), suggested to be due to a different substitution mechanism for trivalent cations at higher Al concentrations [46,47].
431
Carbon isotopic compositions (d13C) of lower mantle diamonds show a narrow range with a mode at 4.5x, which is close to typical mantle compositions ([48] and references therein). Also, all lower mantle diamonds identified so far are Type II, implying negligible nitrogen concentration, in comparison to upper mantle diamonds where ca. 98% have been identified as Type I [1]. It is not clear, however, the degree to which these characteristics reflect those of their source region. Trends between d13C and nitrogen concentration in the worldwide diamond database have been suggested to arise from the evolution of mantle melts (or fluids) during differentiation, where the uptake of nitrogen depends on the rate of diamond growth [49]. These patterns may be complicated by different diffusive relaxation times between carbon and nitrogen isotopes [50]. According to these models the relatively uniform carbon isotopic compositions in lower mantle diamonds can be explained by relaxation of heterogeneities at high temperatures over long residence times [50] and the low nitrogen concentrations can be explained by growth over long periods of time [49]. Kankan diamonds may provide a clue to the origin of carbon in lower mantle diamonds. Carbon isotopic compositions are slightly shifted to isotopically heavier compositions, which was suggested to indicate that diamond formation may have occurred through reduction of subducted carbonates [6]. Magnesite (MgCO3) is known to be stable at conditions throughout the transition zone and into the lower mantle, e.g. [51,52], oceanic carbonates have d13C values that are typically close to 0x [53], and reduction of carbonate could occur according to the reaction MgCO3 ¼ MgO þ C þ O2 :
ð7Þ
Diamond formation according to reaction (7) could account for the high proportion of ferropericlase in lower mantle diamond inclusions (more than half of worldwide occurrences), as well as supply the oxygen needed to oxidise iron during transformation to the perovskite structure. The source region for lower mantle diamonds may undergo significant changes during the crystallisation of individual diamonds. This is illustrated by Fe2 +/Mg partitioning data for diamonds BZ210 and BZ120 from Sa˜o Luiz that show disequilibrium between co-
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existing MgSiO3 and ferropericlase inclusions. We may envisage at least two possible scenarios: (a) ferropericlase was included first, then a partial melting event depleted the source in iron before a second growth stage when MgSiO3 perovskite was included; (b) MgSiO3 perovskite was included first, then melt infiltration enriched the source in iron before a second growth stage when ferropericlase was included. While further data is required to resolve the possible scenarios, these results emphasise the potential heterogeneity of lower mantle diamond sources, and that compositions of lower mantle diamonds do not necessarily reflect those of the bulk mantle.
7. Concluding remarks This study has examined Fe3 + concentrations in inclusions from lower mantle diamonds using Mo¨ssbauer spectroscopy. We calculated Fe2 +/Mg partition coefficients between MgSiO3 and ferropericlase based on these results and empirical relations derived from high-pressure experimental work, and found values for most of the inclusion pairs that were consistent with high-pressure experiments, suggesting that these inclusions equilibrated at lower mantle conditions. Such determinations of Fe2 +/Mg partition coefficients are possible despite the lack of Fe3 + data for ferropericlase, because Fe3 + concentrations are low and hence contribute a relatively small uncertainty to the overall calculation. On the other hand, this study has also shown that Fe3 + concentrations in ferropericlase can be used to qualitatively evaluate oxygen fugacity conditions during diamond formation. More data is clearly needed to understand the mechanism of diamond formation in the lower mantle, but unfortunately techniques such as Mo¨ssbauer spectroscopy are limited to samples with grain size greater than 100 Am, and methods with higher spatial resolution such as electron energy loss spectroscopy require destructive sample preparation. Other methods such as X-ray absorption spectroscopy using synchrotron sources and X-ray emission spectroscopy using the electron microprobe show promise, and
may provide a means for rapid and routine determination of Fe3 +/AFe in ferropericlase in order to construct multiparameter databases for lower mantle diamonds and their inclusions.
Acknowledgements DeBeers Consolidated Mines is thanked for their support. The manuscript was significantly improved through constructive reviews by D. Andrault, P. Cartigny and B. Harte. [BW]
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[12]
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