Influence of low-temperature fluids and post-depositional changes in the siliceous–pelagic sediments of the Central Indian Ocean Basin

In this paper, we discuss the possible reasons for the mass disappearance of radiolarians, hiatus in sedimentation, low-temperature alteration of sediments, and formation of palagonite and phillipsites in the Central Indian Ocean Basin (CIOB). A prominent change in the coarse fraction components was observed in two 5 m long sediment cores retrieved from a water depth of 5000 m near the 79°E fracture in the basin. One core was lifted from the boundary of siliceous-red clay and the other from red clay sediment. In both cores, a distinct antithetical relation exists between occurrence of radiolarians and palagonite grains i.e., in certain down-core sections where radiolarians are present, palagonite grains are absent and vice-versa. This discontinuity in the deposition/dissolution of radiolarians signifies post-depositional variations in the sediment cores and this is confirmed by down-core compositional fluctuations. This study gives a first-hand understanding of the inverse relation between extensive palagonite formation and large-scale absence of radiolarians in the CIOB. The abundance of Ca-poor and K > Na phillipsite crystals in the palagonite-rich zone attest to different stages of palagonitisation of the sediments. The non-preservation of radiolarians in both cores, and elsewhere in the basin, could be due to intra-basinal volcanism and the influence of low-temperature fluids through fracture zones and fissures that are present in the vicinity. The coupled effect of these two processes perhaps led to alteration of the core sediments and the observed changes.