Abstract
Establishing a quantitative link between magmatic processes occurring at depth and volcanic eruption dynamics is essential to forecast the future behaviour of volcanoes, and to correctly interpret monitoring signals at active centres. Chemical zoning in minerals, which captures successive events or states within a magmatic system, can be exploited for such a purpose. However, to develop a quantitative understanding of magmatic systems requires an unbiased, reproducible method for characterising zoned crystals. We use image segmentation on thin section scale chemical maps to segment textural zones in plagioclase phenocrysts. These zones are then correlated throughout a stratigraphic sequence from Saint Kitts (Lesser Antilles), composed of a basal pyroclastic flow deposit and a series of fall deposits. Both segmented phenocrysts and unsegmented matrix plagioclase are chemically decoupled from whole rock geochemical trends, with the latter showing a systematic temporal progression towards less chemically evolved magma (more anorthitic plagioclase). By working on a stratigraphic sequence, it is possible to track the chemical and textural complexity of segmented plagioclase in time, in this case on the order of millennia. In doing so, we find a relationship between the number of crystal populations, deposit thickness and time. Thicker deposits contain a larger number of crystal populations, alongside an overall reduction in this number towards the top of the deposit. Our approach provides quantitative textural parameters for volcanic and plutonic rocks, including the ability to measure the amount of crystal fracturing. In combination with mineral chemistry, these parameters can strengthen the link between petrology and volcanology, paving the way towards a deeper understanding of the magmatic processes controlling eruptive dynamics.
Highlights
The interplay between the chemical and physical processes experienced by magma within the crust is intimately linked to the style and frequency of eruptions observed at the surface (Baker and Holland, 1973; Gertisser and Keller, 2003)
Superpixels within the crystal were compared using an affinity propagation (AP) algorithm, which generates a similarity matrix and groups together similar superpixels. These groups of superpixels are spatially and chemically defined, and their shape and distribution match with zoning patterns observed in the crystal
The disconnect between whole rock and mineral chemistry has been noted in a variety of systems and for a range of mineral phases (Charlier et al, 2005; Kahl et al, 2013; Stock et al, 2020; Ubide et al, 2014)
Summary
The interplay between the chemical and physical processes experienced by magma within the crust is intimately linked to the style and frequency of eruptions observed at the surface (Baker and Holland, 1973; Gertisser and Keller, 2003). Volcanic stratigraphy provides a snapshot of these chemical and physical processes, and holds a plethora of opportunities to collect qualitative and quantitative data within a temporal
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