Nature of the lower crust and Moho at slower-spreading ridges: SloMo Leg 1 (IODP Expedition 360)

IODP Expedition 360, 'SloMo Leg 1', will drill a single deep borehole into gabbros (slowly cooled rock crystallised from basalt) formed by seafloor spreading in the lower ocean crust beneath the ultraslow-spreading SW Indian Ridge. The overall goal of the leg is to better understand how the igneous lower crust is accreted: by what mechanisms do melts migrate, crystallise and evolve; how is plate separation simultaneously accommodated by magma injection and tectonic stretching on faults and shear zones? Drilling for the first time through a magnetic field reversal boundary locked into minerals within the gabbros will allow us to constrain how magnetic anomalies ('magnetic stripes', which were the first definitive evidence for seafloor spreading) are recorded in ocean crust.

As Co-Chief Scientist of Expedition 360 I will play a central role in synthesising the results of onboard (and later post-cruise) scientific results and addressing the broad scale questions outlined above. I will, however, play a full role in undertaking a specific research programme that should contribute significantly in its own right on a number of fronts. There are two distinct strands to my proposed activities:

(1) To reorientate the recovered drill core to the geographical reference frame, by matching distinctive features in core pieces to their representations on images of the borehole wall obtained from probes lowered down the hole. The horizontal orientation of the individual rods of drill core is unknown, so we cannot normally utilise measurements that have a spatial element: e.g. (i) the orientations of mineral veins (to reconstruct the geometry of seawater percolation through cracks), (ii) faults and shear zones (to reconstruct the history of the deformation suffered by the rocks), (iii) boundaries between rock types and orientations of crystals in the gabbros (to reconstruct magma movement), and (iv) the 3D orientation of magnetisation directions recorded in rocks (to understand how magnetic anomalies are recorded in the crust).

We can, however, reorientate core to geographical coordinates if a distinctive feature in it, such as a planar, inclined fracture/vein, can be matched uniquely to its representation in (oriented) images of the borehole wall obtained by electrical or acoustic well logging. We can then restore all spatial measurements from that core piece back to the geographical reference frame. In practice the technique, which needs to be done post-cruise on a dedicated workstation, is time-consuming and requires high core recovery and good quality logs (both of which we expect to obtain). The rewards are nevertheless great and will inform a broad range of studies. I wish to apply my results to all of the four topics outlined above, but especially to item (iv), in specific collaboration with Exp360 scientist Prof A. Morris.

(2) To generate a comprehensive suite of mineral analyses from a selected suite of gabbros to constrain how melt migrates and crystallises in lower crustal magma bodies. In particular I propose to test the hypothesis proposed by myself and close colleagues that significant chemical exchanges occur as melts migrate through the pore spaces of partially crystalline gabbro mushes in the lower crustal magma chamber. This reactive melt migration is poorly documented but potentially of huge significance as a mechanism of modifying melts from the mantle before they are erupted at the seafloor.

I intend to make use of the state-of-the-art new analytical scanning electron microscope facility in Cardiff that, uniquely, allows us to rapidly acquire quantitative image maps of element concentrations of minerals in sections of core. These reveal cryptic variations in composition that we can further probe using laser techniques to measure trace element concentrations in individual crystals. From this we can model the extent of melt-crystal reactions and rigorously distinguish between alternative explanations.

The broader goal of the 'SloMo' project is to drill the first ever 'Mohole': to penetrate the seismological boundary generally regarded as the boundary between the crust and the mantle. This has been a goal of Earth scientists since the 1950s: Project Mohole (1957-66) was the first ever attempt at scientific ocean drilling and, at the time, received a similar level of publicity to the Space Race.

We are drilling at Atlantis Bank because we think it is the ideal place to challenge the assumption, taught in schools worldwide, that the 'Moho' seismic boundary represents the boundary between ocean crust and the mantle. We believe that Moho may instead represent the lower limit of seawater penetration into the mantle; if so, it would overturn our understanding of the basic mechanisms of seafloor spreading and creation of ocean crust, and would force us to fundamentally reassess our estimates of the extent of global scale interactions between the Earth's interior and exterior (e.g. the Carbon Cycle). Moreover, we know that the reactions associated with seawater penetration into mantle rocks support microbial communities, and the SloMo project would offer the first insights into the extent and limits of the biosphere living in the Earth's deep interior.

Expedition 360 (SloMo Leg 1) is the first of the three planned expeditions we believe will be needed to ultimately penetrate the Moho. It is unlikely, though not impossible, that we will drill into mantle rocks on this leg. Even so, it is perfectly possible to communicate the broader impacts of SloMo to the general public. We do not need to reach the Moho to be able to explain why we are starting the quest to get there.

The overall concept of the project is very accessible to the general public and to school children worldwide. All UK schoolchildren learn about plate tectonics in Geography and Science at GCSE (14-16 year-old) level, and many to a much greater level of detail at A-level (16-18 year-old). Geology A-level students explicitly study the seismic layering of ocean crust and concept of the Moho as the crust-mantle boundary in the National Curriculum. We understand curricula are comparable in many countries worldwide.

We believe that SloMo is an ideal topic for A-level (high-) school pupils in the UK and beyond because it provides an excellent example of the scientific method in action: questioning and testing accepted paradigms, and reflecting upon the broader implications and consequences outside of the immediate subject area. It is relatively easy to demonstrate why existing reasoning is flawed. For us still to be challenging topics that are taught to everyone at school level should be genuinely exciting, and make pupils realise that scientific knowledge is not static. We hope it will provide a stimulus for more to go on to study science at university level. We have received enthusiastic support for our ideas from the heads of the bodies responsible for geology school teaching in the UK.

My aims are two-fold: firstly to ensure we engage with schools and the public as broadly as possible during Expedition 360, and secondly to prepare materials post-cruise that we can make freely available as resources for teachers for the longer term. To that end I have insisted that we fill the remaining berths onboard ship with Education officers: four educators and media professionals from the USA, Canada, France and China will sail on the leg.

We plan a media campaign (comparable to my previous engagements e.g. news.bbc.co.uk/1/hi/ wales/6405667.stm, which led to >120 follow-up media articles worldwide) to publicise our onboard activities (via joidesresolution.org). These will include videos, blogs, Q&A sessions and live web sessions with school groups. From these we will ultimately assemble education packages for teachers to ensure the long-term utility of our material.