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The 2007 caldera-forming eruption of Piton de la Fournaise (PdF) erupted the largest volume of magma (210 Mm 3 ) recorded at this volcano in at least three centuries. Major and trace element and Sr–Nd isotope data for bulk-rocks, groundmasses and olivine phenocrysts have been combined with melt inclusion data (major, trace and volatile elements) to track magma evolution over the whole eruptive sequence. We show that each eruptive phase had a distinctive geochemical and petrological signature and that caldera collapse on 5 April was preceded by a marked shift in bulk magma composition and crystal content and size. Aphyric basalt erupted at the beginning of the sequence (February 2007) had relatively high Sr isotope ratio ( 87 Sr/ 86 Sr = 0·70420–0·704180) and low Nd isotopic ratio ( 143 Nd/ 144 Nd = 0·51285–0·51286). Olivine-basalts extruded on 2–5 April just before caldera collapse are less enriched in radiogenic Sr ( 87 Sr/ 86 Sr = 0·70412–0·70416), but characterized by the same Nd isotopic composition. This magma is interpreted as a new deep input, which pressurized the shallow PdF plumbing system and triggered the 2007 activity. Post-collapse oceanite lavas represent the main volume of magma extruded in 2007. Their bulk-rocks and groundmasses have  87 Sr/ 86 Sr (∼0·70418) intermediate between those of February and 5 April, and similar to those of the March 2007 and 2001–2006 lavas. We show that the Steady State Basalts (SSB) commonly erupted at PdF are hybrid melts, which result from multistep mixing between ‘alkaline’ and ‘transitional’ end-members. Our results lead us to propose a new model of the PdF plumbing system to reconcile the petrological, geochemical and geophysical observations: (1) the shallow portion (above sea level) of the PdF plumbing system hosts several small sills, in which magma experiences variable degrees of degassing, cooling and crystallization; (2) oceanite lavas result from the withdrawal of shallow harrisitic mushes stored at low pressures (&lt;48 MPa; &lt;1800–2400 m depth) below both the volcano summit and its eastern flank; (3) water degassing plays a major role in fast magma crystallization at shallow depths. Multistep ascent and periodic extrusion of the shallow magmas is promoted by injections of deeper and hotter basaltic magma, containing up to 1·3 wt % H 2 O and 1630 ppm S. In 2007, the new deep input was the ultimate source of the large excess in sulfur degassing detected by satellites. Lateral draining and intrusion of magma below the eastern flank of the volcano are the cause of major volcano deformation, flank sliding and summit caldera collapse.

The Shallow Plumbing System of Piton de la Fournaise Volcano (La Réunion Island, Indian Ocean) Revealed by the Major 2007 Caldera-Forming Eruption