Carlos Braga, PhD
C. Braga1, M. Hannington1, A. Baxter1, E. Bethell1, J. Jamieson1
1Department of Earth and Environmental Sciences, University of Ottawa, Ottawa, Ontario, Canada
Hydrothermal vents on the global ocean are the modern analogs of ancient volcanogenic massive sulfide deposits now mined on land. Two thirds of the known hydrothermal sites are found along mid-ocean ridges. Of particular interest are vent fields formed under the influence of hotspots: localized sites of excess magmatism associated with mantle plumes. Hotspots are linked to increased crustal thickness (e.g., the Icelandic Plateau), high heat flow, and regional geochemical anomalies, affecting one fifth of the 70,000 km-long mid-ocean ridge system. In some cases, they can promote high-temperature venting (e.g., Axial Seamount). However, the contribution of hotspots to metal endowment over large distances is still unknown.
The Galápagos Spreading Center and its proximal hotspot in the Eastern Pacific offer a natural laboratory to study the regional geology and geochemistry of ridge-hotspot interactions and their effects on the size, composition, distribution and duration of hydrothermal systems. These effects are strongly associated with magmatic and tectonic controls, including magma supply, composition of host rocks, seafloor depth, spreading rate, crustal thickness and permeability. Hotspot activity near the Galápagos Spreading Center is located on the Nazca plate, concentrated on the western Fernandina and Isabela islands.
To understand how melt is being delivered from the hotspot to the spreading center and its hydrothermal systems, we are comparing 4 vent fields along 700 km of the ridge, at distances ranging from 180 to 650 km away from the hotspot. A new regional geological map at 1:1,000,000 scale and covering an area of 380,000 km² is being studied together with the major and trace element geochemistry of ~300 massive sulfide samples collected from the four sites (Iguanas-Pinguinos, Los Huellos, 89° W and 86° W). Preliminary inspection of samples from each site highlights a strong incompatible-element enrichment (e.g., Ba, Au, Cd and Sr) proximal to the hotspot (Los Huellos), which is reflected by abundant barite. Taking into account the range of temperatures and water depths of the hydrothermal vents, we interpret these enrichments in terms of plume-influenced source rocks from which the elements are leached. One interpretation is that the Galápagos hotspot plumbing system is delivering enriched melts to the proximal mid-ocean ridge, controlling the composition of the hydrothermal fluids.