Geochemical heterogeneity along the Vema fracture zone, Indian Ocean: Mixing of melts from the reunion plume and the Central Indian Ridge

Abstract

We present here a scenario of magmatic and tectonic processes associated with the interaction between the Reunion mantle plume and the mantle beneath Central Indian Ridge (CIR) at the Vema Trench (VT) along the Vema Fracture Zone (VMFZ) in the Indian Ocean. The VMFZ offsets the CIR by about 300 km between 8 degrees S and 10 degrees S latitudes and holds probably the deepest point in the Indian Ocean, the Vema Trench, reaching to a depth in excess of 6,500 m. The trench has a width of about 5 km and extends for about 12 km in a NE-SW direction. Petrography and major and trace element concentrations collectively suggest that the rocks recovered from the VT are largely basaltic (pillow, columnar, dikes), composed of plagioclase and clinopyroxene, much evolved, and tholeiitic to calc-alkaline in composition. The REE signatures ([Sm/Yb vs. Sm] and [Ce/Sm] sub(N) vs. [Yb/Sm] sub(N)) hint its source to a spinel lherzolite mantle with additional input from enriched mantle (EM-1). This EM-1 signature generally indicates presence of recycled ancient subducted oceanic crust in the mantle domain wherefrom the plume originates. The detailed petrological and geochemical studies of these rocks suggest dynamic processes involved in transfer of deep mantle-sourced material to mid-ocean ridges leading to geochemical heterogeneity. Trace element ratios (Hf/3-Th-Ta and Thn vs. Nbn) also suggest that the VMFZ basalt preserves the signature of mixing of ridge melt and plume melt.