Abstract
Lattice diffusion coefficients and partition coefficients have been determined for Li, Mg, Al, Sc, Ti, Cr, V, Mn, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, In, Lu, Hf, Ta and U in single crystals of natural magnetite as a function of oxygen fugacity (fO2) at 1150 °C and 1 bar by equilibration with a synthetic silicate melt reservoir. Most experiments were run for twelve hours, which was sufficient to generate diffusion profiles from 25 to > 1000 µm in length. The results were checked at one condition with two additional experiments at 66.9 and 161 h. The profiles were analysed using scanning laser-ablation inductively-coupled-plasma mass-spectrometry. Diffusion coefficients (D) were calculated by fitting data from individual element diffusion profiles to the conventional diffusion equation for one-dimensional diffusion into a semi−infinite slab with constant composition maintained in the melt at the interface. Equilibrium magnetite/melt partition coefficients are given by the ratio of the interface concentrations to those in the melt. Plots of log D as a function of log fO2 produce V-shaped trends for all the investigated elements, representing two different mechanisms of diffusion that depend on (fO2)−2/3 and (fO2)2/3. Diffusion coefficients at a given fO2 generally increase in the order: Cr < Mo ≈ Ta < V < Ti < Al < Hf ≈ Nb < Sc ≈ Zr ≈ Ga < In < Lu ≈ Y < Ni < U ≈ Zn < Mn ≈ Mg < Co < Li < Cu. Thus, Cu contents of magnetites are most susceptible to diffusive reequilibration, whereas the original content of Cr should be best preserved.
Highlights
Trace element concentrations in magnetite vary considerably between different deposit types (Dare et al 2012; Nadoll et al 2014) and can, be used as a tool for fingerprinting mineralisation styles and interpreting primary conditions of crystallisation
The LA−inductively coupled plasma (ICP)−MS at the Research School of Earth Sciences (RSES) consists of an Agilent 7700 × quadrupole ICP− MS coupled to a Resonetics 193 nm excimer laser with a custom-built ablation chamber
A linear trend is observed for K(Sc) as a function of aFe3O04.5∕XFeO1.5 which is supported by data from Wijbrans et al (2015) (Fig. 11b); this suggests that the partitioning of Sc is principally governed by thermodynamic equilibria associated with changes in spinel and melt composition and that there are not substantial changes in the activity coefficients, either in the spinel or melt, with changes in bulk composition
Summary
Trace element concentrations in magnetite vary considerably between different deposit types (Dare et al 2012; Nadoll et al 2014) and can, be used as a tool for fingerprinting mineralisation styles and interpreting primary conditions of crystallisation. Magnetite (ideally F e2+Fe3+2O4) is a common mineral in igneous and metamorphic rocks and has been studied extensively with regard to diffusion. A large volume of early work focused on oxidation kinetics (e.g. Dieckmann 1982). At low temperature, Fe3O4 has an inverse spinel structure, in which both Fe2+ and Fe3+ occupy the octahedral sites in equal proportions, and with F e3+ being the sole component of the tetrahedral sites.
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