Diagenetic Alteration of Hydrothermal Plume Precipitates: Transition Metal and Oxyanion Associations.
Abstract
We evaluate the diagenetic alteration of hydrothermal plume precipitates and discuss implications for the magnitude of transition metal and oxyanion sinks, based on analysis of sediment mineralogy, bulk chemistry, and a four step sequential extraction procedure. All samples were collected from the EXCO (EXchange between the Crust and Ocean) study site at ∼14° S on the east flank of the East Pacific Rise. The sediments are lithologically simple (biogenous carbonate, plume precipitates, minor basaltic debris) and oxic (uniform Fe/Mn). This site thus provides an excellent natural laboratory to investigate the role of plume derived Fe and Mn (hydr)oxides in controlling the fate of divalent transition metals (Cu, Zn, Ni) and oxyanions (V, P, As) in the sediment column, where this study bridges the gap between laboratory experiments and studies conducted in more complex environments. The alteration of Fe (hydr)oxides in the EXCO region is dominated by transformation of ferrihydrite to goethite, where the extent of alteration (goethite-Fe / X-ray amorphous Fe) ranges from 0.3-2.1. The dominant formation of goethite over hematite is in contrast to laboratory studies. We hypothesise that the strong surface adsorption of scavenged seawater species inhibits the structural rearrangement of ferrihydrite to hematite under natural conditions. A preliminary kinetic analysis suggests that the rate of transformation to goethite displays a pseudo second order dependency on the total Fe (hydr)oxide content, in contrast to laboratory data which indicate a first order dependency. This discrepancy may reflect reaction conditions in the sediment column which are dependent on the Fe (hydr)oxide content (e.g. porosity). Cu, Zn and Ni are delivered to the sediment in association with both Fe and Mn (hydr)oxides, where ∼70% of Cu and Zn and ∼10% of Ni are associated with Fe (hydr)oxides. The Fe-associated Cu and Zn are adsorbed to the ferrihydrite surface and incorporated into the lattice structure of goethite during alteration. The Fe-associated Ni most likely represents Ni coprecipitated as Ni-Fe ferrihydrite and incorporated into goethite during alteration. The alteration of hydrothermal Fe (hydr)oxides thus represents a relatively permanent sink (where goethite dissolution is a prerequisite for metal release) for ∼70% of Cu and Zn and ∼10% of Ni. V, P and As are delivered to the sediment in association with Fe (hydr)oxides. The bulk sediment V/Fe is consistent with the plume particle signature, however only ∼30-40% of plume-V is incorporated into goethite during alteration, where the rejected V is most likely retained in the sediment in association with Mn-rich phases. Under oxic conditions, (no loss of hydrothermal Mn), the sediment V/Fe could therefore provide a good tracer of past seawater V content. The bulk sediment P/Fe and As/Fe are variable and lower than the plume particle signature. The loss of plume-P and As is attributed to a decrease in available surface adsorption sites with aging of the Fe (hydr)oxide precipitates, where loss continues until a steady state P/Fe or As/Fe is obtained. The difference between the plume particle P/Fe and the steady state P/Fe indicates a decrease in surface adsorption sites of ∼74%, in good agreement with the ∼60-70% reduction estimated from a comparison of the plume V/Fe to the goethite V/Fe. The remobilisation of plume-P thus impacts the global oceanic P budget, such that the net sink of plume-P is reduced by ∼74% to 4.4-8.0 x 1010 gyr-1, or only 5-9% of the riverine dissolved flux.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFMOS43A0529D
- Keywords:
-
- 4805 Biogeochemical cycles (1615);
- 4825 Geochemistry;
- 3035 Midocean ridge processes;
- 1030 Geochemical cycles (0330);
- 1050 Marine geochemistry (4835;
- 4850)