Uphill diffusion of lithium along phosphorus gradients in olivine from mafic layered intrusions

Abstract

Lithium (Li) concentration and Li isotopes of olivine have been widely adopted to decipher the mantle-crust interaction and short-lived magmatic processes in the shallow magma chambers. However, the diffusion behavior of Li in olivine has not yet been fully understood, which may bias the interpretation of Li concentration and Li isotopes observed in natural olivine. In this study, high-resolution elemental mapping (Li, P, Fe, Mn, Ca, Al, and Ni) combined with in situ Li concentration and Li isotope analyses were conducted for olivine grains from two ca. 260 Ma mafic layered intrusions in SW China, to decode the origin of coupled Li-P zoning and multi-mode diffusion of Li in natural olivine. The 2-D elemental maps and compositional profiles reveal complex, coupled Li-P zoning patterns. The Li-P-rich zones contain 3.5 to 6.1 ppm Li and 187 to 776 ppm P, higher than those of Li-P-poor olivine domains that contain 0.7 to 2.8 ppm Li and 29 to 166 ppm P. Particularly, the Li-P-rich zones in each grain commonly have lower δ⁷Li* than that of the Li-P-poor domains, with the maximum fractionation of Li isotopes in a single grain being up to 15‰. Numerical modeling shows that rapid olivine growth can result in variable degrees of Li and P enrichment in concentration and an increase of δ⁷Li* in the Li-P-rich zone of olivine, which is inconsistent with our observations. Instead, the inverse variations of Li concentration and Li isotopic profiles can be well simulated in an uphill diffusion mode of Li along pre-existing sharp P gradients, accompanying by simultaneous coupled and non-coupled diffusion of Li within a single olivine. The large variation of δ⁷Li* is thus interpreted as kinetic fractionation, which may be caused by dehydration of olivine due to exsolution of a fluid phase from the interstitial liquid of a crystal mush. Three distinct Li-P variation trends of olivine are summarized in this study and can be used to distinguish the process of crystal growth from post-crystallization diffusion. Our results should have important bearings on the understanding of complex crystallization and solidification processes of crustal magma chambers.