Fe-Ti oxide micro-inclusions in clinopyroxene of oceanic gabbro: Phase content, orientation relations and petrogenetic implication

Abstract Fe-Ti-oxide micro-inclusions in clinopyroxene of oceanic gabbro from the mid Atlantic ridge have been studied using electron backscatter diffraction and electron probe microanalyses. A first generation of Fe-Ti-oxide inclusions occurs as needles or elongated plates lying in the (010) plane of the clinopyroxene host. The inclusions show distinct elongation directions following “irrational” planes either nearly parallel to the “c” or nearly parallel to the “a” axis of the clinopyroxene host. The habit planes correspond to inclusion-host interfaces, where densely packed oxygen layers of the inclusion and host phases are coherent across the interface. Both inclusion types have distinct crystallographic orientation relations to the host, which are determined by the nearly parallel alignment of densely packed oxygen layers in the inclusions and the clinopyroxene host. Based on the angle between the two elongation directions primary formation of the inclusion at 800° to 900 °C was inferred. Initially, homogeneous Ti-bearing magnetite was precipitated together with titanian pargasite lamellae due to reaction of early magmatic clinopyroxene with late magmatic fluid or melt. After cooling below 600 °C the Ti-magnetite decomposed into an oriented magnetite + ulvospinel intergrowth. Late stage hydrothermal alteration leads to the corrosion of the magnetite-ulvospinel inclusions and partial replacement by ilmenite as well as newly precipitated homogeneous ilmenite plates that are closely associated with actinolite lamellae lying in the (100) plane of the clinopyroxene host.