Decompression and Heating Induced Amphibole Breakdown in Lava Domes on Dominica, Lesser Antilles

Date of Award

6-2018

Document Type

Restricted (Opt-Out)

Degree Name

Bachelor of Science

Department

Geology

First Advisor

Holli Frey

Language

English

Keywords

breakdown, reaction, esp, lf, cpx

Abstract

Dominica, an island in the Lesser Antilles volcanic arc, is home to nine active volcanic complexes, which have created dozens of lava domes, block and ash flows, and ignimbrite deposits since the Pleistocene. Our study focuses on the breakdown of amphibole at four andesitic-dacitic lava domes on Dominica: Espanol (ESP, 744 ± 44 ka), La Falaise (LF, 84 ± 5 ka), Canot (CAN, < 50 ka), and Patates (MPP, 510 ± 9 y). ESP is on the northwestern coast, and the others are in the southwestern region of the island. Samples from each dome contain plagioclase, Fe-Ti oxides, clinopyroxene, orthopyroxene, quartz, and amphibole. Amphiboles from ESP and MPP are characterized by thin, fine-grained reaction rims, whereas those from LF and CAN exhibit complete breakdown. Amphibole breakdown, which can be induced by heating or decompression, was studied because of its ability to determine ascent paths and rates. Our objective was to determine which process was responsible for the breakdown observed and why the extent of breakdown varied across the island. Six reaction textures were found in the four samples. ESP and MPP each possess a distinct thin rimmed texture, and LF and CAN share four pseudomorph textures. All reaction textures consist of plagioclase, pyroxene, and oxides. Clinopyroxene (cpx) has been shown to be absent in reaction rims caused by extensive heating, so elemental mapping was used to observe the abundance of cpx in each sample. Pseudomorphs in LF and CAN contain abundant cpx, making decompression the probable cause of breakdown. The extent of breakdown in LF and CAN and zoning in their reaction textures imply that they ascended slowly, stalling multiple times. Differences in reaction textures in LF and CAN suggest that multiple melts ascended with unique paths and mixed near the surface before each dome erupted. MPP’s rims contain no cpx, indicating that breakdown was most likely induced by heating and that no stalling occurred during ascent. Elemental maps of ESP show two distinct zones, one with and one without cpx, suggesting two phases of breakdown caused by separate heating and decompression events. The melt of ESP was probably heated before ascending slowly, possibly stalling as it rose. This combination of both heating and decompression induced breakdown in a single crystal has not been described in other studies. Elemental mapping in previously studied volcanoes may reveal that a more complex breakdown history. Our use of a combination of textural and mineralogical analysis, rather than simply observing the thickness of the reaction rim, allowed us to determine ascent rates and paths across Dominica, suggesting that this technique can be employed elsewhere to study how magma rises.

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