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. (C) 2019 Elsevier B.V. All rights reserved.