Do the 85E Ridge and Conrad Rise form a hotspot track crossing the Indian Ocean?

Age-progressive volcanic chains are generally considered as surface expressions of deep-rooted mantle plumes, whereas non-age progressive volcanic groups or solitary volcanic edifices are generally attributed to shallow sources. The Buried Hills, Partly Buried Hills, Afanasy Nikitin Rise and newly discovered Southern Seamount Chain, located between the Kerguelen and Reunion hotspot tracks in the Indian Ocean, form the curved and discontinuous 85 degrees E volcanic track. With the exception of the Afanasy Nikitin Rise, characterized by the most enriched mantle one (EMI) compositions in an ocean basin, no other parts of this volcanic track have been previously sampled. Although some models favor a hotspot origin for the 85 degrees E Ridge track associated with the Crozet, Marion or Conrad Rise plumes, its origin remains highly controversial. We report new 40Ar/39Ar age and geochemical (major, trace element and Sr-Nd-Pb-Hf isotope) data from the 85 degrees E Ridge. Our age data display a progression of decreasing ages (82-66 Ma) from the north to south, suggesting formation by a relatively stationary sub-lithospheric melt anomaly. The geochemistry of the 85 degrees E Ridge can be explained by mixing of Indian-type plume material with Indian mid-ocean-ridge basalt (MORB) and detached continental lithospheric (mantle and/or lower crustal) material in the upper mantle. Plate reconstructions demonstrate that the southern portion of the 85 degrees E Ridge and Conrad Rise could be derived from the Conrad Rise hotspot/plume. A weak, pulsating mantle plume could explain the weak morphological expression and intermittent nature of this hotspot track, compared to the Reunion and Kerguelen tracks.