Magma Chamber History Through Plagioclase Zoning and Textures in the Roseau Valley Ignimbrites of Dominica

Dominica is a volcanic island located in the Lesser Antilles Arc of the Caribbean and is centrally located within this arc. The island primarily consists of andesite and dacite deposited from dome building and explosive eruptions. This study examined the zoning and textural patterns within plagioclase crystals from the Roseau Valley ignimbrites, which are deposits of volcanic ash and pumice. Igneous plagioclase crystals become zoned in sodium and calcium as a response to changes in temperature, pressure, water content, and magma chemistry so can record the history of a magma chamber after eruption. Samples from Goodwill Quarry, St. Aromont, Trafalgar Falls, and Rosalie were studied to determine how zoning patterns change within the different eruptive deposits of the Roseau Ignimbrite. On the SEM, brighter zones typically indicate a higher calcium content while a darker zone indicates that a zone is higher in sodium. The plagioclase zoning patterns were separated into normal, reverse, oscillatory, and irregular zoning while the textural patterns were separated into inclusions, spongy, and resorption textures. Normal, oscillatory, and irregular were all common within the samples, while reverse zoning was quite rare. Irregular zoning was somewhat common (~27% of crystals) but was slightly more common in the Rosalie samples and is most likely a result of a variety of conditions such as magma mixing and rapid ascent. Inclusions, formed when melt and unstable elements from the dissolving crystal are caught within a crystal as it is rapidly ascending and crystallizing, were present in ~20% of crystals, especially in those from St. Aromont and were commonly found together with spongy textures in this sample. Spongy textures were present in all samples but were most abundant in the St. Aromont samples and are likely the result of magma mixing and dissolution. Resorption textures are found in ~10% of the examined crystals and indicate magma mixing or rapid ascent followed by stalling in a new magma chamber. The Roseau Valley ignimbrites may have originated from a single chamber, but that chamber can change over time and can be influenced by other chambers. All of the zoning can occur in a single magma system, but the textures indicate that there had to have been some mixing or interaction between magmatic systems. Spongy and resorption textures specifically, occur in conditions where magma is mixing and changing in composition. The St. Aromont and Rosalie eruptions, at the very least, were therefore affected by injections of new magma or changing conditions within their magma chambers.