Geochemistry of mid ocean ridge basalts (MORB) from the northern Central Indian Ridge between 7°46 and 13°20 S: Implication of mantle heterogeneity influenced by Reunion hotspot plume?

Abstract

Between the Rodrigues Triple Junction (RTJ) and Carlsberg Ridge, The Central Indian Ridge (CIR) is a slow-intermediate spreading-rate (~43mm/year) plate boundary formed during separation of the Mascarene Plateau from the Chagos-Lacadives Ridge. Although several samplings of basaltic rocks were carried out in previous explorations on Indian Ridge, no systematic studies on the petrogenesis of CIR, especially for north of 18°S have been reported yet. We present the major and trace element composition of lavas dredged along the spreading axis of the northern CIR between 7°46 and 13°20 S. The mineralogy of the CIR MORB mainly consists of tiny needleand/or lath-like plagioclase microlites (~45%), sub- to anhedral olivine (~15%), small anhedral clino-pyroxene (~10%), and intersertal/intergranular Fe-Ti oxide glass matrix (~30%). In the whole rock and glass chemistry, the samples show the very similar pattern of compositional variation in SiO2, Al2O3, FeO, and CaO concentration against MgO (6.59 ~ 8.68 wt %), although relatively less compatible elements (TiO2, Na2O, K2O, and P2O5) show linear trend. Plottings of Na8.0, Fe8.0, and CaO/Al2O3 vs. depth are examined to check the varying extents of partial melting and the global correlations. The results broadly agree with the global trend of MORB and fall within the dry MORB domain. Concentration of light rare earth elements (LREE) vary widely in the analyzed rock samples ((La/Sm)N ≈ 0.68 ~ 1.41 and (Nb/Yb)N ≈ 0.42 ~ 1.93). In spider diagram normalized by primitive mantle, the volcanic lavas show systematic increase of incompatible element concentration from south (segment 1) to north (segment 5), which could be attributed to the influence of enriched source in south of the study area. However, basalts from segment 2 are significantly enriched in incompatible elements. Plots of the ratios of several incompatible elements versus La/Sm ratio show a linear trend which could be attributed to the mixing of depleted an