A geochemical study of ridge flank sediments on the East Pacific Rise & the development of uranium as a tracer of low temperature hydrothermal circulation

Abstract

Of the major processes that act to regulate the chemistry of the Ocean-Earth system, hydrothermal circulation and the associated ridge flank sedimentation processes are possibly the least well constrained.  This research focuses on sedimentation processes and low temperature hydrothermal fluid flow across crust aged 0.36Ma at ~14^oS on the east flank of the East Pacific Rise.  The sediments are dominantly oxic and comprise biogenic carbonates, hydrothermal plume fall-out, and locally sourced basaltic debris, with minor barite and scavenged Al.  The Fe/Mn ratio of the plume material decreases with distance from the ridge axis (3.6±0.4wt/wt at 0.36Ma to 2.5±0.3 wt/wt by 4.6Ma), consistent with the slower oxidation kinetics of Mn²⁺ with respect to Fe²⁺.  It is demonstrated that excess barium does not give an accurate estimate of biogenic barite in regions with a significant input of Fe (hydro)oxides.  Comparison of the Al/Ti ratio to other biogenic parameters suggests that scavenged Al reflects a dominant association with the biogenic particle flux.  Early diagenetic alteration of the plume derived Fe (hydr)oxides by dominated by transformation of ferrihydrite to geothite.  Differential behavioural of divalent transition metals and oxyanions during alterations was observed.  Cu and Zn are incorporated into the goethite structure without discrimination.  Ni and Co are strongly associated with the Mn phases.  V is discriminated against during transformation, where rejected V is retained in the sediment.  Significant loss of P appears to occur during transformation of colloidal ferrihydrite to a more structured but still amorphous Fe (hydr)oxide phase.  The distribution of U is a result of post depositional enrichment during early diagenesis followed by remobilisation and loss of U from the sediment column.  The loss of U appears to reflect the influence of basement fluids on the sediment column, where burn-up of the sediment U enrichment is observed, leading to high concentrations of porewater U and the upwards mobilisation of sedimentary U over time.  An assessment of the oceanic U budget suggests that U may provide a good tracer of low temperature hydrothermal circulation.

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