Decompression and heating induced amphibole breakdown on Dominica, Lesser Antilles

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. Amphiboles from ESP and MPP are characterized by thin, fine-grained reaction rims, while those from LF and CAN exhibit complete breakdown. Amphibole breakdown is generally caused by a decrease in water content of the melt, which occurs due to decompression or heating. Our objective was to determine which of these factors 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. Using oxide pair compositions, we determined crystallization temperatures of the matrix and of reaction textures. The temperature limit of amphibole stability in dacite is approximately 950 °C, and though only a handful of the 735 temperatures found were above this threshold, amphibole is sensitive to heating so this did not eliminate heating as a cause of breakdown. Clinopyroxene (cpx) has been shown to be absent in reaction rims caused by extensive heating, and elemental mapping was used to observe the abundance of cpx in each sample. MPP’s rims contain no cpx, indicating that breakdown was most likely induced by heating. 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. 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. These results suggest that lava dome ascent paths on the island are changing over time and space.