sea90E - an oceanographic expedition to the Indian Ocean



	Geochemistry of Volcanic Rocks: by Dr. Fred Frey Elements: The periodic table of elements includes elements from atomic number 1 (hydrogen) to 92 (uranium). Atomic number equals the number of protons in the nucleus of a specific element. All of the elements in the periodic table occur in magmas derived from the earth’s mantle. These magmas are called basalt. Geochemists use basalt erupted on the earth’s surface as indicators of the chemical composition of the earth’s interior. Basalt is erupted in three quite different geologic settings: (a) at divergent zones where plates move apart; the volcanoes that form the oceanic crust at mid-ocean ridges, such as the Southeast Indian Ridge, are an example. The highest areas on the map below are in red and the deepest are in dark blue. So take a look at the light blue of the 90 East Ridge almost in the center of the map. At the base of this is Kerguelen Island and you can see the cracks in the ocean floor that spread out from that area. We know there are volcanos at that Kerguelen area, but did that area move and cause this 90 East Ridge? That is why we are here, collecting rocks! (b) at convergent zones where plates collide; the volcanoes forming the Ring of Fire around the Pacific such as the Andes of South America are an example; (c) the third geologic setting is volcanoes that form within plates rather than at their margins; Hawaiian volcanoes are the most well known example but much of the Ninetyeast Ridge also formed within the Indian plate. Such volcanoes are caused by hotspots in the mantle that form a chain of volcanoes as a plate moves over the hotspot. (Take a look at the Article on Hotspots by Dr. Will Sager.) The basaltic magmas erupted in each of these geologic settings are dominantly, more than 99%, made of oxygen, silicon, iron, magnesium, calcium, aluminum, sodium and titanium. These elements, known as major elements, are abundant because they are produced in large quantities by the nuclear reactions occurring in stars. All other elements in the periodic table also occur in basalt but in very low abundances; therefore they are known as trace elements. Basaltic magma erupted in the three different geologic settings have similar abundances and proportions of major elements, but they have quite different abundances of trace elements. These differences in abundance of trace elements reflect differences in the composition of the mantle that is partially melted to form basaltic magmas in these three different geologic settings. This result was at first very surprising because we know from plate tectonics that the earth’s interior is convecting and over the long history of the earth the mixing resulting from stirring by convection should create a homogeneous mantle. The distinct differences in trace element abundance between basalt erupted in different geologic settings shows that the earth’s mantle is compositionally heterogeneous. Understanding the processes that create the compositional variations within the earth’s interior is an objective of geochemists who study basalt erupted in different geologic settings. An objective of our sampling the Ninetyeast Ridge by dredging (see dredge photo above) is to compare the trace element abundances in basalt of various ages erupted at the Ninetyeast Ridge with basalt erupted at the Southeast Indian Ridge, an example of diverging plates, and with basalt that erupted at two hotspots in the eastern Indian Ocean: The islands of Kerguelen and Amsterdam/St. Paul. Geophysical studies suggest that mantle from each of these regions provided basalt to make the Ninetyeast Ridge; our geochemical studies of the basalt that we recover from the Ninetyeast Ridge will test this hypothesis. What do you think we will find? updated July 21, 2007 from the Indian Ocean
			High.Seas.HIGHTECH/ analyze!ocean@voyages, by lots of us... How to Become a Captain, by the Captain The Search for the Indo-Pacific Humpback Dolphin, by Leslie Nemazi. Rocks Rock! (or ) The Fresh Cut Surface of Beautiful Ocean Rock, by Evelyn Mervine Exploring the Bridge of the RV Roger Revelle Dredging Operations onboard the R/V Revelle, by Amy Eisin Geochemistry of Volcanic Rocks, by Fred Frey Cool Expedition Discoveries Already!, by Will Sager Anatomy of a Seamount Survey, by Will Sager M agnetometers (“Maggie”) and Attracting Sharks, by Rory Wilson Acoustics: Substitute for Superman Vision?, by Will Sager What is Bathymetry?, by Will Sager Hotspots and Ninetyeast Ridge: What’s a hotspot?, by Will Sager