HYDROVOLCANIC ACTIVITY IN THE CENTRAL INDIAN-OCEAN BASIN - DOES NATURE MIMIC LABORATORY EXPERIMENTS

Classical works in the 1960's on spherules (volcanic, extraterrestrial , cosmic etc.) were carried out in order to distinguish these morpholo gically and compositionally so as to understand their genesis yet, in some cases their origin has remained enigmatic and indeterminate. We p resent here the finding of volcanic magnetite spherules up to 475 mu m in diameter which were recovered from the base of a seamount (basemen t age similar to 50 Ma) in the Central Indian Ocean Basin (CIOB). The spherules have quenched and crystalline textures, analogous to cosmic ones but compositionally are Fe-enriched (avg. 74%) together with othe r magmaphile elements. A distinctive lack of a Ni core is noted vis-a- vis Si-enriched areas. The spherules are very similar to those experim entally produced by simulated hydrovolcanic activity involving interac tion of a thermite melt with water-saturated quartzo-feldspathic sand. Our finding seems to indicate such a process to have occurred in natu re at abyssal depth. The plausible mechanism we conceive is the reacti on of Fe-rich lavas or hydrothermal emanations with the abundant surro unding siliceous ooze which is the dominant sediment type in the basin . We regard this is as the first report of an authentic proof for hydr ovolcanism at oceanic depth of 5200 m, which probably occurred similar to 10 ka ago as suggested by the associated radiolarian species. (C) 1997 Elsevier Science B.V.