EVIDENCE FOR MELT TRANSPORT AND CRYSTALLIZATION BELLOW THE CRUST-MANTLE BOUNDARY IN THE SANTA ELENA OPHIOLITE, COSTA RICA

Constraining the melting generation and transport processes in the mantle is fundamental to the understanding of our planet. Well-preserved ophiolite sections around the globe constitute windows into the fossilized melt transport systems that fed the crust and upper mantle. The Santa Elena ophiolite, located in Costa Rica, is a good example of an ancient plumbing system obducted into the surface. Using a combination of techniques, including structural geology, geochemistry, Ar-dating and petrological modeling, we examined the 120 Ma diabase dike swarm that intrudes the peridotite in the Santa Elena ophiolite. Crystallization models require a MORB magma source containing 0.5 wt% H₂O, 53 wt% SiO₂, 3 wt% Na₂O and yielded pressures between 0.4 and 0.8 GPa below the crust-mantle boundary, at depths ranging between 15 and 30 km. Global geochemical comparisons suggest a tectonic scenario that is compatible with back arc basins and slow spreading ridges. We interpret that these sets of conditions may have been achieved during the Late Jurassic-Early Cretaceous spreading between North America and South America, under the influence of the subduction of the Farallon Plate towards the west.