Intragranular to xenolith-scale water heterogeneity in mantle olivine: Insights into cratonic processes

Abstract

This study presents Fourier Transform Infrared (FT-IR) mapping of olivine in mantle xenoliths from the Kaapvaal Craton, introducing methodological innovations for quantifying olivine water content. Our advancements include enabling direct Electron Backscatter Diffraction (EBSD) analysis of FT-IR-measured grains, and developing an automated calculation method to minimize serpentinization effects. This approach enables the calculation of the water content of olivine with high reliability and facilitates a point-to-point correlation between water content and crystallographic orientation. Analysis of garnet lherzolites (100–150 km depth) and spinel peridotites (∼60 km depth) revealed our method's effectiveness for olivine water contents above 40 ppm. Olivine in garnet lherzolites contained 40–210 ppm water, while olivine in spinel lherzolites exhibited contents below the detection limit, indicating heterogeneous water distribution in the Kaapvaal Craton mantle. We observed significant water content heterogeneity across multiple scales: intragranular, inter-grain, and xenolithic. Intragranular water content gradients up to 50 ppm were observed, with high-water olivine (>120 ppm) tending to show gradients aligned with crystallographic axes, suggesting anisotropic water incorporation and diffusion processes. Variations in water content between olivine grains within xenoliths and across different microstructures in garnet lherzolites were also noted. This heterogeneity likely results from complex lithospheric mantle processes, including localized metasomatism and deformation. These findings have important implications for understanding cratonic stability, mantle dynamics, and water-related processes in the Earth's mantle, potentially facilitating small-scale metasomatism or deformation without compromising overall cratonic stability.