Mineral chemistry and alteration characteristics of spinel in serpentinised peridotites from the Northern Central Indian Ridge

Abstract

Serpentinites (frequently cross cut by gabbroic dikelet), collected from Northern Central Indian Ridge (NCIR), Indian Ocean, contain both Cr-rich (Group I) and Cr-poor (Group II) variety of spinel. Based on mineralogy they can be classified as non-residual and residual spinel, respectively. While, non residual spinel (TiO2 up to 0.5 wt %; Group I) display the evidences of peridotite-gabbro interaction, residual spinel (TiO2<0.1 wt%; Cr# ~24; Group II), suggests intermediate degree of partial melting (~10%) of mantle peridotite, which falls between most depleted mantle peridotite (~15%) and most fertile peridotite (~5%). Alteration of spinel porphyroclast is conceded by the presence of relict fresh interior, intermediate ferritchromit zone and secondary magnetite at the margins. Due to the effect of serpentinisation, marginal areas of spinel porphyroclasts display increasing Fe, but decreasing Al and Mg, while their Cr content remains unaffected throughout the spinel.