Investigating the deepest part of a volcano plumbing system: Evidence for an active magma path below the western flank of Piton de la Fournaise (La Réunion Island)

Peripheral diffuse degassing of CO2 from the soil occurs across the western flank of Piton de la Fournaise volcano
(La Réunion Island, Indian Ocean) along a narrow zone. In this area, carbon isotopic analysis on soil gas samples
highlights significant mixing between magmatic and organic end-members. The zones with the strongest magmatic signature (highest δ13C) overlap spatial distribution of hypocenters recorded shortly before and during volcano reactivation and allow discriminating a N135° degassing lineament, with a minimum length of 11 km and
140 ± 20 m-width. Such orientation is in accordance with that of an old dyke network along the rift zone and
with N120–130° and N140–155° lineaments related to the inheritance of oceanic lithosphere structures.
Our findings show that this N135° lineament represents a preferential magmatic pathway for deep magma transfer
below the volcano flank. Moreover, spatial distributions of recent eccentric cones indicate a well-founded possibility that future eruptions may by-pass the shallow plumbing system of the central area of the volcano, taking a lateral pathway along this structure. Our results also confirm that Piton de la Fournaise activity is linked to a laterally
shifted plumbing system and represent a major improvement in identifying the main high-risk area on the densely
populated western flank of the volcano