Magmatic Enclaves and Magma Mixing in Morne Micotrin, Dominica

Morne Micotrin, an andesitic lava dome (59-62 wt% SiO2) at the head of the Roseau Valley in Dominica, Lesser Antilles, contains two distinct enclave types, fine-grained (52-54 wt% SiO2) and coarse-grained (55-56 wt% SiO2). Enclaves provide evidence of magma mingling at depth, which causes disequilibrium in the magma and triggers eruptions. These enclave types differ from one another as well as the host, compositionally and texturally. Coarse-grained enclaves are 3-20 cm in diameter, lack cpx, and contain abundant plagioclase microlites. Cg enclaves are vesicular and margins are diffuse to sharp. Fine-grained enclaves are 1-15 cm and saturated in plagioclase, opx, cpx, and two oxides. Fg enclaves are more vesicular than the host and boundaries are sharp. Two oxides yielded temperatures of 890± 91°C and 774± 168°C and average fO2 values of 1.00± 0.23 and 1.49± 1.22 for the cg enclaves, whereas fg enclaves yield temperatures of 891± 74°C and 794± 60°C and fO2 values of 0.42± 0.23 and 0.60± 0.21. The average host temperature is 851± 24°C, with fO2 of 0.30± 0.15. Oxide exsolution in the enclaves, especially cg, causes the high level of uncertainty associated with these data. Plagioclase in both enclave types is normally- and reversely-zoned and ranges from An40 to An90 in all magma types. Enclave plagioclase exhibit potassic rims, up to 42 wt% orthoclase. Potassic rims in the enclaves suggest the enclaves may have a unique thermal history relative to the host. Opx in all samples is mostly reversely zoned (En50 – En70), with the enclave opx exhibiting Cpx overgrowth. Cpx compositions vary to a greater degree in the enclaves, with up to 20 mol% Wo variations. Whole-rock REE concentrations relative to chondrite reveal that fg enclave compositions follow a unique trend compared to the host and cg enclaves, suggesting a unique fg enclave source. Glass analyses reveal host glass composition is tightly clustered and fg enclave glass compositions tend to cluster with unique compositions relative to the host. The cg enclave glass ranges widely, suggesting cg enclaves may have had changing melt conditions. By clarifying our understanding of enclaves in Micotrin, we are able to better characterize the impacts of the mafic injections that form enclaves and how magma interactions are tied to eruptions.