The White Angel: A unique wollastonite‐bearing,mass‐fractionated refractory inclusion from the Leoville CV3 carbonaceous chondrite

Abstract— We report the study of an unusual compact type A refractory inclusion, named the White Angel, from the Leoville CV3 meteorite. The petrologic, mineral chemical, isotopic, and trace‐element signatures of this once‐molten Ca‐Al‐rich inclusion (CAI), which contains large, equant wollastonite crystals, indicate a short multistage history that occurred very early, before substantial decay of ²⁶Al. Magnesium in the inclusion is isotopically heavy, with F_Mg reaching 18‰/amu, in the range of fractionated and with unidentified nuclear effects (FUN) inclusions. However, the absence of any nuclear anomalies in Ca and Ti and an inferred ²⁶Al/²⁷Al ratio of (5.5 ± 0.9) × 10^?5 indicate that the White Angel belongs to the F inclusions. Silicon and oxygen are also mass fractionated in favor of the heavy isotopes, but to a lesser extent. The O isotopes show a range in ¹⁶O excesses. On an O three‐isotope plot, data points lie on a line parallel and to the right of the carbonaceous chondrite anhydrous mineral mixing line, with wollastonite being the most ¹⁶O‐rich phase. The chondrite‐normalized rare earth and trace‐element pattern of the whole inclusion is the complement of an ultrarefractory pattern indicating that precursor phases of the CAI must have condensed in an Al‐, heavy rare earth element (HREE)‐depleted reservoir. Melting of those precursor phases in an ¹⁶O‐rich environment and evaporation led to mass‐dependent isotopic fractionation of Mg, Si, and O. Partial isotopic exchange with a reservoir containing unfractionated Mg took place at a later stage but before any measurable decay of ²⁶Al. Some minerals (melilite and perovskite) in the White Angel equilibrated oxygen isotopes with a relatively ¹⁶O‐poor reservoir that was also mass‐fractionated toward the heavy isotopes, different from that with which the normal or FUN inclusions interacted.     

Heterogeneous initial Sr isotope compositions of highly evolved volcanic rocks from the Main Ethiopian Rift,Ethiopia

Many individual mineral Sr isotope studies have revealed the complex evolution of highly evolved rocks in upper crustal magmatic systems, casting doubts on the meaning of whole-rock Sr isotopes in such samples. In this paper, whole-rock Sr isotope measurements were replicated (three to 13 times) on six highly evolved peralkaline rhyolites from the Main Ethiopian Rift (MER) to appraise their internal heterogeneity and, thus, the significance of such data. These rocks were all fresh samples of pumice, obsidian and lava. Their maximum Sr contents and ages were 15 μg/g and 1.7 Ma, respectively. Significant small-scale heterogeneities of both Sr isotopes and Rb and Sr contents were observed in most samples, although not necessarily associated with petrological characteristics suggesting possible inheritance processes. Only two, almost crystal-free, obsidian give fairly homogeneous Sr isotope ratios. These results outline the ambiguity of a single whole-rock Sr isotope determination on highly evolved peralkaline rocks, especially when no simultaneous accurate determination of the Rb/Sr is performed. They also suggest that the limitations of Sr whole-rock analyses are not restricted to phenocryst-rich samples as phenocryst-poor obsidian and almost aphyric pumices and lava are also concerned. These data further underscore that unreplicated whole-rock Sr isotope measurements should always be used with great caution in the petrogenetic modeling of highly evolved rocks. However, multiple determinations of whole rock ⁸⁷Sr/⁸⁶Sr in the same samples, combined with other geochemical and isotopic data, may provide constraints on the shallow level evolution of these magmas. It is suggested that selective upper crustal contamination and/or interactions with halogen-bearing hydrous fluids, typical of evolved peralkaline magmas, were probably involved in the late magmatic evolution of these MER rhyolites. The more pervasive character of fluid interaction processes would probably better account for the small-scale association of uncontaminated and contaminated signatures in a single sample. Thus, even fresh samples may have their Rb–Sr isotopic system significantly modified by fluid interactions, not as a secondary process but at the late magmatic stage.     

Analysis of the Microseismicity Induced by Fluid Injections at the EGS Site of Soultz-sous-Forêts (Alsace, France): Implications for the Characterization of the Geothermal Reservoir Properties

In June/July 2000, a hydraulic stimulation experiment took place at the geothermal EGS site of Soultz-sous-Forêts (Alsace, France) in order to enhance the permeability of the fractured granitic massif at 5 km depth. As it is well known that fluid injections tend to induce microseismic events, a downhole and a surface seismological network have been installed to monitor the seismic activity during the stimulation test. 23400 m³ of fluid have been injected in the rock volume through the open-hole section (4400 m–5000 m) of the well GPK2 at increasing rates of 30 l.s⁻¹, 40 l.s⁻¹ and then 50 l.s⁻¹. More than 7200 microseismic events in the magnitude range –0.9 to 2.6 have been precisely located through a simultaneous inversion of the seismic velocity structure and location parameters. The analysis of the behavior of the seismicity relative to the hydraulic parameters gives important information about the geothermal reservoir. It appears that the evolution of the seismicity strongly depends on the variations of the injection rate: An increase or a decrease leads to changes of the velocity structure, the number and magnitude of microseismic events. This involves different hydro-mechanical processes between the fluid flow and the fracture planes, which will control the final shape of the microseismic cloud. Moreover, the study of the variations of the b-value with time suggests that the stimulation experiment produces a large proportion of small earthquakes, but records of events of magnitude higher than 2 indicate that fluid injection could reactivate structures whose dimensions allow the generation of such earthquakes.     

Stream Temperature Modeling and Fiber Optic Temperature Sensing to Characterize Groundwater Discharge

The Ngongotaha Stream was used as a case study to assess the applicability of fiber optic distributed temperature sensing (FODTS) to identify the location of springs and quantify their discharge. Thirteen springs were identified, mostly located within a 115 m reach, five discharged from the right bank and eight from the left bank. To quantify groundwater discharge, a new approach was developed in which the one-dimensional transient heat transport model was fitted to the FODTS measurements, where the main calibration parameters of interest were the unknown spring discharges. The spatial disposition of the groundwater discharge estimation problem was constrained by two sources of information; first, the stream gains ∼500 L/s as determined by streamflow gauging. Second, the temperature profiles of the left and right banks provide the spatial disposition of springs and their relative discharges. FODTS was used to measure stream temperature near the left and right banks, which created two temperature datasets. A weighted average of the two datasets was then calculated, where the weights reflected the degree of mixing between the right and left banks downstream of a spring. The new approach in this study marks a departure from previous studies, in which the general approach was to use the steady-state thermal mixing model (Selker et al. 2006a; Westhoff et al. 2007; Briggs et al. 2012) to infer groundwater discharge, which is then used as an input into a transient model of the general form of equation to simulate stream temperature (Westhoff et al. 2007).     

Consequences of Groundwater-Model Vertical Discretization in Risk-Based Decision-Making

One of the first and most important decisions facing practitioners when constructing a numerical groundwater model is vertical discretization. Several factors will influence this decision, such as the conceptual model of the system and hydrostratigraphy, data availability, resulting computational burden, and the purpose of the modeling analysis. Using a coarse vertical discretization is an attractive option for practitioners because it reduces data requirements and model construction efforts, can increase model stability, and can reduce computational demand. However, using a coarse vertical discretization as a form of model simplification is not without consequence; this may give rise to unwanted side-effects such as biases in decision-relevant simulated outputs. Given its foundational role in the modeled representation of the aquifer system, herein we investigate how vertical discretization may affect decision-relevant simulated outputs using a paired complex-simple model analysis. A Bayesian framework and decision analysis approach are adopted. Two case studies are considered, one of a synthetic, linked unsaturated-zone/surface-water/groundwater hydrologic model and one of a real-world linked surface-water/groundwater hydrologic-nitrate transport model. With these models, we analyze decisions related to abstraction-induced changes in ecologically important streamflow characteristics and differences in groundwater and surface-water nitrate concentrations and mass loads following potential land-use change. We show that for some decision-relevant simulated outputs, coarse vertical discretization induces bias in important simulated outputs, and can lead to incorrect resource management action. For others, a coarse vertical discretization has little or no consequence for resource management decision-making.     

Determining flow patterns and emplacement dynamics from tsunami deposits with no visible sedimentary structure

In the absence of eyewitness reports or clear sedimentary structures, it can be difficult to interpret tsunami deposits or reconstruct tsunami inundation patterns. The emplacement dynamics of two historical tsunami deposits were investigated at seven transects in Okains Bay, New Zealand, using a combined geospatial, geomagnetic and sedimentological approach. The tsunami deposits are present as layers of sand and silt intercalated between soils and become finer and thinner with distance inland. The deposits are attributed to the 1960 and possibly the 1868 tsunamis, based on radiometric dating and correlation with historical records. Measurements of Magnetic Fabric (MF: Anisotropy of Magnetic Susceptibility) and particle size were used to reconstruct the evolution of flow dynamics laterally and vertically. A combination of statistical methods, including spatial autocorrelation testing, Spearman's rank order correlation, Principal Component Analysis (PCA) and K‐means cluster analysis, was applied to examine relationships between MF parameters and sediment texture, and infer depositional hydrodynamics. Flow patterns deduced from MF show the estuary channel acted as a conduit for inundation, with flow commonly aligned sub‐perpendicular to the estuary bed. MF and sediment data suggest deposition occurred from settling during laminar flow. Evidence of both uprush and backwash deposition, as well as wave reflection from infrastructure, was found. Statistical analysis of data showed significant relationships between grain size parameters and MF parameters associated with flow speed and magnetic fabric type. PCA and cluster analysis differentiated samples into two primary hydrodynamic groups: (1) samples deposited from laminar flow; and (2) samples deposited close to the limit of inundation, which includes samples deposited further inland, those affected by flow convergence, and those in the upper part of tsunami deposits. This approach has potential as a tool for reconstructing hydrodynamic conditions for palaeotsunamis and by combining spatial and statistical analyses, large‐scale investigations can be more easily performed. Copyright © 2016 John Wiley & Sons, Ltd.     

L'ouverture et le développement du bassin de Morondava (Madagascar) du Carbonifère supérieur au Jurassique moyen. Données stratigraphiques, sédimentaires, paléontologiques et structurales

In the western part of Madagascar, the Morondava Basin shows the Malagasy Karoo series, made of Late Carboniferous-Mid-Permian (Sakoa), Late Permian-Mid-Triassic (Sakamena) and Late Triassic-Mid-Jurassic (Isalo) sequences. The sedimentary facies are mainly aerial and clastic in the series, and the marine conditions are fully established after Lower Jurassic times, when the strait between Africa and Madagascar was flooded.The Karoo basins where these series were deposited are mainly hemi-grabens. Their filling proceeded from west to east and from south to north. Distinction between the southern and northern part of the Morondava Basin suggests that development of the basin was controlled by old crustal weakness zones trending north-northwest-south-southeast and north-northeast-south-southwest.