Michael Petronis

Magma emplacement into the Lemptégy scoria cone (Chaîne Des Puys, France) explored with structural, anisotropy of magnetic susceptibility, and Paleomagnetic data

The Lemptégy volcano is a small monogenetic scoria cone located in the Chaîne des Puys, Auvergne, France, which erupted about 32,000xa0years ago. A first edifice (Lemptégy 1) formed during a trachybasalt eruption as a group of satellite vents of the Puy de Gouttes scoria cone. A second trachyandesitic edifice (Lemptégy 2) formed soon after and completely covered Lemptégy 1 with an 80-m-high breached cone. Since 1946, the Lemptégy volcano has been quarried for scoria and today offers unprecedented three-dimensional exposure of the subvolcanic plumbing system. To map the internal flow architecture of the plumbing system and to study the subvolcanic deformation of Lemptégy 2, structural mapping, petrographic observations, anisotropy of magnetic susceptibility (AMS), rock magnetic, and paleomagnetic data were collected. Field structural mapping and thin section study of tension gashes, Riedel shears, striations as well as ductile shear zones and bubbles allow the direction and sense of the magma flow to be determined. Twenty AMS sites were established in ten dikes (one to four sites in each dike) with 504 specimens analyzed and 479 specimens used to infer magma flow patterns. Structural data, the maximum susceptibility axis (K1), and the imbrication of the magnetic foliation (K1–K2) planes indicate both upward and downward sense of flow, as well as flow toward and away from the central vent. Rock magnetic experiments reveal that a cubic Fe–Ti oxide phase, likely low-Ti titanomagnetite, is the principal magnetic phase carrying both the remanence and anisotropy. Paleomagnetic data from some sites yield statistically distinct, at the 95xa0% confidence level, remanence directions while at other sites the data are indistinguishable at the 95xa0% confidence level. The paleomagnetic results, observed steeply tilted scoria layers, internal unconformities, and faults show that as each dike was emplaced, it displaced earlier dikes evidencing subvolcanic deformation. The Lemptégy 2 volcano shares similarities in terms of inferred eruption style and structures with other scoria cones, such as Cerro Negro (Nicaragua), and thus provides an excellent field laboratory to investigate active scoria cones world-wide.

Dykes and structures of the NE rift of Tenerife, Canary Islands: a record of stabilisation and destabilisation of ocean island rift zones

Many oceanic island rift zones are associated with lateral sector collapses, and several models have been proposed to explain this link. The North–East Rift Zone (NERZ) of Tenerife Island, Spain offers an opportunity to explore this relationship, as three successive collapses are located on both sides of the rift. We have carried out a systematic and detailed mapping campaign on the rift zone, including analysis of about 400 dykes. We recorded dyke morphology, thickness, composition, internal textural features and orientation to provide a catalogue of the characteristics of rift zone dykes. Dykes were intruded along the rift, but also radiate from several nodes along the rift and form en échelon sets along the walls of collapse scars. A striking characteristic of the dykes along the collapse scars is that they dip away from rift or embayment axes and are oblique to the collapse walls. This dyke pattern is consistent with the lateral spreading of the sectors long before the collapse events. The slump sides would create the necessary strike-slip movement to promote en échelon dyke patterns. The spreading flank would probably involve a basal decollement. Lateral flank spreading could have been generated by the intense intrusive activity along the rift but sectorial spreading in turn focused intrusive activity and allowed the development of deep intra-volcanic intrusive complexes. With continued magma supply, spreading caused temporary stabilisation of the rift by reducing slopes and relaxing stress. However, as magmatic intrusion persisted, a critical point was reached, beyond which further intrusion led to large-scale flank failure and sector collapse. During the early stages of growth, the rift could have been influenced by regional stress/strain fields and by pre-existing oceanic structures, but its later and mature development probably depended largely on the local volcanic and magmatic stress/strain fields that are effectively controlled by the rift zone growth, the intrusive complex development, the flank creep, the speed of flank deformation and the associated changes in topography. Using different approaches, a similar rift evolution has been proposed in volcanic oceanic islands elsewhere, showing that this model likely reflects a general and widespread process. This study, however, shows that the idea that dykes orient simply parallel to the rift or to the collapse scar walls is too simple; instead, a dynamic interplay between external factors (e.g. collapse, erosion) and internal forces (e.g. intrusions) is envisaged. This model thus provides a geological framework to understand the evolution of the NERZ and may help to predict developments in similar oceanic volcanoes elsewhere.

Structural regulation of luminescent and mangetic properties of MOFS

Two large series of metal organic frameworks (MOFs) were synthesised and structurally characterized. The first series represents anionic frameworks constructed from Zn2+ with 1,3,5-benzenetricarboxylate (BTC) anions and amino-cations. An influence of various cations such as NH4+, MeNH3+, Me2NH2+, Et3NH+, and n-Bu4N+ was charactrized by the structural changes of the anionic Zn-BTC connectivity within the frameworks, and the fluorescence of the corresponding MOFs. The 13 ZnBTC MOFs with 1, 2 and 3 dimensional structures were obtained and it was clearly demonstrated that cations are acting as structure directing agents. The second series represents MOFs based on the same cationic-anionic structure {Zn-BTC}{ Me2NH2+} that was doped with different metals (Co, Cu, Ni, Mn, Ca, Mg and Gd). The X-ray diffraction studies did not reveal strutural differences between doped materials, however their luminescent and magnetic properties were found to be different that might lead to their potential practical applications.