Petrographic, REE, fluid inclusion and stable isotope study of magnesite from the Upper Triassic Burano Evaporites (Secchia Valley, northern Apennines): contributions from sedimentary, hydrothermal and metasomatic sources

Sparry and microcrystalline magnesite are minor constituents of the Upper Triassic Burano Evaporite Formation of the northern Apennines in Italy. Petrography and geochemistry of magnesite suggest three modes of formation. (1) Evaporitic precipitation of stratified microcrystalline magnesite layers associated with sulfate and carbonate rocks. Most REE are below ICP-MS detection limits. '¹⁸O is +20.2‰ (SMOW) and '¹³C is -2.6‰ (PDB). (2) Hydrothermal infill of Fe-rich (9.78 wt% FeO) lenticular sparry magnesite. This type of magnesite is characterized by very low LREE concentrations, whereas HREEs are relatively high. The fluid inclusion composition is NaCl-MgCl₂-H₂O, salinity is ~30 wt% NaCl equiv., and total homogenization temperatures range from 204-309 °C; '¹⁸O is +17.5‰ and '¹³C is +1‰. (3) The partial or total replacement of dolostones by lenticular sparry magnesite. LREEs are lower in magnesite compared with the partly replaced dolostones. Magnesite yields '¹⁸O and '¹³C compositions of +17.3 to +23.6‰ and +0.5 to +1.4‰, respectively, whereas the partly replaced dolostones yield '¹⁸O and '¹³C values of +25.0 to +26.2 and +1.3 to +1.9, respectively. Complete replacement of dolostones produced massive lenticular sparry magnesite rock containing ooids and axe-head anhydrite relicts; LREEs are depleted compared to unaffected dolostones; '¹⁸O and '¹³C compositions range from +16.4 to +18.4‰ and +0.4 to +0.9‰, respectively. These data and the association between fracture-filling and replacive magnesite suggests a metasomatic system induced by hydrothermal circulation of hot and saline Mg-rich fluids. These processes probably occurred in the Oligocene-Miocene, when the Burano Formation acted as main detachment horizon for the Tuscan Nappe during the greenschist facies metamorphism of the Apuane complex. Thrusting over the Apuane zone produced large scale fluid flow focused at the Tuscan Nappe front. Sources of Mg-rich fluids were metamorphic reactions in the Apuane complex and dissolution of Mg-salts at the thrust front. Considering a maximum tectonic burial depth of 10 km, as inferred from the geometry of the chain, the pressure-corrected temperature of magnesite precipitation (380 to 400 °C) and the calculated fluid composition ('¹⁸O=+13.3ǃ.2‰) are in the range of the published Apuane metamorphic temperatures (300-450 °C) and fluid compositions ('¹⁸O=7-16‰). The results of this study support the hydrothermal-metasomatic model for the formation of sparry magnesite deposits at the expense of dolostone units involved in thrusting and low-grade metamorphism, as proposed for the Northern Graywacke Zone (Alps) and the Eugui deposit (western Pyrenees).     

How does co-registration affect geomorphic change estimates in multi-temporal surveys?

ABSTRACT

High-Resolution Topography (HRT) data sets are becoming increasingly available, improving our ability and opportunities to monitor geomorphic changes through multi-temporal Digital Terrain Models (DTMs). The use of repeated topographic surveys enables inferring the sediment dynamics of hazardous geomorphic processes such as floods, debris flows, and landslides, and allows us to derive important information on the risks often associated with these processes. The topographic surveying platforms, georeferencing systems, and processing tools have seen important developments in the last two decades, in particular Light Detection And Ranging (LiDAR) technology used in Airborne Laser Scanning (ALS) and Terrestrial Laser Scanning (TLS). Moreover, HRT data, produced through these techniques, changed a lot in terms of point cloud density, accuracy and precision over time. Therefore, old “legacy” data sets and recent surveys can often show comparison problems, especially when multi-temporal data are not homogeneous in terms of quality and uncertainties. In this context, data co-registration should be used to guarantee the coherence among multi-temporal surveys, minimizing, on stable areas, the distance between corresponding points acquired at different epochs. Although several studies highlight that this process is fundamental to properly compare multi-temporal DTMs, it is often not addressed in LiDAR post-processing workflows. In this paper we focus on the alignment of multi-temporal surveys in a topographically complex and rugged environment as the Moscardo debris-flow catchment (Eastern Italian Alps), testing various co-registration methods to align multi-temporal ALS point clouds (i.e. years 2003, 2009 and 2013) and the derived DTMs. In particular, we tested the pairwise registration with manual correspondences, the Iterative Closest Point (ICP) algorithm and a mathematical model that allows aligning simultaneously a generic number of point clouds, the so-called Generalized Procrustes Analysis (GPA), also in its GPA-ICP variant. Then, to correct the possible small inaccuracies generated from the gridding interpolation process, a custom-developed DTM co-registration tool (GRD-CoReg) was used to align gridded data. Both alignment phases (i.e. at point cloud and DTM level) proved to be fundamental and allowed us to obtain proper and reliable DTMs of Difference (DoDs), useful to quantify the debris mobilized and to detect the spatial and temporal patterns of catchment-scale erosion and deposition. The consistency of DoDs data was verified through the comparison between the erosion estimate of DoDs and the volumes of debris-flow events measured by the monitoring station close to the Moscardo torrent catchment outlet. The GPA-ICP algorithm followed by the GRD-CoReg tool proved to be the most effective solution for improving DoDs results with a decrease of systematic trend due to vertical and horizontal uncertainties between surveys, especially at steep slopes. The net volume difference (i.e. the sediment output from the catchment) of the 2003–2013 period changed from 3,237,896 m³ to 135,902 m³ in DoDs obtained from not co-registered and co-registered DTMs. The volume of debris flows measured at the catchment outlet during the same time interval amounts to 169,660 m³. The comparison with debris-flow volume measures at the monitoring station shows, therefore, that the DTMs obtained from the co-registration processes generate more reliable DoDs than those obtained from the raw DTMs (without the alignment).     

The effects of mechanical forcing on the mean meridional circulation and transfer properties of the atmosphere

Experiments using a quasi-geostrophic model and the ECMWF T21 spectral model with and without orography are performed to investigate the effects of mechanical forcing on the mean meridional circulation. Results show that mechanical forcing intensifies the horizontal poleward heat flux and redistributes the eddy angular momentum in the vertical, and that this changes significantly the intensity and location of the mean me-ridional circulation centres.It is shown how the mean meridional circulation is set up in such a way to satisfy both the dynamical and thermodynamical transport requirements of the model atmosphere. Whenever external forcing changes the eddy fluxes, the Coriolis torques from the upper horizontal branches of the mean meridional circulations change to balance the extra divergence of eddy momentum flux, and additional adiabatic heating is produced by the vertical branches of the toroids to balance the extra divergence of eddy heat flux. The mean meridional circula-tion is, therefore, confirmed to be very sensitive to mechanical forcing, and can be used as an efficient tool to quantitatively diagnose the adequacy of the orographic representation of numerical forecasting and general cir-culation models.     

Response of a harbour opened to a sea of variable depth

Wave-induced oscillations in a circular harbour with protruding breakwaters and narrow mouth are analytically investigated by using a variational method. The effects of the bottom topography of the contiguous external sea are studied, especially as far as Helmholtz resonance is concerned; it has been found that the amplitude of the resonance peak is quite sensitive to the depth variation, while the related resonance wavenumber is more stable.     

On the stability of equivalent modons

In the framework of the equivalent barotropic dynamics, we show that the kinetic energy and enstrophy a small scale perturbation superimposed on a large scale solitary wave do not grow in time as long as the perturbation scale remains much less than that of the wave. This property is proved to hold for the equivalent modons of Flierl et al. which therefore turn out to be resistant to small scale perturbations. These conclusions appear to explain the numerical results of McWilliams et al. on non divergent modons.     

Santite,a new mineral phase from Larderello,Tuscany

A new borate mineral has been found in a sample from Larderello, mixed with larderellite and lesser amounts of sassolite. The mineral is never well crystallized, but it constantly appears as very small, clear and colourless grains; its refractive indices are: =1.422, =1.435, =1.480. Single-crystal X-ray photographs show that the mineral is orthorombic with cell dimensions: =11.10 Å, b=11.18 Å, c=9.08 Å. X-ray powder diffraction and partial chemical analyses carried out on the whole sample (it being impossible to separate a sufficient quantity of pure material) confirm the foregoing data, according to which the mineral may be considered as the natural equivalent of the synthetic compound KB₅O₈ · 4H₂O. It is likely that it originated from the reaction between potassium-bearing hot solutions and primary deposits of larderellite. It is named for the Tuscan naturalist Giorgio Santi.This new mineral has been approved by the Commission on New Minerals and New Mineral Names of the International Mineralogical Association.This research was supported by Consiglio Nazionale delle Ricerche, Roma.     

Thermal modelling of the Larderello geothermal field (Tuscany,Italy)

We present a 3-D thermal model of the Larderello geothermal field (Tuscany) to evaluate (1) the extent and contribution of the heat transfer mechanisms (conduction vs. convection) at the intermediate-upper crust levels, (2) the variability of the heat and mass fluxes entering from below and (3) the crucial role of the formation permeability. The model, composed by three main layers, considers the upper 10 km of the crust to better constrain the simulations with experimental data from borehole, fluid inclusion studies and hypocentral distributions. Several sets of simulations were carried out with different bottom boundary temperatures and different formation permeabilities for the two deeper layers. The results indicate that the present temperature (T) and pressure distributions in the Larderello field require deep reservoir rocks with higher permeability than the overlying capping units and underlying intermediate crust. Permeability values of 1 mDarcy for the reservoir rocks are enough to allow fluid convection, if the temperature at 10 km depth is as high as 500 ± 50°C. The presence of localized zones with formation permeability 50–100 times higher than the surrounding rocks strongly favours the migration of over-pressurized fluids, which episodically break through the overburden, feeding the presently exploited geothermal fields.     

Fatal landslides in Europe

Landslides are a major hazard causing human and large economic losses worldwide. However, the quantification of fatalities and casualties is highly underestimated and incomplete, thus, the estimation of landslide risk is rather ambitious. Hence, a spatio-temporal distribution of deadly landslides is presented for 27 European countries over the last 20  years (1995–2014). Catastrophic landslides are widely distributed throughout Europe, however, with a great concentration in mountainous areas. In the studied period, a total of 1370 deaths and 784 injuries were reported resulting from 476 landslides. Turkey showed the highest fatalities with 335. An increasing trend of fatal landslides is observed, with a pronounced number of fatalities in the latest period from 2008 to 2014. The latter are mostly triggered by natural extreme events such as storms (i.e., heavy rainfall), earthquakes, and floods and only minor by human activities, such as mining and excavation works. Average economic loss per year in Europe is approximately 4.7 billion Euros. This study serves as baseline information for further risk mapping by integrating deadly landslide locations, local land use data, and will therefore help countries to protect human lives and property.