Date/heure
Date(s) - 25 mai 2023
13 h 00 min - 14 h 00 min
Catégories
Jeudi 25 Mai 2023, 13h00, amphithéâtre du CRPG | Jabrane Labidi (IPGP)
Hydrothermal 15N15N abundances in Guadeloupe vents: monitoring the composition of arc melts through time
The abundance and isotope composition of nitrogen in arc sources help constraining the deep nitrogen cycle. However, fresh glasses with mantle-derived volatiles are rare or absent on arc volcanoes. One may study hydrothermal gases, since they are more plentiful than the small quantities of gas accessible from mantle-derived rocks. However, gases are often more contaminated by air than glasses, and their 15N/14N ratios are ambiguous. We will present 15N15N data that allows tracing the provenance of nitrogen in gas mixtures. The approach is based on the recent finding that atmospheric N2 has an enrichment in 15N15N of nearly 20‰ relative to volcanic N2. We will present new 15N15N data acquired on two vents from the Soufrière de Guadeloupe. Samples were taken regularly on the “South-crater” and “Napoleon” vents, from October 2018 to April 2021 (n=33). We find temporal variation for nitrogen 15N15N that covers most of the published variations for other volcanoes. Samples with atmospheric 15N15N values show variable 15N/14N ratios, almost always resulting in light d15N values. The light d15N are similar to estimates for mantle values, yet the 15N15N data preclude a mantle origin. The data establish that a reservoir of fractionated nitrogen contributes to the Soufriere vents throughout our period of investigation. Volcanic nitrogen is also observed to feed the vents, on the basis of near-zero 15N15N enrichments. It has a positive d15N and appears constant through time. The implications of our work for the origin of high-temperature nitrogen in the Guadeloupe vents will be discussed.