Janet L. Morton

Active hydrothermal vents and sulfide deposits on the southern Juan de Fuca Ridge

Massive-sulfide deposits rich in zinc and silver were recovered from the Juan de Fuca Ridge 500 km west of Oregon in September 1981. The samples recovered are composed largely of zinc sulfide, with lesser amounts of iron, lead, and copper sulfide. Most of the deposits occur at a series of hydrothermal vents within a relatively continuous depression in the center of a smooth 1-km-wide valley along the ridge axis. The depression appears to be formed by collapse of a lava lake possibly modified by extensional faulting. The axial valley floor outside the depression is underlain by fresh, glassy, ferrobasalt sheet and lobate flows. Two types of sulfide-mineral deposits were dredged from one of the hydrothermal vents: (1) angular slabs of dark-gray zinc sulfide; and (2) subrounded fragments of porous light-gray zinc sulfide. The samples contain fresh sphalerite, zoned wurtzite, pyrite, and minor amounts of marcasite, galena, and chalcopyrite-cubanite. The spreading rate of the Juan de Fuca Ridge and the composition of the sulfide samples are generally similar to the East Pacific Rise lat 21° N site; however, the texture and geologic setting of the sulfide deposits are significantly different.

Relic magma chamber structures preserved within the Mesozoic North Atlantic crust

The North Atlantic Transect seismic reflection data, collected southwest of Bermuda, have been reinterpreted following post-stack migration and reveal two major intracrustal reflections. The shallower of these two events, located ∼1 s below the igneous basement, is a subhorizontal, undulating surface that in some places is continuous for as much as 10 km. On the basis of its position within the section and its laterally discontinuous nature, we believe that this upper crustal reflection corresponds to the intermittently sharp contact between the sheeted dikes and the underlying isotropic gabbro. A second set of lower crustal reflections, dipping ∼20°-40° eastward, is also prominent on the migrated profile and terminates downdip against the subhorizontal reflection Moho. Several lines of evidence argue against these features being either artifacts or out-of-the-plane events. Instead, their presence may be ascribed either to crustal-penetrating fault zones or to mafic-ultramafic cumulate layers frozen into the oceanic crust at the time of formation at the paleo-spreading center. Because of the laminated character of these events and their typical occurrence within 1.0 to 1.5 s of the reflection Moho, we prefer a compositional versus a structural interpretation for their origin. The gradual thinning in the crust approaching the fracture zones is shown to be more complex than was originally inferred; although the interpretation that the crust gradually thins toward fracture zones may still apply in a few localities, significant departures are recognized elsewhere. Similarly, the improved image on the migrated profile documents an increase in complexity across the localized region directly surrounding the Blake Spur fracture zone. An interpretation advocating crustal thickening in this narrow zone is proposed as an alternative to the crustal-thinning model of Mutter and others.

East Pacific Rise at lat 19°S: Evidence for a recent ridge jump

A detailed ANGUS (Acoustically Navigated Geological Undersea Surveyor) photographic and bathymetric survey of the East Pacific Rise (EPR) near lat 19°S reveals a small jump of the ridge axis to the west. The axial block in this region consists of two parallel ridges 3 km apart and separated by a 200-m-deep valley. South of lat 19°06′S the plate boundary is a single, narrow (

Distribution and Composition of Massive Sulfide Deposits at Escanaba Trough, Southern Gorda Ridge

Massive sulfide deposits were photographed and sampled at two sites (southern or SESCA and northern or NESCA) on the sediment-covered Escanaba Trough segment of Gorda Ridge, a slow-spreading midocean ridge located offshore of northern California within the U.S. Exclusive Economic Zone. Massive sulfide ledges, mounds, and chimneys in various stages of oxidation and erosion are spatially associated with the margins of small (1 km wide, 100 m high) sediment hills at SESCA and NESCA, and with faulted sediment on the north flank of a volcanic hill at NESCA. The sediment hills are uplifted above volcanic edifices that rise through sediment filling the axial valley. Two types of massive sulfide have been dredged from Escanaba Trough. Porous aggregates of pyrrhotite-rich sulfide contain small amounts of intergrown isocubanite, chalcopyrite, sphalerite, and loellingite. In some samples, pyrrhotite is partly to completely replaced by marcasite. Pyrrhotite-rich massive sulfide generally contains low amounts of base and precious metals, but barite encrustations on some of the samples are enriched in Zn, Pb, Ag, and Au. This sulfide type is largely derived from mound and talus deposits associated with the sediment hills at SESCA and NESCA. Zoned polymetallic sulfide samples from a single dredge at the NESCA site comprise the second massive sulfide type. These polymetallic samples are fragments from the wall of a sulfide structure with a large fluid channelway. Proceeding from the inner to outer wall, the mineral zonation includes isocubanite, sphalerite, arsenopyrite, galena, tetrahedrite, a Pb-Sb suifosalt, and argentite. Polymetallic sulfide samples are enriched in Pb, As, Sb, Ag, Hg, and Sn relative to massive sulfide recovered from other spreading axes. Other dredge samples include sulfide-cemented sediment-clast breccia and sediment containing abundant interstitial galena and hydrocarbon. The composition of massive sulfide from both areas indicates extensive interaction between hydrothermal fluids and the sediment in Escanaba Trough. Pyrrhotite-rich sulfide probably forms above sites of diffuse, sluggish discharge, whereas polymetallic sulfide structures formed in response to focused, high-velocity discharge at a “black smoker” type of vent.

Detailed Structural Mapping of an Ocean Rift, Using Acoustic-Transponder Positioning: Problems of Net Maintenance (1981–86) and New Experiments Possible with Long-Life Transponders

Since September 1981, the U.S. Geological Survey has continuously maintained an acoustic-transponder net on the axis of the Juan de Fuca Ridge off west-central Oregon. This transponder net has provided position control for deep-tow vehicles, samplers, and manned submersibles as part of a long-term program to study the structural, volcanic and hydrothermal processes along the 2,200m- deep ridge crest. Field operations in the area each fall utilize a net of four or five limited-lifespan transponders (both recoverable and expendable units); between operations, a skeletal net of one to three transponders is left. In addition to the difficulty of guaranteeing ship time each year for refurbishing the transponder net, servicing the net includes other problems: (1) time lost to deployment, net calibration, and recovery each year, (2) net rotation ambiguity resulting when only one transponder survives to the next field season; and (3) difficulties in reoccupation of sample localities using separate net calibration for each successive field season. We have anchored benchmark floats at several sampling sites to aid in future site reoccupation and to determine absoulute accuracy between successive net calibrations. Long-life (5 + years) transponders, capable of measuring round-trip travel times to 10 µs (Spiess and Boegeman, Eos., v. 65, p. 851, 1984), would allow or facilitate several new experiments, including: (1) geodetic determination of rifting rate, (2) monitoring of changes in mineral and faunal assemblages at hydrothermal vents, and (3) detection of sea-floor deformation.