Crustal recycling and mantle heterogeneity beneath the Central Indian ridge, Indian Ocean

This study presents a new database including mineral chemistry, whole rock geochemistry and Sr-Nd isotopic compositions for basalts recovered from the Central Indian Ridge (CIR) with a view to evaluate melt generation processes, source characteristics and tectonic controls on Indian Ocean mantle evolution. The salient geochemical characteristics of the subalkaline, tholeiitic to transitional Central Indian Ridge Basalt (CIRB) are marked by relatively higher abundances of LILE and LREE over HFSE, depletion in HFSE abundances with respect to primitive mantle compositions corroborated by negative Nb, Zr, Hf anomalies and Nb/Nb∗<1, Zr/Zr∗<1, Hf/Hf∗<1; depleted to moderate enrichment of LREE and MREE over HREE and trace element variations reflecting distinct deviation from N-MORB to E-MORB with very low Nb/Y (avg.: 0.08) and Zr/Y (avg.: 3.00) values with respect to OIB (Nb/Y: 1.66, Zr/Y: 9.66). The CIRB samples showing Ba/Nb>6, Rb/Nb>0.6, Nb/U<42 and Ce/Pb<22 conform to the BABB filter thereby preserving past subduction signals and validating the evidence for a subduction-modified source mantle. The Sr-Nd isotopic ratios yielded the following ranges: ⁸⁷Sr/⁸⁶Sr: 0.702810–0.703518 and ¹⁴³Nd/¹⁴⁴Nd: 0.512831–0.513118 (Ɛ_Nd: +3.77 to +9.37), reflecting an enriched heterogeneous mantle end-member source mantle. Isotopic mixing calculations identify major HIMU-EM1 signature with minor EM2 component for CIR basalts attesting to the chemical heterogeneity of the Indian Ocean mid oceanic ridge (MOR) mantle. The major HIMU-EM1 isotopic trend, distinct deviation from DMM-N-MORB with lower Nb/Y, Nb/Yb, Nb/U, Nb/Th and higher Ba/Nb, Ba/Th, Ba/La, Nd/Hf, Ce/Nb for CIRB in comparison with OIB and N-MORB suggest that OIB component or deep mantle plume sources had no influence on the compositional diversity of the mantle source. Instead, these features attribute the fertility of CIRB source mantle to: (i) convection driven recycling of ancient, subducted, metasomatized oceanic crust (HIMU) and (ii) delamination, dispersion and dilution of Gondwanan SCLM, LCC and UCC. This study equates the chemical evolution of the Indian Ocean MOR mantle and its pervasive heterogeneity with polychronous tectonic events involving cyclic amalgamation and disintegration of supercontinents synchronized with ocean basin closure and opening that systematically recycled ancient lithospheric components into the mantle.