Cruise Log: August 4, 2008
The TAG hydrothermal site The TAG hydrothermal mound (named for the 1972 US-lead ‘Trans-Atlantic Geotraverse’ cruise that found it) is one of the largest massive sulfide deposits found on the oceans to date. With an estimated 30,000-60,000 metric tons of copper in the deposit, the TAG site is comparable in size to some of the largest ore deposits found on land. However, at a water depth of 2¼ miles beneath the North Atlantic, TAG is inaccessible to all but the most specialized of vehicles. TAG is unique in the fact that, as well as extensive exploration and sampling of its black-smoker fluids and chimneys by submersibles, TAG has also been drilled by the Ocean Drilling Program (ODP). In 1994, ODP Leg 158 drilled several cores into the mound which offered a stunning insight into the internal structure of an active hydrothermal system. The bulk of the mound consists of a heterogeneous assemblage of cemented angular rock fragments (called breccias) composed of iron sulfide, copper-iron sulfide, quartz, and anhydrite (calcium sulfate) minerals. Anhydrite will precipitate from seawater at temperatures of 150°C or greater, but will dissolve below that. TAG appears to have experienced a complex history reflecting multiple cycles of active growth of the mound, separated by periods of inactivity where anhydrite re-dissolved, causing collapse. As hydrothermal activity renewed again each time, fragments of previously formed sulfide minerals became cemented in the anhydrite that formed as seawater was heated in the subsurface. The sheer size of the TAG ore deposit implies that hydrothermal activity has been occurring here for a very long period of time as such large mounds are rare on mid-ocean ridges. It is thought that TAG sits at the confluence of large long-lived faults that have intermittently allowed hydrothermal circulation and activity to persist at one locale. Resembling a large two-tiered wedding cake, TAG has two main areas of venting, the ‘Kremlin’ area and the infamous ‘Black Smoker Complex’. The lower temperature ‘Kremlin’ area is located on the southeastern side of the mound and is named for its small, bulbous chimneys diffusely venting fluids around 260-300°C. The ‘Black Smoker Complex’ sits atop the highest tier of TAG on the northwestern side and is violently venting 363°C fluids from a large sulfide structure. The venting there is so strong that it is almost impossible to see the top of the complex due to the plethora of ‘booming’ black smokers. Fluid sampling here is surprisingly difficult as often the orifices from which fluids are venting cannot be seen, and ROV pilots must ‘feel’ around for the hottest fluids using temperature probes attached to the samplers (see picture). Years of investigation of TAG have revealed that the black smoker fluids at both areas are actually related and ultimately originate from the same source fluid. At the Black Smoker Complex, the fluids travel directly from the high-temperature reaction zone beneath TAG to vent directly into seawater without any mixing. However, at the Kremlin area, cold seawater is entrained into the subsurface of the mound and so the original source fluid becomes modified by mixing with the seawater here. One of the key reactions occurring is the premature precipitation of iron sulfide minerals beneath the mound from dissolved iron and hydrogen sulfide (H2S) in the fluids, which generates some acidity and causes the Kremlin fluids to be slightly more acidic and have higher Zn contents than the Black Smoker Complex fluids, despite being cooler. The integration of observations provided by the ODP drilling as well as the extensive sampling of the TAG site over the past two decades have offered us an amazing glimpse at the inner workings of hydrothermal systems and their relevance to ore deposits that formed in the geological past. Bibliography |
Measuring temperature of a TAG black smoker
Old ODP drilling cone
Sulfide sampling in the Kremlin area of TAG
Using the "Ghostbuster" sensor on a TAG black smoker |
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