Olivine exit interviews – gaining insights into magma assembly from olivine populations

Most eruptions tap magmas from a range of pressures and storage environments within the crust and potentially even in the mantle. In this context, early crystallizing phases, such as olivine, provide a unique perspective into the deeper parts of many eruptions. When considering large olivine populations, compositional spectra of those populations serve as proxies for the different batches of magma that constitute the sometimes complex assembly of an eruption. While major element compositions in olivine provide links to melt compositions, minor and trace elements fingerprint deviations from simple liquid lines of descent. Moreover, Fe-Mg isotope signatures demonstrate whether magmas equilibrated diffusively or whether some mineral zoning retains growth histories prior and during assembly.

To reveal the full picture of the assembly of an eruption and potentially changes during the eruption it is essential to collect olivine “interviews” from a large population as they exit the volcano. Here I show that such magma assembly prior to eruption varies greatly from one eruption to the next and that simple monogenetic cones may contain much more complex histories as they quickly pass the crust and may extract crystal cargo from a variety of locations and conditions, while many volumetrically larger more long-lived systems retain a simpler story despite their polybaric magma storage.