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Unravelling the plumbing system of the oldest ignimbrite in Martinique using chemical segmentation of plagioclase and thermobarometry

Aurelie Germa 1, Tom Sheldrake2, Luca Caricchi2, Abigail Martens1, Sebastian Flöter2, Charline Lormand2

  • Affiliations:  1: School of Geosciences, University of South Florida, Tampa, U.S.A.; 2 : Département des sciences de la Terre, Université de Genève, Geneva, Switzerland 

  • Presentation type: Poster

  • Presentation time: Monday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 38

  • Programme No: 1.7.25

  • Theme 1 > Session 7

Abstract

In Martinique, the Roches-Genty episode (~3 Ma) crops out as a thick (> 50m) stratigraphic sequence of pyroclastic falls and flow deposits, rich in large (>2 cm) pumices and displaying characteristics typical of an ignimbrite. Despite the singularity of these deposits, their chemistry, petrography, pre-eruptive conditions, and eruptive dynamics remain unstudied. The possibility of ignimbrites in Martinique has implications for volcanic hazards in the Eastern Caribbean. We combine physical volcanology, petrography, and geochemistry to provide insights into the evolution of the plumbing system of the oldest ignimbrite in Martinique. Chemical maps of major and minor elements were generated by electron microprobe analysis to classify the chemical and textural complexity of pumice from different sub-units of the Roches-Genty episode. We identify eight chemical zoning groups in plagioclase populations (An55-95), and measure the Fracture Index and Vesicle Number Densities, parameters that can be used to calculate decompression rates. Thermobarometry suggests that pyroxene and amphibole crystallized in the upper crust (<3 kbar). Throughout the eruption, bulk pumice composition changes from dacite to andesite, and the plagioclase composition varies from An55-85 to An95. Also, the variety of plagioclase zoning group combinations decreases from 28 (complex) to 2 (simple). We suggest that the early Plinian phases mixed various parts of a complex plumbing system, draining a differentiated dacitic magma stored in the upper crust. This was rapidly followed by a series of column collapse or crater overflow events responsible for the deposition of pumice-rich pyroclastic density currents and surges, involving more primitive andesitic magma.