Day 28: Sunday 1 April 2007
Sampling area: Dredging
Ship's position at midnight: 12º57N, 44º56W
The last week has been really busy on board the James Cook; especially for the geologists who have been working non-stop on all the seafloor rocks we are now happily in possession of!! We’ve had both success and failure with the rock drill and dredging. After some initial teething problems with the dredging the scientists and the deck crew have a hopefully now foolproof technique which is working well and giving excellent results. The dredges generally take around 3 hours from initial deployment to retrieving the dredge. The dredge consists of a metal net with an open front with a pipe attached behind it as a second trap for samples. Once on board, both parts of the dredge are emptied of all the samples and sediment and taken into the deck lab to be processed.
We begin by washing the sediment off the samples and having an initial look to determine if the rock is mantle or volcanic in origin. Samples are then selected to be cut into two on the rock saw, we cut all the large samples and a representative selection of the smaller samples as well as any which catch our eye. We need to cut the rocks so that we have a fresh surface to look at the different minerals in the rocks because they are often coated on the outside by manganese and other seafloor weathering minerals. Each type of rock has a distinctive mineral assemblage and texture and these features are used to name and identify the rocks. These key features of the rocks are described and the sample labelled and bagged up ready to be analysed in more detail back at our different universities. The PhD students, especially Sam and myself have been benefiting greatly from the vast pools of knowledge that Bram, Chris and Jack have about seafloor rocks and have been learning continuously from them. It has been hard work with dredges coming up all the time and needing to completely process the previous dredge samples before the next arrives on deck, however it has been worth it and has been an invaluable experience for those of us at the start of our careers. Although I must admit when I started my PhD I never expected that I would spend my time cut, washing and blow drying rocks, and I definitely never expected to say that sentence!!
So what have we found? So far we have retrieved a range of mantle rocks and a range of volcanic rocks. The mantle rocks we’ve retrieved, called peridotites, have a broad range of textures and compositions with a range from quite fresh to completely serpentinised. Serpentinisation is the alteration process that happens to mantle rocks because they are unstable at seafloor temperatures and pressures and they react with seawater to create a new mineral called serpentine. This mineral has a range of colours and forms and usually creates an attractive looking mesh-like network of veins throughout the rock. Serpentine is very soft and may help to lubricate the faults that bring these mantle rocks to the seafloor. This mineral can also be found on land where pieces of ocean floor have been emplaced onto the land (called an ophiolite), an example of this is The Lizard, Cornwall where serpentinite has been mined in the past and used to carve decorative pieces.
The volcanic rocks we have are generally basaltic pillow lavas, with some dolerites. We have some quite large pieces of intact pillows (10’s of cms in size), which the initial shape of the pillow can still be seen. Pillow lavas form when lava erupts slowly onto the seafloor and becomes quenched by the seawater as soon as it comes into contact with it, this creates a very thin layer of glass around the lava and forms pillow shaped pieces of volcanic rock. The glass that forms is very useful for analysis and so we have been chipping the glass away from the rest of the rock ready for later analysis. The lava may contain gases and these escape from the lava and leaves behind tiny holes in the rock called vesicles, these are a common feature of the pillow lavas we have found.