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, formed by the subduction of the North American Plate beneath the Caribbean Plate. Previous studies have linked various pyroclastic deposits to vents now capped by lava domes. 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. These deposits likely originated from the Wotten Waven caldera, now occupied by the Micotrin dome (Sigurdsson, 1972). Past studies have shown ignimbrite deposits within this valley span from 20 to 70 ka (Howe et al., 2014). However, we have dated an ignimbrite deposit in the Roseau Valley ~6 ka, based on U-Th analyses of zircon rims. The U-Th ages of five different deposits within the valley show polymodal distributions, suggesting multiple periods of crystallization within the magma chamber. However, the young deposit at Casso, a ~5 m thick sequence of ash located on the southern ridge of the Roseau Valley and an ignimbrite on the eastern side of the island near Rosalie, have distinctive zircon populations. The relationship between these deposits within the Roseau Valley 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. The older deposits tend to be more evolved than the younger deposits, suggesting that the magma chamber may be reversely zoned. Apatite records changes in volatile composition and recent work suggests volatile saturation may trigger eruptions (Stock et al., 2016). Apatite crystals from seven pumice clasts were analyzed by BSE, CL, and elemental mapping. The crystals were ~35-50 um and did not exhibit any zoning in BSE. Elemental mapping of the crystals showed their cores to be enriched in chlorine, and rims enriched in fluorine. This may indicate changing volatile composition of the melt during crystallization and/or ascent. CL zoning textures indicate a complex crystallization history, with some crystals containing inherited, irregularly shaped cores. The zoning in the apatites suggests that the conditions in the magma chamber are dynamic and unique for different deposits. This has implications for magma storage conditions and potential for eruption.