Petrological Characteristics and Genesis of the Central Indian Ocean Basin Basalts

The Central Indian Ocean Basin （CIOB） basalts are plagioclase-rich, while olivine and pyroxene are very few. The analyses of 41 samples reveal high FeOT （～10-18 wt%） and TiO 2 （～1.4-2.7 wt%） indicating a ferrobasaltic composition. The basalts have high incompatible elements （Zr 63-228 ppm; Nb ～1-5 ppm; Ba ～15-78 ppm; La ～3-16 ppm）, a similar U/Pb （0.02-0.4） ratio as the normal midoceanic basalt （0.16±0.07） but the Ba/Nb （12.5-53） ratio is much larger than that of the normal midoceanic ridge basalt （～5.7） and Primitive Mantle （9.56）. Interestingly almost all of the basalts have a significant negative Eu anomaly （Eu/Eu＊=0.78-1.00） that may have been a result of the removal of feldspar and pyroxene during crystal fractionation. These compositional variations suggest that the basalts were derived through fractional crystallization together with low partial melting of a shallow seated magma.

Physical Properties,Morphology and Petrological Characteristics of Pumices from the Central Indian Ocean Basin

About 400 pumice clasts collected from the Central Indian Ocean Basin（CIOB）were studied for their morphology and were classified based on their shape and size.A majority of the samples range between1 cm and 36 cm and in the Zinggs shape diagram plot in the equant and oblate fields.The Corey Shape Factor for most of the samples is close to 0.7,which is common for volcaniclastic material. The physical properties such as density,specific gravity,void ratio,porosity,moisture content and degree of saturation,were determined for 30 pumice samples.Density varies from 0.21 to 0.74 g/cm^3 specific gravity 1.84 to 3.27,void ratio 2.21 to 10.67,porosity 67%to 91%,moisture content during sinking 0.44 to 2.35 and degree of saturation varies from 26.5%to 86%.Binocular and electron microscopy studies reveal that 60%of the vesicles are elongated,30%are spherical and 10%are fibrous.Petrography of the pumices exhibits vitrophyric texture with phenocrysts of feldspars and clinopyroxenes.X-ray diffractrogram and mineral analyses confirm plagioclase to be a major phase, while quartz and orthoclase are not uncommon.Todorokite is commonly present in the ferromanganese oxide coating present over some of the pumices.This paper also delves into some details concerning the controversial origin of the pumices and glass shards in the CIOB.

Implications of REE incorporation and host sediment influence on the origin and growth processes of ferromanganese nodules from Central Indian Ocean Basin

This study presents new major,trace and REE data for thirty-five ferromanganese nodules recovered from areas representing three different sediment types(siliceous,red clay and their transition zone)in the Central Indian Ocean Basin(CIOB)to address their genetic aspects,classification,growth rate,nature of host sediments and influence of REE in the processes of nodule formation.The nodules from CIOB are mostly either hydrogenetic(metals coming from oxygenated bottom water)and diagenetic(metals coming from suboxic sediment pore water)or a combination of both,depending on the source of supply of metal.However,a number of biogeochemical processes mediate this supply of metals which again changes from time to time,making the nodule growth process highly dynamic.This study suggests that at the initial stage of nodule growth,host sediments do not play much role in controlling the growth processes for which REEs can enter both Mn and Fe oxyhydroxide phases equally.Thus,the bottom water signature is imprinted in these early formed nodules irrespective of their host sediment substrate but with gradual growth and burial in the sediment,the main mode of metal enrichment becomes diagenetic through sediment pore water.This tends to increase the concentration of Mn,Ni and Cu over other elements which are retained in the sediment fraction.Among the REEs,Ce concentration of the nodules shows significant positive anomaly due to variation in redox potential and hence its magnitude can be used to get an idea about the metal enrichment procedure and the genetic type of the nodules.However,based on host sediment only,not much difference is found in the magnitude of Ce anomaly in these nodules.On the other hand,discrimination diagram,based on HFSE and REY chemistry,indicates that most of these nodules are of diagenetic origin under oxic condition with a trend towards hydrogenetic field.Further,the genetic type of the ferromanganese nodules from the CIOB are more effectively differentiated by a combination of their major and trace element concentrations rather than solely based on their REE or HFSE chemistry or host sediment substrate.

Geochemical characteristics of REY-rich pelagic sediments from the GC02 in central Indian Ocean Basin

As a potential mineral resource, the clay minerals enriched in rare earth elements including yttrium（REY） in the deep sea have been attracting great attention. However, the enrichment mechanism of REY remains unclear. To understand the geochemical characteristics and factors controlling REY enrichment in zeolite clay in the deep sea, we conducted mineral identification by XRD, major and trace element measurements by XRF and REY analyses by ICP-MS on a 1.4-m-long sediment core（GC02） located in the Central Indian Oceanic Basin（CIOB）. The main findings include：（1） the core sediments in GC02 possess elevated REY contents and exhibited a strong negative Ce anomaly, an apparent MREE bulge and positive Y anomaly. These were comparable with typical REY-rich clays in the Pacific Ocean, indicating the similar REY enrichment mechanism and the presence of REY-rich clays in the CIOB;（2） in comparison with the dataset from the Wharton Basin and DSDP site 213, the higher content of REY and stronger PAAS（Post Archean Australian Shale） normalization patterns in the GC02 sediments were likely caused by the weaker impact of terrigenous materials of GC02. The CIOB was suggested to be a promising place hosting REY rich pelagic sediments.