Sub-solidus evolution of plagioclase-hosted Fe-Ti oxide micro-inclusions from oceanic gabbros, Mid-Atlantic ridge (10–13°N)

Abstract

Iron‑titanium oxide minerals are highly reactive under changing P-T-fO₂ conditions, and their chemical compositions and phase relations are important petrogenetic indicators. In this study, we use optical-, scanning electron-, and scanning transmission electron microscopy to investigate the sub-solidus and hydrothermal evolution of plagioclase-hosted FeTi oxide micro-inclusions from two types of oceanic gabbro dredged from the Mid-Atlantic Ridge at 10–13°N and exhibiting different types of hydrothermal alterations. Initially, the micro-inclusions formed as oriented, needle-shaped titanomagnetite, by precipitation from Fe- and Ti-bearing plagioclase. In most of the studied gabbros, such inclusions evolved into magnetite-ilmenite intergrowths by high-temperature oxidation above the Curie temperature of magnetite. Further evolution of the FeTi oxide micro-inclusions occurred during hydrothermal alteration at temperatures below the Curie temperature. Under low-intensity hydrothermal alteration, the micro-inclusions underwent minor dissolution and recrystallization. In contrast, intensive hydrothermal alteration under relatively reducing conditions resulted in substantial recrystallization and formation of magnetite-ulvospinel micro-inclusions, which were subsequently transformed into fine-grained magnetite-ilmenite ± ulvospinel aggregates. The phase contents and the microstructures of the plagioclase-hosted FeTi oxide micro-inclusions, formed during the sub-solidus and hydrothermal evolution, provide insights into their evolution during cooling under changing redox conditions. These features are crucial for interpreting paleomagnetic measurements of Fe-rich gabbros, which often feature plagioclase hosting FeTi oxide micro-inclusions.