Bull Volcanol (2009) 71:1077–1089 DOI 10.1007/s00445-009-0287-5 RESEARCH ARTICLE The November 2002 eruption of Piton de la Fournaise, Réunion: tracking the pre-eruptive thermal evolution of magma using melt inclusions Nathalie Vigouroux & A. E. Williams-Jones & Paul Wallace & Thomas Staudacher Received: 19 November 2007 /Accepted: 29 April 2009 /Published online: 6 June 2009 # Springer-Verlag 2009 Abstract The November 2002 eruption of Piton de la estimate eruption temperatures. The November 2002 melt Fournaise in the Indian Ocean was typical of the activity of inclusion compositions suggest that they were at temper- the volcano from 1999 to 2006 in terms of duration and atures between 1,070°C and 1,133°C immediately before volume of magma ejected. The first magma erupted was a eruption and quenching. This relatively wide temperature basaltic liquid with a small proportion of olivine phenocrysts range may reflect the fact that most of the melt inclusions (Fo81) that contain small numbers of melt inclusions. In were from olivine in lava samples and therefore likely subsequent flows, olivine crystals were more abundant and underwent minor but variable amounts of post-eruptive 2+ richer in Mg (Fo83–84). These crystals contain numerous melt crystallization and Fe loss by diffusion due to their and fluid inclusions, healed fractures, and dislocation relatively slow cooling on the surface. In contrast, melt features such as kink bands. The major element composition inclusions in tephra samples from past major eruptions of melt inclusions in this later olivine (Fo83–84)isoutof yielded a narrower range of higher eruption temperatures equilibrium with that of its host as a result of extensive post- (1,163–1,181°C). The melt inclusion data presented here and entrapment crystallization and Fe2+ loss by diffusion during in earlier publications are consistent with a model of magma cooling. Melt inclusions in Fo81 olivine are also chemically recharge from depth during major eruptions, followed by out of equilibrium with their hosts but to a lesser degree. storage, cooling, and crystallization at shallow levels prior to Using olivine–melt geothermometry, we determined that expulsion during events similar in magnitude to the relatively melt inclusions in Fo81 olivine were trapped at lower small November 2002 eruption. temperature (1,182±1°C) than inclusions in Fo83–84 olivine (1,199–1,227°C). This methodology was also used to Keywords Piton de la Fournaise . Volcanic plumbing system . Melt inclusions . Editorial responsibility: M. Clynne Olivine cumulates . Post-entrapment modifications . A. E. Williams-Jones Volatiles . Magma evolution Department of Earth and Planetary Sciences, McGill University, Montreal, QC, Canada Introduction P. Wallace Department of Geological Sciences, University of Oregon, Eugene, OR, USA The 530-ka Piton de la Fournaise shield volcano is the current manifestation of the Réunion hot spot, which T. Staudacher produced the Deccan Traps in India at about 65 Ma Observatoire Volcanologique du Piton de la Fournaise, IPGP, Réunion, France (Courtillot et al. 1986). It is located at the southern end of the Mascarene Basin, encompassing the islands of Maur- Present address: itius and Rodrigues. Piton de la Fournaise rests on the south * N. Vigouroux ( ) flank of the older and now extinct Piton des Neiges, which Department of Earth Sciences, Simon Fraser University, Burnaby, BC, Canada V5A 1S6 forms most of present day Réunion Island (Fig. 1), and is e-mail: [email protected] one of the world’s most active volcanoes. 1078 Bull Volcanol (2009) 71:1077–1089 The anatomy of Piton de la Fournaise is similar to that of the early phase (first day) of the November 2002 eruption Kilauea in that a central conduit links a shallow magma crystallized from the magma hosting it, which was more chamber with the summit craters, and flank eruptions are evolved and cooler than the magma erupted during past fed by lateral dyke intrusions connected to the central events studied by Bureau et al. (1998b). The bulk of the conduit (Peltier et al. 2007). Fissures from historic flank olivine, which was ejected in increasing proportions during eruptions are mostly distributed along an arcuate rift zone the later stages of the 2002 eruption (late phase) and is less extending from the northeast of the caldera to the southeast, evolved, appears to have been entrained in this lower passing through the central craters (Peltier et al. 2005). temperature magma. This caused it to experience a larger Recent models based on seismicity and deformation of the amount of pre-eruptive cooling than olivine of similar volcano prior to eruption suggest that Piton de la Fournaise composition ejected during the earlier events studied by has a magma chamber with a volume of ~300×106 m3 Bureau et al. (1998b). These results are consistent with the located between sea level and 500 m above sea level (a.s.l.; model of crystallization, accumulation, storage, and erup- Sapin et al. 1996; Nercessian et al. 1996; Peltier et al. tion proposed previously by Bureau et al. (1998b, 1999), in 2007). A deeper magma chamber, located at 5–6 km below which they used melt inclusions to reveal the crystallization sea level (b.s.l.), has also been postulated (Battaglia et al. and storage of olivine Fo83–85 at a variety of depths ranging 2005) and olivine crystallization has been shown to occur from ~5 km b.s.l. to the near surface, prior to eruption from as deep as 15 km b.s.l. (Bureau et al. 1998a, b). Finally, a the central conduit area of the volcano. They showed that crystal cumulate zone, evident as a high-density plug on the the volatile contents found in the melt inclusions from a basis of seismic data, is likely to be present within the single eruption could not be reconciled with a plausible central cone above sea level (Nercessian et al. 1996; Sapin degassing trend and therefore must represent trapping of et al. 1996) and to extend to the Moho at a depth of about individual melt batches with variable degassing histories. 12.5 km b.s.l. (Gallart et al. 1999). Information on the depth of crystallization and accumu- Background geology lation of magma within the volcano is crucial to further understanding the plumbing system of Piton de la Four- The summit area of Piton de la Fournaise volcano is naise. Melt inclusions provide an effective tool for this enclosed on three of its sides by the 4,500-year-old Enclos purpose because they trap small pockets of magma at Fouqué, the remnant of the youngest of three calderas, different stages during the crystallization of crystals in whereas its east flank is open to the Indian Ocean. The response either to undercooling (Faure et al. 2003)or summit rises to 2,631 m a.s.l. and is crowned by two changes in the chemical environment (e.g., magma mixing) intersecting craters, Bory and Dolomieu, the latter one (Sobolev 2007). Analysis of the chemical composition, being the larger and representing the current center of including volatile contents (H2OandCO2), of melt activity (Fig. 1). inclusions, yields information on the depth of crystalliza- Most of the historic eruptions have been from vents tion and storage, as well as trends in the chemical evolution located inside the walls of the current caldera, at various of the magma. At Piton de la Fournaise, melt inclusion elevations along the rift zone (summit to 1,000 m a.s.l.) but studies of olivine from both prehistoric (~3,000–50 ka) and three of the last four large-volume (>50×106 m3) deep- historic (1931, 1977, 1998) eruptions have shown that seated eruptions (1977, 1986, 1998) have had at least one olivine crystallization and accumulation occurs over a wide vent located outside of the caldera rim along an extension range of depths from the Moho to approximately sea level of the intracaldera rift zone. Prior to the collapse of the (Bureau et al. 1998b, 1999). The samples analyzed in these current caldera (Enclos Fouqué), the southeast and the studies were from large-volume, mostly olivine-rich mag- northeast rift zones were active along the margins of the old mas erupted from vents located outside the current caldera caldera rim (Plaine des Sables collapse; Fig. 1) and a few and rift system (prehistoric eruptions) and intracaldera vents vents/eruptive centers were preserved after the sector active in historical times (1931, 1998). collapse that created the current caldera (e.g., Piton For this study, we analyzed olivine-hosted melt inclu- Manapany; Bureau et al. 1998b). sions from the moderate-volume (8×106 m3) flank eruption The Piton de la Fournaise volcano is still in its shield of November 2002, a seismically shallow event (down to building stage, erupting lavas that range from aphyric 1.5 km b.s.l.) characteristic of the activity at Piton de la basalts to oceanites, which are olivine-rich basalts equiva- Fournaise from 1999 to 2006. Using the compositions of lent to the Hawaiian picrites (Upton and Wadsworth 1966). these inclusions after correction for post-entrapment mod- Most of the olivine erupted at Piton de la Fournaise ranges ifications, we have characterized the thermal history of the in composition from Fo81 to Fo87 (Albarède et al. 1997; magma from which the olivine crystallized and in which it Bureau et al. 1998a, b, 1999). However, basalts containing erupted. Results of this study suggest that olivine ejected in Fo83–84 olivine have been the dominant lava type during the Bull Volcanol (2009) 71:1077–1089 1079 Fig. 1 Digital elevation model of the summit area of Piton de la Fournaise. Inset shows location on Reunion Island. The two summit craters (Bory and Dolomieu) are within the youngest caldera (L’Enclos Fouqué).