Geochemical and petrographic characterisation of serpentinised peridotite from the Indian Ocean: Insights into mantle processes and tectonic evolution

Abstract

The present study explores the intriguing serpentinised peridotites from the Central Indian Ridge, discovered during the Sagar Kanya SK-195 cruise (2003). Detailed petrographic analysis revealed high-relief olivine and clinopyroxene, with orthopyroxene showing high-temperature deformation, along with opaque Cr-spinel and magnetite minerals. Mesh and brucite textures are prominent. Degree of serpentinisation was confirmed by the presence of antigorite, lizardite, and chrysotile, observed using Electron Microprobe Analysis EPMA and BSE imaging. Geochemical data indicated a harzburgitic protolith with iron-rich lizardite, and significant Fe enrichment in chromiferous spinel. Spinel, a quantitative melting indicator of mantle residues, exhibits compositional diversity and relative proportions of Cr, Fe, and Al in spinel-group minerals. REE patterns showed a distinct U-shape with a positive Eu anomaly, suggesting the absence of plagioclase feldspar. Major element analyses, supported by MgO/SiO₂ vs. Al₂O₃/SiO₂ trends, indicated 25 – 30% of partial melting, and a negative magnesium loss trend was also observed. Abyssal peridotite samples revealed the effect of regional geological processes and environments. Spinel Cr# and Mg# ratios and variations in Th, Zr, and Nb concentrations reflected complex melt-rock interactions. Characteristics of serpentinised peridotites suggest a heterogeneous origin influenced by multiple tectonic episodes and shared source material, enhancing understanding of serpentinisation, mantle melting, and the geochemical evolution of oceanic peridotites.