A 29-year natural experiment constrains Fe−Mg diffusion rates in olivine

Fe−Mg diffusion in olivine is the most widely used chronometer for extracting the time scales of magmatic processes that drive pre-eruptive volcanic unrest. Yet experimental calibrations of diffusion rates disagree: melt−solid diffusion couples yield rates up to ten times faster than the solid−solid couples underlying canonical values, and whether this discrepancy applies to natural magmas remains untested. The 1959 eruption of Kīlauea Iki (Hawaiʻi, USA) created a rootless lava lake that was repeatedly drilled and sampled over 29 yr as it cooled, preserving petrological snapshots of diffusive re-equilibration on decadal time scales that are inaccessible in the laboratory. Here, we model olivine from drill cores collected between 1960 and 1988 by coupling a 3-D lava-lake cooling model to diffusion models of olivine populations re-equilibrating with their host melt. Matching the measured compositions requires Fe−Mg diffusivities 5−15 times faster than canonical values, independently confirming the faster melt−solid rates and implying that many published pre-eruptive time scales are correspondingly too long.