Abstract
AbstractThe phenomena of reactive percolation of enriched asthenospheric melts and pervasive melt-rock interactions at mid oceanic ridge-rift systems are the principal proponents for mantle refertilization and compositional heterogeneity. This study presents new mineralogical and geochemical data for the abyssal peridotites exposed along the Vema and Vityaz fracture zones of the Central Indian Ridge (CIR) to address factors contributing to the chemical heterogeneity of CIR mantle. Cr-spinel (Cr#: 0.37-0.59) chemistry classifies these rocks as alpine-type peridotites and corroborates a transitional depleted MORB type to enriched, SSZ-related arc-type magma composition. HFSE and REE geochemistry further attests to an enriched intraoceanic forearc mantle affinity. The distinct boninitic signature of these rocks reflected by LREE>MREE<HREE and PGE compositions substantiates refertilization of the CIR mantle harzburgites by boninitic melt percolation concomitant to initiation of oceanic subduction. The mineral chemistry, trace, and PGE signatures of the CIR peridotites envisage (i) replenishment of depleted sub-ridge upper mantle by impregnation of subduction-derived boninitic melts, (ii) tectonic transition from mid oceanic ridge-rift to an embryonic suprasubduction zone, and (iii) initiation of spontaneous intraoceanic subduction along submarine transform faults and fracture zones of slow-spreading CIR owing to the weakness and mechanical instability of older, denser, and negatively buoyant Indian Ocean lithosphere.
Highlights
The tectonic and magmatic processes contributing to the heterogeneous nature of the upper mantle posit important constraints on the composition and differentiation of the earth at various scales
This study presents petrology, mineral chemistry, bulkrock major, trace, and platinum group elements (PGE) data for the abyssal mantle peridotites exposed along the Vema and Vityaz fracture zones of the northern portion of the Central Indian Ridge (CIR) located in the Indian Ocean to address their petrogenetic evolution, geodynamic implications, and factors contributing to the refertilization of the Indian Ocean ridge mantle
Several studies have demonstrated that major elements like Al, Ti, Fe, and P, high field strength elements (HFSEs), REEs, and transition metals like Cr, Ni, Sc, V, and Y are relatively immobile during hydrothermal alteration and greenschist to amphibolite grade metamorphism [56, 57]
Summary
The tectonic and magmatic processes contributing to the heterogeneous nature of the upper mantle posit important constraints on the composition and differentiation of the earth at various scales. Geochemical signatures of magmas in oceanic realms, owing to minimal effects of contamination during ascent, record imprints of multiple episodes of melt replenishment, meltrock interaction, fractional crystallization in open magma systems, and crustal recycling from paleotectonic events. These features provide a natural archive to probe into the compositional anomalies of the melting domains within the mantle. The studied peridotites from the CIR represent the upper mantle section of the oceanic lithosphere tectonically exhumed on the ocean floor along the Vema and Vityaz transform faults and fracture zones. The mineralogical and geochemical attributes of these mantle peridotites explicitly track the magmatic response to melt-percolation, meltrock interaction, and mantle replenishment processes operative beneath the ridge system in an extensional tectonic regime
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