Diffusion and mobility of electrically conducting defects in olivine

 Electrical conductivity of lherzolite (65% olivine), measured as a function of time after changes in the oxygen fugacity (f _o2) of the surrounding CO₂/CO atmosphere, is used to infer the diffusivity of the point defects responsible for conduction in olivine. A total of 63 equilibration runs at temperatures of 900, 1000, 1100, and 1200 ^∘C were fit using nonlinear parameter estimation to recover time constants (directly related to diffusivity) and conductivity steps. An observed f _o2 dependence in the time constants associated with re-equilibration implies two defect species of fixed diffusivity but with f _o2-dependent concentrations. Although the rate-limiting step may not necessarily be associated with a conducting defect, when time constants are converted to diffusivities, the magnitudes and activation energies agree extremely well with the model for magnesium vacancies (the slower species) and small polarons (holes localized on Fe³⁺) derived by Constable and Roberts (1997). This earlier study used an independent method of simultaneous modeling of thermopower and electrical conductivity as a function of f _o2 and temperature, on data from a different type of sample (a dunite). We observe that at high f _o2 where polarons dominate over magnesium vacancies in the defect population, re-equilibration is dominated by magnesium vacancy diffusion, and vice versa (at low f _o2 magnesium vacancies dominate and re-equilibration proceeds at the faster rate associated with polaron mobility). We interpret this to suggest association between the cation vacancies and polarons, as has been suggested by Tsai and Dieckmann (1997), making the concentration of the minority defect the rate-limiting step in the oxidation/reduction reactions. Received: 18 October 2000 / Accepted: 7 May 2002     

Experimental determination of Ni diffusion coefficients in olivine and their dependence on temperature, composition, oxygen fugacity, and crystallographic orientation

Diffusion couple experiments were carried out with San Carlos olivine (Fo₉₀) and NiFe alloys (Ni₁₀₀, Ni₉₇Fe₃, Ni₉₀Fe₁₀) or other olivine compositions (Fo₁₀₀, Fo₂₅) in order to determine the dependence on temperature, oxygen fugacity, composition and crystallographic orientation of Ni diffusion coefficient (D_Ni) in olivine. Experiments at 1 atmosphere total pressure cover a temperature range of 900-1445°C with run durations from 48 to 2155 h at different oxygen fugacities. In an Arrhenius plot the best fit for all data for Fo₉₀ yields an activation energy (E_D) of 220 ± 14 kJ/mol and an fO₂ dependence of (1/4.25)·Δ log fO₂ = Δ log D_Ni. The relationship between diffusion coefficients along different crystallographic axes at 1200°C is given by D_[001] ≈ 6·D_[100] ≈ 6·D_[010]. D_Ni depends strongly on the major element (i.e. Fe/Mg) composition of olivine and decreases by about 1 order of magnitude as the olivine composition changes from Fo₃₅ to Fo₉₀. Thus, experimental investigations in Fe-free systems cannot be applied to natural samples. For calculation of residence times or cooling rates the present Ni data yield shorter timescales compared to those obtained using diffusion data published until now.In addition to Ni diffusion coefficients, Fe-Mg, Mn and Ca diffusion data were obtained from some of the same diffusion couples (Fo₉₀-Fo₁₀₀). It is found that the activation energies, E_D[Ni] ≅ E_D[Fe-Mg] ≅ E_D[Mn] ≤ E_D[Ca]. All diffusion coefficients are strongly dependent on the major element composition of olivine.     

Manganese olivine II: point defect relaxation

To investigate the point defect chemistry and the kinetic properties of manganese olivine Mn₂SiO₄, the point defect relaxation time (τ) characterizing the rate of re-equilibration of electrical conductivity following a change in oxygen fugacity was measured for single crystals oriented for electrical conduction along the [010] direction. The experiments were carried out at temperatures T = 1173–1473 K and oxygen fugacities with the MnSiO₃ activity controlled at unity. The value of τ, which ranges from 130 to 1463 s, increases with decreasing temperature. At 1273 K, the value of τ in the regime is a factor of ∼1.8 smaller than that in the regime. The point defect relaxation time was used to calculate the chemical diffusivity (). Values of lie in the range 2.2× 10^{− 10} − 2.5×10^{− 9} m²/s. For the regime, a semi-log plot of vs 1/T yields a concave downward curve. Based on these results combined with those from Part I of this work for the point defect structure and electrical conductivity in Mn-olivine, it is proposed that the relaxation rate of electrical conduction is determined by the coupled diffusion of manganese vacancies and electron holes which rate-control the relaxation process at low and high temperatures, respectively. Deconvolution of the curve into two straight-line segments yields values for the mobilities and the diffusivities for Mn vacancies and electron holes. These results, combined with the measured electrical conductivity data, were used to calculate the concentrations of Mn vacancies and electron holes. These results in conjunction with those published for other transition-metal silicate olivines reveal the following: The mobility of electron holes in Mn-olivine is about two orders of magnitude smaller than that in Fe-olivine and is somewhat larger than that in (Mg_0.9Fe_0.1)-olivine. The mobilities of metal vacancies in these three olivines are similar both in magnitude and in temperature dependence. The concentrations of the majority point defects are the highest in Fe-olivine crystals and the lowest in Co-olivine crystals, while those in Mn-olivine and (Mg_0.9Fe_0.1)-olivine crystals lie in between. The deviation from stoichiometry in the cation sublattice for a transition-metal silicate olivine is about two orders of magnitude lower than that for the corresponding transition-metal oxide. Received: 10 January 1996 / Revised, accepted: 29 May 1997     

Simultaneous modeling of thermopower and electrical conduction in olivine

 Measurements of conductivity and thermopower as a function of oxygen fugacity (ƒ_{O 2}) are used to derive a model for conduction in olivine. Thermopower at 1000–1200 °C is between 50 and 400 μV/K and has a positive ƒ_{O 2} dependence, and electrical conductivity exhibits approximately a ¹/₁₁ power dependence on ƒ_{O 2}. However, small polarons, considered to be the conducting defect in olivine at these temperatures, would produce a larger thermopower than observed, with a negative ƒ_{O 2} dependence, as well as ¹/₆ power dependence of conductivity on ƒ_{O 2}. At least one other conducting defect species must be invoked to explain the observed magnitude and ƒ_{O 2} dependence of thermopower. An electron/polaron model cannot be made to fit the conductivity and thermopower data well, but a polaron/magnesium vacancy model fits the data if a constant polaron or magnesium vacancy term is included. Concentrations from our fits are consistent with predictions from theoretical models, and our analysis predicts a transition from polaron dominance in conduction to magnesium vacancy dominance at around 1300 °C, as has been previously inferred from other data. Received April 2, 1996 / Revised, accepted September 6, 1996     

Experimental determination of the diffusion coefficient for calcium in olivine between 900°C and 1500°C

Experiments have been carried out to determine the temperature, oxygen fugacity (fO₂) and compositional dependence of the tracer diffusion coefficient (D) of calcium in olivine. These data constrain the diffusion coefficient over the temperature range 900 to 1500°C for the three principal crystallographic axes. Well constrained linear relationships between the reciprocal of the absolute temperature and log(D) exist at any given oxygen fugacity. There is a strong dependence of the diffusion coefficient on oxygen fugacity with D ∝ fO₂^(1/3). This makes a knowledge of the T-fO₂ path followed by geological samples a prerequisite for modelling Ca diffusion in olivine. The best fitting preexponential factor (Do) and activation energy (E) to the Arrhenius equation log (D) = log [Do exp(−E/RT)] + 0.31Δ log fO₂ for Ca diffusion in olivine at a given oxygen fugacity (fO₂*) are given by:diffusion along [100]: log [Do (m²/s)] = −10.78 ± 0.43; E = 193 ± 11 kJ/moldiffusion along [010]: log [Do (m²/s)] = −10.46 ± 0.37; E = 201 ± 10 kJ/moldiffusion along [001]: log [Do (m²/s)] = −10.02 ± 0.29; E = 207 ± 8 kJ/molwhere Δ log fO₂ = log[fO₂*] − log[10⁻¹²] with fO₂* in units of bars. There is no measurable compositional dependence of the diffusion coefficient between Fo₈₃ and Fo₉₂. Diffusion in Fo₁₀₀ has a much higher activation energy than in Fe-bearing olivine and has a weaker fO₂ dependence.     

西藏罗布莎蛇绿岩中不同产出的纯橄岩及成因探讨

罗布莎蛇绿岩中的纯橄岩有三种产出情况,除了与豆荚状铬铁矿伴生的薄壳状纯橄岩外,还有产在方辉橄榄岩底部被认为是堆晶岩的厚层状纯橄岩和方辉橄榄岩中的透镜状纯橄岩。厚层状纯橄岩约700～1000m厚,以橄榄石富镁(Fo93～95),单斜辉石低铝富镁(Al2O30.47%～0.85%,Mg#95～97),铬尖晶石高铬低镁(Cr#值平均77,Mg#平均51)为特征。该纯橄岩中的浸染状铬铁矿也是高铬低镁型,但Mg#值(平均59)高于厚层状纯橄岩的副矿物铬尖晶石。薄壳状纯橄岩与厚层状纯橄岩成分相近,其橄榄石Fo92～94,单斜辉石Al2O3<1%和Mg#95～97;铬尖晶石的Cr#值平均71,Mg#值平均52。与薄壳状纯橄岩伴生的块状铬铁矿为高镁高铬型,但Mg#值(平均68)相对更高些,Cr#值平均79。透镜状纯橄岩的特征是橄榄石Fo(91～92)和铬尖晶石Cr#(60左右)均低于前两类纯橄岩,但单斜辉石的Al2O3(1.41%～1.71%)则高于前两者。透镜状纯橄岩的矿物成分与方辉橄榄岩重叠,两者为渐变过渡关系。研究对比表明,罗布莎厚层状纯橄岩不同于经典的蛇绿岩的超镁铁质堆晶岩,认为将其成因解释为拉斑玄武质熔体与地幔橄榄岩的反应较为合理。透镜状纯橄岩与方辉橄榄岩存在成生联系,可能是地幔橄榄岩高度部分熔融的产物,或熔体和方辉橄榄岩在原位发生反应的产物;薄壳状纯橄岩成因与厚层状纯橄岩相同,但与其相伴的块状铬铁矿是否由拉斑玄武质熔体与方辉橄榄岩反应形成,值得商榷。     

Frequency dependent electrical properties of dunite as functions of temperature and oxygen fugacity

Using impedance spectroscopy, we have measured the electrical properties of two dunites and a single crystal olivine sample from 1000 to 1200° C as a function of oxygen fugacity (f _{o 2}). Two conduction mechanisms with resistances that add in series are observed for the dunites corresponding to grain interior and grain boundary conduction mechanisms. The conductivities for each mechanism were determined by analyzing the data using a complex nonlinear least squares fitting routine and the equivalent circuit approach. The grain interiors display a conductivity dependent on f _{o 2} to the 1/5.5–1/7 power, consistent with other determinations, and interpreted as indicating small polaron transport (Fe _Mg ^· ). The grain boundaries demonstrate a weaker f _{o 2} dependence that is dependent on temperature and material. Under certain conditions the f _{o 2} dependence of the grain boundary conductivity is negative. This result indicates that oxygen ion transport is probably not the dominant grain boundary charge transport mechanism; however, an unequivocal determination of the grain boundary mechanism has not been achieved. In some dunites the grain boundaries are more conductive than the grain interiors; in other dunites they are more resistive than grain interiors. The grain boundaries do not enhance the total conductivity of any of the materials of this study but are the controlling mechanism in some instances. Measurement of the complex electrical response at frequencies as low as 10^-4 Hz is required to determine the role of grain boundaries on the overall electrical properties of polycrystalline dunite.     

Thermodynamic analysis of Fe and Mg partitioning between plagioclase and silicate liquid

 Thermodynamic analysis of Fe- and Mg-bearing plagioclase and silicate liquid was carried out based on reported element partitioning data between plagioclase and silicate liquid in reduced conditions, solution properties of ternary feldspar, standard state properties of plagioclase endmembers and solution properties of multicomponent silicate liquid. Derived mixing properties of Fe- and Mg-bearing plagioclase are in harmony with estimated results from synthetic experiments in the systems CaAl₂Si₂O₈-CaFeSi₃O₈ and CaAl₂Si₂O₈-CaMgSi₃O₈. Based on the determined solution properties of the plagioclase, a computer program to calculate the element partition relationships between Fe- and Mg-bearing plagioclase and multicomponent silicate liquid was developed. The FeO, MgO and MgO/(MgO + FeO) in plagioclase predicted from known liquid compositions and pressure are in agreement with measurements within 0.2 wt%, 0.1 wt% and 0.1 (mol ratio), respectively. The Fe³⁺ content in plagioclase crystallized at high oxygen fugacity can be estimated with this program. The Fe³⁺/total Fe ratio in plagioclase crystallized near the quartz-fayalite-magnetite buffer ranges from 0 to 0.5, which is consistent with previous study on natural plagioclase in submarine basalt. Derived solution properties of the Fe- and Mg-bearing plagioclase are also used to calculate equilibrium composition relationship between olivine and plagioclase. Change of X _Fo in olivine coexisting with plagioclase affects MgO and FeO contents in plagioclase greatly. The present model predicts X _Fo of coexisting olivine from the chemical composition of plagioclase to ±0.1 accuracy at given pressure and temperature. Received: 27 March 1998 / Accepted: 30 September 1999     

The importance of defining chemical potentials,substitution mechanisms and solubility in trace element diffusion studies: the case of Zr and Hf in olivine

The diffusion, substitution mechanism and solubility limits of Zr and Hf in synthetic forsterite (Mg₂SiO₄) and San Carlos olivine (Mg_0.9Fe_0.1)₂SiO₄ have been investigated between 1,200 and 1,500 °C as a function of the chemical potentials of the components in the system MgO(FeO)–SiO₂–ZrO₂(HfO₂). The effect of oxygen fugacity and crystallographic orientation were also investigated. The solubilities of Zr in forsterite are highest and diffusion fastest when the coexisting three-phase source assemblage includes ZrSiO₄ (zircon) or HfSiO₄ (hafnon), and lower and slower, respectively, when the source assemblage includes MgO (periclase). This indicates that Zr and Hf substitute on the octahedral sites in olivine, charge balanced by magnesium vacancies. Diffusion is anisotropic, with rates along the crystal axes increasing in the order a < b < c. The generalized diffusion relationship as a function of chemical activity (as \(a_{{{\text{SiO}}_{2} }}\)), orientation and temperature is: \(logD_{\text{Zr}} = \frac{1}{4}loga_{{{\text{SiO}}_{2} }} + logD_{0} - \left( {\frac{{368 \pm 17\;{\text{kJ}}\;{\text{mol}}^{ - 1} }}{{2.303\;{\text{RT}}}}} \right)\) where the values of log D ₀ are ?3.8(±0.5), ?3.4(±0.5) and ?3.1(±0.5) along the a, b and c axes, respectively. Most experiments were conducted in air (fO₂ = 10^?0.68 bars), but one at fO₂ = 10^?11.2 bars at 1,400 °C shows no resolvable effect of oxygen fugacity on Zr diffusion. Hf is slightly more soluble in olivine than Zr, but diffuses slightly slower. Diffusivities of Zr in experiments in San Carlos olivine at 1,400 °C, fO₂ = 10^?6.6 bars are similar to those in forsterite at the same conditions, showing that the controls on diffusivities are adequately captured by the simple system (nominally iron-free) experiments. Diffusivities are in good agreement with those measured by Spandler and O’Neill (Contrib Miner Petrol 159:791–818, 2010) in San Carlos olivine using silicate melt as the source at 1,300 °C, and fall within the range of most measurements of Fe–Mg inter-diffusion in olivine at this temperature. Forsterite–melt partitioning experiments in the CaO–MgO–Al₂O₃–SiO₂–ZrO₂/HfO₂ show that the interface concentrations from the diffusion experiments represent true equilibrium solubilities. Another test of internal consistency is that the ratios of the interface concentrations between experiments buffered by Mg₂SiO₄ + Mg₂Si₂O₆ + ZrSiO₄ or Mg₂SiO₄ + ZrSiO₄ + ZrO₂ (high silica activity) to those buffered by Mg₂SiO₄ + MgO + ZrO₂ (low silica activity) agree well with the ratios calculated from thermodynamic data. This study highlights the importance of buffering chemical potentials in diffusion experiments to provide constraints on the interface diffusant concentrations and hence validate the assumption of interface equilibrium.     

Application of the cBΩ model to the calculation of diffusion parameters of He in olivine

The validity of the thermodynamic cBΩ model is tested in terms of the experimentally determined diffusion coefficients of He in a natural Fe-bearing olivine (Fo₉₀) and a synthetic end-member forsterite (Mg₂SiO₄) over a broad temperature range (250–950 °C), as reported recently by Cherniak and Watson (Geochem Cosmochim Acta 84:269–279, 2012). The calculated activation enthalpies for each of the three crystallographic axes were found to be (134 ± 5), (137 ± 13) and (158 ± 4) kJ mol^?1 for the [100], [010] and [001] directions in forsterite, and (141 ± 9) kJ mol^?1 for the [010] direction in olivine, exhibiting a deviation of <1 % with the corresponding reported experimental values. Additional point defect parameters such as activation volume, activation entropy and activation Gibbs free energy were calculated as a function of temperature. The estimated activation volumes (3.2–3.9 ± 0.3 cm³ mol^?1) of He diffusion in olivine are comparable with other reported results for hydrogen and tracer diffusion of Mg cations in olivine. The pressure dependence of He diffusion coefficients was also determined, based on single experimental diffusion measurements at 2.6 and 2.7 GPa along the [001] direction in forsterite at 400 and 650 °C.     

P–T–fO2 controls on a partly inverse chromite-bearing ultramafic intrusive: an evaluation from the Sukinda Massif, India

The chromites from the alpine type ultramafic intrusive of Sukinda, India, display a typical partly inverse spinel form and occur in two distinct zones: Brown Ore Zone (BOZ) and Grey Ore Zone (GOZ). The host ultramafites are mostly altered and are represented by the serpentinite, tremolite-talc(chlorite) schist, talc-serpentine schist and chlorite rock. The less altered variants are dunite, harzburgite and websterite. A dyke of orthopyroxenite runs through the main ultramafic body.The composition of olivine (Fo₉₂), orthopyroxene (En_92–89) and Al₂O₃ contents of the parental liquid (10.40–11.45%) determined from chromites, suggest that the parent melt is of boninitic affinity. The chemical plot of TiO₂ content against cr# of chromites corroborates a boninitic parental melt. The Fe–Mg partitioning in olivine and chromite depicts the temperature for chromitites as 1200 °C. A compositional plot of mg# and cr# suggests crystallization at high pressure conditions, corresponding to the kimberlite xenolith field. From the P–T diagram of pyrolite melting and mineral assemblage, the pressure of crystallization is stipulated to be ≥1.2 GPa. The f_O2 values estimated from Fe³⁺/Cr+Al+Fe³⁺ ratios range from 10^−8.3 to 10^−9.3 for the GOZ and 10^−7.1 to 10^−7.3 for the BOZ. The f_O2 values together with the pressure range suggest crystallization at upper mantle conditions. The heterogeneity in chemical composition and f_O2 conditions for the GOZ and BOZ could be linked to heterogeneity in the upper mantle.     

Determination of the uranium valence state in the brannerite structure using EELS, XPS, and EDX

In this study, the valence states of uranium in synthetic and natural brannerite samples were studied using a combination of transmission electron microscopy-electron energy loss spectroscopy, scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX), and X-ray photoelectron spectroscopy (XPS) techniques. We used a set of five (UO₂, CaUO₄, SrCa₂UO₆, UTi₂O₆, and Y_0.5U_0.5Ti₂O₆) U standard samples, including two synthetic brannerites, to calibrate the EELS branching ratio, M₅/(M₄ +M₅), against the number of f electrons. The EELS data were collected at liquid nitrogen temperature in order to minimise the effects of electron beam reduction of U⁶⁺ and U⁵⁺. Test samples consisted of three additional synthetic brannerites (Th_0.7U_0.3Ti₂O₆, Ca_0.2U_0.8Ti₂O₆, and Th_0.55U_0.3Ca_0.15Ti₂O₆) and three natural brannerites from different localities. The natural brannerite samples are all completely amorphous, due to cumulative alpha decay events over geological time periods (24–508 Ma). Our U valence calibration results are in reasonable agreement with previous work, suggesting possibly a non-linear relationship between the branching ratio and the number of f electrons (and hence the average valence state) of U in solids. We found excellent agreement between the nominal valence states of U and the average valence states determined directly by EELS and estimated by EDX analysis (with assumptions regarding stoichiometry) in two of the three synthetic brannerite test samples. The average U oxidation states of the five synthetic brannerite samples, as derived from XPS analyses, are also in good agreement with those determined by other techniques. The average valence states of U in three amorphous (metamict) natural brannerite samples with alpha decay doses ranging from 3.6×10¹⁶ to 6.9×10¹⁷ /mg were found to be 4.4, 4.7, and 4.8, consistent with the presence of U⁵⁺ and/or U⁶⁺ as well as U⁴⁺ in these samples. These results are in general agreement with previous wet chemical analyses of natural brannerite. However, the average valence states inferred by SEM-EDX for two of the natural brannerite samples do not show satisfactory agreement with the EELS determined valence. This may be due to the occurrence of OH^– groups, cation vacancies, anion vacancies, or excess oxygen in the radiation-damaged structure of natural brannerite.     

Olivine/Fe-metal equilibrium under high pressure: an ATEM investigation

San Carlos olivine samples enclosed in soft iron capsules were annealed in an uniaxial split-sphere apparatus, at pressures ranging from 4.6 to 9.0 GPa and temperature ranging from 1310o to 1595 oC. We estimated the annealing fO₂, theoretically controlled by the olivine/Fe-metal equilibrium, to be 1 to 2 log units above the fO₂ of the iron/wustite buffer. Samples were investigated by analytical transmission electron microscopy (ATEM) in order to verify that olivine and Fe capsule did equilibrate during the annealings. TEM imaging of the olivine bulk shows a and c dislocations confined in the (010) plane, and small (0.5 μm) spatially coupled precipitates of (1) Al-rich spinel and (2) enstatite (volumic proportion of precipitates ≃60 ppm). These coupled precipitates are surrounded by split c dislocation loops. Olivine composition profiles, determined by ATEM near the Fe-capsule/olivine contact, reveal a weak loss of Ni from the olivine matrix toward the capsule, as expected in such reducing conditions. These profiles also reveal a marked incorporation of Fe from the capsule into the olivine matrix. These observations, and their interpretation in terms of olivine point defect chemistry, lead to the following conclusions: (1) the starting olivine contained a high concentration of vacancies on octahedral sites (≥1000 ppm per site); such a high vacancy concentration is expected in San Carlos olivine which equilibrated in nature at relatively high fO₂; (2) the olivine/Fe-metal equilibrium did control fO₂ during the annealings, that resulted in a rapid re-equilibration of olivine at the beginning of the runs to the lower fO₂ imposed by the Fe capsule; this led to a strong decrease of the octahedral vacancy concentration in olivine. (3) Such a fO₂ decrease promoted in olivine the coupled precipitation of both types of Al-rich spinel and enstatite precipitates. These observations show that the use of Fe-capsule in high pressure experiments is an efficient method for controlling fO₂ when studying olivine, and more generally Fe-bearing silicates. Received: 5 November 1997 / Accepted: 5 May 1998     

Lamellar pyroxene-spinel symplectites in lunar olivine from the Luna 24 regolith

Cr-Ca lamellae in a magnesian olivine grain (section 1611) from the Luna 24 regolith were investigated in detail by electron microprobe analysis (EMPA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was found that the lamellae are parallel to the (100) plane of oxygen closest packing in olivine and consist of regular vermicular intergrowths of two phases, diopside (Di) and chromite (Chr), in the volume proportion Di: Chr ≈ 3: 1. The bulk chemical composition of the lamellae is approximated as Ca₂Mg₂Fe²⁺(Cr³⁺)₂Si₄O₁₆. They are identical in phase composition to type A, F, and E symplectites from Apollo lunar samples [9]. Based on morphology and phase composition, the lamellar aggregates in the olivine grain from the Luna 24 regolith were classified as pyroxene (Px)-spinel (Spl) symplectites of a lamellar type, the formation of which was related to olivine oxidation at IW ≤ logfO₂ ≤ QFM. The obtained data indicate a solid-phase mechanism of lamella formation and the existence of a lamellar precursor phase, which transformed subsequently into the Px-Spl symplectite. It was supposed that uvarovite-knorringite garnet produced by the oxidation of olivine at high pressures and t > 800°C could be the transitional phase during symplectite formation. The subsequent conversion of the garnet into the low-pressure assemblage of Px-Spl symplectites could occur via cellular decomposition in accordance with the reaction Ca₂MgCr₂Si₃O₁₂ + (Mg,Fe)₂SiO₄ = 2CaMgSi₂O₆ + FeCr₂O₄. The reported results are the first data of a detailed nanomineralogical investigation of lamellar Px-Spl symplectites in lunar olivine.     

Cations in olivine,Part 2: Diffusion in olivine xenocrysts,with applications to petrology and mineral physics

Diffusivities for calcium, iron, magnesium, manganese and aluminum have been measured for St. John's olivine undergoing cation exchange with synthetic basaltic melts. The variety of temperature, pressure and fO₂ conditions under which the diffusivities were measured complement the equilibrium-partitioning study of calcium in olivine-bearing basalts by Jurewicz and Watson, 1988. Olivine was found to be anisotropic with respect to the diffusion of calcium, iron, magnesium and manganese. This anisotropy is a weak function of temperature, but strongly dependent upon oxygen fugacity.Because diffusion is independent of olivine composition over the small range of compositions used in this study, it could be shown that the absolute values of the diffusion coefficients were also functions of temperature and fO₂. At near-atmospheric total pressure and an oxygen fugacity of 10^–8atm, D _Fe>D _Mn>D _Ca and D _MgD _Mn for a range of geologically reasonable temperatures. These relative diffusivities were shown to change with oxygen fugacity. The power-law dependence of diffusion on oxygen partial-pressure was determined for each cation and the results are consistent with the range of values given by Stocker (1978) and by other workers.For Ca and Fe, the effect of hydrostatic pressure on diffusion appears to be weak, at least for transport parallel to the c crystallographic direction. Unfortunately, no true activation volumes (or other pressure-related parameters) could be computed because the oxygen fugacity was not held constant over changes in pressure, and because accurate post-experiment reconstruction of sample orientation was not possible. Al was found to enter high-pressure olivines at concentrations of up to 0.14 weight percent, thus allowing aluminum diffusion to be characterized. The diffusivity of aluminum is, within error, the same as iron at 20 kb at 1430° C at the ambient fO₂ of our piston-cylinder cells. This correspondence suggests that diffusion of Al may depend on transport of either Fe or of Fe ⁺³ defects. While the results of these experiments are generally consistent with results published elsewhere, there are important inconsistencies. Tracer diffusion and interdiffusion in pure, ordered, olivine endmembers (e.g., tephroite and forsterite) showed significantly higher activation energies. This discrepancy could reflect the role of Fe⁺³ defects in diffusion; however, it may also suggest that order-disorder phenomena may be significant factors influencing diffusion in analog systems.The results of this study are applied to four petrologic problems: (1) calculation of rates of equilibration for olivine xenocrysts; (2) calculation of closure temperatures for the CaO/MgO olivine/basalt geothermometer (Jurewicz and Watson 1988); (3) delineation of an intrinsic-/O₂ geobarometer; and (4) investigation of the dependence of olivine dissolution upon crystallographic orientation. In addition, it is demonstrated that diffusion-exchange experiments are useful for studying the dominant point-defect mechanisms for cation diffusion.Currently, a visiting scientist with Air Force Wright Aeronautical Laboratories Materials Laboratory (MLLM), Wright-Patterson AFB, OH 45433     

Development of orthopyroxene-Fe/Mg ferrite symplectites by continuous olivine oxidation

The development of orthopyroxene-Fe/Mg ferrite symplectites associated with olivine is discussed with respect to the chemical reactions by which they form. Previously proposed reactions are presented graphically and the differences between them are reviewed. With the exception of exsolution, these are all discontinuous reactions in the sense that olivine is replaced by the two-phase symplectite assemblage.Olivine-hosted symplectites developed in the margins of lherzolite xenoliths from Kauai, Hawaii, demonstrate a reaction mechanism which has not been previously documented from natural samples. Original Fo₉₀ olivine in these samples oxidized to a new assemblage consisting of orthopyroxene (En_92–95)-Fe/Mg ferrite (Mf_35–50) symplectites developed within more magnesian olivine (Fo_92–96) hosts. Thus, by this mechanism, olivine of a different composition persists as part of a final three-phase assemblage. As oxidation advanced, the compositions of all three product phases became continuously more magnesian and the stoichiometric coefficients of the orthopyroxene and Fe/Mg ferrite continuously increased, whereas those of the product olivine decreased in the mass-balance equations. These characteristics demonstrate that the reaction was controlled by oxygen diffusion into the xenoliths from the highly oxidized alkali picrite melt in which they were entrained. Thermodynamic calculations suggest that a gradient in oxygen fugacity of 10^0.9 bars existed across the xenolith rims and resulted in compositional gradients of 4 mol% fayalite and ferrosilite and 15 mol% magnetite.     

Titanium solubility in olivine in the system TiO<Subscript>2</Subscript>–MgO–SiO<Subscript>2</Subscript>: no evidence for an ultra-deep origin of Ti-bearing olivine

The finding of ilmenite rods in olivine from orogenic peridotites has sparked a discussion about the processes of incorporation and exsolution of titanium in olivine. We have experimentally investigated the solubility of Ti in olivine as a function of composition, temperature and pressure in the synthetic TiO₂–MgO–SiO₂ system. Experiments at atmospheric pressure in the temperature range 1,200–1,500°C showed that the highest concentration of TiO₂ is obtained when olivine coexists with spinel (Mg₂TiO₄). The amount of TiO₂ in olivine in the assemblages olivine + spinel + periclase and olivine + spinel + ilmenite at 1,500°C was 1.25 wt.%. Changes in the coexisting phases and decreasing temperature result in a significant reduction of the Ti solubility. Olivine coexisting with pseudobrookite (MgTi₂O₅) and a Ti–Si-rich melt at 1,500°C displays a fourfold lower TiO₂ content than when buffered with spinel. A similar decrease in solubility is obtained by a decrease in temperature to 1,200°C. There is a negative correlation between Ti and Si and no correlation between Ti and Mg in Ti-bearing olivine. Together with the established phase relations this suggests that there is a direct substitution of Ti for Si at these temperatures, such that the substituting component has the stoichiometry Mg₂TiO₄. The unit cell volume of olivine increases systematically with increasing TiO₂ content demonstrating that the measured TiO₂ contents in olivine are not caused by micro-inclusions but by incorporation of Ti in the olivine structure. Least squares fitting of 20 olivine unit cell volumes against the Ti content yield the relation: V (ų)=290.12(1) + 23.67(85) N_Ti. The partial molar volume of end-member Mg₂TiO₄ olivine (N_Ti=1) is thus 47.24±0.13 cm³. The change of the Ti solubilty in olivine coexistent with rutile and orthopyroxene with pressure was investigated by piston cylinder experiments at 1,400°C from 15 to 55 kbar. There is no increase in TiO₂ contents with pressure and in all the experiments olivine contains ~0.2 wt.% TiO₂. Moreover, a thermodynamic analysis indicates that Ti contents of olivine coexisting with rutile and orthopyroxene should decrease rather than increase with increasing pressure. These data indicate that the ilmenite exsolution observed in some natural olivine does not signify an ultra-deep origin of peridotite massifs.     

Fe-rich tholeiitic liquids and their cumulate products in the Pleasant Bay layered intrusion, coastal Maine

Fe-rich tholeiitic liquids are preserved as chilled pillows and as the chilled base of a 27 meter thick macrorhythmic layer in the Pleasant Bay mafic-silicic layered intrusion. The compositions of olivine (Fo₁) and plagioclase (An₁₃₋₈) in these extremely fine grained rocks suggest that they represent nearly end stage liquids that formed by fractionation of tholeiitic basalt. Their major element compositions (∼17.5 wt% FeO_T and 54 wt%SiO₂) closely resemble highly evolved glasses in the Loch Ba ring dike and some recent estimates of end-stage liquids related to the Skaergaard layered intrusion, and are consistent with recent experimental studies of tholeiite fractionation. Their trace element compositions are consistent with extensive earlier fractionation of plagioclase, olivine, clinopyroxene, ilmenite, magnetite and apatite. The mineral assemblage of the chilled rocks (olivine, clinopyroxene, quartz, ilmenite and magnetite), apatite saturation temperatures, and very low Fe³⁺/Fe²⁺indicate conditions of crystallization at temperatures of about 950 °C and f _{O 2} about two log units below FMQ. Cumulates that lie about 3 meters above the chilled base of the macrorhythmic layer contain cumulus plagioclase, olivine, clinopyroxene, ilmenite, apatite and zircon. This mineral assemblage and the Fe-Mg ratio in clinopyroxene cores suggest that this cumulate was in equilibrium with a liquid having a composition identical to that of the chilled margin which lies directly beneath it. The high FeO_T and low SiO₂ concentrations of this cumulate (23.3 and 45.8 wt%, respectively) are comparable to those in late stage cumulates of the Skaergaard and Kiglapait intrusions. This association of a chilled liquid and cumulate in the Pleasant Bay intrusion suggests that late stage liquids in tholeiitic layered intrusions may have been more SiO₂-rich than field-based models suggest and lends support to recent experimental studies of tholeiite fractionation at low f _O2 which indicate that saturation of an Fe-Ti oxide phase should cause FeO_T to decrease in the remaining liquid. Received: 17 January 1997 / Accepted: 10 June 1997     

Fe–Mg interdiffusion rates in clinopyroxene: experimental data and implications for Fe–Mg exchange geothermometers

Chemical interdiffusion of Fe–Mg along the c-axis [001] in natural diopside crystals (X _Di = 0.93) was experimentally studied at ambient pressure, at temperatures ranging from 800 to 1,200 °C and oxygen fugacities from 10^?11 to 10^?17 bar. Diffusion couples were prepared by ablating an olivine (X _Fo = 0.3) target to deposit a thin film (20–100 nm) onto a polished surface of a natural, oriented diopside crystal using the pulsed laser deposition technique. After diffusion anneals, compositional depth profiles at the near surface region (~400 nm) were measured using Rutherford backscattering spectroscopy. In the experimental temperature and compositional range, no strong dependence of D ^Fe–Mg on composition of clinopyroxene (Fe/Mg ratio between Di₉₃–Di₆₅) or oxygen fugacity could be detected within the resolution of the study. The lack of fO₂-dependence may be related to the relatively high Al content of the crystals used in this study. Diffusion coefficients, D ^Fe–Mg, can be described by a single Arrhenius relation with $$D^{{{\text{Fe}} - {\text{Mg}}}} = 2. 7 7\pm 4. 2 7\times 10^{ - 7} {\text{exp(}}-3 20. 7\pm 1 6.0{\text{ kJ}}/{\text{mol}}/{\text{RT)m}}^{ 2} /{\text{s}}.$$ D ^Fe–Mg in clinopyroxene appears to be faster than diffusion involving Ca-species (e.g., D ^Ca–Mg) while it is slower than D ^Fe–Mg in other common mafic minerals (spinel, olivine, garnet, and orthopyroxene). As a consequence, diffusion in clinopyroxene may be the rate-limiting process for the freezing of many geothermometers, and compositional zoning in clinopyroxene may preserve records of a higher (compared to that preserved in other coexisting mafic minerals) temperature segment of the thermal history of a rock. In the absence of pervasive recrystallization, clinopyroxene grains will retain compositions from peak temperatures at their cores in most geological and planetary settings where peak temperatures did not exceed ~1,100 °C (e.g., resetting may be expected in slowly cooled mantle rocks, many plutonic mafic rocks, or ultra-high temperature metamorphic rocks).     

Megacrysts and xenoliths in kimberlite,Elliott County,Kentucky: A mantle sample from beneath the Permian Appalachian Plateau

Two kimberlite pipes in Elliott County contain rare ultramafic xenoliths and abundant megacrysts of olivine (Fo_85–93), garnet (0.21–9.07% Cr₂O₃), picroilmenite, phlogopite, Cr-poor clinopyroxene (0.56–0.88% Cr₂O₃), and Cr-poor orthopyroxene (<0.03–0.34% Cr₂O₃) in a matrix of olivine (Fo_88–92), picroilmenite, Cr-spinel, magnetite, perovskite, pyrrhotite, calcite, and hydrous silicates. Rare clinopyroxene-ilmenite intergrowths also occur. Garnets show correlation of mg (0.79–0.86) and CaO (4.54–7.10%) with Cr₂O₃ content; the more Mg-rich garnets have more uvarovite in solution. Clinopyroxene megacrysts show a general decrease in Cr₂O₃ and increase in TiO₂ (0.38–0.56%) with decreasing mg (0.87–0.91). Clinopyroxene megacrysts are more Cr-rich than clinopyroxene in clinopyroxene-ilmenite intergrowths (0.06–0.38% Cr₂O₃) and less Cr-rich than peridotite clinopyroxenes (1.39–1.46% Cr₂O₃). Orthopyroxene megacrysts and orthopyroxene inclusions in olivine megacrysts form two populations: high-Ca, high-Al (1.09–1.16% CaO and 1.16–1.18% Al₂O₃) and low-Ca, low-Al (0.35–0.46% CaO and 0.67–0.74% Al₂O₃). Three orthopyroxenes belonging to a low-Ca subgroup of the high-Ca, high-Al group were also identified (0.86–0.98% CaO and 0.95–1.01% Al₂O₃). The high-Ca, high-Al group (Group I) has lower mg (0.88–0.90) than low-Ca, low-Al group (Group II) with mg=0.92–0.93; low mg orthopyroxenes (Group Ia) have lower Cr₂O₃ and higher TiO₂ than high mg orthopyroxenes (Group II). The orthopyroxene megacrysts have lower Cr₂O₃ than peridotite orthopyroxenes (0.46–0.57% Cr₂O₃). Diopside solvus temperatures indicate equilibration of clinopyroxene megacrysts at 1,165°–1,390° C and 1,295°–1,335° C for clinopyroxene in clinopyroxene-ilmenite intergrowths. P-T estimates for orthopyroxene megacrysts are bimodal: high-Ca, high-Al (Group I) orthopyroxenes equilibrated at 1,165°–1,255° C and 51–53 kb (± 5kb) and the low-Ca, low-Al (Group II) orthopyroxenes equilibrated at 970°–1,020°C and 46–56 kb (± 5kb). Garnet peridotites equilibrated at 1,240°–1,360° C and 47–49 kb. Spinel peridotites have discordant temperatures of 720°–835° C (using spinel-olivine Fe/Mg) and 865°–1,125° C (Al in orthopyroxene).Megacrysts probably precipitated from a fractionating liquid at >150 km depth. They are not disaggregated peridotite because: (1) of large crystal size (up to 1.5 cm), (2) compositions are distinctly different from peridotite phases, and (3) they display fractionation trends. The high mg, low T orthopyroxenes and the clustering of olivine rims near Fo_89–90 reflect liquid changes to higher MgO contents due to (1) assimilation of wall-rock and/or (2) an increase in Fe³⁺/Fe²⁺ and subsequently MgO/FeO as a result of an increase in f _o.