Following magma: The pathway of silicic magmas from extraction to storage during an ignimbrite flare-up, Taupō Volcanic Zone, New Zealand

Abstract

Very large silicic magma systems can generate tens to thousands of cubic kilometres of very evolved, rhyolitic magmas that erupt in caldera-forming events. These magma systems may incorporate multiple bodies of magma mush, from which eruptible, crystal-poor magma is extracted and stored. We examine the pressure of magma extraction from the mush and storage of eruptible magma bodies from seven caldera-forming eruptions in the central Taupō Volcanic Zone, New Zealand, during a 350 ka to 240 ka ignimbrite flare-up. For the first time, we use rhyolite-MELTS geobarometry to calculate extraction and storage pressures for the same pumice clasts (representing magma parcels), revealing that each eruption was fed by multiple, compositionally distinct, mush and eruptible magma bodies. Some of the eruptible bodies were stored contiguously with the magma mush; but, on average, our calculations reveal that individual parcels of eruptible magma were stored ∼6 km shallower than their extraction depths, demonstrating that magmas migrate an appreciable vertical distance in the build-up to eruption. The flare-up includes three different geometries of magmatic system: 1) a “mature” type in which multiple eruptible magma bodies sit adjacent to each other in the shallow crust (∼100 MPa) and are in contact with the top of a vertically extensive, heterogeneous mush system; 2) a “mush model” type in which a shallow eruptible magma body is directly in contact with a shallow mushy cumulate; and 3) a “vertically discretised” type in which eruptible magma bodies are stored at different depths and may also be extracted from separate mush bodies. The flare-up began with a “mature” type system, with the very large (>2200 km3 magma) Whakamaru Group eruptions. Following Whakamaru, the eruptions were smaller (erupting 50 – 150 km3 magma), and initially had deep, “vertically discretised” magma systems. The subsequent eruptions reveal the vertical expansion of a new system, until the last eruption (Ohakuri) had a “mature”-type organisation. This sequence of flare-up eruptions hints at how very large silicic magma systems can spatially and vertically mature over a geologically short timeframe (∼100 kyrs) from a patchwork of vertically discretised magma bodies to a mature magma system with a shallow storage zone and compositionally heterogeneous magma mush beneath.