Studies of hydrothermal circulation within partly buried basement on the eastern flank of the Juan de Fuca Ridge (JFR) have shown that ridge-flank geochemical fluxes are potentially important for the global budgets of some elements. There are major uncertainties in these flux calculations, however, because the composition of these basement fluids is strongly dependent on temperature and because they may be modified by interaction with the overlying terrigenous sediments, either by diffusive exchange with basement or during upwelling to the seafloor. To better understand the nature and temperature control of basalt-fluid and sediment-fluid reactions at the JFR flank, we have conducted laboratory experiments between 51 and 350°C and at 400 bars pressure. K, Rb, and Si are leached from basalt between 150 and 351°C, and Sr and U are taken up. The direction of exchange of Li and Ca with basalt varies with temperature. Li and Sr are removed from fluid at 150°C, but isotope studies show that there is simultaneous release of both elements from basalt, indicating that uptake is controlled by the formation of secondary minerals. Moreover, our experiments confirm that Sr isotope exchange with oceanic crust occurs at moderate temperature and is not confined to high-temperature axial hydrothermal systems. Our data and field data from the JDR flank indicate that uptake of U into basalt at moderate temperature could remove between 9.9 and 106mol U yr-1 from the oceans. This is higher than a recent estimate based on measurements of U in altered ocean crust (5.7 ± 3.3 × 106 mol yr-1), which concords with arguments that the Δelement/heat ratios of JDR flank fluids are too large to be representative of average global flank fluids. K, Ca, Sr, Ba, Li, Si, and B are leached from terrigenous sediments between 51 and 350°C, and U is taken up. Cs and Rb are removed from the fluid below 100°C and leached from the sediment at higher temperature. Sr isotope data show that Sr is preferentially mobilised from volcanic components within terrigenous sediments, which may lead to an overestimation of the ridge-flank Sr isotope flux at the JDR if there is exchange of sediment pore fluids with basement.
|Original language||English (US)|
|Number of pages||11|
|Journal||Geochimica et Cosmochimica Acta|
|State||Published - Feb 15 2003|
Bibliographical noteFunding Information:
This work has benefited from discussions with Mike Berndt and Mark Rudnicki. We thank Rick Knurr (University of Minnesota) for ICP-MS analyses, M. R. Palmer (University of Bristol) for access to a thermal ionization mass spectrometer, and H. Elderfield (University of Cambridge) for Sr isotope analyses. M. Mottl and four anonymous reviewers are thanked for their comments, which greatly improved the initial version of this article. This work was carried out while Rachael H. James was in receipt of a Natural Environment Research Council (NERC)/British Mid-Ocean Ridge Initiative (BRIDGE) fellowship (BRIDGE 76) at the University of Bristol, and support for R. James at the University of Minnesota was provided by the NERC through the BRIDGE programme.