Jean De Bremond d'Ars

Volcanism in a compressional Andean setting: A structural and geochronological study of Tromen volcano (Neuquén province, Argentina)

We document evidence for growth of an active volcano in a compressional Andean setting. Our data are surface structures and 39Ar-40Ar ages of volcanic products on Tromen volcano. Tromen is an active back-arc volcano in the Andean foothills of Neuquen province, Argentina. Its volcanic products are unconformable upon Mesozoic strata of the Neuquen basin. The volcano straddles a N-S trending pop-up, which formed during E-W shortening. The main underlying structures are eastward verging thrusts. Their traces curve around the eastern foot of the volcano. Minor folds and faults also occur in the volcanic cover of Tromen, as a result of E-W shortening. New 39Ar-40Ar ages for these volcanic rocks are younger than 2.27 ± 0.10 Ma and show that Tromen has been active almost continuously from the late Pliocene to the Holocene. We conclude that volcanism and thrusting have been coeval and that magma must have reached the surface in a tectonic setting of horizontal compression. Our results have wider implications for magmatic processes in such settings.

Chemical exchange in migmatites during cooling

Abstract The chemistry of the leucosome components in limited-size migmatitic associations is generally believed to reflect the composition of the partial melt that could have been extracted to yield an anatectic magma. Several well-documented studies of migmatites indicate that the distributions of some trace elements in leucosome/melanosome pairs are inconsistent with equilibrium partial melting but are rather in agreement with chemical reequilibration during cooling. In migmatites, the liquid fraction does not leave the system. This may induce an alteration of the chemical composition of the whole migmatite system by extensive chemical exchange between the liquid and the solid residue during cooling. If this is the case, the leucosome and melanosome components are not representative of the chemical composition of the liquid and solid residue respectively, at the time of partial melting. A rough evaluation of the residences time of the melt fractions in cooling in-situ migmatitic bodies yields minimum estimates of 1 to 10 Ma. For such time spans the characteristics distances of diffusion distances of diffusion-induced chemical transfer exceed the sizes of individual mineral grains, which is consistent with a re-equilibration process. The release of latent heat of crystallization in cooling migmatitic systems and the presence of an interstitial liquid phase are critical in determining the extent and scale of chemical exchange. The mechanism involves (1) reducing the cooling rate and (2) promoting the efficiency of chemical diffusion through the liquid phase. Thus, some doubt is cast on the ability of small-scale migmatitic systems to provide relevant information about chemical mass balance during partial crustal melting.

Gravity instabilities in magma chambers: rheological modelling

Abstract The physical study of gravity instabilities in magma chambers provides a method to estimate the rheological properties of magmas. Many layered intrusions exhibit gravity instability structures, such as magmatic load casts. We present here a study based on structures occuring in veined diorites in the layered plutonic complex of Guernsey. Wavelengths of gravitational perturbations are determined by spectral analysis. We relate the wavelengths to the rheological properties of the magmas by a perturbation enhancement model. The time scale for the gravity instabilities to develop is calculated to be about 10 2 s, which is about the characteristic time scale for solidification in these rocks; this is why these structures are preserved in the rock. The calculated length and time scales are compatible with the geometry of the observed structures and the crystallization kinetics of magma chambers, showing that the structures are appropriately modeled as gravity instabilities. This mechanical approach, together with petrological study and direct computation of magma viscosity, is likely to provide several constraints on petrogenetic modelling. The fact that the observed structures can be interpreted as gravity phenomena is clear evidence that the observed layering is a primary igneous feature rather than the result of any metasomatic process.

Muon dynamic radiography of density changes induced by hydrothermal activity at the La Soufrière of Guadeloupe volcano

Imaging geological structures through cosmic muon radiography is a newly developed technique which shows a great potential in volcanology. Here we demonstrate that muon radiography permits to detect and characterize mass movements in shallow hydrothermal systems of low-energy active volcanoes like the La Soufrière lava dome. We present an experiment conducted on this volcano during the Summer 2014 and bring evidence that very important density changes occurred in three domains of the lava dome. Depending on their position and on the medium porosity the volumes of these domains vary from 1 × 106 m3 to 7 × 106 m3. However, the total mass budget remains approximately constant : two domains show a mass loss (Δm∈ [−0.8;−0.4] × 109 kg) and the third one a mass gain (Δm∈ [1.5; 2.5] × 109 kg). We attribute the negative mass changes to the formation of steam in shallow hydrothermal reservoir previously partly filled with liquid water. This coincides with the emergence of new fumaroles on top of the volcano. The positive mass change is synchronized with the negative mass changes indicating that liquid water probably flowed from the two reservoirs invaded by steam toward the third reservoir.

Monitoring temporal opacity fluctuations of large structures with muon tomography : a calibration experiment using a water tower tank

Usage of secondary cosmic muons to image the geological structures density distribution significantly developed during the past ten years. Recent applications demonstrate the method interest to monitor magma ascent and volcanic gas movements inside volcanoes. Muon radiography could be used to monitor density variations in aquifers and the critical zone in the near surface. However, the time resolution achievable by muon radiography monitoring remains poorly studied. It is biased by fluctuation sources exterior to the target, and statistically affected by the limited number of particles detected during the experiment. The present study documents these two issues within a simple and well constrained experimental context: a water tower. We use the data to discuss the influence of atmospheric variability that perturbs the signal, and propose correction formulas to extract the muon flux variations related to the water level changes. Statistical developments establish the feasibility domain of muon radiography monitoring as a function of target thickness (i.e. opacity). Objects with a thickness comprised between ≈50 ± 30 m water equivalent correspond to the best time resolution. Thinner objects have a degraded time resolution that strongly depends on the zenith angle, whereas thicker objects (like volcanoes) time resolution does not.