Exploring the Origin of Pyroxenites using Spinel Compositions of Western Rift Ugandan Xenoliths

Abstract:

Pyroxenite is recognized increasingly as an important lithological component contributing to mantle-derived volcanism. Pyroxenite is not a primary mantle lithology and, instead, can be generated through a variety of processes (crustal recycling, melt-rock interaction, fractional crystallization, etc.). Mineral compositions can provide insight into the origin of pyroxenites. However, few studies have explored spinel chemistry as a means to understand pyroxenite petrogenesis. This study investigated the composition of spinel minerals contained in pyroxenite and rare peridotite xenoliths from the Western Rift of Uganda in the East African Rift System to understand pyroxenite origins. Petrographically, the pyroxenite xenoliths record a combination of three textures: (1) granular without compositional zoning, (2) granular with subtle zoning and areas of glassy melt pockets or coarsely crystallized channels, (3) coarse-grained cumulate textures. Spinel compositions were measured on grains encased in olivine or, in olivine-free samples, clinopyroxene. Compositionally, spinel ranges between Al-rich and Cr-rich endmembers. When viewed by locality and within individual samples, spinel record significant heterogeneity in both Mg# (0.14-0.63) and Cr# (0-0.59). All of the spinel, including those within the peridotite, contain moderate to high TiO₂ contents (0.74-19.64 wt. %); cumulate-textured pyroxenite contains the highest TiO₂ abundance, consistent with a magmatic origin. Petrogenetically, the spinel can be used to group the pyroxenite xenoliths into two populations: melt-rock reaction pyroxenites and magmatic cumulate pyroxenites. Melt rock reaction pyroxenites contain spinel with a negative Cr₂O₃ vs. Al₂O₃ correlation, consistent with partial melting of mantle materials; spinel in magmatic cumulates record a positive correlation and contain some of the lowest Cr₂O₃ abundances. Texturally, the melt-rock reaction pyroxenites are dominated by granular textures, though some samples have localized channels of coarser crystals. These channels contain spinel with compositions similar to magmatic cumulates. The variation in pyroxenite spinel populations demonstrates that the xenoliths are sampling different portions of the lithosphere that may correspond to different depths.