The first 9 Ma years in the life of an East Pacific Rise segment

Abstract

The understanding of mid-ocean ridge processes is dominated by the results of studies on zero-age basalts from the present-day axes. Little is known about the processes that occur as the crust moves off-axis in terms of the influence of low temperature alteration on the mineralogical and chemical composition of the rocks, as well as the magmatic evolution of the spreading axis during the past on a several million year time scale. This thesis is focused on basalts dredged within a narrow flow-line corridor perpendicular to the present day ridge axis extending 9 Ma eastward of the East Pacific Rise. Backtracking these samples to their original on-axis eruption location and comparison of their geochemical data with samples from the present day ridge axis, results in a report of the first 9 Ma in the life of an East Pacific Rise segment.The backtracked samples show a surprisingly good and significant agreement with samples from the present day ridge axis and can be divided into two geographical groups separated by a magmatic border at 14°24 S ( related by differences in the amount of garnet present in the melting residue and by crystal fractionation, suggesting that they are derived from the same long-lived magma chamber).Information about the influence of low temperature alteration over several million years on the geochemical composition of the oceanic crust, is of fundamental importance when examining the changes on the surface of the upper crust, for crust-ocean mass balances and for an understanding of the input to the subduction factory. Fresh rocks were sampled at the present day ridge axis, whereas off-axis rocks contain features of seawater dominated low temperature alteration. Secondary minerals, sealing of fractures and the occurrence of more intense alteration rinds on older rocks show evidence for a slight change in redox conditions from an oxidizing, water dominated to a more reducing, rock-dominated environment with time.