Mineral Chemistry Evidence for Open-System Magma Evolution in Northern Dominica

Understanding why volcanic systems transition between effusive dome-building eruptions and explosive ignimbrite-forming events with similar bulk compositions is critical for interpreting eruptive behavior and assessing volcanic hazards. This study investigates crystal-scale evidence for magma evolution in andesitic eruptions from the island of Dominica in the Lesser Antilles volcanic arc. Dominica is characterized by both explosive ignimbrite deposits and younger dome-forming eruptions. This research focuses on mineral chemistry and zoning patterns in plagioclase and pyroxene crystals from the Pointe Ronde ignimbrite in northern Dominica. Using scanning electron microscopy with energy-dispersive spectroscopy, mineral compositions were analyzed. Chemical zoning and microlite compositions share insight into magmatic evolution prior to eruption. Zoning in phenocrysts records changes in temperature, pressure, and/or magma composition during crystallization, allowing reconstruction of processes such as magma recharge and mixing. Microlites formed during magma ascent preserve information about late-stage crystallization not captured in bulk-rock compositions. Three thin section samples from Pointe Ronde were analyzed in different zones of the ignimbrite. Plagioclase crystals exhibit reverse zoning, with Ca-rich rims surrounding Na-rich cores, indicating changes in magmatic conditions during crystallization and suggesting mixing or magma recharge prior to ascent. Clinopyroxene compositions plotted on a Wo-En-Fs ternary diagram reveal compositionally distinct populations. Crystals from PR-5B and PR-5D cluster along more Mg-rich, En-rich compositions, while PR-4A microlites show relatively Fe-rich (Fs-rich) compositions compared to PR-4A phenocrysts. Mg# distributions also vary among samples, with sample 4A displaying lower Mg# values (~0.65) and a narrower compositional range compared to the more Mg-rich populations in samples 5B and 5D (~0.69-0.75). These differences indicate evolving magmatic conditions and chemical heterogeneity within the magma system. Identifying magma mixing or multiple magma sources within the magmatic system beneath Dominica helps pinpoint eruption triggers and processes that influence eruption style and transitions from effusive to explosive activity. Improved understanding of these processes contributes to more accurate volcanic hazard assessments and may provide additional time for the evacuation of at-risk communities.