Middle Valley - Juan de Fuca Ridge
Leg 139 was located in the Middle Valley, an axial rift valley of the northern Juan de Fuca Ridge,
which receives hemipelagic turbidites from the nearby continental margin. Operations were
conducted in four distinct hydrologic environments within the rift; an area of fluid recharge (Site
855), a large massive sulfide deposit (Site 856) , an area of high heat flow where the igneous crust
is sediment-sealed and high-temperature fluids reside in a hydrothermal "reservoir" (Site 857), and
a vent site at the seafloor (Site 858).
The sediment filling the rift provides a relatively low-permeability seal over the underlying, more
permeable, basement. Hydrothermal circulation is controlled strongly by basement structure and
sediment thickness variations. While vertical flow is probably impeded by clay-rich layers,
significant horizontal flow appears to be permitted by continuous coarse-grained layers. The
concentrations of sodium, potassium, and calcium in sediment pore waters were virtually identical
at 200-400 mbsf at the sealed site and in the hot water venting at the seafloor, 2 km distant,
providing evidence for lateral flow. The basement high that has localized upflow and discharge at
the vent may also be the source or destination for the water flowing laterally at the sealed site.
Another indication of horizontal fluid flow was observed at the vent. Hole 858G, drilled 11 m into
basement, was left with a bottom casing seal in place, to return to equilibrium. After two weeks, the
temperature in the upper 25 m of the hole had recovered almost completely to that measured in situ.
Below this depth, temperatures dropped (100°C at 60 m) then rose again toward the inferred in-situ
formation temperature (280°C at 150 m). This temperature inversion is believed to have resulted
from invasion of cold seawater into the sediment section below an inferred impermeable cap
(between 25 and 50 mbsf) via the path provided by an incompletely cemented Hole 858F, 10 m to
the north. Hole 858G was then deepened 175 m into basement and a temperature profile, measured
two days later, revealed flow of cold seawater down the hole and into the formation. To counter the
tendency for water to flow into the hot, permeable formations, and to provide a means to observe
true formation pressure and temperature over a long period of time, instrumented hydrologic seals
were installed in two reentry holes at the sealed and vent sites. The first visit to the instrumented
seals was made during an Alvin program two weeks after Leg 139; temperatures and pressures at
the sealed site were slowly recovering from the large drilling disturbance but, at the vent site,
continued to reflect downward fluid flow into the formation through the nearby Hole 858F.