Reactive melt migration in the lower oceanic crust and its implications for the evolution of mid-ocean ridge basalt

Mid-ocean ridge basalt (MORB) is the most abundant magma on Earth. It is generated beneath mid-ocean ridges by decompression melting of upwelling mantle, and, following processing in lower crustal magma chambers, erupted onto the seafloor. For more then four decades igneous petrologists and geochemists have relied upon MORB as their major window into the mantle, deriving its composition, melting processes and melt migration mechanisms from the erupted lavas. However, this approach assumes that modification of melts in crustal magma chambers occurs exclusively by fractional crystallisation, and can thus be easily corrected for. Within the last decade evidence has emerged that melt may react extensively with existing cumulus crystals as it migrates through mid-ocean ridge magma chambers. This process can strongly modify both the major and trace element compositions of the melt, leading to significant deviations from fractional crystallisation trends. In a recent paper, I was the first to reconstruct the potential effect of reactive melt migration in the lower crust on magma compositions, arguing that the process may impart strong chemical signals on MORB. If so, this requires a fundamental reassessment of MORB petrogenesis and its use as a messenger from the mantle. Thus, in order to understand MORB petrogenesis, and its implications for mantle studies, a robust model that fully characterises the nature and extent of reactive flow in oceanic magma chambers is required. However, no systematic study of reactive melt migration has been done to date, with data limited to a small number of samples from only two locations. As a result, no data exist to robustly assess the extent, nature and spatial distribution of reactive melt migration, hampering an understanding of MORB petrogenesis and its use for studying the mantle. This proposal seeks to determine the nature and extent of reactive melt migration in mid-ocean ridge magma chambers, and test the hypothesis that it plays an important role in MORB petrogenesis. This will be achieved by conducting a systematic, high-resolution study of what has previously been shown to represent a complete, discrete intrusion within an ancient mid-ocean ridge magma chamber (ODP Hole 735B, Southwest Indian Ridge). I will obtain extensive textural and mineral chemical analyses, acquiring the first systematic dataset on reactive melt migration in the lower oceanic crust. The melt-rock reaction history as deduced from the rock record will then be modelled, allowing its role in MORB evolution to be quantified. Combined, the data and models will provide an unprecedented view of reactive melt migration through mid-ocean ridge magma chambers and its role in the evolution of MORB. Ultimately, this will determine the fidelity of MORB as recorders of mantle properties and processes.