The control of oceanic crustal age and redox conditions on seafloor alteration: Examples from a quantitative comparison of elemental mass transport in the South and Northwest Pacific

Abstract

Oceanic crust interacts with infiltrating seawater, resulting in the formation of secondary minerals. Sediment cover on oceanic plates can potentially change the redox conditions of the underlying basaltic crust, which may influence which secondary minerals form and what elemental mass transport occurs during alteration. However, quantitative estimates of the variations in seafloor alteration using altered samples from different sites have not previously been carried out, and the factors controlling seafloor alteration remain unclear. We present a novel approach for the quantitative analysis of element mobility during seafloor alteration, based on a regional dataset of whole-rock compositions of altered basalts sourced from different drilling sites and alteration systems. Protolith reconstruction models (machine-learning-based element mobility analyses) were applied to the compositions of samples of basaltic crust from the South and Northwest Pacific. Our analyses show that older altered basalt has higher element mobility. In particular, Rb and K are enriched relative to their estimated contents in the protolith by a factor of 100 and 10, respectively, due to the formation of secondary minerals under oxidizing conditions. In the oxidizing settings of the South Pacific, enrichment in Ba and U was observed in samples with intense oxidation. In contrast, under relatively reducing conditions in the Northwest Pacific, alteration was associated with the formation of carbonate veins and U enrichment. The differences in quantitative element mobility and secondary mineralization demonstrate that sediment thickness and crustal age control redox conditions and the duration of seafloor alteration.