Lithium concentration gradients in feldspar and quartz record the final minutes of magma ascent in an explosive supereruption

Pre- and syn-eruptive time-scales of magmatic processes in large-scale explosive eruptions can be quantified from compositional gradients established between (and within) crystals and their host melt using models of diffusive relaxation. The faster the elemental diffusion rate, the shorter the time periods that are measurable. Here we document Li compositional gradients from crystals in the rhyolitic magma of the ~ 27 ka Oruanui supereruption (Taupo, New Zealand). In plagioclase feldspar and quartz from pumices in the first-erupted material, and loose feldspar crystals from late-erupted ignimbrite, the crystal rims display a ~ 50% increase in Li concentration over equilibrium values in the crystal cores. At appropriate magmatic temperatures, these gradients represent time-scales of 125 to 720 s, equivalent to decompression rates of 30–180 kPa/s and rise rates of frothy magma of 4–21 m/s. We infer that these short time-scales reflect changes in lithium partitioning behaviour during decompression, and cannot be due to changes in Li concentration accompanying degassing and the growth of bubbles (which act in the opposite sense). The Li partitioning changes are due to the breakdown of the liquid–vapour equilibrium at the critical pressure of 22 MPa, where the vapour phase transitions from a supercritical to a subcritical state. This breakdown causes Li to partition from melt into crystals, inducing a short-lived increase in effective Li activity of about 50%. The system is then quenched rapidly by cooling in air or water on eruption. These results capture syn-eruptive processes on unprecedentedly short time-scales in a type and size of eruption where information concerning such late-stage processes has never before been obtainable.