PII S0016-7037(01)00712-8 Evidence for basaltic Sr in midocean ridge-flank hydrothermal systems and implications for the global oceanic Sr isotope balance DAVID A. BUTTERFIELD, 1, *BRUCE K. NELSON, 2 C. GEOFFREY WHEAT, 3 MICHAEL J. MOTTL, 4 and KEVIN K. ROE 1 1 Joint Institute for the Study of the Atmosphere and Oceans, Box 357941, University of Washington, Seattle, WA 98195, USA 2 Department of Geological Sciences, University of Washington, Seattle, WA 98195, USA 3 Global Undersea Research Unit, P.O. Box 757220, University of Alaska, Fairbanks, AK 99775, USA 4 Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822, USA (Received March 10, 2000; accepted in revised form June 8, 2001) Abstract—Previous models and calculations of the global mass balance of Sr in the oceans have shown that the input of unradiogenic basaltic Sr from on-axis midocean ridge hydrothermal systems is much less than needed to balance the input of radiogenic Sr delivered to the oceans by rivers. The implication is that either the oceans are far from steady state with respect to Sr isotope balance (and that the 87 Sr/ 86 Sr ratio of seawater is increasing at unprecedented rates) or that there is a significant missing source of basaltic Sr. It has long been recognized that off-axis hydrothermal fluxes might significantly affect the mass and isotopic balance of Sr and other elements in the oceans, but nearly all previous work has concluded that the 87 Sr/ 86 Sr ratio of pore fluids in ridge-flank hydrothermal areas is virtually indistinguishable from the seawater ratio or is dominated by authigenic carbonates. In contrast, we report here the 87 Sr/ 86 Sr ratios of warm springs, sediment pore fluids, and basement reservoir fluid with a clear basaltic signature from the eastern flank of the Juan de Fuca ridge (JFR). Fluids venting from Ocean Drilling Program Hole 1026B on the Juan de Fuca east flank have relatively stable Sr isotope and major element composition for the 3 yr following drilling. These results and similar results recently reported by Elderfield et al. (1999) indicate that low-temperature ridge-flank hydrothermal circulation has an important 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 hydrothermal flux) are accurate, then the rate of increase of the 87 Sr/ 86 Sr ratio in seawater (0.000054 per million years) can be accommodated if ridge flanks on a global scale deliver fluids to the ocean with  ( 87 Sr/ 86 Sr)/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 calcite veins in the uppermost basaltic ocean crust, the average  ( 87 Sr/ 86 Sr)/heat ratio of low-temperature fluids is in the range required to balance the oceanic Sr isotope budget. Although the 87 Sr/ 86 Sr ratios of the JFR flank fluids in this study overlap with fluid properties inferred from some calcite veins in the upper oceanic crust, the magnitudes of the  ( 87 Sr/ 86 Sr)/heat ratios of nearly all of the JFR flank fluids are too large 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 © 2001 Elsevier Science Ltd 1. INTRODUCTION 1.1. Significance of Midocean Ridge-Flank Fluxes The contribution of hydrothermal fluids to the deep ocean was recognized in the mid-1960s by Bostrom and Peterson (1966), who found unusually high concentrations of Fe, Mn, Ba, Zn, Pb, and other elements in sediments on the flanks of the East Pacific Rise. Dasch et al. (1971) subsequently recognized a small volcanogenic Sr isotope signature in these same sedi- ments. After the composition of high-temperature hydrothermal fluids was reasonably well established, Palmer and Edmond (1989) measured and compiled Sr concentration and isotopic composition data for most of the major rivers of the world, and used this to constrain the flux of Sr from ridge-crest hydrother- mal systems. The 87 Sr/ 86 Sr ratio of seawater at present is 0.70918 and, based on the marine carbonate record from 2.5 Ma to present, is increasing at a rate of 0.000054 per million years (Hodell et al., 1990). Changes in the 87 Sr/ 86 Sr ratio of seawater reflect the shifting balance between input of radio- genic Sr ( 87 Sr/ 86 Sr  0.710) dissolved in rivers and unradio- genic Sr ( 87 Sr/ 86 Sr  0.703) derived from the reaction of seawater with volcanic ocean crust. Henceforth we will refer to the latter simply as basaltic Sr. The rate of increase throughout the Cenozoic era reflects an excess of radiogenic Sr from rivers over basaltic Sr from hydrothermal inputs. Palmer and Edmond (1989) concluded that estimates of on-axis midocean ridge (MOR) hydrothermal Sr flux derived from independent esti- mates of on-axis MOR heat flux were too low (by a factor of 2 to 5) to match the present rate of change of 87 Sr/ 86 Sr in the oceans. Similarly, the Mg removal by ridge-axis hydrothermal circulation based on heat and water fluxes is insufficient to balance the Mg input from rivers (Mottl and Wheat, 1994). Palmer and Edmond (1989) called for much higher on-axis fluxes to balance both the Sr and Mg budgets in the ocean. Thus, either the geophysical estimates of ridge-axis heat flux are too low by a factor of 2 or more (very unlikely), or there are significant off-axis hydrothermal fluxes. Given the apparent discrepancy in the Sr isotopic budget and the geologic evidence for low-temperature alteration of oceanic crust (e.g., Staudigel et al., 1981; Elderfield and Gieskes, 1982), investigations of Sr isotope composition in ridge-flank pore fluids have been carried out. Hess et al. (1991) analyzed Sr * Author to whom correspondence should be addressed (butterfield@pmel.noaa.gov). Pergamon Geochimica et Cosmochimica Acta, Vol. 65, No. 22, pp. 4141– 4153, 2001 Copyright © 2001 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/01 $20.00 + .00 4141