An experimental study of Fe-Mg partitioning between olivine and orthopyroxene at 1173, 1273 and 1423 K and 1.6 GPa

The partitioning of Mg and Fe²⁺ between coexisting olivines and orthopyroxenes in the system MgO-FeO-SiO₂ has been investigated experimentally at 1173, 1273, 1423 K and 1.6 GPa over the whole range of Mg/Fe ratios. The use of barium borosilicate as a flux to promote grain growth, and the identification by back-scattered electron imaging of resulting growth rims suitable for analysis by electron microprobe, results in coexisting olivine and orthopyroxenene compositions determined to a precision of±0.003 to 0.004 in molar Fe/(Mg+Fe). Quasi-reversal experiments were performed starting with Mg-rich olivine and Fe-rich orthopyroxene (low K_D) and vice versa (high K_D), which produced indistinguishable results. The distribution coefficient, K_D, depends on composition and on temperature, but near Fe/(Mg+Fe)=0.1 (i.e. mantle compositions) these effects cancel out, and K_D is insensitive to temperature. The results agree well with previous experimental investigations, and constrain the thermodynamic mixing properties of Mg-Fe olivine solid solutions to show small near-symmetric deviations from ideality, with between 2000 and 8000 J/mol. Multiple non-linear least squares regression of all data gave a best fit with (implying 5450 J/mol at 1 bar) and , but the two W _G parameters are so highly correlated with each other that our data are almost equally well fit with , as obtained by Wiser and Wood. This value implies , apparently independent of temperature. Our experimental results are not compatible with the assessment of olivine-orthopyroxene equilibria of Sack and Ghiorso.     

Experimental Study on the Solubility of Cr^2 in Olivine,Orthopyroxene and Spinel Solid Solutions

Experiments have been performed on the system MgO-SiO2-Cr-O at 0-2.88 GPa and 1100-1450℃,focusing on the stability of Cr^2 in olivine(O1),orthopyroxene(Opx) and spinel(Sp) and its partitioning between these phases.Analytical reagent grade chemicals,MgO,SiO2,Cr2O3.and Cr were used to make starting mixtures.Excess Cr(50%) was then added in these mixtures to ensure that the resultant phases were in equilibrium with the metal Cr.Flux of BaO B2O3(%) was added for facilitating experimental equilibrium and crystal growth.Cr was used as capsule material.All phases in the product were identified by X-ray and analyzed by electron microprobe,The contents of CrO in the different phases(O1,Opx and Sp)were calculated according to stoichiometry.The obtained results of calculation indicate that Cr^3 in Ol and Opx is negligible.The experimental results show;(a) with increasing temperature and decreasing pressure,Cr^2 solubility in Ol,Opx and Sp increases;(b) with in creasing temperature,the partitioning coefficient of Mg and Cr^2 between Ol and Opx decreases,that between Opx and Sp increases,and that between Ol and Sp remains almost unchanged;(c) the effect of pressure on all partitioning coefficients is negligible.     

An experimental study of Fe-Mg partitioning between garnet and olivine and its calibration as a geothermometer

The partitioning of Fe and Mg between coexisting garnet and olivine has been studied at 30 kb pressure and temperatures of 900 ° to 1,400 °C. The results of both synthesis and reversal experiments demonstrate that K _D (= (Fe/Mg)_gt/(Fe/Mg)_OI) is strongly dependent on Fe/Mg ratio and on the calcium content of the garnet. For example, at 1,000 °C/30 kb, K _D varies from about 1.2 in very iron-rich compositions to 1.9 at the magnesium end of the series. Increasing the mole fraction of calcium in the garnet from 0 to 0.3 at 1,000 ° C increases K _D in magnesian compositions from 1.9 to about 2.5.The observed temperature and composition dependence of K _D has been formulated into an equation suitable for geothermometry by considering the solid solution properties of the olivine and garnet phases. It was found that, within experimental error, the simplest kind of nonideal solution model (Regular Solution) fits the experimental data adequately. The use of more complex models did not markedly improve the fit to the data, so the model with the least number of variables was adopted.Multiple linear regression of the experimental data (72 points) yielded, for the exchange reaction: 3Fe₂SiO₄+2Mg₃Al₂Si₃O₁₂ olivine garnet 2Fe₂Al₂Si₃O₁₂+3Mg₂SiO₄ garnet olivine H ° (30kb) of –10,750 cal and S ° of –4.26 cal deg^–1 mol^–1. Absolute magnitudes of interaction parameters (W _ij ) derived from the regression are subject to considerable uncertainty. The partition coefficient is, however, strongly dependent on the following differences between solution parameters and these differences are fairly well constrained: W _FeMg ^ol -W _FeMg ^gt 800 cal W _CaMg ^gt -W _CaFe ^gt 2,670 cal.The geothermometer is most sensitive in the temperature and composition regions where K _D is substantially greater than 1. Thus, for example, peridotitic compositions at temperatures less than about 1,300 ° C should yield calculated temperatures within 60 °C of the true value. Iron rich compositions (at any temperature) and magnesian compositions at temperatures well above 1,300 °C could not be expected to yield accurate calculated temperatures.For a fixed K _D the influence of pressure is to raise the calculated temperature by between 3 and 6 °C per kbar.     

Intracrystalline relationships in olivine,orthopyroxene, clinopyroxene and spinel from a suite of spinel lherzolite xenoliths from Mt. Noorat,Victoria, Australia

A detailed crystal chemical study of coexisting olivine, orthopyroxene, clinopyroxene and spinel from selected Victorian (Australia) lherzolite suites was carried out by means of single crystal x-ray diffraction and electron probe microanalysis to obtain actual site occupancies. The aim of this study was primarily to characterise the intracrystalline configurations and related cation ordering on sites in major mantle constituents. The results demonstrate that cation ordering on sites is subject to distinctive crystallographic controls which depend on the petrological evolution of the suite. Mg-Fe²⁺ ordering in M1–M2 pyroxene sites depends on variations of the smaller cations, mainly Al^vi, Ti⁴⁺, Fe³⁺, and related configurations of M 1. Pressuresensitive Al^vi is crucial to Fe²⁺, the more ordered clinopyroxene showing high Al^vi configurations which tend to exclude the larger bivalent cations and yield small polyhedral volumes for M 1, M 2, T sites and the unit cell. Conversely, the coexisting orthopyroxene, characterised by lower Al^vi configuration and higher M 1 and unit cell volumes, is relatively more disordered. Olivine is consistent with the coexisting clinopyroxene, the more disordered crystals coexisting with more disordered clinopyroxene, while Al-Mg order in the coexisting spinel shows the reverse relationship. Estimated temperatures of apparent equilibration based on current geothermometers are not considered realistic. Assumptions of ideal cation mixing on sites in pyroxene and spinel are not supported.     

Compositional controls on spinel clouding and garnet formation in plagioclase of olivine metagabbros,Adirondack Mountains,New York

Olivine metagabbros from the Adirondacks usually contain both clear and spinel-clouded plagioclase, as well as garnet. The latter occurs primarily as the outer rim of coronas surrounding olivine and pyroxene, and less commonly as lamellae or isolated grains within plagioclase. The formation of garnet and metamorphic spinel is dependent upon the anorthite content of the plagioclase. Plagioclase more sodic than An_38±2 does not exhibit spinel clouding, and garnet rarely occurs in contact with plagioclase more albitic than An_36±4. As a result of these compositional controls, the distribution of spinel and garnet mimics and visually enhances original igneous zoning in plagioclase. Most features of the arrangement of clear (unclouded) plagioclase, including the shells or moats of clear plagioclase which frequently occur inside the garnet rims of coronas, can be explained on the basis of igneous zoning. The form and distribution of the clear zones may also be affected by the metamorphic reactions which have produced the coronas, and by redistribution of plagioclase in response to local volume changes during metamorphism.Authors listed alphabeticallyPublished by permission of the Director, New York State Museum, Journal Series Number 299     

Partitioning data pertaining to Fe-Mg ordering around trace cations in olivine and low-Ca pyroxene

We have used trace element partitioning data available in the literature to investigate nonideality of the cations of Yb, Sm, Gd, Ca, Mn, Sc, Ni, and Al in silicate melt, olivine, and low-Ca pyroxene. Results are consistent with ordering of Mg and Fe around trace cations in olivine and pyroxene. On the basis of these data, we suggest there is an increasing tendency for Fe to congregate in the vincinity of the trace cation as the size of the trace cation increases. These results are important both in achieving a better understanding of trace element behavior in crystals and in constraining the temperature and compositional dependence of trace element partitioning.     

Electron microscopical study of garnet exsolution in orthopyroxene

By means of electron-optical methods the structure and orientation of lamella-like garnet exsolutions in an orthopyroxene from the eclogite of Gilsberg, Saxony Granulite Complex, are investigated. It can be shown that besides the macroscopic and microscopic submicroscopic exsolution lamellae also exist; they have widths up to nearly 5 nm. The intergrowth conditions are such shat (100)_op is parallel to {100}_gr and [001]_op parallel to 〈110〉_gr. The lamellae are coherent, but sometimes extended dislocation arrangements are found in microscopic lamellae which are caused by a deformation. These investigations point to formation of the rock as a result of a solid state reaction under high pressure conditions with subsequent diffusive-dissociative exsolution of garnet from primary pyroxene.     

High pressure breakdown of antigorite to spinifex-textured olivine and orthopyroxene, SE Spain

The prograde, high pressure, transition from antigorite serpentinite to enstatite-olivine rock occurs along a tectonically undisturbed profile at Cerro del Almirez, SE Spain. The reactant assemblage is antigorite + olivine with tremolite rimming precursor diopside. The product assemblage of tremolite + chlorite + enstatite + olivine has a spinifex-like texture with arborescent or radiating olivine elongated parallel to [001] and with radially grown enstatite. Product enstatite is very poor in Al₂O₃. Due to numerous oriented submicroscopic inclusions of chromian magnetite, product olivine has a brownish pleochroism and a bulk chromium content similar to precursor antigorite. Titanian clinohumite with a fluorine content of 0.45–0.50 wt% persisted beyond the breakdown of antigorite. The partitioning of iron and magnesium amongst the silicate phases is almost identical to that at lower pressures. Average K_d values Mn/Mg and Ni/Mg are 0.17 and 0.70 for antigorite-olivine pairs and 1.83 and 0.22 for orthopyroxene-olivine pairs, respectively. These data are useful in discriminating generations of olivine grown on each other. From the field data a phase diagram topology for a portion of the system CaO-MgO-SiO₂-H₂O is derived. This topology forms the basis for extrapolations into inaccessible P-T regions. Received: 6 February 1998 / Accepted: 24 March 1998     

Experimental study of Fe-Mg distribution between biotite and orthopyroxene at P=490 MPa

Partitioning of Mg and Fe between coexisting biotite and orthopyroxene has been experimentally determined at temperatures 700, 750 and 800° C and 490 MPa total pressure in the system KAlO₂-MgO-FeO-SiO₂-H₂O. Oxygen fugacity was controlled by the QFM buffer. Starting materials were synthetic minerals of differing Fe/(Fe+Mg) values. Run products were analyzed for partitioning of components by a microprobe. Orthopyroxene was established to be notably inhomogeneous, whereas biotite was essentially homogeneous. To establish equilibrium relations, statistical treatment of the results of each experiment in addition to the whole complex of experimental data was applied. The regression equations for isotherms of the Fe-Mg partitioning between the minerals studied have been obtained. As a result, the equation for a two-dimensional regression may be written as: $$\begin{gathered} Y = (A + A_1 t + A_2 t^2 )(X - X^4 ) + (B + B_1 t + B_1 t^2 )(X^2 - X^4 ) + \hfill \\ (C + C_1 t + C_1 t^2 )(X^3 - X^4 ) + X^4 {\text{ where }}Y = X_{{\text{Opx}}}^{{\text{Fe}}} ;{\text{ X}} = {\text{X}}_{{\text{Bi}}}^{{\text{Fe}}} ; \hfill \\ t = 1000/T,K, \hfill \\ \begin{array}{*{20}c} {A = {\text{ }}4.59398,} & {A_1 = - {\text{ }}8.29838,} & {A_2 = {\text{ }}4.97316,} \\ {B = - 11.13731,} & {B_1 = {\text{ }}28.19304,} & {B_2 = - 20.98240,} \\ {A = {\text{ }}8.25072,} & {C_1 = - 20.80485,} & {C_2 = {\text{ }}15.35967} \\ \end{array} \hfill \\ {\text{ }}\sigma = 0.0143{\text{ }} \hfill \\ \end{gathered}$$ . This equation enables extrapolation of partitioning isotherms over a wide range of temperatures.     

The solubility of alumina in orthopyroxene coexisting with garnet

The solubility of alumina in complex orthopyroxenes crystallised in equilibrium with garnet has been determined over the pressure-temperature range 8–30 Kb and 800–1250° C. The results are in good agreement with predictions made using the simple thermodynamic model of Wood and Banno (1973). The model has been refined using a combination of the new data on orthopyroxenes of about En₆₀Fs₄₀ (with variable calcium content) and previously published data on more magnesian systems. Given analyses of coexisting Orthopyroxene and garnet in natural assemblages it is possible to calculate a P-T curve for the rock which should, in most cases, be accurate to within 2 or 3 kbar.     

Experimental determination of Mn-Mg mixing properties in garnet,olivine and oxide

We have measured the mixing properties of Mn-Mg olivine and Mn-Mg garnet at 1300° C from a combination of interphase partitioning experiments involving these phases, Pt-Mn alloys and Mn-Mg oxide solid solutions. Activity coefficients of Mn dilute in Pt-Mn alloys at 1300° C/1 atm were measured by equilibrating the alloy with MnO at known f _{O 2}. Assuming that the log f _{O 2} of the Mn-MnO equilibrium under these conditions is-17.80 (Robie et al. 1978), we obtain for _Mn: log_Mn = –5.25 + 3.67 X_Mn + 11.41X² _Mn Mixing properties of (Mn,Mg)O were determined by reversing the Mn contents of the alloys in equilibrium with oxide at known f _{O 2}. Additional constraints were obtained by measuring the maximum extent of immiscibility in (Mn,Mg)O at 800 and 750° C. The data are adequately described by an asymmetric (Mn,Mg)O solution with the following upper and lower limits on nonideality: (a) W_Mn = 19.9kj/Mol; W_Mg = 13.7kj/Mol; (b) W_Mn = 19.9kj/Mol; W_Mg = 8.2kj/Mol; Olivine-oxide partitioning was tightly bracketed at 1300° C and oxide properties used to obtain activity-composition relations for Mn-Mg olivine. Despite strong M2 ordering of Mn in olivine, the macroscopic properties are adequately described by a symmetric model with: W^ol = 5.5 ± 2.5 kj/mol (1-site basis) Using these values for olivine, garnet-olivine partitioning at 27 kbar/1300° C leads to an Mn-Mg interaction parameter in garnet given by: W^gt = 1.5 ± 2.5kJ/mol (1-site basis) Garnet-olivine partitioning at 9 kbar/1000° C is consistent with the same extent of garnet nonideality and the apparent absence of excess volume on the pyrope-spessartine join indicates that any pressure-dependence of W^Gt must be small. If olivine and garnet properties are both treated as unknown and the garnet-olivine partitioning data alone used to derive W^Ol and W^Gt, by multiple linear regression, best-fit values of 6.16 and 1.44 kJ/mol. are obtained. These are in excellent agreement with the values derived from metal-oxide, oxide-olivine and olivine-garnet equilibria.     

The distribution of chromium among orthopyroxene,spinel and silicate liquid at atmospheric pressure

A series of experiments has been carried out in which a synthetic silicate melt, of composition equivalent to a “U-type” Bushveld Complex parent liquid, was equilibrated with bronzitic orthopyroxene and chromite spinel between 1334 and 1151°C over a range of oxygen fugacities between the nickel-nickel oxide and iron-wüstite buffers. The partition coefficient for Cr between bronzite and melt increases with falling temperature along a given oxygen buffer, and decreases with falling oxygen fugacity at a given temperature, showing an overall range from 1.1 to 11.7. The Cr content of the melt in equilibrium with spinel (Cr solubility) decreases with falling temperature and increases with lower oxygen fugacity. This variation may be quantified in terms of temperature-dependent solubility of Cr³⁺ combined with a changing ratio of Cr³⁺ to Cr²⁺ in the melt. Iso-oxidation curves for Cr are approximately parallel to Fe-Si-O buffer curves and to curves for equal Fe³⁺/Fe²⁺ determined by Hill and Roeder (1974), and agree within experimental error with the results of Schreiber and Haskin (1976). When the changing oxidation state of Cr is allowed for, the orthopyroxene partition coefficient may be expressed as the sum of the temperature-dependent coefficient for Cr³⁺ and a partition coefficient of about 1 for Cr²⁺.The experimental results are used to construct a series of model curves for liquid and bronzite compositional variations during fractional crystallization of a Bushveld parent liquid. Trends for Cr variation are shown to depend critically upon oxygen fugacity, and on whether the liquid is saturated with chromite. The position of the peritectic between chromite and orthopyroxene is shown to be very sensitive to oxygen fugacity within one and a half log units of the QFM buffer. This observation may explain the contrasting distribution of chromite seams in the Bushveld Complex, where chromite occurs within bronzitite-norite sequences, and in the Stillwater Complex in which chromite is restricted to olivine cumulate layers.     

Infrared spectra of olivine polymorphs: α, β phase and spinel

Infrared (IR) absorption spectra are presented for olivine (α) and spinel (γ) phases of A₂SiO₄ (A=Fe, Ni, Co) and Mg₂GeO₄. IR spectra of β phase (“modified spinel”) Co₂SiO₄ and of α Mg₂SiO₄ are also included. These results provide reference spectra for the identification of olivine high-pressure polymorphs. Isostructural and isochemical correlations are used to support a general interpretation of the spectra and to predict the spectrum of γ Mg₂SiO₄. A γ Mg₂GeO₄ sample equilibrated at 1,000° C shows evidence of partial inversion, but one equilibrated at 730° C does not. This suggests that partial inversion could occur in silicate spinels at elevated temperatures and pressures, however no evidence of inversion is seen in the ir spectra of the silicates in this study.     

The stability of ortho- and clinopyroxenes,olivine, and garnet in kimberlitic magma

It is generally accepted that the composition of ultrabasic nodules and their quantitative proportions do not significantly change during their transportation with kimberlitic magma to the Earth’s surface. We performed an experimental study of the relative stability of olivine, garnet, and pyroxenes in kimberlite melt at high pressure and temperatures (4 GPa, 1300–1500 °C). The study has shown that the loss in weight of minerals and, correspondingly, the rate of their dissolution in kimberlite melt differ considerably. The following sequence of the dissolution rates of minerals has been established: Cpx ≥ Opx > Gar > Ol. Pyroxenes are characterized by the most rapid dissolution, and olivine is the most stable mineral. The assumption is made that clinopyroxenites and websterites disintegrate more rapidly than dunites and lherzolites in kimberlitic magma.     

Temperature and pressure dependence of Fe-Mg partitioning between garnet and phengite,with particular reference to eclogites

Experimental data combined with data from natural rocks have been used to calibrate a geothermometer based on the distribution of Fe²⁺ and Mg between coexisting garnets and phengites. The pressure effect on the K _D -value appears to be considerable. The calculated thermometer is expressed as $$T(K) = \frac{{3685 + 77.1P(kb)}}{{InK_D + 3.52}}.$$ The use of this \(K_{D_{(FeO/MgO)} }^{ga + ph}\) geothermometer on eclogites with low Fe₂O₃ content, gives P-T values which are in good accordance with those obtained by other methods. The problems that arise when Fe³⁺ is present in larger amounts, are discussed.