Magnetite-orthopyroxene symplectites in gabbros of the Urals: A structural track of olivine oxidation

Abstract

The chemical compositions of magnetite-orthopyroxene symplectites (MOS) and rock-forming minerals—olivine (Ol), clinopyroxene (Cpx), and magnetite(Mt)—have been studied in 20 samples of olivine-bearing rocks in the Urals, including troctolite, olivine gabbro, and gabbronorite. MOS are orthopyroxene (Opx) monocrystals up to 500 μm in size containing myrmekite-like magnetite intergrowths up to 20–30 μm in width. According to the microprobe examination, the dark-colored minerals are characterized by a high Fe mole fraction F = Fe/(Fe + Mg) = 0.20–0.50, whereas F = 0.33–0.65 is typical of the bulk rock compositions. The plagioclase varies in composition from An₉₀ to An₅₀. No significant compositional difference has been established between the MOS and rock-forming minerals. The F _opx and F _Ol are closely correlated (linear trend, r = 0.97); F _Ol/F _Opx is ∼1.2. Similarly, a positive correlation between F _Opx and F _Cpx is noted (linear trend, r = 0.90); F _Opx/F _Cpx is ∼1.2. The crystallization temperature of the Ol-Opx-Cpx assemblage is roughly estimated at 700–800°C. A high positive correlation (r = 0.95) is established between the TiO₂ contents in the magnetites from the MOS (Mt1) and host rock (Mt2). The Mt1/Mt2 ratio reaches ∼0.8, implying that Mt1 contains somewhat less TiO₂ than Mt2. Hence, the rock-forming and MOS minerals make up an equilibrium assemblage. As follows from the structural pattern, symplectites were formed as products of the reaction between olivine and oxygen in the solid state with the entire volume of the rock involved in the oxidation; i.e., the distance of the diffusion was significant. Free oxygen appeared as a product of the dissociation of the water penetrating into the hot gabbro and ultramafic rocks at the initial stage of the tectonic extension and high-temperature hydration. According to the redox state of dunite coexisting with gabbro, the oxygen fugacity is estimated at +2.7 log units of fO₂ relative to the QFM buffer. The structure and products of the olivine oxidation were eventually obliterated in the course of the hydration.