Evidence for basaltic Sr in midocean ridge-flank hydrothermal systems and implications for the global oceanic Sr isotope balance

input of unradiogenic basaltic Sr from on-axis midocean ridge hydrothermal systems is much less than needed to balance the input of radiogenic Sr deli vered to the oceans by rivers. The implication is that either the oceans ar e far from steady state with respect to Sr isotope balance (and that the Sr -87/Sr-86 ratio of seawater is increasing at unprecedented rates) or that t here is a significant missing source of basaltic Sr. It has long been recog nized that off-axis hydrothermal fluxes might significantly affect the mass and isotopic balance of Sr and other elements in the oceans, but nearly al l previous work has concluded that the Sr-87/Sr-86 ratio of pore fluids in ridge-flank hydrothermal areas is virtually indistinguishable from the seaw ater ratio or is dominated by authigenic carbonates. In contrast, we report here the Sr-87/Sr-86 ratios of warm springs, sediment pore fluids, and bas ement reservoir fluid with a clear basaltic signature from the. eastern fla nk of the Juan de Fuca ridge (JFR). Fluids venting from Ocean Drilling Prog ram Hole 1026B on the Juan de Fuca east flank have relatively stable Sr iso tope and major element composition for the 3 yr following drilling. These r esults and similar results recently reported by Elderfield et al. (1999) in dicate that low-temperature ridge-flank hydrothermal circulation has an imp ortant effect on the Sr isotope balance in the oceans. If published values for the other major sources of Sr input to the oceans (rivers and axial hyd rothermal flux) are accurate, then the rate of increase of the Sr-87/Sr-86 ratio in seawater (similar to0.000054 per million years) can be accommodate d if ridge flanks on a global scale deliver fluids to the ocean with Delta (Sr-87/Sr-86)/heat ratios one third to one half of the ratio found in warm JFR basement fluids. Based on published Sr and O isotope signatures of calc ite veins in the uppermost basaltic ocean crust, the average Delta (Sr-87/S r-86)/heat ratio of low-temperature fluids is in the range required to bala nce the oceanic Sr isotope budget. Although the Sr-87/Sr-86 ratios of the J FR flank fluids in this study overlap with fluid properties inferred from s ome calcite veins in the upper oceanic crust, the magnitudes of the Delta ( Sr-87/Sr-86)/heat ratios of nearly all of the JFR flank fluids are too larg e to be representative of the average global flank fluid flux; the same has been argued on the basis of the extremely high implied Mg flux. Copyright (C) 2001 Elsevier Science Ltd.