Sampling mantle rocks with the Oman Drilling Project – Part 2

This blog post was written by Elliot Carter, who is currently on board a ship somewhere in the Pacific Ocean, taking part in the Oman Drilling Project. You can read Elliot’s first post about the project here, and follow the project on Twitter, @OmanDrillProj.

Crushing the old fashioned way

Crushing the old fashioned way

Hello again from Chikyu and the Oman Drilling Project!

As we pass the halfway point of our stay aboard I thought it was time to write a little more about my experiences on board a scientific drill ship.

After the flurry of activity of the first few days, things have settled into a routine – one largely revolving around mealtimes. For those of us on the day shift, things kick off with the sampling meeting where the different scientific teams (geochemistry, igneous petrology, paleomagnetism…) discuss and decide on small sections of the core to sample and subject to a variety of tests. These include producing microscope slides to study the textures and mineralogy, small cubes to be analysed for the tiny magnetic minerals they host and – my focus – blocks for geochemical analysis. Much to the horror of many, our first act after sawing and cleaning the geochem samples is to wrap them in plastic bags and bash them to smithereens between two hammers. It’s somehow rather heartening that such simple efficiency remains in use, even in the 21st century.

Once crushed, we powder and split the sample and start on the fun part of turning it into data. One portion is melted into a glass bead (my favourite thing to watch) and a pellet is pressed from the powder, both for X-ray fluorescence analysis telling us the elements it is made from. Part is heated to 1000°C and weighed before and after to determine its water and CO2 content. Finally, a small sample of powder is dissolved in acid allowing us to measure extremely rare, or “trace”, elements on a mass-spectrometer. This entire process takes about two days to complete so we have a constant rotation of jobs to do. It’s fairly extraordinary to be able to do such a range of analysis aboard and gives us a huge array of data that can be quickly distributed to all the scientists working on the project.

Fuchsite listvenite

Fuchsite listvenite

The rocks we are working on are also fairly extraordinary. They are part of the Oman Ophiolite which consists of huge expanses of oceanic crust and mantle which were tectonically thrust up onto the Arabian continental margin around 90 million years ago. The focus of our work are rocks from the mantle which have entirely reacted with carbon dioxide, changing them from peridotite (a rock mostly composed of olivine Mg2SiO4) to listvenite (composed of magnesite MgCO3 and quartz SiO2). The mountains sampled by the Oman Drilling Project alone are estimated to store over a billion tons of carbon dioxide and understanding how this has happened naturally in the past and how it might be made to happen by humans in the future is one of the primary goals of the project. The potential of these rocks for carbon capture and storage could have huge implications for our understanding and potential mitigation of climate change.

The process of carbonation in the Oman mantle sections has resulted in some truly bizarre rocks. Even the most jaded geologists on board can’t help but be taken aback by both the appearance and freshness of these rocks which encompass a vibrant array of colours from deep red to bright green. Particularly lovely are patches of fuchsite, a rare chromium-bearing mica present in listvenite. Exactly why it is there and whether it is common in the mantle is another fascinating open question.

Melting rocks in glass beads

Melting rocks in glass beads

While the listvenites are easy on the eyes, they are proving rather less easy to handle, describe and analyse. Time after time we are finding that routine methods have to be adapted. Saws need new blades to cope with the ultra-hard quartz veins, potent acids fail to dissolve the rocks, powders cake and bind to containers (and yet crumble when we try to press them into pellets). Dealing with these pitfalls is part of the fun, or at least so we tell ourselves. The learning experience for us and for the wider technical staff of the ship is definitely crucial if Chikyu is going to realise its founding aim to drill to the mantle.

After two weeks in port, tomorrow we head out to sea, leaving behind the now familiar sights of Shimizu and Mt. Fuji. With it comes the loss of what limited internet there is aboard (which has imparted an element of urgency to many of our email exchanges). In compensation though, we get to see the ship in action, testing its drilling rig and submersing camera systems in deep water. Meanwhile, work continues on the cores, with the added spice of some potentially quite big waves as we skirt the edges of the typhoon currently lurking in the Western Pacific.

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About Sarah Crowther

I'm a Post Doc in the Isotope Geochemistry and Cosmochemistry group. I study xenon isotope ratios using the RELAX mass spectrometer, to try to learn more about the origins and evolution of our solar system. I look at a wide range of samples from solar wind returned by NASA's Genesis mission to zircons (some of the oldest known terrestrial rocks), from meteorites to presolar grains.
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