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Hydrothermal Plumes Over Spreading-Center Axes: Global Distributions and Geological Inferences

Edward T. Baker

NOAA Pacific Marine Environmental Laboratory, Seattle, Washington, USA

Christopher R. German

Institute of Oceanographic Sciences, Wormley, Surrey, UK

Henry Elderfield

Department of Earth Sciences, Cambridge University, Cambridge, UK

Seafloor Hydrothermal Systems: Physical, Chemical, Biological, and Geological Interactions, Geophysical Monograph 91, AGU, 47-71, 1995.
Copyright ©1995 by the American Geophysical Union. Further electronic distribution is not allowed.

DISCUSSION

Exploration Implications

The studies described above document that about 10% of the 60,000 km of global spreading center systems have been examined at least cursorily for chemical or physical evidence of hydrothermal plumes. Perhaps one-third of that fraction has been mapped in enough detail to confidently locate all the major vent fields on a particular segment (e.g., Figure 7, 10, 11, 12, 15). The principal result of these systematic explorations is that hydrothermal venting is present, albeit at widely differing levels of activity, across the entire spectrum of ridge crest spreading rate, topographic expression, magmatic budget, and other geological and geophysical characteristics. Many other generalizations have crystallized from plume studies over the last decade and are discussed herein and in other papers within this volume [e.g., Lupton, this volume; Lilley et al., this volume; Kadko et al., this volume; McDuff, this volume], but here we wish to emphasize an important operational result that remains underappreciated by the ridge-crest community at large: the utility of plumes as exploration tools for seafloor vents.

In the heady early days of hydrothermal discovery, many workers believed that the chemistry of hydrothermal solutions was simply a function of exit temperature, and that this function could be established empirically by careful sampling at a few sites [e.g., Edmond et al., 1982]. Further discoveries demonstrated a much more complex universe of chemical variations [e.g., Von Damm, 1990], emphasizing the need for continued exploration and the compilation of as large a hydrothermal data base as possible. (We leave unwritten the many biological, physical, and geological reasons for vent field exploration.)

The search for new vent fields has historically followed an inefficient path: A tectonic segment, or segment portion, is chosen on geological considerations. A remote or manned search for geologic or biologic visual clues of active venting is then mounted to pinpoint discharge locations. Finally, vent fluids are sampled from a submarine and plume waters from a surface ship. Consideration of the several studies reviewed in this paper demonstrates that a more efficient approach would postpone detailed seafloor exploration until after systematic plume mapping had revealed the relative magnitude and approximate location of discharge sources. Moreover, chemical and physical observations of plume waters can provide first-order information on the composition of the source fluids, which are likely indicative of the present magmatic and hydrothermal conditions of the vent field.

The efficacy of plume reconnaissance can be documented from several recent examples. On the EPR near 10°N, an expensive and exhaustive (~107 m2) video survey by the Argo imaging system between 9°09' and 9°54'N in 1989 [Haymon et al., 1991], followed by a submersible program in 1991 [Haymon et al., 1993], located virtually all discharge sources along the surveyed ridge crest. A schematic map of these sources compared with the plume distribution from a chemical and hydrographic water-column survey [Lupton et al., 1993; Baker et al., 1994] shows excellent agreement, at roughly the kilometer scale, even for isolated single smokers (Figure 11). Various plume tracers, especially CH4 and Mn [Mottl et al., 1995], identify the location and chemical character of unusual fluids rich in volatiles (e.g., CH4, 3He, H2S) and low in chlorinity (and thus dissolved metals) associated with a recent dike intrusion and lava eruption at 9°54'N [Haymon et al., 1993]. Plume measurements can thus provide first-order information on the magmatic and hydrothermal history of the vent field independent of seafloor sampling.

While the above example is an after-the-fact plume success story, instances of plume mapping directing the discovery of seafloor discharge sources are common. On the CoAxial segment of the JDFR, plume surveying immediately after the remote detection of seismic activity in 1993 directed seafloor imagery mapping by a remotely operated vehicle towards the eventual discovery and sampling of a new lava eruption [Embley et al., 1995]. On the MAR, systematic plume surveying between 27° and 30°N in 1993 discovered optical and chemical evidence for three possible hydrothermal sites, including a strong signal at 29°10.1'N, 43°10.3'W [Murton et al., 1994]. The Broken Spur vent field was confirmed at this site three months later during an Alvin dive series [Murton et al., 1994].

Fig. 21. Comparison of the global extent of plume studies and seafloor vent fluid sampling. Shaded areas on spreading centers mark the locations of multi-segment plume mapping investigations on the Reykjanes Ridge, Mid-Atlantic Ridge, northeast Pacific ridges, Gulf of California, northern and southern East Pacific Rise, and the north Fiji Basin. Numbers refer to Table 1 and mark sites of vent fluid collection and analysis along spreading centers; fluid discharge sites noted only from visual observations are not included.

The number of fluid discharge sites sampled by submersible or remote vehicle is steadily growing but remains only a tiny fraction of all global sources (Table 1, Figure 21). Plume mapping and sampling has allowed us to expand our knowledge of the distribution and geologic setting of hydrothermal venting beyond the limits imposed by camera and submersible operations. Figure 21 graphically compares the relative coverage of vent and plume data sets. The expansion of hydrothermal information afforded by plume studies is essential for regional-scale issues such as the effect of hydrothermal cooling on heat transport within the crust [Phipps Morgan and Chen, 1993], the movement of melt within the crust [Purdy et al., 1992], magmatic differentiation [Sinton et al., 1991], and the effect of hydrothermal heating on abyssal waters [Hautala and Riser, 1993].

If your browser cannot view the following table correctly, click this link for a GIF image of Table 1.

TABLE 1. Sites of Hydrothermal Vent Fluid Collection


Map No. Site Reference

1 Lucky Strike (37°17'N) Colodner et al. [1993]
2 Broken Spur (29°N) James et al. [in press b]
3 TAG (26°N) Campbell et al. [1988]
4 Snakepit/MARK (23°N) Campbell et al. [1988]
5 Explorer Ridge (49°45'N) Tunnicliffe et al. [1986]
6 Middle Valley Butterfield et al. [1994b]
7 Endeavour (48°) Butterfield et al. [1994a]
8 Axial Volcano (46°N) CASM [1985]
9 Cleft--North (45°) Butterfield and Massoth [1994]
10 Cleft--South (44°40'N) Von Damm and Bischoff [1987]
11 Escanaba Trough Campbell et al. [1994]
12 Guaymas Basin (27°N) Von Damm et al. [1985a]
13 EPR (21°N) Von Damm et al. [1985b]
14 EPR (13°N) Michard et al. [1984]
15 EPR (11°N) Bowers et al. [1988]
16 EPR (10°N) Von Damm et al. [1991]
17 Galapagos (86°W) Edmond et al. [1979]
18 EPR (17°-19°S) Charlou et al. [1994]
19 Okinawa Trough (27°30'N) Sakai et al. [1990]
20 Mariana Trough Campbell et al. [1987]
21 Manus Basin Lisitsyn et al. [1993]
22 Woodlark Basin Binns et al. [1993]
23 N. Fiji Basin (17°S) Grimaud et al. [1991]
24 Lau/Valu Fa (22°S) Fouquet et al. [1991]

Geological Implications

As an example of the kind of linkage between geological and hydrothermal data made possible by plume studies, we examine a simple but crucial hypothesis: A functional relationship exists between axial hydrothermal heat flux and ridge-crest spreading rate, a convenient analogue of the time-averaged magmatic budget. Incorporation of hydrothermal cooling into models of the genesis of oceanic crust [e.g., Phipps Morgan and Chen, 1993] requires generalizations of this kind. To use plume distributions to test this hypothesis, we assume that hydrothermal heat output is proportional to plume incidence, the percentage of ridge-axis length overlain by a significant hydrothermal plume. Simply put, more hydrothermal cooling produces more plumes. (We operationally define a "significant" plume as one with plume tracer values greater than three standard deviations above the background (non-plume) mean.) Our assumption of the equivalence of heat flux and plume incidence is reasonable over the narrow crest and shallow axial depressions characteristic of ridges spreading faster than ~60 mm yr-1 [Baker and Hammond, 1992; Baker, 1994; Baker et al., 1994]. On these ridges, plumes are advected off axis quickly and plume incidence closely matches the extent of the vent fields. On slow-spreading ridges with deep axial valleys, the high bounding walls trap plumes so water over an entire segment can become "contaminated" with hydrothermal anomalies.

Fig. 22. (a) Full-rate seafloor spreading vs. axial magma chamber depth from seismic observations [Purdy et al., 1992] (solid circles; LAU, Valu Fa Ridge in the eastern Lau Basin; JDFR, Cobb segment on the Juan de Fuca Ridge; NEPR, northern East Pacific Rise, 9°-13°N; SEPR, southern EPR, 14°-20°S) and from the Phipps Morgan and Chen [1993] crustal thermal model (solid line). (b) Full-rate seafloor spreading vs. plume incidence (solid squares) and hydrothermal cooling in the neovolcanic zone according to a crustal thermal model (Y. J. Chen and J. Phipps Morgan, The effect of magma emplacement geometry, spreading rate, and crustal thickness on hydrothermal heat flux at mid-ocean ridges, submitted to Geophys. Res. Lett., 1994) (dashed line). Plume incidence data available for the Reykjanes Ridge (RR) [German et al., 1995], Mid-Atlantic Ridge (MAR) (11°-40°N) [German et al., 1994], JDFR [Baker and Hammond, 1992], NEPR (8°40-13°10'N) [Bougault et al., 1990; Baker et al., 1994], and SEPR (13°50'-18°40'S) [Baker and Urabe, 1994]. "Plume incidence" is the percentage of axis length overlain by a significant (plume tracer values greater than background mean plus three standard deviations) plume. The heavy solid line is a least-squares fit forced through the origin: slope = 0.375, r2 = 0.98. Hydrothermal cooling is the steady state convective heat flux from a neovolcanic zone 2 km wide centered on the spreading axis.

Continuous surveys from the JDFR (Figure 7) [Baker and Hammond, 1992], the northern EPR (Figures 10, 11) [Bougault et al., 1990; Baker et al., 1994], and the southern EPR (Figure 12) [Baker and Urabe, 1994] have yielded direct measures of plume incidence. On slow-spreading ridges, in the current absence of conclusive continuous surveys, we approximate plume incidence from estimates of vent field frequency derived from discontinuous plume surveys along the MAR and Reykjanes Ridge [German et al., 1994; German et al., 1995]. If we assign a reasonable plume size of 10 km to each of the 15 suspected sites of venting on the MAR between 11° and 40°N [German et al., 1995], we calculate a 6% plume incidence. Similarly, the one vent field found on the Reykjanes Ridge [German et al., 1994] yields a 2% plume incidence. When data from all five regions are plotted together, a strong linear relationship between plume incidence and spreading rate emerges (Figure 22).

It is vital to realize that Figure 22 is valid only over long time or space scales. Vigorous hydrothermal activity can occur on segments of any spreading rate, so an instantaneous observation at a particular location may not be representative of its long-term hydrothermal history. We obtain a representative hydrothermal history of a particular ridge-crest location only by observing it through geologic time, which is impractical, or by simultaneously observing many individual segments spreading at the same rate. Figure 22 thus implies that, over long time scales, (1) one-third of the global ridge axis spreading at ~100 mm yr-1 (for example) will be hydrothermally active at any instant, and (2) any area on that ridge-axis portion will be hydrothermally active one-third of the time.

How does the plume vs. spreading rate relationship compare to other observed and modeled parameters? Axial magma chamber (AMC) depth from seismic observations [Purdy et al., 1992] shows a rapid decrease from 60 to 120 mm yr-1 followed by only a slight further shallowing at superfast rates. This same trend has been reproduced by the crustal thermal model of Phipps Morgan and Chen [1993], in which the supply of heat by magma injection and the removal of heat by hydrothermal cooling is balanced to produce a steady state magma lens at crustal depths corresponding to the seismic observations at various spreading rates (Figure 22). In contrast to the non-linear relationship between magma chamber depth and spreading rate, the relationship between spreading rate and the hydrothermal cooling required by the crustal thermal model (Y. J. Chen and J. Phipps Morgan, The effect of magma emplacement geometry, spreading rate, and crustal thickness on hydrothermal heat flux at mid-ocean ridges, submitted to Geophys. Res. Lett., 1994) is approximately linear (Figure 22). The common linear trend between spreading rate and both plume incidence and hydrothermal cooling supports our initial assumption that plume incidence is roughly proportional to hydrothermal heat flux from the neovolcanic zone of ridge crests.

Still at issue is whether our meager coverage of the global ridge crest is representative and the trends described above are general. Over 3900 km of the southern EPR spreads faster than 140 mm yr-1 [DeMets et al., 1990], and we have a single snapshot of hydrothermal activity along only an eighth of that distance. Other spreading-rate portions of the midocean ridge crest are similarly undersampled, especially the slow-spreading ridges. A confident understanding of the balance between global (e.g., spreading rate) and local (e.g., magma supply, crustal permeability) controls on the distribution and vigor of hydrothermal activity awaits continued exploration.


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