Geochemical evolution of Juan de Fuca Ridge flank hydrothermal fluids: evidence from veins and porefluids, IODP Leg 301.

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IODP ABSTRACT (typed Sep 04) NE/C513242/1 - Teagle The earth's surface (crust) is made up of tectonic plates, which move around very slowly. Under the middle of the oceans the plates are moving away from each other, and new crust forms as volcanic eruptions fill the gap with molten rock. These underwater volcanoes form chains under the oceans known as mid-ocean ridges. As these volcanoes erupt underwater the seawater cools the lava very quickly. When rocks cool quickly they crack. Seawater is able to pass through the cracks, down deeper into the crust where the rocks are still hot. As this happens the cold seawater cools these rocks as well, and the seawater heats up as a result. If the water gets hot enough it expands and becomes less dense, so it rises back to the seafloor. Along mid-ocean ridges seawater as hot as 400 degrees Celsius has been found coming out of cracks in the rock. These hot fluids are known as 'hydrothermal fluids', but they don't only remove heat from the rock. They can also exchange chemicals with the rock. Some elements are left behind in the rock, such as magnesium. Others are dissolved out of the rock, into the water, and transported into the ocean, such as silica. This process can produce major changes in the chemistry of the oceans and the rocks, which are very important if we want to know how the chemistry of the oceans has changed since they formed. When elements are left behind by the hot seawater passing through the rock they are often left as mineral 'veins' that fill the cracks the fluids passed through. Common vein minerals include clay, carbonate and quartz. The chemistry of the veins depends on the chemistry of the fluid they formed from. The chemistry of the veins can be measured, and the results are used to calculate the chemistry of the fluids. The Integrated Ocean Drilling Program (IODP) is an international scientific project that uses a drill ship, the 'JOIDES Resolution', to drill rocks from beneath the oceans. In June 2004 IODP is taking the ship to a mid-ocean ridge: the Juan de Fuca Ridge, in the Pacific, west of Canada. It will drill volcanic rocks that formed along the ridge and were cooled by hydrothermal fluids. We will investigate the chemistry of these rocks, and the mineral veins that formed within them. This will allow us to understand the chemistry of the hot fluid, and how it changes as the rocks cool and get older. From our analyses of the altered rocks and minerals we will be able to estimate how these hydrothermal fluids affect the chemistry of the oceans.