The interaction between K-rich melts and mantle minerals in Fernando de Noronha (Brazil) mantle xenoliths: a 3D X-ray computed microtomography and geochemical study

In this work, we present the direct evidence of mantle metasomatism that occurred in the Fernando de Noronha archipelago. We performed both textural (optical microscopy and 3D computed microtomography) and chemical analysis (EMPA and LA-ICP-MS) on mineral phases and K-rich glasses inside xenoliths to understand the nature, dynamics, and composition of FN mantle section and to give new insight into the formation and evolution of potassic rich melts. We selected 5 samples particularly rich in glassy patches, that are characterized by the presence of several euhedral crystals inside, which are mainly olivine, clinopyroxene, spinel and plagioclase, with the sporadic occurrence of apatite and amphibole. 3D Throughout 3D X-ray computed microtomography (μ-CT), we studied in three dimensions the texture of the samples, defining quantitatively parameters that have petrological implications. The interconnectivity of the observed glasses, for example, has been discovered to be significantly high in all our xenoliths, testifying extensive percolation of the metasomatizing melts. We based the 3D textural study on a multiscale approach structured in steps of incremental resolution: conventional X-ray μ-CT with 23.33 μm of voxel size, conventional μ-CT with a voxel size ranging from 10.9 to 6.7 μm depending on the sample, and synchrotron radiation X-ray μ-CT reaching voxel sizes of 2.5 and 0.9 μm. As the attached manuscript (AppendixA, Venier et al., due to submission) testifies, this reveals to be the best way to analyze a representing volume of the sample without losing the small-scale details (visible at a resolution below the μm). Major, minor and trace elements analyses allow us to characterize the mineral chemistry of phases and the chemistry of glasses. The latter appear as potassic in composition and are completely different from their hosting basanite, proving they are not the effect of magma infiltration during the ascent. Trace elements are far more sensible to petrological processes, rare earth elements (REE) and incompatible elements (IE) patterns allow us to understand how glasses induced metasomatism in the mantle phases and partial assimilation of orthopyroxene when alkalinity reached a sufficient level. Moreover, trace elements allow disproving some theories, such as the carbonate metasomatism evoked by Kogarko et al. (2001) and strengthened what was stated by Rivalenti et al. (2000). As a matter of fact, our samples confirmed what the isotopic composition of mantle xenolith’s Cpx previously suggested: the potassic source that contaminated the mantle beneath FN is the same that originated the potassic series of Remédios formation and is due to contamination of the oceanic lithosphere as a consequence of lithospheric stretching during the opening of the Atlantic ocean. Combining optical microscopy, 3D multiscale textural analysis and chemical analysis of both major, minor and trace elements we were able to find an exhaustive model for the mantle section beneath the Fernando de Noronha archipelago, giving direct evidence of texture and chemical composition of the metasomatizing melt that percolated our xenoliths.