Diffusion-controlled spherulite growth in obsidian inferred from H<Subscript>2</Subscript>O concentration profiles

Spherulites are spherical clusters of radiating crystals that occur naturally in rhyolitic obsidian. The growth of spherulites requires diffusion and uptake of crystal forming components from the host rhyolite melt or glass, and rejection of non-crystal forming components from the crystallizing region. Water concentration profiles measured by synchrotron-source Fourier transform spectroscopy reveal that water is expelled into the surrounding matrix during spherulite growth, and that it diffuses outward ahead of the advancing crystalline front. We compare these profiles to models of water diffusion in rhyolite to estimate timescales for spherulite growth. Using a diffusion-controlled growth law, we find that spherulites can grow on the order of days to months at temperatures above the glass transition. The diffusion-controlled growth law also accounts for spherulite size distribution, spherulite growth below the glass transition, and why spherulitic glasses are not completely devitrified. An erratum to this article can be found at     

Comparative behavior of Sr, Nd and Hf isotopic systems during fluid-related deformation at middle crust levels

We have carried out a comparative Rb-Sr, Sm-Nd and Lu-Hf isotopic study of a progressively deformed hercynian leucogranite from the French Massif Central, belonging to the La Marche ductile shear zone, in order to investigate the respective perturbation of these geochronometers with fluid induced deformation. The one-meter wide outcrop presents a strongly deformed and mylonitized zone at the center, and an asymmetric deformation pattern with a higher deformation gradient on the northern side of the zone. Ten samples have been carefully collected every 10 cm North and South away from the strongest deformed mylonitic zone. They have been analyzed for a complete major, trace element data set, oxygen isotopes, Rb-Sr, Sm-Nd and Lu-Hf isotopic systematics.We show that most of major and trace elements except SiO₂, alkaline elements (K₂O, Rb), and some metal transition elements (Cu), are progressively depleted with increasing deformation. This depletion includes REE + Y, but also HFS elements (Ti, Hf, Zr, Nb) which are commonly considered as immobile elements during upper level processes. Variations in elemental ratios with deformation, e.g. decrease in LREE/MREE- HREE, Nd/Hf, Th/Sr, increase in Rb/Sr, U/Th and constant Sr/Nd, lead to propose the following order of element mobility: U ? Th > Sr = Nd ? Hf + HREE. We conclude in agreement with previous tectonic and metallogenic studies that trace element patterns across the shear zone result from circulation of oxidizing F-rich hydrothermal fluids associated with deformation. A temperature of the fluid of 470-480 °C can be deduced from the δ¹⁸O equilibrium between quartz-muscovite pairs.Elemental fractionation induces perturbation of the Rb-Sr geochronometer. The well-defined ⁸⁷Rb/⁸⁶Sr-⁸⁷Sr/⁸⁶Sr correlation gives an apparent age of 294 ± 19 Ma, slightly younger than the 323 ± 4 Ma age of leucogranites in this area. This apparent age is interpreted as dating event of intense deformation and fluid circulation associated with mass transfer, and exhumation of the ductile crust shortly after the leucogranite emplacement. Sm-Nd and Lu-Hf isochron-type diagrams do not define any correlation, because of the low fractionated Sm/Nd and Lu/Hf ratios. Isotopic data demonstrate that only the Lu-Hf geochronometer system is not affected by fluid circulation and gives reliable T_DM age (1.29 ± 0.03 Ga) and ε_Hf signatures. By contrast, the Sm-Nd geochronometer system gives erroneous old T_DM ages of 2.84-4 Ga. There is no positive ε_Nd-ε_Hf correlation, because of decreasing ε_Nd values with deformation at constant ε_Hf values. However, ε_Nd-ε_Hf values remain in the broad ε_Nd-ε_Hf terrestrial array, which strongly indicates that fluid-induced fractionation can contribute to the width of the terrestrial array. The strong ε_Hf negative values of the leucogranite are similar to metasedimentary granulitic xenoliths from the French Massif Central and confirm the generation of the leucogranite by several episodes of reworking of the lower crust.     

Mineralisation and structural changes during the initial phase of microbial degradation of pyrogenic plant residues in soil

The microbial recalcitrance of char accumulated after vegetation fires was studied using pyrogenic organic material (PyOM) with increasing degrees of charring, produced from rye grass (Lolium perenne) and pine wood (Pinus sylvestris) at 350 °C under oxic conditions. Solid state ¹³C and ¹⁵N nuclear magnetic resonance (NMR) spectroscopy confirmed increasing aromaticity and the formation of heterocyclic N with prolonged charring. After mixing with a mineral soil, the PyOM was aerobically incubated for 48 days at 30 °C. To account for the input of fresh litter after a fire event, unburnt rye grass residue was added as a co-substrate. The grass-derived PyOM showed the greatest extent of C mineralisation. After 48 days incubation, up to 3.2% of the organic C (OC) was converted to CO₂. More severe thermal alteration resulted in a decrease in the total C mineralisation to 2.5% of OC. In the pine-derived PyOM, only 0.7% and 0.5% of the initial C were mineralised. The co-substrate additions did not enhance PyOM mineralisation during initial degradation. ¹³C NMR spectroscopic analysis indicated structural changes during microbial degradation of the PyOM. Concomitant with a decrease in O-alkyl/alkyl-C, carboxyl/carbonyl C content increased, pointing to oxidation. Only the strongly thermally altered pine PyOM showed a reduction in aromaticity. The small C losses during the experiment indicated conversion of aryl C into other C groups. As revealed by the increase in carboxyl/carbonyl C, this conversion must have included the opening and partial oxidation of aromatic ring structures. Our study demonstrates that plant PyOM can be microbially attacked and mineralised at rates comparable to those for soil organic matter (SOM), so its role as a highly refractory SOM constituent may need re-evaluation.     

New insights into the genesis of volcanic-hosted massive sulfide deposits on the seafloor from numerical modeling studies

Numerical computer simulations have been used to gain insight into the evolution of marine hydrothermal systems and the formation conditions of massive sulfide deposits in ancient and modern submarine volcanic terrains. Simulation results have been used to gain a better understanding of the formation of massive sulfide ore deposits, their location, zonation, size, and occurrence in various geotectonic settings.Most hydrothermal fluid discharging at the seafloor exhibits temperatures ranging from 200 °C to about 410 °C and average fluid discharge velocities of 1 to 2 m/s in agreement with seafloor observations. Mass calculations imply that average massive sulfide deposits may form in ~ 5000 years while giant deposits take longer than 5000 years to accumulate; supergiant deposits either need much longer time to form (> 35,000 years) or at least 100 ppm of metal in solution. Results indicate that supergiant deposits may only form in certain geotectonic environments where longevity and preservation potential of the hydrothermal system are high. An additional process (mineral precipitate cap) is proposed here to explain the zinc content of massive sulfide deposits. This cap would prevent the widespread dissolution of anhydrite and the ‘wash-out’ of zinc by subsequent hydrothermal fluid discharge.     

Microbial reduction of ferrous arsenate: Biogeochemical implications for arsenic mobilization

In reduced aqueous environments, the presence of As in solution is a function of both biotic and abiotic mechanisms. Recent studies have demonstrated a significant release of As(III) through the microbial reduction of dissolved and mineral-bound As(V), which raises health concerns when the greater comparative mobility and toxicity of As(III) is considered. These release mechanisms do not operate in isolation but occur in concert with a number of removal processes, including secondary mineralization and sorption to other natural substrates. Thermodynamic and applied experimental studies have shown that ferrous arsenates, such as symplesite [Fe(II)₃(As(V)O₄)₂·8H₂O], may provide a significant sink for Fe(II) and As(V). In this study, the stability of a representative ferrous arsenate phase in the presence of the arsenate-reducing bacterium Shewanella sp. strain ANA-3 is examined. The reduction of ferrous arsenate by ANA-3 results in the release of aqueous As(III) and, subsequently, the progressive nucleation of a biogenic ferrous arsenite phase proximal to the microbial cells. The valence states of secondary solid-phase products were verified using X-ray absorption spectroscopy (XAS). Electron microscopy reveals that nucleation occurs on cellular exudates which may imply a role of extracellular reduction through c-type cytochromes as investigated in recent literature. These observations provide new insights into the reduction mechanisms of ANA-3 and the biogeochemical cycling of As(III) in natural systems.     

Roadheader performance prediction using genetic programming (GP) and gene expression programming (GEP) techniques

Roadheading machines play a vital role in excavation operation in tunneling and mining industries notably when selective mining is required. Roadheaders are more effective in soft to medium rock formations due to a higher cutting rate in such strata. A precise prediction of machine’s performance is a crucial issue, as it has considerable effects on excavation planning, project’s cost estimation, machine specification selection as well as safety of the project. In this research, a database of machine performance and some geomechanical parameters of rock formations from Tabas coal mine project, the largest and fully mechanized coal mine in Iran, has been established, including instantaneous cutting rate (ICR), uniaxial compressive strength, Brazilian tensile strength, rock quality designation, influence of discontinuity orientation (Alpha angle) and specific energy. Afterward, the parameters were analyzed through genetic programming (GP) and gene expression programming (GEP) approaches to yield more accurate models to predict the performance of roadheaders. As statistical indices, coefficient of determination, root mean square error and variance account were used to evaluate the efficiency of the developed models. According to the obtained results, it was observed that developed models can effectively be implemented for prediction of roadheader performance. Moreover, it was concluded that performance of the GEP model is better than the GP model. A high conformity was observed between predicted and measured roadheader ICR for GEP model.     

Late Cretaceous (Cenomanian-Turonian) ammonites from southern Morocco and south western Algeria

On the basis of nine lithostratigraphical profiles, 23 cephalopods taxa (nautilus and ammonites) are described from the Preafrican Trough and the Kem Kem region. Among them, a new species is proposed: Spathites (Jeanrogericeras) asflaensis nov. sp. This fauna is placed in its biostratigraphical framework and correlated with the standard zonation of the late Cenomanian–early Turonian. Moreover, stratigraphic correlations are proposed for the whole Maghreb from the Tarfaya Basin in the west to central Tunisia in the east. Selected taxa reveal the paleogeographical context of the western Tethys; a complex distribution of emerged areas and epicontinental seas impacted by an important marine trangression that constantly modified the costaline.     

Topical Collection: Groundwater-based agriculture in the Mediterranean

This essay introduces a collection of articles that explore the future of groundwater-based agriculture in the Mediterranean from an interdisciplinary perspective, in a context of declining water tables due to intensive groundwater use. The imminent crisis that many groundwater economies face due to very rapid and intense global change may have severe irreversible social, economic and environmental consequences, but could also be the opportunity to make a clear break with current agricultural development models and move towards more sustainable agricultural practices. The Mediterranean region is, therefore, an interesting case for the future of intensive groundwater use, as innovative ideas and practices may emerge and inspire similar groundwater-based agricultural systems around the world.     

Anthropization of groundwater resources in the Mediterranean region: processes and challenges

A comprehensive overview is provided of processes and challenges related to Mediterranean groundwater resources and associated changes in recent decades. While most studies are focused thematically and/or geographically, this paper addresses different stages of groundwater exploitation in the region and their consequences. Examples emphasize the complex interactions between the physical and social dimensions of uses and evolution of groundwater. In natural conditions, Mediterranean groundwater resources represent a wide range of hydrogeological contexts, recharge conditions and rates of exploitation. They have been actively exploited for millennia but their pseudo-natural regimes have been considerably modified in the last 50 years, especially to satisfy agricultural demand (80% of total water consumption in North Africa), as well as for tourism and coastal cities. Climate variability affects groundwater dynamics but the various forms of anthropization are more important drivers of hydrological change, including changes in land use and vegetation, hydraulic works, and intense pumpings. These changes affect both the quantity and quality of groundwater at different scales, and modify the nature of hydrogeological processes, their location, timing, and intensity. The frequent cases of drastic overexploitation illustrate the fragility of Mediterranean groundwater resources and the limits of present forms of management. There is no easy way to maintain or recover sustainability, which is often threatened by short-term interests. To achieve this goal, a significant improvement in hydrogeological knowledge and closer collaboration between the various disciplines of water sciences are indispensable.     

Mud‐rich delta‐scale compound clinoforms in the Triassic shelf of northern Pangea (Havert Formation,south‐western Barents Sea)

In recent years it has become clear that many shallow‐marine heterolithic and mudstone‐dominated successions are deposited as mud belts forming part of subaqueous deltas that are related to major fluvial sources either upstream or along shore. Here the Havert Formation is presented as an ancient example of this kind of system. The Havert Formation in the south‐western Barents Sea represents shelf margin clinoforms consisting predominantly of heterolithic deposits. Sediments were mainly derived from the east (Ural Mountains), but a smaller system prograded northward from Fennoscandia. The Havert Formation holds a lot of interest due to: (i) its stratigraphic position, directly above the Permo–Triassic boundary and contemporaneous to the emplacement of the Siberian Traps; (ii) the fact that it represents the first siliciclastic input in the south‐western Barents Sea and it shows interaction between Uralian‐derived and Fennoscandian‐derived sediments; and (iii) its hydrocarbon potential. This study is focused on a detailed sedimentological analysis of cored intervals of the (Ural‐derived) Havert Formation, in combination with seismic interpretation, well‐log correlations and palynological analysis of the Havert and overlying Klappmyss formations. The cored intervals belong to the shelf environment of the Havert shelf‐margin clinoforms (300 to 500 m thick). This sedimentological analysis distinguishes six facies associations, spanning from tidally‐influenced channels at the shoreline to mud‐rich subaqueous platform and foresets of the subaqueous delta. Seismic lines and well‐log correlations show the larger‐scale evolution of the Ural‐derived Havert Formation, characterized by episodes of low‐accommodation and high‐accommodation. The palynological analyses provide the first detailed study of the Havert Formation in the Nordkapp Basin, revising its depositional age in the region as Induan to early Olenekian (Smithian). Furthermore, they strengthen the environmental interpretation; palynofacies present on the shelf record flora of tidally‐influenced coastal plains, whereas the palynofacies in the deep‐water slope contain only amorphous organic matter.