Quantitative chemical mapping of plagioclase as a tool for the interpretation of volcanic stratigraphy: an example from St Kitts, Lesser Antilles

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 volcanoes. The study of chemical zoning in minerals can be exploited for such a purpose due to its ability to fingerprint successive events or states within a magmatic system as the crystals grow. However, to move beyond a qualitative understanding of a magmatic system an unbiased quantification of a large number of zoned crystals is required. We apply an image segmentation approach to thin section scale chemical maps to segment textural zones in plagioclase and correlate these zones between crystals from a stratigraphic sequence from St Kitts, Lesser Antilles. Mapping crystals at the thin section scale allows us to assess chemical and textural complexity within an individual sample and between different samples. By working on a stratigraphic sequence with this methodology we can quantify chemical and textural complexity in time, in this case specifically on a millennial timescale. Furthermore, recurring zoning patterns observed from this “crystals-eye view” of the magmatic system have the potential to show temporally repetitive processes experienced by the magma at depth. When coupled with textural quantification from crystal size distributions, these data show that variations in whole-rock geochemistry are more likely due to subtle variations in the assemblage of recycled phenocrysts, rather than long-term variations in chemistry of the juvenile magma. The evolution of microlite chemistry, however, unveils a temporal trend towards less chemically evolved magma.