Abstract

The cocrystallization coefficients of Cr, V, and Fe (DMe/Fe) in magnetite and sulfide minerals (pyrite, chalcopyrite, and Fe-containing sphalerite) in multiphase associations are determined in hydrothermal-growth experiments with internal sampling at 450 °C and 100 MPa (1 kbar). The results are compared with previous data on DMn/Fe. Magnetite and pyrite are characterized by the highest DMe/Fe values for both Cr (1.2 and 2) and V (6.6 and 1.1). These minerals also show the highest mineral/solution distribution coefficients of Cr and V. For V and Cr in chalcopyrite, much lower DMe/Fe values (0.03 and 0.04, respectively) were obtained, which, however, are slightly higher than those for Mn in magnetite (0.01). Although the deposition of magnetite and iron sulfides has no significant effect on the evolution of Mn in solution and Mn–Fe partitioning, crystallization of magnetite and pyrite favors a decrease in Cr and V contents relative to Fe content in solution.

The data obtained can be used to reconstruct the chemical composition of paleofluids. Spinel minerals with close contents of Mn, V, and Cr can form through a hydrothermal process provided that the solutions are highly enriched in Mn relative to Fe and have V and Cr contents close to the Fe one. Such solutions seem to be exotic. Usually, a magnetite-forming hydrothermal fluid contains V and Cr as millionths of Fe, while the Mn content in it can be of the same order of magnitude as the Fe content. The data obtained may be of interest for reconstructing the evolution of the chemical composition of the World Ocean in different geologic periods.

The study has shown that the bulk distribution coefficient of variable-valence elements between mineral and hydrothermal solution varies over a wide range of values even at constant pressure, temperature, and solution composition and can be used only for qualitative estimation of the element compatibility. In contrast, the bulk cocrystallization coefficient of chemically similar elements is less dependent on physicochemical conditions, has a nearly three times lower variation coefficient, and permits an element partitioning analysis in heterogeneous mineral–fluid systems.

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