Date of Award
Union College Only
Bachelor of Science
geology, volcanology, petrology, magma, magma mixing, magma mingling, enclaves
Morne Micotrin is a lava dome located at the head of the Roseau Valley in Dominica, Lesser Antilles, and has an explosive past; the valley was filled with ~3 km3 of pyroclastic deposits from 65-25 ka. The andesitic dome (59-62 wt% SiO2) contains two distinct types of enclaves, coarse-grained and fine-grained, which differ texturally and compositionally from the host and each other. Enclaves provide evidence of magma mingling and mixing processes at depth, which may be introducing heat and volatiles into the magma chamber and triggering eruptions. Fine-grained enclaves (52-54 wt% SiO2) are 1-15 cm in diameter and saturated in plagioclase, opx, cpx, and two oxides. Fine-grained enclaves are vesicular and lack microlites and matrix glass. Coarse-grained enclaves (55-56 wt% SiO2) are larger, 3-20 cm, lack cpx phenocrysts, and contain plagioclase microlites. Coarse-grained enclaves are vesicular as well, a characteristic not observed in the host, and the margins are diffuse compared to the sharp fine-grained enclave margins. Coarse-grained enclave oxides are highly exsolved, and two oxides yielded temperatures of 890± 91°C and 774± 168°C, whereas fine-grained enclave oxides yielded temperatures of 891± 74°C and 794± 60°C. Both enclave types yielded higher fO2 values than the host (average host yield fO2 values of 0.30± 0.15, coarse-grained enclaves yield fO2 values of 1.00± 0.23 and 1.49± 1.22, fine-grained enclaves yield fO2 values of 0.42± 0.23 and 0.60± 0.21, all relative to DNNO). Mineral zoning in the enclaves is more complex than in the host. Plagioclase is normally and reversely zoned in all samples and ranges from An40 to An90 in the host and enclaves. Many enclave plagioclase exhibit potassic rims, up to Or20. Opx in all samples display mostly reverse zoning (En50-70). Enclave opx are characterized by cpx overgrowth. Cpx compositions are more varied within the enclaves, with up to 20 wt% Wo variation. Bulk whole-rock rare earth element concentrations compared to chondrite reveal that fine-grained enclave compositional patterns are not parallel to the trends of the host and coarse-grained enclaves, and glass inclusion analyses reveal a wide range in coarse-grained enclave glass compositions. These data are interpreted and applied to a model of enclave formation at Morne Micotrin. Fine-grained enclaves are likely the result of magma mingling processes, whereas coarse-grained enclaves appear to be formed by hybridization between the Micotrin host andesite and an intruded mafic magma. Continued seismic and gas monitoring of Micotrin may give insight into the timescales of these open-system processes, and allow for a better understanding of the hazards associated with this volcanic center.
Hickernell, Sarah, "Magma Mixing: Magmatic Enclaves in Morne Micotrin, Dominica" (2018). Honors Theses. 1619.