The scientists have been hoping to collect peridotite – mantle rock – to help explain how the Marion rise formed. Peridotite is a dense igneous rock with coarse grains. It is made mostly of olivine (a dense silicate containing magnesium and iron) and pyroxene (like olivine but less dense- the lighter, white patches in the picture below). If we find lots of peridotite in a location, there hasn’t been much melting there otherwise the peridotite would be covered up by basalt.
The mantle is under a lot of pressure deep below the ocean floor. As it rises, as a consequence of the tectonic plates spreading apart, the pressure decreases and when the peridotite is less than 100 km below the ocean floor it starts to melt due to decompression melting. The first melts from the peridotite come from melting of the pyroxene and olivine in the rock. This melt is less dense than the host peridotite, so it rises quickly to the surface, separating from the peridotite that didn’t melt. Some erupts as basaltic lava adding to the ridge. The rest cools slowly -in magma chambers below the surface making new oceanic crust. The thickness of the crust depends on the amount of melt extracted from the mantle. When peridotite is on the surface, there’s no crust in that area – it’s mantle.
Variations in the chemistry of the basalts we collect can reveal variations in the temperature, and composition of the underlying mantle. Those observations are used to infer the extent of melting and the physical nature of the mantle. Once we have the actual peridotites from the mantle, we can analyze their chemical composition to learn about the temperatures and pressures at which they were formed.
Peridotites rarely make it to the Earth’s surface without being altered or changed by metasomatism, that’s a chemical transformation by fluids, that indicates hydrothermal activity. The black areas in the picture above are made of serpentine and fine particles of magnetite- the altered remains of what was once olivine.
Multi beam maps are used extensively to examine the structure of the ocean floor and to predict where basalts or peridotites could be collected. Variation in topography reflects what’s going on geologically and therefore the types of rocks in various places. We use the rocks we collect by dredging as evidence to calibrate our ability to interpret the maps. Dredge 8 was predicted to have mostly basalt, which it did, and dredge 9 was full of peridotites just as predicted!
There are a lot of happy geologists around here right now!