Date of Award

6-2017

Document Type

Open Access

Degree Name

Bachelor of Science

Department

Geology

First Advisor

Holli Frey

Language

English

Keywords

pyroclastic, sea, island, deposit, samples, volcanic

Abstract

The island of Dominica is located in the center of the Lesser Antilles island arc. Located in the southwestern part of the island, 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 partially capped 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 wholerock chemistry of deposits throughout the valley is similar, and ranges from 58-65 wt% SiO2. Apatite crystals from five pumice clasts were analyzed by BSE, CL, elemental mapping, and laser ablation ICP-MS. The crystals were ~35-50 cm in length and did not exhibit any zoning in BSE. Most crystals showed zoning in CL, suggesting differences in REE and/or trace element chemistry. Zoning patterns show “streaky” CL in two samples and three other samples have grains with irregularly shaped cores. The “streaky” crystals also have cores enriched in chlorine and rims enriched in fluorine. This may have implications for pre-eruption magma chamber conditions, as recent work suggests apatite may record volatile saturation as an eruption trigger (Boyce and Hervig, 2008; Stock et al., 2016). The apatite laser ablation and textural data showed the samples have distinctive compositions and histories. In one sample, the apatites have significantly higher oxygen fugacity, LREE/HREE ratios, and darker cores than the other samples, but have similar silica content and chronology to other samples, showing that the differences in apatite chemistry cannot be due to fractional crystallization processes. The grains with irregular cores contain slightly lower Sr/Y ratios, and may be inherited cores from the incorporation of non-erupted residual material as new melts ascend. Overall the apatite textures and chemistry are variable between samples, suggesting the different eruptive events originate from chemically distinct magma and have unique ascent paths and crystallization histories. This work further suggests that there is not one large magma chamber beneath Dominica, but numerous semi-related bodies supplying magma to the surface with the potential for transient vertical connectivity (Cashman et al, 2017). The most recent reactivation of the Micotrin volcanic center in the past 6 ka may have been caused by a mafic injection, triggering an eruption resulting in the RI-13 andesitic pyroclastic deposit.

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