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 o ceanic crust on the mid-ocean ridge flank have been examined on a well-stud ied 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 conce ntration gradients from altered values in the sediment section to seawater- like values in basaltic basement. In general, the basement fluids become mo re geochemically evolved with distance from the ridge and broadly follow ba sement temperature which ranges from similar to 16 degrees to 63 degrees C. Although thermal effects of advective heat exchange are only seen within 2 0 lan east of where basement is exposed near the ridge crest, chemical reac tivity extends to all sites. Seawater passing through oceanic crust has rea cted with basement rocks leading to increases in Ca2+ and decreases in alka linity, Mg2+, Na+, K+, SO42- and delta(18)O. Sr isotope exchange between se awater and oceanic crust off axis is unequivocally demonstrated with endmem ber Sr-87/Sr-86 approximate to 0.707. Evidence of more evolved fluids is se en at sites where rapid upwelling of fluids through sediments occurs. Chlor inities of the basement fluids are consistent with post-glacial seawater an d 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 glac ial chronometer, and from radiocarbon dating of basal fluids. All three met hods reveal fluid flow with C-14 ages less than 10,000 yr and particle velo cities of similar to 1-5 m/yr, in agreement with thermally constrained volu metric flow rates through a similar to 600 m thick permeable layer of simil ar to 10% porosity. Delta(elemant)/Delta(heat) extraction ratios are simila r to values for ridge-crest hydrothermal systems. (C) 1999 Elsevier Science B.V. All rights reserved.