Minutes to Millennia: Diffusion Methods in Subduction-Related Volcanism

Abstract:

Diffusion methods have several advantages as relative geochronometers in volcano-related magmatic processes: diffusion stops on eruption, locking in short timescale information; methods are relatively easy to implement; and suitable material is generally abundant. Such methods also pose significant, accompanying challenges: the need for accurate melt palaeothermometry, uncertainties in diffusion parameters, and sometimes even a cryptic connection between mineral zonation and timescale. As all timescales are relative to an event, timing is not absolute, and care must be taken in interpretation. Yet for all the difficulties, diffusion tools are seeing more widespread usage. This has come about because of the potential of diffusion methods to interrogate certain pre-eruptive processes operating over timescales of relevance to human timescales and responses, having direct bearing on hazard mitigation procedures.

In studying subduction zone systems we have a wide range of minerals to choose from but will, in subduction-related, andesitic-to-rhyolitic systems, usually be operating away from the relatively well-constrained system of olivine, and instead be dealing with mineral phases that offer different challenges, such as plagioclase, quartz, sanidine, amphibole, orthopyroxene, Fe-Ti oxides and mica. Timescales here span a wide range across different mineral-element combinations, from minutes for Li in plagioclase and quartz to days by Fe-Ti oxides, years by orthopyroxene Fe-Mg and decades to millennia with plagioclase, sanidine and quartz.

This contribution will focus on the interpretation of diffusion signals in minerals found in subduction-related volcanic systems. To understand what any modelled timescale means, the process which formed the relevant zonation pattern is absolutely key, yet often elusive. Variations in P, T, X conditions really drive the crystallisation process yet certain zonation patterns are non-unique. This ambiguity necessitates painstaking petrological and geochemical detective work to understand where the zoning came from to begin with, and which parameter or parameters were truly driving the zonation. The rewards are insights into short-lived processes occurring in the build up to, or process of, eruption, which are not accessible via other mechanisms.