Recent eruptions on the CoAxial segment of the Juan de Fuca Ridge: Implications for mid-ocean ridge accretion processes

R. W. Embley,¹ W. W. Chadwick,² M. R. Perfit,³ M. C. Smith,³ and J. R. Delaney⁴

¹Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Hatfield Marine Science Center, Newport, Oregon 97365
²Cooperative Institute for Marine Resources Studies, Oregon State University, Hatfield Marine Science Center, Newport, Oregon 97365
³Department of Geology, University of Florida, Gainesville, Florida, 32611
⁴School of Oceanography, University of Washington, Seattle, WA 98195

Journal of Geophysical Research, 105(B7), 16,501–16,525 (2000).
Copyright ©2000 by the American Geophysical Union. Further electronic distribution is not allowed.

9. Southern CoAxial Segment and the "Source Site"

The southern third of the CoAxial axial valley (south of about 46°14N) is characterized by a distinct median volcanic ridge (Plate 1 and Figure 11). The ridge summit is covered with young pillow lavas with small pockets of sediment and small areas of sheet flows at its southern end (~46°07N) where it broadens. Seafloor between the median ridge and the AVNRZ (dashed line on Figure 11) consists of heavily sedimented sheet flows (Plate 3g). A young pillow mound on the axis of the AVNRZ centered at 46°10N, 129°55W (Figure 11) mapped by the towed camera probably represents a single eruption from a recent diking episode originating at or near the summit of Axial Volcano. No SeaBeam anomaly was found at this site, so it apparently precedes all of the historic CoAxial eruptions.

Figure 11. Bathymetric map of southern CoAxial segment including the Source site. See Figure 5 caption for details. Location of Figure 12 is shown by box.

CTDT tows along this part of the CoAxial segment in October 1993 encountered a hydrothermal plume centered around 46°09N (Figure 11) that was clearly separate from the plumes originating at the known Floc and Flow sites to the north [Baker et al., 1998] . The high-temperature vent site subsequently discovered on Alvin dive 2681 in October 1993 became known as the "Source site" because it was postulated that this site may have represented the magma source region that generated the 1993 dike injection [Butterfield et al., 1997].

The Source vent field is located on the west side of the neovolcanic ridge of the CoAxial segment (Figure 12) along a fissure/fault system. This ridge is the shallowest part of the CoAxial neovolcanic zone, rising to ~2100 m in the vicinity of the vent site and thus is the most likely position of a long-term hydrothermal system on this segment [Ballard and Francheteau, 1982; Crane, 1985]. The vent site consists of four major vents aligned at about N022°E along a west facing scarp (Plate 3h). The chimneys are spaced ~10–20 m apart and the entire vent field is <100 m in length. Less vigorous venting also occurs on small chimneys west of and slightly deeper than the large ones. The small chimneys are oriented along trends about N040°W and intersect the primary vents (Plate 3i). This pattern suggests that the locations of the vents may be controlled by intersection of two structural trends such as has been documented at the Endeavour Main Field [Delaney et al., 1992]. The metal-poor vents emit a clear or grayish "smoke" with a maximum temperature of 294°C. Butterfield et al. [1997] concluded that these fluids have a relatively long crustal residence time compared to other hydrothermal systems on the JdFR, but the mechanism for this is unclear.

Figure 12. Bathymetry of "Source" site with camera, dive tracks, and locations of sites discussed in text. Side-lit from west. Contour interval is 5 m.

The only other active hydrothermal activity found nearby was an an area of diffuse venting a few tens of meters on a side about 3.5 km to the south (Figure 12). Small mounds of iron oxides had a fluid temperature only ~4°C above ambient. A line of extinct sulfide chimneys was found several hundred meters northwest of the diffuse vent. These Zn-rich chimneys lie along a small fault aligned along N020°E and extend for at least 100 m along the same trend as the active high-temperature vent field to the north (Figure 12). The presence of these older chimneys indicates that this site has had a long-term history of high-temperature venting. To date, no other high-temperature activity (either past or present) has been found along the length of the CoAxial segment.

Several observations lead one to the conclusion that the Source site predated and was probably not affected by the 1993 seismic swarm. First, the initial seismic activity was to the north and northwest of the site (Figure 1); there were no SOSUS-detected events located along this ridge. Second, the submersible dives and camera tows across and along the ridge yielded no geologic or biologic evidence of very recent tectonic events or young venting such as observed at the Floc site. Last, analyses of a time series of vent fluid samples from this site taken in 1993, 1994, and 1995 [Butterfield et al., 1997] do not show the rapid changes in temperature and chlorinity that characterize other recent eruption sites [Butterfield et al., 1997; Butterfield and Massoth, 1994; Von Damm et al., 1995]. Considering these data, it now seems unlikely that the Source site was significantly perturbed by the 1993 event.

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