Leader: Laurent Michon (IPGP- La Réunion U.) Participants & coll.: V. Famin (IPGP- La Réunion U.), J. Bernard (BRGM), F. Paquet (BRGM), X. Quidelleur (GEOPS, Paris Sud), S. Zaragosi (EPOC), S. Jorry (IFREMER), N. Gardiner (Monash U.), M. Danisik (Curtin U.), V. Clouard (GET/OMP), P. Bachèlery (LMV), F. Nauret (LMV, UCA), L. Gurioli (LMV, UCA); C. Berthod (LMV)

WP2 is sub-divided in three thematic tasks. It is multidisciplinary, including both fieldwork and laboratory analyses on subaerial and submarine samples and numerical modelling. For this WP, we already made two field campaigns (2018) during which we measured microtectonic structures in Mayotte and collected a few samples in Mayotte and Anjouan. We will complete the sampling in April and May 2019 for dating and petro-geochemical purposes. These campaigns have been funded by internal financial supports and by the MAYVOLTE (INSU Tellus) project. Submarine samples will be collected during the SISMAORE R/V cruise.

* Task 2.1: Chronostratigraphy of the recent volcanism of the Comoros archipelago (Leader: L. Michon)

Our first objective is to determine a precise chronostratigraphy of the recent activity (<0.2 Myrs) on the different islands of the archipelago and on the submarine domain in order to assess the frequency of the eruptions and the temporal relationship between mafic and differentiated eruptions. This chronostratography will also enable to date the periods of deformation. Ages of the volcanic eruptions will be obtained with different isotopic methods depending on the type of material. A total of 20 K/Ar dating will be carried out on mafic subaerial and submarine rock samples in GEOPS by the PhD student with X. Quidelleur. Ten of these datings are already funded by the MAYVOLTE project. The differentiated products will be dated with U/Pb and U-Th/He methods by N. Gardiner and M. Danisik, respectively, with their own financial support. Finally, five radiocarbon ages will be undertaken on charcoals sampled below the unaltered tephra deposits of Anjouan, assuming that the age of the eruptions falls in the 50 ka range of this method. Our dating strategy is to determine the most recent formations by determining 5 K/Ar ages per island (5 14C ages for Anjouan, 4 U/Pb and U-Th/He ages for Mayotte and 5 K/Ar ages for the submarine domain). Note that if the SISMAORE data are not available, 2 K/Ar ages will be reallocated to constrain the age of the oldest part of the Karthala edifice in Grande Comore, and of the volcanic activity of the other islands. The SISMAORE sediment cores located close to MSS may contain one or several layers of volcanic glass or debris that may be subject to dating and geochemical analyses. The core logging will be handled in T3.2 and results on volcanic content will integrate T 2.1. Vintage sediment cores from former surveys in the area [MD104/MOZOPHARE (1996) and MD136/VIGO (2004)] will be sampled at core repository locations for further analyses.

• Task 2.2. Tectonics and volcano-tectonics relationship (Leaders: V. Famin)
  

Task 2.2 aims at inferring to which extent the tectonic setting and structural inheritance control the magma migration and volcanic development. At the scale of Mayotte, a cross interpretation of the micro tectonic field measurements with the available 3D geophysical model of the island (Geomayotte survey, 2010, BRGM) should help us to better define the spatial extensions of the dykes and/or brittle faults and their relationship with the volcanic edifice(s). At crustal scale, we intend to complete our preliminary dataset of faults and magma intrusions to determine the paleo stress fields that affected the different volcanic edifices and integrate them at regional scale. New data will be acquired by the PhD thesis on Anjouan, Moheli and Grande Comore (field campaigns are funded outside the project, only the cost of the PhD thesis is asked here). Microtectonic data will be measured and processed. At the scale of the archipelago, our goals are to study the role of the edifice load and crustal faults on magma migration and to integrate the local deformation in the regional geodynamic. We will follow the methodology developed by Clouard et al. (2003) on the Tarava volcanic chain in central Pacific. The stress fields determined from the field campaigns will be implemented in numerical models.

• Task 2.3. Origin and transfer of the lavas of the Comoros (Leaders: L. Michon)

T2.3 aims at determining the magma sources and transfer dynamics of the magmas involved in the recent eruptions of the Comoros archipelago. We propose to constrain the pressure and the temperature of their sources together with their geochemical signature by combining i) bulk rock analysis of major and trace elements, and Sr-Nd-Pb isotopes on dated samples, together with ii) noble gas signature obtained after crystal crushing and the iii) microanalysis of the minerals, melts and fluid inclusions for thermobarometric purposes. These data will provide strong constraints on the magma sources, their contribution in terms of deep fluids able to percolate along lithospheric discontinuities and the potential occurrence of lithospheric assimilation. In this task, we will determine the mutual age relationships between mafic eruptions and differentiated ones and compare the chemistry, petrology and thermobarometry of mingled basalts with that of scoria cones.