Short-timescale diffusion clocks for studying conduit processes

Abstract:

Syneruptive pressure-temperature-time (P-T-t) paths of magma in volcanic conduits exert a strong influence on eruptive style via feedbacks between degassing, decompression, and crystallization that trigger rapid changes in magma rheology and ascent rate. Conduit models have greatly advanced our understanding of these processes; however, such models rely on many simplifying assumptions, and their ability to reproduce behaviors in complex natural systems is hard to assess. We are developing short-timescale diffusion “clocks” that exploit chemical gradients in fast-diffusing species (e.g., the diffusion of H in olivine and the diffusion of MgO in olivine-hosted melt inclusions) to constrain P-T-t paths of basaltic magmas during the final seconds to hours of their ascent and eruption. Recent advances in these techniques include the development of a chronometer that uses water concentration gradients along the crystallographic ‘a’ direction in olivine phenocrysts to estimate syneruptive magma decompression rates (Barth et al. 2019; Newcombe et al. 2020a). We have now developed the capability to model 2D and 3D water diffusion in melt-inclusion- bearing olivines with realistic crystal morphologies (Mutch and Newcombe, this meeting). Another diffusion clock exploits MgO zonation in olivine-hosted melt inclusions to constrain syneruptive thermal histories (Newcombe et al. 2014, 2020b). Coupling these thermal histories (from MgO diffusion) with decompression rates (from water diffusion) provides estimates of the P-T-t path of magmas before and during eruption. Application of this combined approach to Seguam and Fuego (both arc magmas containing ~4 wt% H2O) reveals less cooling in the conduit than predictions of isentropic conduit models, which may reflect the importance of open-system degassing and/or degassing-driven crystallization in the minutes to hours preceding eruptions.