Sepentinized Peridotite Spinel Composition: Northern Central Indian Ridge at 6°39

Exposures of serpentinized peridotites on the seafloor at slow-spreading ridges have been interpreted either as accretion of ridge segments in a magma-starved condition along the non-transform setting or as preferential outcrops at ridge offsets in transform fault setting. Here we present the mineral chemistry and geochemistry of serpentinites and serpentinized spinel peridotites recovered from an off axis region (corner high) at south of Vityaz transform fault (6°39'S), Northern Central Indian Ridge. Our purpose is to use mineral chemical data of serpentine and spinel to investigate the effect of low temperature alteration processes and degree of partial melting. Serpentine composition shows presence of high Mg-rich lizardite and chrysotile pseudomorphs and these rocks mostly preserve `mesh rim', `window' and `hourglass' textures, representing extensive hydration during low temperature hydrothermal alteration. In thin section, serpentine veins (mainly lensoidal, pinch and swell or anastomosing) are common, sometime crosscutting the `mesh rim' textures to attest to the intensity of serpentinization process. In one sample, a 1.9 cm-thick feldspathic vein crosscut the serpentinite as a porphyroblast and this indicates discontinuity in magmatic crust caused due to less magma input at off-axis region facilitate the intrusion of short-living feeder dykes of highly fractionated late magmatic liquids within the peridotite. In addition, in hand specimen, presence of smaller-scale striations analogous to slickenlines on serpentinite surfaces suggests low-angle faulting, which could have enhanced pervasive serpentinization during their subsequent emplacement. Individual serpentine grain displays very low Ca content (0.01 wt%) suggesting possible absence of any secondary Ca-rich phases also verified by very low Sr content (< 0.5 ppm). High Zn, Mn and U values (max up to 24, 165 and 107 ppm respectively) may suggest their enrichment in view of late stage hydrothermal input. Positive Eu anomaly (Eu/Eu* +3.38) coupled with negative Ce anomaly (~ -0.44 to -1.05) with total REE enrichment [(La/Sm)N 4.40 and (Yb/Sm) N 1.34-2.29] indicates significant hydrothermal input. High Nb/ La ratio (~ 1.06-6.34) further test the same connotation. Limited data on composition of individual spinel porphyroclast exhibits substantial variation in their Mg# (mole [Mg/ Mg+Fe2]) and Cr# (mole [Cr/ Cr+Al]) (~ 0.63-0.72 and 0.27-0.44 respectively) suggesting variability in the degree of melting (Cr# corresponds to 10.9-15.8 % of melting) of parent magma to generate the present peridotite. Very low TiO2 content (< 0.1 wt%) of the spinel grains also confirms their residual nature. Magnetite sometimes partly replaces the Cr-spinel, occurring as dusty clusters, and tend to concentrate along mesh rim and late stage serpentine veins. Present petrological observations on serpentine mineral chemistry allow us to demonstrate their mineralogical, textural and chemical changes and help to decipher the following interpretations (i) these rocks probably suffered the last stage or advanced serpentinization process for a long period of time and thus favorably induced to initiate the formation of low temperature mineral phases (mainly lizardite and chrysotile), (ii) furthermore, spinel chemistry suggests variable degree of partial melting of the parent magma which might reflect the heterogeneity in melt extraction at this slow-spreading ridge environment.