USING TRACE ELEMENT GEOCHEMISTRY OF APATITE PHENOCRYSTS TO FINGERPRINT TEPHRAS IN DOMINICA

The 750 km² island of Dominica contains the highest concentration of volcanic centers in the Lesser Antilles Arc. Previous studies have shown that Dominica is the home of multiple crystal-rich, andesite-dacite domes and associated pyroclastic flows <100 ka including voluminous ignimbrite deposits: Roseau, Grande Savanne, Grand Bay. In addition, the Wesley Ignimbrite and the Pointe Ronde ignimbrite associated with Grande Savanne and Morne Diablotins were also sampled and analyzed. Mineral separation was performed according to traditional heavy liquid methods to extract apatite phenocrysts from seven locations. Twenty grains from each location were extracted and dissolved in dilute nitric acid before being analyzed for major and REE on the ICP-MS. Samples with inappropriate Ca/P ratios were not considered to be apatites. Otherwise, data within a single ignimbrite cluster well. Geochemical data was used to fingerprint different tephras by comparing Ce/Ca to Sr/Y and Dy/Yb to Dy/Dy^*. The unwelded and welded samples taken from the Layou Ignimbrite show strong clustering in both plots. Ignimbrites from the north including Grande Savanne, Wesley, and Point Ronde show similar apatite chemistry confirming that the three deposits emanate from the Morne Diablotins Volcanic Center. There is also a separation between the pumice samples from Morne aux Diables, which is inferred to be older than the deposits observed to the south. Different samples from within the Roseau ignimbrite are indistinguishable, and partially overlap samples from the Grand Bay Ignimbrite. Additional samples from Roseau including the Casso Estate and Saint Aromant, and Grand Bay will be analyzed to determine if they group with the Roseau apatites. Previous studies have indicated that the pyroclastic deposits observed in Layou and Roseau have the same source, located in the Morne Trois Pitons National Park. Whole-rock, major, and trace element geochemistry, phase assemblages, and mineral chemistry are strikingly similar between units, so it has been difficult to identify source regions and unique tephra deposits. REE element plots elucidated some differences, but apatite analyses allow us to fingerprint individual tephra and show unequivocally that the Layou and Roseau deposits are not related and are unlikely to be sourced from the same vent.