Volcanic Apatite Chemistry and Zoning in the Roseau Valley, Dominica, Lesser Antilles

The island of Dominica is located in the center of the Lesser Antilles island arc.  The Roseau Valley contains a thick and complex stratigraphy of pyroclastic flows that terminate in the sea on the western side of the island near the capital of Roseau, likely originating from the Wotten Waven caldera, now occupied by the Micotrin dome (Sigurdsson, 1972). We have dated several ignimbrite deposit using U-Th analyses of zircon rims. The ages of five deposits within the valley show polymodal distributions, suggesting multiple periods of crystallization (~6 ka - >250 ka). The youngest deposit at Casso and an ignimbrite on the eastern side of the island near Rosalie, have distinctive zircon populations. The relationship between these deposits and the underlying magma chamber are poorly understood. The whole-rock chemistry of deposits throughout the valley is similar, and ranges from 58-65 wt% SiO2. Apatite crystals from seven pumice clasts were analyzed by BSE, CL, elemental mapping, and laser ablation ICP-MS. The crystals were ~35-50 um and did not exhibit any zoning in BSE. Many crystals showed zoning in CL, suggesting differences in REE and/or trace element chemistry. Zoning patterns showed “streaky” CL in two samples and two other samples had grains with irregularly shaped cores. The “streaky” crystals also had cores enriched in chlorine and rims enriched in fluorine. This may have implications for pre-eruption magma chamber conditions, as recent work suggests volatile saturation may trigger eruptions (Stock et al., 2016). The laser ablation data showed each deposit has a distinctive composition, with some samples showing sizeable variability. The grains with irregular cores contained lower Sr/Y ratios, and may represent a compositional continuum derived from two older deposits. This work further suggests that there is not one large magma chamber beneath Dominica, but numerous semi-related bodies supplying magma to the surface.