Micro-CT investigation of garnet-spinel clusters in mantle peridotite xenoliths

We have investigated the origin of garnet in garnet-spinel peridotite mantle xenoliths from two well known localities, Pali-Aike (Patagonia) and Vitim (Siberia). We used micro-CT scanning to examine the textures of garnet-spinel clusters and the relationship between garnet and spinel. CT scanning yields different mineral modes in terms of spinel and garnet than observed in thin-section, because distribution of these phases is inhomogeneous, and a larger volume is visible in a CT scan than in a thin section. We conclude that garnet formed after spinel in both localities, but the xenoliths record a complex tectonic history, in which the original garnets in garnet peridotites were transformed to spinel pyroxene clusters in spinel peridotites, via lithospheric thinning. This stage was followed by deformation of the spinel peridotites, producing a lineation. A final stage of isobaric cooling caused garnet to re-grow around the spinel clusters. Aspect ratios of garnets differ from those of garnet-spinel clusters; the latter are more similar to those of spinel pyroxene clusters. Kelyphite is a secondary feature resulting from heating and rapid decompression during entrainment in the host alkali basaltic lavas and has no relationship with the formation of the garnet-spinel clusters