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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°50
N, 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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