U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
North Cleft Segment: Hydrothermal Vents
A 6-year time series of CTD "tow-yos" along the southern Juan de Fuca Ridge has shown that the Cleft segment is producing hydrothermal plumes that originate from two distinct sources [Baker, this issue]. The southern vent fields include several black smoker sites lying between 44°38'N and 44°41.5'N (Figure 1) within the "cleft" [U.S. Geological Survey Juan de Fuca Study Group, 1986; Normark et al., 1987]. As discussed above, the northern vent sites lie between 44°53'N and 45°03'N, where towed camera and submersible observations reveal extensive diffuse venting (<60°C) and several black smokers, including Pipe Organ, Monolith, and Fountain vents (Figure 5a). There is no unequivocal photographic evidence of active venting (e.g., chemosynthetic biological communities) along the neovolcanic zone between the southern and northern sites, although some small yellow/orange deposits and small temperature anomalies have been recorded (by NOAA and USGS camera systems) [Kappel and Normark, 1988; R.W. Embley, unpublished data, 1989]. However, there has not been any detailed mapping in this region (Figure 1, inset).
The most extensive venting on the northern Cleft segment occurs along the fissure system at the eastern boundary of, and within about 5 km north and south of, the YSF. North of 44°59.5'N, venting becomes less vigorous and more widely spaced and no active venting has been found north of about 45°04'N.
The southernmost venting is within the inner graben between 44°53'N and 44°55.3'N and consists of low-temperature vents. The densest concentration of vents occurs within a shallow trough within 100 m of the base of the eastern side of the inner graben (Figure 6i and Plate 4a). Sporadic venting can be followed for hundreds of meters along fractures and lava push-ups identifiable by characteristic white "staining" on broken lava surfaces (Plate 4b). Two isolated inactive sulfide chimneys occur about 100 m east of the active vents (Plate 4c). The extent of venting along the west side of the graben is less well known, because of the difficulty of maneuvering a towed camera near the steep wall, but some vents are confirmed here by several temperature anomalies and photographs.
Plate 4. Photographs of vents at north Cleft site. Same abbreviations as Plate 3. (a) (TC) Luxuriant Vestimentiferan "bushes" at base of inner graben at 44°55'N site (8 m). (b) (ALVBC) Recent fissures (possibly small graben) at inner graben site (5 m). (c) (TC) Inactive sulfide chimney (~56 m high) located about 100 m east of line of most active venting (Plate 4). A few (apparently) alive Vestimentiferan were found at the base of the chimney during a subsequent Alvin dive. Sulfide samples taken here are composed mostly of pyrite and silica (6 m). (d) (ALVHH) Marker 6 at Tripod site, photo taken in September 1988. Note bare basalt and bacterial mats along fracture. (e) (ALVHH) Marker 6 at Tripod, photo taken in September 1991. Note extensive coating of iron-silica precipitate over previously bare basalt. This depositional event took place sometime between September 1988 and September 1990 (an Alvin dive in 1990 observed but did not obtain useable photographs) (34 m). (f) (ALVBC) Bacterial mats coating spires at edge of primary fissure. (g) (ALVHH) Organ pipe high-temperature vent. Tops of 56 m chimneys. (h) (TC) Extensive hydrothermal precipitate on young lineated sheet flows. (i) (ALVBC) Old sulfide chimney at contact between young sheet flow and older lobate flow. Chimney is hollow. Black coating is either lava spatter or manganese.
The (known) high-temperature vents and the highest density of diffuse vents along the northern Cleft segment occur along the fissure system on the east side of the YSF and along its continuation immediately to the north (Figure 5a). Across any particular cross section, all the venting is located either within the main fissure system, or within a few tens of meters from the fissure. Although no high-temperature vents were found during the first submersible dives at the northern Cleft site in 1988, extensive low-temperature diffuse vents (up to 60°C) were mapped along several hundred meters of the main fissure system. Bacterial mats were extensive, and there were also thousands of globules of mucous-like polymers associated with the mats [Milligan and Tunnicliffe, this issue], reminiscent of the extensive bacterial floccules at 910°N found immediately after an eruption in 1991 [Haymon et al., 1993]. At the Tripod Vent (named for a large tripod at the site), the 1988 dives documented extensive venting through posteruptive fractures in the glassy lava. At that time there was only a small amount of hydrothermal sediment (Plate 4d). By 1990, however, the lavas at this site had been extensively covered with iron-rich sediment [Massoth et al., this issue; Butterfield and Massoth., this issue] (Plate 4e), the maximum vent fluid temperature had decreased from 60°C to 25°C, and the vent fauna had drastically decreased in numbers. On a subsequent visit in 1991, the venting had become so diffuse that it was no longer possible to obtain a vent fluid sample for analysis.
The vents along the eastern edge of the YSF north of the Tripod site are all diffuse vents with maximum temperatures <60°C and most are <30°C. Between the Marker 4 vent and the Cavern vent, all the vents lie along a fissure system that was traversed by Alvin in 1988, 1990, and 1991 (Figure 4f). Two or three closely spaced fissures occur in this area, but only one is actively venting on any cross section.
The Pipe Organ vent is the only known high-temperature vent located along the eastern edge of the YSF. This vent, about 100 m south of the Tripod Vent, consists of six to 10 narrow, fragile, pipelike chimneys venting water up to 260°C (Plate 4g). The chimney field is entirely confined within and emerging from the YSF (and so postdates it) and has no substantive sulfide mound surrounding it [Koski et al., this issue]. It lies about 10 m west of the contact with the age 2 pillow mounds. The fluids sampled from the Pipe Organ vents have high iron-manganese ratios that distinguish it from the high-temperature vents north of the sheet flow [Butterfield and Massoth, this issue].
The Monolith vent is the most robust of the high-temperature vents found at the north Cleft segment. It lies just north of the YSF where the main fissure system cuts into the base of the LRR (Figure 5). The Monolith is an agglomeration of chimneys that is about 4 m in diameter and 5 m high, and vents water up to 327°C and immediately west of Monolith is the steep, east facing slope of the LRR. North of Monolith, the active fissure cuts across the slope of the LRR (Figure 5). Several smaller vents lie on the same trend within along the fissure to the north. The morphology and mineralogy of the Monolith vent has led Koski et al. [this issue] to propose that it is an older, lower-temperature structure that has been rejuvenated by an increased flow rate and temperature (possibly from the megaplume/NPM eruption events). Another, smaller high-temperature vent (Fountain vent) is found about 600 m north of Monolith along the 020° trend. This is the only one of the three active high-temperature vents that has an extensive biological community associated with it. Several diffuse vents and groups of extinct sulfide chimneys are found along the same trend within a few hundred meters north of the Fountain vent (Figure 3b).
Abundant yellowish deposits (Plate 4h), sometimes exceeding ~1 × 103 m2 in area, and whitish stains coat the YSF in places (Figure 4h). These have not been extensively sampled, but some of them contain elemental sulfur, boehmite and Fe-Si hydroxides. Since most of these sample locations are not in presently active venting areas, they probably formed near the time the YSF erupted. This material may represent products deposited by diffuse venting from an immature hydrothermal system like that observed after the recent eruption on the EPR [Haymon et al., 1993]. Also, the presence of the mineral boehmite [Howard and Fisk, 1988; I. Jonasson, personal communication, 1989] on some of the fractures in areas not presently venting indicates that high-temperature venting had at one time been considerably more widespread than during the first submersible visit in 1988.
Several dozen extinct chimneys have been mapped within 100200 m of the primary fissure system. In some places they are found at the contact between the young sheet flow and older lavas. These chimneys commonly form chains that parallel the primary fissure system for short distances. At two sites the young lava is observed to directly onlap a chimney (Plate 4i). At another site a chimney had been broken by a fracture that cut through the chimney. Most of these older chimneys are characterized by a high silica content and a dominant pyritic mineralogy, implying a formation temperature below 200°C [Koski et al., this issue]. These older chimneys represent a pre-YSF, less robust phase of hydrothermalism.
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