[S-CG45] Petrology, Mineralogy & Resource Geology

[SCG45-P09] Factors Controlling the Water Content in the Mantle beneath the Kaapvaal Craton: A Kimberley Pipeline Case Study

*Wenzhao DONG¹, Yui Kouketsu¹, Katsuyoshi Michibayashi¹, Takenori Kato² (1. Department of Earth and Planetary Sciences, GSES, Nagoya University, 2.Institute for Space–Earth Environmental Research, Nagoya University)

Keywords:Kaapvaal Craton, Mantle peridotite, Water content, Chemical composition, P-T equilibrium

Olivine is the principal mineral in Earth's mantle, and its water content, reflected as OH-concentration, is vital for interpreting mantle dynamics. Traditional methods for quantifying olivine's water content have often neglected the comprehensive screening of grains and the impact of serpentinization, both essential for precise hydration assessment. Current techniques still face challenges in accurately determining water content and integrating the influence of geochemical and crystallographic factors that govern mantle hydration. Addressing these methodological gaps, we introduce an advanced approach that initially applies Fourier Transform Infrared Spectroscopy (FTIR) mapping to capture an extensive array of infrared spectrograms, which are then analyzed with a MATLAB program for meticulous calculation and visualization of olivine's water content. Our method reveals that olivine grains in mantle peridotite from the Kimberley Pipeline exhibit water contents ranging from 60 to 210 ppm. Notably, the coarse porphyroclastic microstructure contains the highest water content, suggesting a transitional microstructure influenced by metasomatism or deformation. Subsequent to water content determination, electron backscatter diffraction (EBSD) analysis on the same olivine grains correlates water content with crystal orientation. Although some connections were observed, the complex patterns and potential trends require further elucidation. Our combined approach, including electron probe microanalysis (EPMA) for chemical composition, indicates no direct correlation between CaO, MnO, NiO concentrations, or equilibrium P-T conditions with olivine's water content. A moderate linear correlation between grain size and water content is discerned, yet it is not sufficiently strong to be a primary determinant of olivine's water content. According to the diagrams of Mg# versus water content and pressure-depth profiles in relation to hydration, we propose that different microstructures undergo transformations via metasomatism and deformation, with metasomatism being a predominant factor affecting olivine's water content. This comprehensive methodological framework enhances our understanding of the factors controlling olivine's water content and the impact of metasomatic processes, offering critical insights for future research into the mechanisms of mantle hydration and microstructural evolution.