The skeleton in the closet: New views of major and minor element zoning in olivine and implications for magma crystallization and storage.

Abstract:

Olivine composition and zoning patterns are widely used to reconstruct the evolution of mafic magmas from source to surface and to extract timescales of magmatic processes. Deciphering the olivine zoning code is challenging because the contributions of growth and diffusion may overlap. A detailed multi-element (Fe, Mg, P, Al, Ni) examination of zoning in an exceptional olivine with skeletal morphology allows unequivocal discrimination between these two processes using simple but powerful geometrical arguments. Olivine crystals initially grow diagonally from corner locations, whereas diffusion follows mutually-perpendicular crystal lattice orientations. Generating the zoning patterns for our case-study olivine required at least 4-5 months of diffusive re-equilibration of Fe-Mg, further demonstrating that crystal morphologies produced by rapid growth can survive at magmatic temperatures for extended periods. No significant major element zoning is preserved after rapid growth, lending further credibility to timescales retrieved via diffusion modeling. Extending multi-element approaches to decoding olivine zoning patterns can help determine whether the kinetic relationship between growth- and diffusion-induced zoning recognized herein is widely applicable. Such studies will improve our understanding of timescales of magma storage, solidification, mixing and/or transit towards the surface.