Day 28: Monday 2 April 2007
Sampling area: Dredging
Ship's position at midnight: 12º57N, 44º56W
The Start of a New Month Brings a Gem of a Find
Diamonds may be a ‘girls best friend’ but they are also the shape of pure carbon while under the enormous pressures encountered deep within the Earth’s mantle. Most diamonds are found in volcanic ‘kimberlite’ pipes that cut through ancient continental crust. But we weren’t expecting to find any diamonds in the mantle that is exposed on the seafloor out here, at 13°N on the Mid-Atlantic Ridge.
This is because the mantle pressures are too low and the carbon concentrations too scarce to make diamond. However, as dawn broke at the start of our second month at sea, we were amazed to find small, clear crystalline shapes in the gravel from the latest dredge. In amongst fragments of fresh mantle peridotite, sparkles and glints of blue-white light caught the eye of Bob Spencer, the Deck PO on the 4-8 watch. Photographed below are the small, clear crystal sifted from the gravel. Without access to geochemical analyses, we cannot be sure of the nature of these crystals. However, a carefully controlled scratch test by Glyn Collard, the 2nd Engineer, proved them considerably harder than a pint beer-glass. Their refractive index is much higher than either water or gin, and their shape is also reminiscent of the cubic crystal structure of diamond. In a subsequent dredge we found more of the same, and even some quite large ones (although these are a bit worn and dirty on the outside). All of the crystals are clear, inclusion free, and white in colour; so only of the best quality. While we await the outcome of chemical analyses, the ‘samples’ are being kept locked up in the ship’s safe just in case the gleam of ‘promised riches’ causes the ship’s company to mutiny.....