Low Temperature Alteration of Basalts During the Last 9 Ma at 14o15'S on the South East Pacific Rise

Little is known about the influence of low temperature alteration on the mineralogical and chemical composition of the oceanic crust once it leaves the spreading axis. Yet this crust is one of the major inputs to subduction zones, its composition is therefore highly relevant for the evolution of mantle, crust and biosphere. In addition, any secondary minerals contain a large amount of H20 (i.e. clays), in consequence they play an important role as a water depot and therefore affect the deydration reactions during subduction. For a better comprehension of the alteration effects it is important to understand the effects of the initial low temperature alteration. To determine these features, electron microprobe, X-ray diffraction and ICP-MS analyses have been carried out on basalts from the eastern flank of the East Pacific Rise at 14^o15'S. The investigation concentrates on the products of a single spreading segment in a corridor perpendicular to the SEPR over a period from 0 to 9 Ma. Fresh rocks were sampled at the ridge axis (0-0.015 Ma), whereas off-axis basalts contain features of seawater generated alteration, which increases in intensity with time. Celadonite is the main alteration component in 0.12-4.6 Ma old rocks, whereas phillipsite is more abundant in rocks older than 4.6 Ma. The secondary minerals show evidence for a slight change in redox condition with time, from oxidizing, water-dominated to more reducing, rock-dominated environment. Iron-oxyhydroxide and celadonite are the first alteration products filling voids and veins and replacing olivine, partly replaced by saponite under more reducing conditions. The Fe necessary for the formation of these minerals is furnished by the dissolution of glass and the breakdown of olivine. Phillipsite is present in fractures and veins in rocks older than 1 Ma. Analyses also indicate an illite-smectite mixed layer which is believed to be an intermediate between saponite and celadonite and small amounts of a chlorite/smectite mixed layer. The presence of calcite is due to contamination with foraminifera and/or sediment. All samples are characterized by the lack of hydrothermal related minerals. We conclude that the alteration took place under seawater-dominated conditions at low temperature. A comparison of ICP-MS and XRF analyses from altered whole rock samples and their appendant fresh glass chips provide a record of element flow during alteration. Off-axis basalts show a significant increase of Rb, Cs and Ba which are supplied by seawater and incorporated in or on secondary minerals. The enrichment of U is connected to the oxidative conditions. K2O is also gained in all altered off-axis basalts, which is believed to be the result of the formation of celadonite.