Diffusion and partition coefficients of minor and trace elements in magnetite as a function of oxygen fugacity at 1150 ºC

Sievwright, Robert; O’Neill, Hugh St. C.; Tolley, J.A.; Wilkinson, Jamie J.; Berry, Andrew J.

doi:10.1007/s00410-020-01679-z

Cited by 33 publications

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“…where , and are the Cr content, thickness and density of melt at i-th increment, respectively, and the is the partition coefficient of Cr between magnetite and melt, and here is assumed as 200 based on recent experimental measurements 24 . The density of magnetite ( ) is estimated as ~ 4955 kg•m - 3 49 .…”

Section: Methodsmentioning

confidence: 99%

“…where r is the radius of magnetite grain, t is the time and DCr-mt is the diffusion coefficient of Cr in magnetite (~2×10 -15 m 2 •s -1 ) 24,37 . On the other hand, during the convective diffusion, a compositional gradient of Cr persists within a thin diffusive boundary layer around the magnetite grain, and the mass transfer (dMCr) of an element Cr via this compositional boundary layer within a short time interval (dt) can be approximated as 40 :…”

Section: Methodsmentioning

confidence: 99%

“…Here we assume that the MML is crystallized from a thick magma sheet, and that magnetite is the sole crystalline phase during the formation of the crystal mush of the MML (Methods). In Figure 1 we show models of fractional crystallization of magnetite from a homogeneous body of melt containing 57 ppm Cr 3,25 , with fO2 between zero and one log unit below the quartz-fayalite-magnetite oxygen buffer (i.e., -1 < QFM < 0) 20,26-27 using a magnetite-melt partition coefficient of 200 at 1150 °C 14,24 . If the initial thickness of the overlying magma is set to 120 m as shown (Table 1), on the order of the vertical distance to the next magnetite layer up section 17,20,28 , then the exponential decrease in Cr between 10 and 40 cm is reproduced by the model but in the upper portions of all of the profiles the model predicts much greater depletion of Cr than is observed.…”

Section: Fractional Crystallization Models Vertical Profiles Of Cr Distribution In the MML Shownmentioning

confidence: 99%

“…However, Kruger and Latypov 18 recently proposed that this mushy zone had negligible thickness, being deposited directly onto a solid substrate in situ to form magnetite adcumulates with little or no initial interstitial melt. The highly compatible behavior of Cr in magnetite 24 makes it sensitive to magmatic processes (e.g., crystallization and reactive melt infiltration), and the quasi-exponential vertically decreasing Cr gradients in monomineralic magnetitite layers have been widely attributed to fractional crystallization at or near the magma chamber floor [10][11][12][14][15]18 . Failure of a simple one-stage Rayleigh fractionation model to match observed compositional profiles (e.g., Fig.…”

Section: Introductionmentioning

confidence: 99%

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Magnetite layer formation in the Bushveld Complex of South Africa

Yao

Mungall

2021

Preprint

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The great economic significance of layered mafic-ultramafic intrusions like the Bushveld Complex of South Africa results from the existence within them of some layers highly concentrated in valuable elements. Here we address the origins of the Main Magnetite Layer, a globally important resource of Fe-Ti-V-rich magnetite. Previous models of in situ fractional magnetite crystallization require frequent ad hoc adjustments to the boundary conditions. An alternative model of rapid deposition of loose piles of magnetite crystals followed by compositional convection near the top of the pile and infiltration of the pile from beneath by migrating intercumulus melt fits observations without any adjustments. The data admit both explanations, but the latter model, with the fewest unconstrained interventions, is preferable. The choice of models has pivotal ramifications for understanding of the fundamental processes by which crystals accumulate and layers form in layered intrusions.

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Fractional Crystallization Models Vertical Profiles Of Cr Distribution In the MML Shownmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Magnetite layer formation in the Bushveld Complex of South Africa

Yao

Mungall

2021

Preprint

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“…Similarly, both iron and oxygen diffusion have been demonstrated to vary nonlinearly with f O 2 (see review by Van Orman and Crispin (2010)). Diffusion of Fe, Ti, and many other trace element cations in magnetite are well described by a V-shaped trend (Dieckmann and Schmalzried 1977;Sievwright et al 2020…”

Section: Effect Of Oxygen Fugacity On Dislocation Creep Rates In Magnmentioning

confidence: 99%

High-temperature creep of magnetite and ilmenite single crystals

Till

Rybacki

2020

Phys Chem Minerals

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We performed deformation experiments on dry natural single crystals of magnetite and ilmenite to determine the rheological behavior of these oxide minerals as a function of temperature, orientation, and oxygen fugacity. Samples were deformed at temperatures of 825–1150 $$\,^{\circ }$$ ∘ C to axial strains of up to 15–24% under approximately constant stress conditions up to 120 MPa in a dead-load-type creep rig at ambient pressure in a controlled gas atmosphere. Oxygen fugacity ranged from 10$$^{-9.4}$$ - 9.4 to 10$$^{-4}$$ - 4 atm. Ilmenite creep was insensitive to oxygen fugacity, while magnetite displayed a strong, non-monotonic oxygen fugacity dependence, with creep rates varying as $$f_{O_{2}}^{-0.7}$$ f O 2 - 0.7 and $$f_{O_{2}}^{0.4}$$ f O 2 0.4 at more reducing and more oxidizing conditions, respectively. Dislocation creep rates of magnetite single crystals were weakly dependent on crystallographic orientation with stress exponents that varied between 2.8 and 4.3 (mean 3.5 ± 0.4). Magnetite compressed parallel to <100>, <110>, and <111> axes exhibited apparent activation energies of 315±5, 345±30, and 290±5 kJ/mol, respectively. We estimated $${f_O}_2$$ f O 2 -independent magnetite activation energies of 715 ± 150, 725 ± 145, and 690 ± 150 kJ/mol for <100>, <110>, and <111> orientations, respectively, in the region of negative $${f_O}_2$$ f O 2 -dependence. Ilmenite single crystals were compressed parallel, normal, and inclined to the c-axis. Stress exponents of 3.4, 4.3, and 3.9 indicate dislocation creep with activation energies of 420 ± 35, 345 ± 30, and 360 ± 40 kJ/mol, respectively, for these orientations. Mechanical anisotropy in ilmenite is notably higher than in magnetite, as expected from its lower crystal symmetry. Constitutive equations were formulated for ilmenite and magnetite creep.

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Fractionation of trace and platinum-group elements during metamorphism of komatiitic chromites from the early Archean Gorumahishani greenstone belt, Singhbhum Craton (eastern India)

Banerjee

Mondal

Reisberg

et al. 2022

Contrib Mineral Petrol

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It is well established that the major and minor element contents of chromites are subject to change during greenschist to amphibolite facies metamorphism. During upper amphibolite facies metamorphism, chromite can be completely converted to chrome magnetite.However, not all elements are affected to the same degree, the concentrations of +2 ions (e.g. Zn, Co, Mn) being particularly vulnerable to modification. The degree to which trace elements, particularly the platinum-group elements (PGE), are affected has not been closely examined. The compositions and textures of chromites from komatiites of the Gorumahishani greenstone belt of the Singhbhum Craton (India) have experienced a range of metamorphic conditions from greenschist to amphibolite facies, providing the opportunity to study the changes of trace and platinum-group element composition with metamorphic grade. Five types of altered chromites are identified from the komatiitic suite of rocks in the ~120 km long greenstone belt. The type-I chromites are non-porous and characterized by the least modified cores. These chromites are mostly present in the northern Maharajgunj-Tua Dungri section where rocks show metamorphism from greenschist to greenschist-amphibolite transition facies. The type-II and type-III chromites are porous and mostly found in the southern Kapili section of the greenstone belt where rocks show metamorphism up to the mid-amphibolite facies. Type-IV and type-V chromites are completely modified to ferritchromit and chrome magnetite, respectively and are present in the komatiitic rocks from the entire greenstone belt. The central cores of the type-I and type-II grains have relatively higher concentrations of mobile trace elements (e.g. Zn, Co, and Mn) with higher Mg# [Mg/(Mg+Fe 2+ )], lower Cr# [Cr/(Cr+Al)] and lower Fe 3+ /R 3+ (R 3+ = Fe 3+ +Cr 3+ +Al 3+ ) ratios than their respective rims. Significantly higher concentrations of the immobile trace elements (e.g. Ti and V) in the cores of the type-II grains relative to their chrome magnetite rims from the Kapili section, and to the type-I varieties from other sections might be due to the metamorphism of the komatiitic rocks under higher grade conditions (amphibolite facies). In situ LA-ICPMS analysis for PGE reveal a relatively higher concentration of Ru and Rh in the rims of the type-I chromites than in the cores which is due to the diffusion of these elements from the normal spinel structure of the cores towards the bivalent octahedral sites of the inverse spinel structure of the chrome magnetite rims during metamorphic processes. The lower concentrations of Os, Ir, Ru, and Rh in the cores of the type-II chromites from the Kapili section might be related to the metamorphism of the rocks under higher grade conditions that facilitated the diffusion of these elements to associated sulfide or platinum-group mineral or alloy phases. The calculated partition coefficients of Sc,

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Diffusion and partition coefficients of minor and trace elements in magnetite as a function of oxygen fugacity at 1150 ºC

Cited by 33 publications

References 46 publications

Magnetite layer formation in the Bushveld Complex of South Africa

Magnetite layer formation in the Bushveld Complex of South Africa

High-temperature creep of magnetite and ilmenite single crystals

Fractionation of trace and platinum-group elements during metamorphism of komatiitic chromites from the early Archean Gorumahishani greenstone belt, Singhbhum Craton (eastern India)

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