What is the source of baseflow in agriculturally fragmented catchments? Complex groundwater/surface‐water interactions in three tributary catchments of the Wabash River,Indiana, USA

Some conceptual models suggest that baseflow in agriculturally fragmented watersheds may contain little, if any, groundwater. This has critical implications for stream quality and ecosystem functioning. Here, we (a) identify the sources and flowpaths contributing to baseflow using ²²²Rn and ⁸⁷Sr/⁸⁶Sr and (b) quantify mean apparent ages of groundwater and baseflow using multiple isotopic tracers (CFC, SF₆, ³⁶Cl, and ³H) in 4 small (0.08 to 0.64 km²) tributary catchments to the Wabash River in Indiana, USA. ²²²Rn activities and ⁸⁷Sr/⁸⁶Sr ratios indicate that baseflow in 3 catchments is sourced primarily from groundwater; baseflow in the fourth is dominated by a source similar to agricultural run‐off. CFC‐12 data indicate that springs in 1 catchment are discharging significant proportions of water that recharged between 1974 (42 ± 2 years) and 1961 (55 ± 2 years). Those same springs have ³⁶Cl/Cl ratios between 1,381.08 ± 29.37 (×10^?15) and 1,530.64 ± 27.65 (×10^?15) indicating that a substantial proportion of the discharge likely recharged between 1975 (41 years) and 1950 (66 years). Groundwater samples collected from streambed mini‐piezometers in a separate catchment have CFC‐12 concentrations indicating that a large proportion of the recharge occurred between 1948 (68 ± 2 years) and 1950 (66 ± 2 years). Repeat sampling conducted in September 2015 after above‐average summer rainfall did not show significant decreases in mean apparent age. The relatively old ages observed in 3 of the catchments can be explained by geological complexities that are likely present in all 4 catchments, but overwhelmed by flow from the shallow phreatic aquifer in the fourth catchment.     

Découverte d'un diatrème de kimberlite diamantifère à Séguéla en Côte-d'Ivoire

The Séguéla area in Ivory Coast is known for its diamond-bearing field related to dykes of kimberlite and lamproite. These dykes, devoid of any deformation and metamorphism, crosscut the Birimian formations. Their N 170° orientation is controlled by the tectonized contact between the Archean and the Paleoproterozoic shields of the West-African craton. Discovery of a diatreme, with its pipe breccias and well-preserved maar sediments, below the present-day colluvia, attests for the probable recent geological age (Cretaceous?) of the kimberlitic activity. To cite this article: A. Pouclet et al., C. R. Geoscience 336 (2004).     

An experimental study of Fe–Ni exchange between sulfide melt and olivine at upper mantle conditions: implications for mantle sulfide compositions and phase equilibria

The behavior of nickel in the Earth’s mantle is controlled by sulfide melt–olivine reaction. Prior to this study, experiments were carried out at low pressures with narrow range of Ni/Fe in sulfide melt. As the mantle becomes more reduced with depth, experiments at comparable conditions provide an assessment of the effect of pressure at low-oxygen fugacity conditions. In this study, we constrain the Fe–Ni composition of molten sulfide in the Earth’s upper mantle via sulfide melt–olivine reaction experiments at 2 GPa, 1200 and 1400 °C, with sulfide melt \(X_{{{\text{Ni}}}}^{{{\text{Sulfide}}}}=\frac{{{\text{Ni}}}}{{{\text{Ni}}+{\text{Fe}}}}\) (atomic ratio) ranging from 0 to 0.94. To verify the approach to equilibrium and to explore the effect of \({f_{{{\text{O}}_{\text{2}}}}}\) on Fe–Ni exchange between phases, four different suites of experiments were conducted, varying in their experimental geometry and initial composition. Effects of Ni secondary fluorescence on olivine analyses were corrected using the PENELOPE algorithm (Baró et al., Nucl Instrum Methods Phys Res B 100:31–46, 1995), “zero time” experiments, and measurements before and after dissolution of surrounding sulfides. Oxygen fugacities in the experiments, estimated from the measured O contents of sulfide melts and from the compositions of coexisting olivines, were 3.0?±?1.0 log units more reduced than the fayalite–magnetite-quartz (FMQ) buffer (suite 1, 2 and 3), and FMQ ??1 or more oxidized (suite 4). For the reduced (suites 1–3) experiments, Fe–Ni distribution coefficients \(K_{{\text{D}}}^{{}}=\frac{{(X_{{{\text{Ni}}}}^{{{\text{sulfide}}}}/X_{{{\text{Fe}}}}^{{{\text{sulfide}}}})}}{{(X_{{{\text{Ni}}}}^{{{\text{olivine}}}}/X_{{{\text{Fe}}}}^{{{\text{olivine}}}})}}\) are small, averaging 10.0?±?5.7, with little variation as a function of total Ni content. More oxidized experiments (suite 4) give larger values of K_D (21.1–25.2). Compared to previous determinations at 100 kPa, values of K_D from this study are chiefly lower, in large part owing to the more reduced conditions of the experiments. The observed difference does not seem attributable to differences in temperature and pressure between experimental studies. It may be related in part to the effects of metal/sulfur ratio in sulfide melt. Application of these results to the composition of molten sulfide in peridotite indicates that compositions are intermediate in composition (\(X_{{{\text{Ni}}}}^{{{\text{sulfide}}}}\)?~?0.4–0.6) in the shallow mantle at 50 km, becomes more Ni rich with depth as the O content of the melt diminishes, reaching a maximum (0.6–0.7) at depths near 80–120 km, and then becomes more Fe rich in the deeper mantle where conditions are more reduced, approaching (\(X_{{{\text{Ni}}}}^{{{\text{sulfide}}}}\)?~?0.28)?>?140 km depth. Because Ni-rich sulfide in the shallow upper mantle melts at lower temperature than more Fe-rich compositions, mantle sulfide is likely molten in much of the deep continental lithosphere, including regions of diamond formation.     

Toward a rapid probabilistic seismic vulnerability assessment using satellite and ground-based remote sensing

Natural hazards such as earthquakes threaten millions of people all around the world. In a few decades, most of these people will live in fast-growing, inter-connected urban environments. Assessing risk will, therefore, be an increasingly difficult task that will require new, multidisciplinary approaches to be tackled properly. We propose a novel approach based on different imaging technologies and a Bayesian information integration scheme to characterize exposure and vulnerability models, which are among the key components of seismic risk assessment.     

Importance of the oxidation/maturation pair in the evolution of humic coals

The natural thermal evolution of type III coals (Humic origin) is expressed during diagenesis by a loss of oxygen as CO₂ and H₂O. Other phenomena such as oxidation can cause extensive geochemical modifications and may complicate the effects of simple maturation.Humic coals from the Jurassic in southeastern Utah were studied by elemental analysis, Rock-Eval pyrolysis and infrared spectroscopy. In a van Krevelen diagram (atomic H/C vs atomic O/C), the samples fall within the envelope defined by 860 reference humic coals covering the entire range of diagenesis. Nevertheless, various criteria (geochemical, petrographic, geological and microscopic) cast doubt upon the interpretation that such a distribution of coal composition results from thermal maturation.The same criteria indicate the intervention of redox phenomena. Comparison of our results with those from artificial and natural oxidation shows that these coals were subjected to an oxidation process different from ordinary late alteration. This process was probably due to circulation of highly oxidizing saline water causing oxygen fixation and the transformation of carboxyls into carboxylate anions. The cations that were fixed are oxygenated and certainly contain calcium, but also uranium and perhaps several other cations (V, Mo, Fe...). Emphasis is placed on possible mechanisms that cause such phenomena.     

North-south transfer zones and paleo-morphological reconstruction of the Xylokastro area (Corinth Gulf, Greece)

A large number of studies have documented the structural and sedimentary architecture of the Corinth Gulf, especially the major E–W trending normal faults important in accommodating the main extensional strain and associated growth of the Gilbert fan deltas. However, the role of several N–S oriented structures that crop out at the surface and are detectable at depth by seismic reflection and gravity surveys are not clearly understood. Based on new geological data, collected near Xylokastro, we describe the N–S oriented tectonic activity of this area. The mapped inland faults correspond to off-shore structures identified in re-interpreted seismic lines and appear to have acted as a major transfer zone during the opening of the Corinth Gulf. Reconstruction of the paleo-topography based on sedimentary facies and the environment of clay mineral formation suggest these transfer zones played an important role in controlling both local structural relief and depositional conditions of the Gilbert fan deltas.     

Equilibrium interface segregation in the diopside-forsterite system II: Applications of interface enrichment to mantle geochemistry

Segregation of incompatible elements at grain interfaces may have considerable influence on the physical and chemical properties of mantle rocks. Using a recently developed predictive model to estimate the interface enrichment of elements based on their mineral/melt partitioning (Hiraga and Kohlstedt, companion paper), we consider interface enrichment for a simplified model peridotite consisting of olivine, orthopyroxene, and clinopyroxene. Our calculated results reveal the following: (1) Significant amounts of heavy alkali elements and rare gases likely reside at grain-grain interfaces, whereas interface concentrations of less incompatible are less pronounced. (2) The contribution of the chemical components stored at interfaces to whole-rock chemistry strongly depends on mineral mode and, most importantly, on grain size. (3) Grain size reduction resulting from dynamic recrystallization can increase the total storage of highly incompatible elements on grain interfaces and thereby will diminish their concentration in mineral grains. (4) Analysis of Cs concentrations in mantle clinopyroxenes potentially provides estimates of the grain size of mantle rocks. (5) Transport through peridotite will be dominated by diffusion along interfaces rather than through grain interiors for elements less compatible than Lu.     

Absarokites from the western Mexican Volcanic Belt: constraints on mantle wedge conditions

We have investigated the near liquidus phase relations of a primitive absarokite from the Mascota region in western Mexico. Sample M.102 contains ~11.6 wt% MgO, Mg#=0.73 and the lava contains Fo₉₀ olivine phenocrysts, indicating near equilibrium with the mantle. High-pressure experiments on a synthetic analogue of the absarokite composition containing low and high H₂O abundances of (~2 and ~5 wt%, respectively) were performed in a piston cylinder apparatus over the pressure range of 1.2 to 2.0 GPa. The composition containing ~2 wt% H₂O is multiply saturated with olivine and orthopyroxene at 1.6 GPa and 1,400 °C. At the same pressure, clinopyroxene appears 30 °C below the liquidus. At an H₂O content of ~5 wt% the multiple saturation with olivine and orthopyroxene occurs at 1.7 GPa and 1,300 °C. Assuming a batch-melting process, we suggest that the primitive absarokite was segregated from a depleted lherzolite or harzburgite residue at ~50 km, placing the depth of origin well within the mantle wedge beneath the Jalisco Block. A low degree (<5 %wt%) batch-melt of an original metasomatized depleted lherzolite or harzburgite source would contain the observed trace element abundances found in M.102. The liquidus phase relations are not consistent with the presence of non-peridotitic veins at the depth of last equilibration. Therefore, we propose that the Mascota absarokites segregated at an apparent melt fraction of less than 5% from a depleted peridotitic source. Melting first began at a greater depth as a small degree H₂O- and trace element- rich melt of a metasomatized peridotite that ascended into the overlying wedge and re-equilibrated with shallower, hotter mantle.Editorial responsibility: J. Hoefs