Minerals Report - International Seabed Authority

nevertheless be detected by relying on chemical indicators of dissolved
volatiles/gases, most notably methane, which has led to the detection of both
the Steinaholl and Menez Gwenn hydrothermal areas on the shallow
Reykjanes Ridge and the shallow Mid Atlantic Ridge near the Azores Triple
Junction, respectively [6, 24].
The first mechanism by which such clear fluids might be formed is
through a process called “phase separation”. In much of the deep ocean the
pressure and temperature conditions encountered at and below the seafloor
are such that, despite the high temperature achieved, vent-fluids remains as a
single liquid phase because of the confining pressure (typically 200-300 times
atmospheric pressure). In sufficiently shallow systems, however, the
confining pressure is reduced such that, at depths below approximately 1800-
1700m a two-phase stability field for seawater-like fluids is encountered
leading to separation into i) a dense, metal-laden brine-like fluid which is
retained within the ocean crust and ii) a vapour phase, close to fresh, distilledwater
in composition enriched in many of the dissolved gases initially present
in the original vent-fluid. It is this latter “vapour-phase” fluid which is
erupted from the seabed (examples are Axial Volcano, Juan de Fuca Ridge;
Steinaholl, Reykjanes Ridge; Menez Gwen and parts of Lucky Strike, and the
Mid-Atlantic Ridge near the Azores Triple Junction). In such cases, it is
anticipated that the conjugate metal-laden brine phase is retained within the
ocean crust where saturated solutions may be formed leading to extensive
solid-phase sulphides precipitation. No such systems have yet been
positively identified in the geological record, however, and the potential for
recovery – from what would almost certainly be a rock hosted hydrothermal
system – would appear to be highly problematic.
Following the above approach we have been able to demonstrate that
with only on the order of 30 days of scientific ship time it is possible to both
conduct a preliminary broad-brush survey, locating all likely hydrothermal
sources along a 200 km length of mid-ocean ridge crest, and to complete the
near-filed follow-up survey work required, in at least one area, to more
precisely locate the exact source of venting for more detailed seafloor
investigations. Such investigations include camera surveys, manned or
unmanned research for active polymetallic sulphides deposits along all the
mid-ocean ridges that span the deep ocean basins throughout the Area.
INTERNATIONAL SEABED AUTHORITY 397