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Seafloor eruptions and evolution of hydrothermal fluid chemistry

D. A. Butterfield,1 I. R. Jonasson,2 G. J. Massoth,3 R. A.Feely,3 K. K Roe,1 R. E. Embley,4 J. F. Holden,5 R. E. McDuff,5 M. D. Lilley,5 and J. R. Delaney

1Joint Institute for the Study of Atmosphere and Ocean, University of Washington, Seattle, WA 98195
2Geological Survey of Canada, Ottawa, Ontario, Canada
3Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, WA 98115
4Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Newport, OR 97365
5School of Oceanography, University of Washington, Seattle, WA 98195

Philosophical Transactions of the Royal Society of London A 355, 369-386 (1997).
Copyright ©1997 by the Royal Society. Further electronic distribution is not allowed.

1.  Introduction

Seawater evolves into hydrothermal fluid through a series of reactions as it is heated below the seafloor (see review by Seyfried & Mottl 1995). Magnesium and sulphate are removed from solution and many other elements are extracted from the rock through dissolution and exchange reactions. Hydrothermal fluids commonly pass through two-phase conditions in their subseafloor reaction path (Massoth et al. 1989; Butterfield et al. 1990, 1994 ; Von Damm et al. 1995) and separate into a low-chlorinity vapour phase and a high-chlorinity liquid or brine phase. The differing physical properties of vapour- and brine-like fluids (density, viscosity, surface tension) provide a mechanism to segregate them and produce vents on the seafloor with a wide range of chlorinities (Goldfarb & Delaney 1988; Butterfield et al. 1990; Fox 1990). To a first approximation, the ratios of most major elements to chloride are changed very little by the phase separation process (Berndt & Seyfried 1990), while gases are enriched in the low-chlorinity phase. Most of the variation in major element composition between different mid-ocean ridge (MOR) hydrothermal fluids can be explained by phase separation and segregation of brine and vapour. Different source rock chemistry, reaction zone conditions, kinetic factors and continued reactions in the segregated fluids are required to explain the remaining variation.

It has been recognized since seafloor hydrothermal vents were first sampled that they represent an important part of the geochemical cycles of many elements (Edmond et al. 1979; Von Damm et al. 1985; Palmer & Edmond 1989). Attempts to extrapolate to global chemical fluxes (using independent estimates of MOR fluxes of heat or helium and their correlations to elemental concentrations) have been tempered by an ever-increasing range of endmember fluid concentrations and by the lack of information on how fluid concentrations vary with time (see recent discussion by Von Damm 1995).

Large changes over time in chemical composition of vent fluids were not seen until 1990, when hydrothermal systems affected by volcanic activity were first sampled and it became apparent that rapid and significant changes in the style of venting and fluid compositions occurred immediately following an eruption (Haymon et al. 1993; Butterfield & Massoth 1994; Von Damm et al. 1995). Seafloor volcanic eruptions clearly have dramatic and interconnected consequences including formation of megaplumes (event plumes), microbiological blooms and rapid evolution of the temperature and composition of vent fluids (Haymon et al. 1993; Embley & Chadwick 1994; Baker 1995; Baker et al. 1995; Embley et al. 1995; Lupton et al. 1995; Von Damm et al. 1995; Holden 1996; Charlou et al. 1996). With the evidence accumulating from seafloor eruption events at N. Cleft, EPR at 9°50N, CoAxial segment, and the EPR near 17°30S (Auzende et al. 1996), it is possible to describe how hydrothermal systems change immediately following a volcanic event and to estimate how hydrothermal fluxes are affected. In this paper, we present the first detailed chemical data on hydrothermal fluids from the CoAxial segment of the Juan de Fuca Ridge, and relate it to a general model of hydrothermal response and chemical evolution of fluids following a volcanic event.


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