Timing of melt migration and pyroxenite formation in the mantle

Abstract:

Layered pyroxenites are the dominant type of ultramafic heterogeneities associated with exhumed peridotites. Although they usually represent less than 5% in volume of exposed mantle rocks, they are widely observed in various tectonic environments such as the sub-continental, sub-arc and sub-oceanic mantle. Here we show that mm-scale chemical variations across layered pyroxenites and peridotites can provide a wealth of information on the timing and pathways of migrating melts in the Earth’s mantle, and on temperatures and timing of equilibration of lithospheric peridotites. We describe a novel approach that uses a combination of diffusion and temperature calculations with isotope ages, on multiple pyroxenite layers formed by melt migration in the Lherz massif, to reconstruct the timing of melt migration. We show that after melt fronts froze into the lithosphere, they were preserved in the lithospheric mantle for hundreds of Ma. To constrain the timing of melt migration, we determine the trace element composition of the percolating melt; we use isotopes to estimate the maximum age at which minerals reacted and precipitated from that melt; based on the melt percolation age, and the age of exhumation, we obtain a maximum amount of time between the end of melt percolation event and the exhumation age. In parallel, we calculate closure temperatures based on major elements and rare-earth element thermometers. Then, we use partition coefficients and mineral diffusivities to retrieve the cooling rates recorded by the thermometers. Finally, we will discuss the critical relationship between ‘heterogeneities’ in the source of basalts and the presence of veins in the lithospheric mantle.