Concluding remarks
The results presented in this poster explore why iron in chimney sulfides is isotopically
lighter then the "earth-moon average". Equilibrium modeling attempted to put this fractionation
in the context of speciation changes due to changes in temperature, pH, and bulk chemical
composition of the fluids in vent environments. This work is influenced by the recent results
of Anbar et al. (2000) and Bullen (2001, in press) who maintain that abiotic processes
fractionate iron isotopes in the natural environment. In particular these authors believe
that equilibrium processes, involving changes in speciation and fractionation amongst various
pools of iron complexes, are responsible for observed isotope fractionation.
The current work characterizes basic elements of the Finn chimney sample from the Mothra vent
field and measures the iron isotopic composition of pyrite, chalcopyrite, and sphalerite from
this sample. Results suggest that sulfide minerals in the chimney are isotopically-lighter
than Johnson and Beard's (1999) "earth-moon average" and that some variation exist between
the three major sulfide minerals.
One possible explanation for this behavior, consistent with work currently underway at other
laboratories, is that changes in iron speciation during cooling of hydrothermal fluids and
mixing with seawater affects from which pool precipitation reactions take iron.
Future work will examine in more detail:
- The isotopic composition of sulfide minerals in other chimneys from different locations,
- The mechanism of iron substitution into sphalerite and its effect on isotope composition,
- Modeling interactions between fluids and vent minerals and the effect that overprinting
and hydrothermal reworking have on isotope composition and bulk fluid composition.
Acknowledgments
Special thanks to Deborah Kelley (University of Washington; vent sample),
David Butterfield (University of Washinton; fluid chemistry) for assistance with
sample-related concerns. Great thanks also to Hal Helgeson, Bill McKenzie, Laurent
Richard, Doug Larowe, and the rest of Prediction Central (University of California,
Berkeley) for help with the computer codes (SUPCRT 92 and "The Geochemist's Workbench")
and the mixing calculations. Of course, this work could not have been completed without
the resources of the Center for Isotope Geochemistry (University of California, Berkeley)
especially Tom Owens and Julie Bryce, without whom life would have been much more difficult.
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