Abstract
Oxide-rimmed, spherical structures interpreted as former gas bubbles have been discovered within a chromitiferous taxitic lithology of the Norilsk-Talnakh intrusions. These rocks are represented by variable grain size, presence of reworked country-rock xenoliths and millimeter- to centimeter-scale irregular spinel-rich aggregates, patches, or disrupted seams and stringers. They contain spherical and subspherical agglomerations that we interpret as amygdules, partially or completely filled with low-temperature hydrothermal minerals and locally with magmatic phases including sulfide globules. In places these amygdules form clusters that are interpreted as former bubble foams.
The wetting relationships visible between vapor bubbles, silicate melt, sulfide liquid, and oxide were investigated in detail using 3-D μX-ray tomography and detailed 2-D X-ray fluorescence maps. They also reveal short-range spatial variability in silicate and oxide minerals, reflecting small-scale advanced fractionation of silicate melt.
Three possible mechanisms are considered for the formation of these bubble-spinel foams: (1) the abundant spinel allows for the in situ nucleation of vapor bubbles, (2) the vapor bubbles ascend through the magma and collect spinel, or (3) the vapor bubbles cause rapid nucleation of chromite within these layers. Although none of these mechanisms can be exclusively ruled out, the texture and chemistry of the Norilsk-Talnakh chromitiferous taxitic lithology is most indicative of in situ nucleation of vapor bubbles on spinel surfaces (i.e., mechanism 1).
Partitioning of moderately siderophile elements among olivine, silicate melt, and sulfide melt: Constraints on core formation in the Earth and Mars