JC007: Drilling the Mid-Atlantic Ridge


Cruise diary

Day 23: Satruday 31 March
Sampling area: TOBI survey
Ship's position at midnight: 13 21.6376N, 44 54.91477W

Bramley writes:

What’s in a piece of rock?

Since our earliest involvement with the Sea, Mankind has imagined an ancient realm of unfavomable darkness, where nothing stirs and nothing changes. A place locked in time from which mythical gods emerge to torment those navigators foolish or brave enough to venture across its endless surface. Even today, when we think of the deep ocean, we think of processes that are infinitely slow: the fine rain of sediment accumulating at one centimetre per thousand years on the seafloor, layers of mud dating back to the birth of the ocean itself, and the creeping motion of the Earth’s crustal plates as they crawl across the surface of the globe. Yet in this abyssal darkness, volcanoes burst into life, showering white-hot molten lava across the seabed, incinerating everything in its path and causing gysers of superheated water to gush upwards into the depths above.  Elsewhere, as we are now discovering, the Earth’s mantle these volcanoes extrudes onto the surface odf the sea floor forming large corrugated mountains.

But what of these volcanoes and the mantle rocks: what are they made of, and what do their composition tell us about their formation? Ironically, the ocean crust forms the youngest areas of the Earth’s surface. What seems like an ancient environment dating back to the beginnings of our planet is actually in a constant state of renewal. Whereas the ancient interiors of the continents contain rock that dates back over 3,500 million years, the oldest ocean crust is a mere 160 million years old. Why? Because new ocean crust is continuously created at the mid-ocean ridges, a line of volcanoes that encircles the global ocean, stretching for over 70,000 km. The rate of crustal growth is between 20 and 150mm per year. So, after one million years, an ocean like the Atlantic will widen by about 20 km. But this doesn’t mean that the Earth is expanding. For every millimetre by which the ocean crust grows, an equivalent amount is destroyed. The destruction occurs along subduction zones, the antithesis to mid-ocean ridges, where the world’s most destructive earthquakes and tsunamis are born. And it is this history of crustal recycling, stretching back to the birth of the planet 4,600 million years ago, that the volcanoes of the mid-ocean ridges record in their lavas.

“Why not use lavas from volcanoes on land?” you may ask. Because they erupt through hundreds of kilometres of ancient continental crust, they become contaminated on route to the surface. But the mid-ocean ridges are erupting through a few kilometres of their own lava crust, and hence preserve a clean record of their origins.

To unlock their mystery, all that is required are small fragments of glass and crystals from the mantle rock. Held within are the elements that tell the story of cooling and crystallization of the molten rock, its origin deep below the crust during the melting of the mantle, and further back in time, over hundreds of millions of years to the fate of ancient oceans past, the formation of the continents and the origin of the early Earth.

Before we can begin this geological journey, we first have to collect our fragments of rock. Although seemingly the simplest of operations, it is often the most tricky, and certainly the most expensive. Finding and recovering rocks from 3 or 4km below the ocean surface requires an ocean-going research ship with accurate navigation, precision station keeping, sonar imaging and detailed multibeam-bathymetry. This is team work:  the skills of the officers on the Bridge holding the ship in position, often to within 200m of the volcanic target; the technical and engineering support team ensuring the sonar is giving the best and clearest images of the seafloor; and the deck crew running out the winches at their maximum speed whilst keeping a keen eye on the wire tension and proximity of the sampler to the bottom.

Perhaps the least technical aspect of the operation is the rock sampler itself. Affectionately referred to as the ‘dredge’, this devise is simply a heavily weighted steels bucket with a chain-mail bag. Just think of it as an ocean-going geological hammer! Once on station, the dredge is deployed on the a steel rope and the whole package accelerated downwards at 60 metres per minute. It is then drasgged over a few hundred metres of seafloor. Recovery is made at a similar rate, and the round trip to the seafloor in 3500m of water takes about 90 minutes.

Once on board, the dredge is emptied and the rocks taken to the wet lab, a laboratory full of sinks, saws, bright lights and sample tables . The rocks are washed, sawn in half, sorted, described, numbered, curated and bagged for analysis back at the NOC.

For the rock samples, this is just the beginning of its journey back through time. In the laboratory in Southampton, the freshest fragments are picked from each sample. Once mounted on a glass slide, the fragments are polished to 120 microns thick. The analysis for major elements involves placing the sample in an electron micro-probe where a 20,000 volt electron beam is fired at a spot only 15 microns in diameter. This causes secondary X-Rays whose energy, wavelength and amplitude indicate the concentration of the major elemental constituents of the rock or mineral. These are measured as a percentage of the total mass of the material and tell the history of cooling and crystallization of the magma before it erupted, or the melting and history of the mantle. The trace elements are far lower in concentrations, and measured in parts per million. To analyse these, we vaporize a 20 micron spot of the sample using a high-powered laser beam. The vaporised rock is then sucked into an argon plasma at 8,000 degrees centigrade. At this temperature, the molecules of rock or mineral are striped to their individual elements, then these are stripped of their outer electrons. The resulting charged particle beam is passed through an intense magnetic field that separates out each elemental mass. The number of atoms of each mass is then counted, often at a rate of several million per second, and their abundances and ratios measured. The trace element concentrations tell us about the melting history of the mantle, while the isotopic ratios of elements like strontium, neodymium, hafnium and lead tell us about the recycling of earlier ocean crust into the mantle and the origin and evolution of the Earth itself. All this from a tiny piece of rock.

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February 2007