U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
Ridge sections apparently underlain by mantle melt anomalies, or hotspots, make up an intriguing subset of the global ridge system. Three hotspot-associated ridge sections have been systematically surveyed for plume distributions: the Reykjanes Ridge from 57.75° to 63.15°N, almost a full radial transect of the Iceland hotspot; the SEIR from 35.6° to 40.2°N, a 445 km section passing over the top of the Amsterdam-St. Paul hotspot; and 35.7°–38°N on the MAR, a short section on the southern fringe of the Azores hotspot. The first two sections appear unusually deficient in hydrothermal activity compared with other sections of similar magma budget (with the exception of the 27°–30°N MAR section, of which only 50 km has been densely surveyed). German et al. [1994] collected 174 optical/chemical profiles using a CTDO-rosette along 750 km of the Reykjanes Ridge, at intervals of 4–18 km. Despite this intense sampling, only the Steinahóll vent field at 63.1°N was located, an Fs of only 0.013 (Figure 5a). Based on the ~10 km lateral extent to which this Steinahóll plume could be traced, these results translate to a ph = 0.012 (Figure 5b). On the SEIR section, two certain and two possible plumes were detected on 58 MAPR profiles, corresponding to an Fs of 0.45–0.90 and a ph value of 0.03–0.07 (Figure 5).
Mantle upwelling beneath both these ridge sections has abnormally thickened the oceanic crust to at least ~10 km [Scheirer et al., 2000; Smallwood and White, 1998]. An early hypothesis for the Reykjanes Ridge hydrothermal results [German et al., 1994, 1996c], consistent with the first sidescan sonar images of the Reykjanes Ridge [Parson et al., 1993], proposed that thermal gradients beneath the present-day ridge may be sufficiently high, compared to elsewhere along the MAR, to markedly reduce the depth of brittle fracturing of the ocean crust and consequent penetration of seawater. This hypothesis is consistent with a crustal thermal model [Phipps Morgan and Chen, 1993; Chen, 2003] that explains the combination of a shallow magma chamber at 57.74°N on the Reykjanes Ridge [Sinha et al., 1997] and the lack of hydrothermal plumes [German et al., 1994] as the result of a positive mantle-temperature anomaly (
T 
40°–70°C) and inefficient hydrothermal cooling. The model calculations for a shallow, steady state magma chamber on a slow-spreading ridge require that convective cooling by hydrothermal discharge be only ~25% of that expected on a slow-spreading ridge not affected by a hotspot. Similar model calculations have not been run for the Amsterdam-St. Paul SEIR section, but its unusually low ph value suggests that thickened crust there may similarly depress hydrothermal cooling.
The 35.7°–38°N section of the MAR seems to contradict this hypothesis, since Fs here is 2–3 times higher than expected for its magmatic budget (Figure 5a). A gravity analysis along this section of the MAR, however, found crustal thicknesses south of 38°N to be increased by <2 km relative to normal (~6 km) crust, shrinking to no increase south of 37°S [Escartín et al., 2001]. While this area was underlain by a melt anomaly at 5–10 Ma, at present there is no crustal thickness or mantle temperature anomaly signature here [Cannat et al., 1999; Escartín et al., 2001]. Instead, moderate crustal thickness variations suggest magma focusing at the segment centers, especially at Lucky Strike and Menez Gwen [Escartín et al., 2001]. Magma focusing, combined with fracturing of the ridge axis and establishment of short 2nd order segments linked by broad, ultramafic-exposing NTOs that accommodate the oblique orientation of the plate boundary [Parson et al., 2000], may explain the relatively high Fs found here.
Determining if these results signify a systematic hotspot effect or simply a present-day sampling artifact will require a broader consideration of other hotspot-affected ridges. Of the 47 hotspots listed by Richards et al. [1988], only a few lie within 500 km of a ridge axis; beyond that distance hotspot influence upon ridge morphology appears negligible [Ito and Lin, 1995]. According to this criterion, possibly influential hotspots, besides Iceland and the St. Paul-Amsterdam system, include Galápagos, Easter, Jan Mayen, Azores, Ascension, Tristan de Cunha, Bouvet, and Crozet (Figure 1). Bouvet and Crozet are near the ultraslow spreading SWIR where the incidence of venting is already low and so the hypothesized "hotspot effect" on venting may be difficult to discern at these locations. Tristan de Cunha is distant from the MAR axis, while the effect of the Easter Island mantle plume may be further complicated by tectonic interactions with the adjacent microplate. We conclude, therefore, that the most attractive candidates deserving of future, detailed plume/vent surveys are the Galápagos, Jan Mayen, and Ascension hotspots, together with more detailed observations around the Iceland, Azores, and Amsterdam-St. Paul hotspots.
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