Fluid and geochemical transport through oceanic crust: a transect across the eastern flank of the Juan de Fuca Ridge

The geochemical implications of thermally driven flow of seawater through oceanic crust on the mid-ocean ridge flank have been examined on a well-studied 80 km transect across the eastern flank of the Juan de Fuca Ridge at 48 degrees N, using porewater and basement fluid samples obtained on ODP Leg 168. Fluid flow is recognised by near-basement reversals in porewater concentration gradients from altered values in the sediment section to seawater-like values in basaltic basement. In general, the basement fluids become more geochemically evolved with distance from the ridge and broadly follow basement temperature which ranges from similar to 16 degrees to 63 degrees C. Although thermal effects of advective heat exchange are only seen within 20 lan east of where basement is exposed near the ridge crest, chemical reactivity extends to all sites. Seawater passing through oceanic crust has reacted with basement rocks leading to increases in Ca2+ and decreases in alkalinity, Mg2+, Na+, K+, SO42- and delta(18)O. Sr isotope exchange between seawater and oceanic crust off axis is unequivocally demonstrated with endmember Sr-87/Sr-86 approximate to 0.707. Evidence of more evolved fluids is seen at sites where rapid upwelling of fluids through sediments occurs. Chlorinities of the basement fluids are consistent with post-glacial seawater and thus a short residence time in the crust. Rates of lateral flow have been by estimated by modelling porewater sulphate gradients, using Cl as a glacial chronometer, and from radiocarbon dating of basal fluids. All three methods reveal fluid flow with C-14 ages less than 10,000 yr and particle velocities of similar to 1-5 m/yr, in agreement with thermally constrained volumetric flow rates through a similar to 600 m thick permeable layer of similar to 10% porosity. Delta(elemant)/Delta(heat) extraction ratios are similar to values for ridge-crest hydrothermal systems. (C) 1999 Elsevier Science B.V. All rights reserved.