The 3D anatomy of magma transport at fast-spreading ocean ridges

Plate tectonics is the most important discovery in Earth Science and is a unique characteristic of our planet. It involves formation of new tectonic plates by seafloor spreading and their recycling back into the deep Earth at subduction zones. This process continuously repaves two-thirds of the Earth's surface. The formation of new oceanic crust represents the largest magmatic system on Earth, and involves the cooling and solidification of magma (supplied from below by partial melting of the Earth's mantle) along the 70,000 km global network of seafloor spreading axes. Understanding the details of how ocean crust forms is therefore critical to understanding the exchange of heat and mass from the solid Earth to the oceans and atmosphere. Since the rocks of the deep oceans are largely inaccessible, scientists trying to understand how magma builds new crust at spreading axes employ geophysical (seismic) experiments to investigate the sub-seafloor. Results are then compared to and combined with observations made on oceanic rocks in ophiolites (fragments of oceanic crust and upper mantle that have been pushed onto the continents and exposed above sea-level) to develop scientific models of seafloor spreading.

In the search for magma chambers along the East Pacific Rise (EPR), the most magmatically active spreading axis on Earth, geophysicists have discovered thin (10's m thick) lens-shaped magma chambers (known as 'axial melt lenses') at the top of the lower crust that extend along the EPR. These are thought to sit on top of mushes made up of crystals surrounded by small amounts of magma, that feed melt upwards into the overlying melt lens. More detailed experiments have shown that the physical properties of these melt lenses change along the EPR axis, suggesting that the proportion of melt to mush along the EPR varies on a range of length-scales. Upwards expulsion of magma from the melt lens happens periodically via forceful intrusion of sheets of magma (forming so-called "sheeted dyke complexes"), leading to eruption of lava on to the seafloor.

This geophysical picture of the magmatic plumbing system of seafloor spreading axes (based mostly on decades-old inferences from seismic experiments) is incomplete, however, and lacks any constraints on the pathways followed by magma migrating into and out of axial melt lens systems. Lateral variations in seafloor morphology and erupted lava compositions suggest that there must be significant along-axis (3D) transport and evolution of melt, but how extensively this occurs, at what level(s) within the crust, and by what mechanisms remain unknown. These questions have broad implications for the overall process of melt generation and delivery from the mantle and formation of ocean crust, and can only be answered by quantifying melt transport trajectories along a spreading axis in detail and by combining this with determinations of magma geochemistry.

This project addresses these questions by directly determining the migration pathways followed by magma as it enters and exits from an axial melt lens system that has been mapped out along a 100 km complete spreading segment preserved in the Oman ophiolite. This provides the world's only on-land analog for fast-spreading axes like the EPR. We will use a technique called 'anisotropy of magnetic susceptibility' or 'AMS' to measure the 3D preferred alignments of crystals resulting from the flow of magma during the formation of crustal rocks. We will then combine these observations with geochemical analyses of rock compositions to establish whether and how 3D spatial variations in magma flow regimes along a fast-spreading axis control the geochemical evolution of magmas during crustal construction. This novel approach will allow us to develop a comprehensive model for the anatomy of the magma systems responsible for forming two-thirds of the Earth's surface, testing and challenging the predictions of remotely-sensed seismic investigations.