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Gravity changes occurring during the initial stage of the 2011-2012 El Hierro submarine eruption are interpreted in terms of the preeruptive signatures during the episode of unrest. Continuous gravity measurements were made at two sites on the island using the relative spring gravimeter LaCoste and Romberg gPhone-054. On 15 September 2011, an observed gravity decrease of 45 μGal, associated with the southward migration of seismic epicenters, is consistent with a lateral magma migration that occurred beneath the volcanic edifice, an apparently clear precursor of the eruption that took place 25 days later on 10 October 2011. High-frequency gravity signals also appeared on 6-11 October 2011, pointing to an occurring interaction between a magmatic intrusion and the ocean floor. These important gravity changes, with amplitudes varying from 10 to À90 μGal, during the first 3 days following the onset of the eruption are consistent with the northward migration of the eruptive focus along an active eruptive fissure. An apparent correlation of gravity variations with body tide vertical strain was also noted, which could indicate that concurrent tidal triggering occurred during the initial stage of the eruption.

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Summary We test an innovative inversion scheme using Green's functions from an array of pressure sources embedded in finite-element method (FEM) models to image, without assuming an a-priori geometry, the composite and complex shape of a volcano deformation source. We invert interferometric synthetic aperture radar (InSAR) data to estimate the pressurization and shape of the magma reservoir of Rabaul caldera, Papua New Guinea. The results image the extended shallow magmatic system responsible for a broad and long-term subsidence of the caldera between 2007 February and 2010 December. Elastic FEM solutions are integrated into the regularized linear inversion of InSAR data of volcano surface displacements in order to obtain a 3-D image of the source of deformation. The Green's function matrix is constructed from a library of forward line-of-sight displacement solutions for a grid of cubic elementary deformation sources. Each source is sequentially generated by removing the corresponding cubic elements from a common meshed domain and simulating the injection of a fluid mass flux into the cavity, which results in a pressurization and volumetric change of the fluid-filled cavity. The use of a single mesh for the generation of all FEM models avoids the computationally expensive process of non-linear inversion and remeshing a variable geometry domain. Without assuming an a-priori source geometry other than the configuration of the 3-D grid that generates the library of Green's functions, the geodetic data dictate the geometry of the magma reservoir as a 3-D distribution of pressure (or flux of magma) within the source array. The inversion of InSAR data of Rabaul caldera shows a distribution of interconnected sources forming an amorphous, shallow magmatic system elongated under two opposite sides of the caldera. The marginal areas at the sides of the imaged magmatic system are the possible feeding reservoirs of the ongoing Tavurvur volcano eruption of andesitic products on the east side and of the past Vulcan volcano eruptions of more evolved materials on the west side. The interconnection and spatial distributions of sources correspond to the petrography of the volcanic products described in the literature and to the dynamics of the single and twin eruptions that characterize the caldera. The ability to image the complex geometry of deformation sources in both space and time can improve our ability to monitor active volcanoes, widen our understanding of the dynamics of active volcanic systems and improve the predictions of eruptions.

The Bonga complex is composed of a central carbonatite plug (with a ferrocarbonatite core) surrounded by carbonatite cone sheets and igneous breccias of carbonatitic, fenitic, phoscoritic and lamprophyric xenoliths set in a carbonatitic, lamprophyric or mingled mesostase. To reconstruct the dynamics of the complex, the pyrochlore composition and distribution have been used as a proxy of magmatic-hydrothermal evolution of the complex. An early Na-, F-rich pyrochlore is disseminated throughout the carbonatite plug and in some concentric dykes. Crystal accumulation led to enrichment of pyrochlore crystals in the plug margins, phoscoritic units producing high-grade concentric dykes. Degassing of the carbonatite magma and fenitization reduced F and Na activity, leading to the crystallization of magmatic Na-, F- poor pyrochlore but progressively enriched in LILE and HFSE. Mingling of lamprophyric and carbonatite magmas produced explosive processes and the formation of carbonatite breccia. Pyrochlore is the main Nb carrier in mingled carbonatites and phoscorites, whereas Nb is concentrated in perovskite within mingled lamprophyres. During subsolidus processes, hydrothermal fluids produced dolomitization, ankeritization and silicification. At least three pyrochlore generations are associated with late processes, progressively enriched in HFSE, LILE and REE. In the lamprophyric units, perovskite is replaced by secondary Nb-rich perovskite and Nb-rich rutile. REE-bearing carbonates and phosphates formed only in subsolidus stages, along with late quartz; they may have been deposited due to the release of the REE from magmatic carbonates during the hydrothermal processes.