Cracking the olivine zoning code; distinguishing between crystal growth and diffusion

• Published in: Geology (Boulder) 2015-10, Vol.43 (10), p.935-938
• Main Authors: Shea, Thomas, Lynn, Kendra J, Garcia, Michael O
• Format: Article
• Language: English
• Subjects: crystal growth; crystal zoning; Crystals; Diffusion; Geology; Magma; magmas; magmatism; Mineralogy; Minerals; Morphology; nesosilicates; olivine; olivine group; orthosilicates; phenocrysts; silicates; Solidification
• ISSN: 0091-7613; 1943-2682
• DOI: 10.1130/G37082.1

Olivine composition and zoning patterns are widely used to reconstruct the evolution of mafic magmas from source to surface and to extract time scales of magmatic processes. Deciphering the olivine zoning code is challenging because the contributions of growth and diffusion may overlap. A detailed multielement (Fe, Mg, P, Al, and 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 rapidly and diagonally from corner locations, whereas diffusion effectively tracks mutually perpendicular crystal lattice orientations. Generating the zoning patterns for our case-study olivine required at least 4-5 months of diffusive reequilibration 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 time scales retrieved via diffusion modeling. Extending multielement 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 time scales of magma storage, solidification, mixing, and/or transit toward the surface.