The geochemistry of transitional sediments recovered from the Galapagos Hydrothermal Mounds Field during DSDP Leg 70—implications for mounds formation

Abstract

A total of 58 sediment samples have been analysed from holes drilled in the Galapagos Hydrothermal Mounds Field during Leg 70 of the Deep Sea Drilling Project. The sediments were selected, on the basis of physical and chemical characteristics, as being transitional in nature between pure nontronite of hydrothermal origin and typical pelagic sediment. Analyses were carried out for Li, Be, K, Mg, Ca, Si, Al, Mn, Fe, Sr, Ti, V, Cr, Co, Ni, Cu, Zn, Mo and Pb. The variations in chemical composition of the samples indicate that substantial volume changes have occurred in the pelagic sediment blanket during nontronite formation, a process which occurs predominantly within pre-existing sediments by precipitation over a redox gradient and dissolution of the biogenic component of the pelagic sediment. The elements Ca, Sr, Mn, Ni and Zn are lost overall from the pelagic sediment blanket whilst K, Mg, Fe, Si and possibly Mo and Pb also, are added. Li, Be, Al, Ti, V, Cr and Co remain immobile during the process of nontronite formation. Mass balance calculations for nontronite formation show that the overall morphology of the mounds can be explained satisfactorily in terms of volume changes which have occurred due to dissolution of biogenic sediment from and addition of nontronite to the sediment blanket.