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

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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.