Minerals Report - International Seabed Authority

anomalies from the original vent-fluid, a range of chemicals are typically at
least one million-fold enriched over seawater in vent-fluids meaning that,
even at the top of the rising hydrothermal plume, they are still 100 times more
concentrated than normal seawater. Prime examples of this are: iron (Fe),
manganese (Mn), methane (CH4) and a particular isotope of helium (He-3).
Strong potential exists to prospect for enrichments of any or all of these tracers
in the deep water column above ridge crests, therefore, to detect evidence for
new sites of hydrothermal activity.
4.2. Chemical Prospecting
From a “purist” point of view, the ideal tracer is helium (He3) because
it is geochemically inert once it has erupted from a vent-field and therefore its
trace in the water column can persist over very long distances - for example
one plume from the South Pacific could be detected at constant latitude over
2000km West, away from the ridge-crest (Lupton & Craig, 1981). In practice,
however, dissolved helium (He3) measurements in seawater require a
systematic water-sampling programme at sea followed by laborious analysis
in a specialist noble gas mass spectrometry laboratory on shore - which
means, typically, that any evidence for a new hydrothermal site could not be
found until 6-12 months after the survey was completed! This is far from
ideal. Dissolved Manganese (Mn) and methane (CH4), by contrast, offer much
greater promise. Although one can sample for these tracers, as for helium
(He3), for subsequent shore based analysis it is also possible to routinely take
the necessary laboratory instrumentation for dissolved Mn and (CH4) analyses
to sea. Thus, sampling equipment can be lowered to the seafloor beneath a
survey vessel, a series of samples from different (near-bottom) water depths
can be collected in the order of an hour or two, and a complete analysis of
those samples can be expected within a matter of hours, rather than months,
of the samples arriving on deck, greatly accelerating the survey potential of a
research cruise (see e.g. Klinkhammer et al., 1986; Charlou et al., 1988; Gamo
et al., 1996). Perhaps the simplest and most elegant tracer that can be
exploited, however, is Iron (Fe). The dissolved Fe erupted from a
hydrothermal vent does not remain in solution. Instead, it is quantitatively
precipitated out before it reaches the top of a buoyant hydrothermal plume as
a combination of sulphide and oxide mineral particles. Because they are so
very fine-grained, however, these particles do not immediately sink to the
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