Hydrothermal Plume Mapping Along the Hotspot-affected Galapagos Spreading Center Finds High-Temperature Vent Sites are Anomalously Scarce
Abstract
Systematic searches for hydrothermal activity along midocean ridges (MORs) demonstrate that the spatial density of hydrothermal activity is a robust linear function of spreading rate. This trend argues that the availability of mantle heat is the first order control on the distribution of seafloor vent fields. However, some crustal thermal models predict that the thicker, hotter, more ductile crust associated with hotspots substantially reduces convective hydrothermal cooling, explaining observations of axial magma chambers (AMC) at shallower depths than found on normal MORs. In Dec-Jan 2006 we tested this hypothesis by mapping hydrothermal plumes overlying the hotspot-affected Galapagos Spreading Center (GSC) from 95°-89.6°W, using a dual-pass, side-scan deep tow with an array of plume sensors spanning 50- 250 m above bottom. The western GSC near 91°-92.5°W has axial-high morphology, shallow and quasi-continuous AMC, and thick (8 km) crust, changing to a transitional morphology, deeper and more discontinuous AMC, and normal (6 km) crust from 93° to 95°W. The eastern GSC, 90.5°- 89.6°W is also an axial high and presumably has crustal characteristics similar to the western GSC at 91°-92.5°W. We identified hydrothermal plumes by anomalies in light backscattering (NTU) from a vertical array of MAPR sensors along the tow line, plus redox potential (Eh) measured continuously in-situ on the tow body at a nominal elevation of 100 m. Many plumes were subsequently confirmed by CTD tows and sampling. Only three areas of extensive and intense plumes were observed: 90.52°-90.63°W, 91.78°- 91.96°W, and 94°-94.1°W. Maximum plume rise at the latter two sites exceeded 200 m, indicative of high-temperature venting that was confirmed by camera tows. Some 25 other NTU and Eh anomalies were detected along ~1000 km of trackline, but none were >5 km in length. The primary result of our survey is that hydrothermal plumes were scarce for a ridge spreading at ~60 mm/yr. Total plume incidence (ph), the fraction of ridge crest overlain by plumes, was 0.076, about a third of that expected for a normal ridge section of similar spreading rate. While the overall low ph is consistent with the hypothesis that the Galapagos hotspot has stifled convective cooling, the distribution of plumes points to a more complex relationship between geology and venting. Where the crust is thickest and the AMC shallowest (91°-92.5°W) ph = 0.124; where the crust thins and the AMC deepens (92.5°-95°W) ph = 0.065. One extensive vent field was active in each of these areas. The eastern GSC section is morphologically similar to the 91°-92.5°W section but has a lower ph = 0.067 and no evidence of high-temperature vents. The GSC joins MORs over the Iceland (Reykjanes Ridge) and Amsterdam/St Paul (SEIR) hotspots as locations of significantly reduced high-temperature venting. The importance of low-temperature, diffuse venting as a cooling agent on hotspot-affected ridges remains unresolved.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V14A..06B
- Keywords:
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- 3015 Heat flow (benthic);
- 3035 Midocean ridge processes;
- 3037 Oceanic hotspots and intraplate volcanism;
- 8424 Hydrothermal systems (0450;
- 1034;
- 3017;
- 3616;
- 4832;
- 8135)